M e mo t o D e s ig n e r s 7-1 * J u n e 1994

Bridge Bearings

Introduction
AASHTO defines a bearing as “a structural device that transmits loads while
facilitating translation and/or rotation ” . 1 In the past Caltrans has used a variety of
bearings with varying degrees of success. These include rockers, rollers, pins, pots,
steel girder hangers, PTFE/elastomeric, and elastomeric pads. Of all the bearings
mentioned, the reinforced elastomeric bearing (introduced in 1955) has been the most
widely used the past four decades.

As design trends have shifted toward designs that favor structures with longer frames
and fewer joints for seismic reasons, not to mention the widespread use of curved and
skewed bridges, the demands on bearings have increased. Provisions must be made
for large longitudinal displacements due to temperature, prestress shortening, shrink-
age, creep and seismic activities, as well as rotations produced by changes in camber,
live load, and misalignment of bearing seats due to construction tolerances. In short,
Designers need a selection of bearing types to handle varying demands.

Increased demands on bearings have led to the development of “new” bearings (post
World War II). Improvements in engineering materials, particularly plastics and
elastomers are largely responsible for the innovative designs and refinements made
in the past three decades. The three “new” bearing types most widely used today in
the United States are pot, spherical and disk. Collectively, these bearings are known
as High Load Multi-Rotational bearings. Of the three bearing systems mentioned,
spherical bearings have the greatest rotation capacity and most trouble-free mainte-
nance record. Pot bearings have been troublesome in the past and are still not
considered trouble-free. Disk bearings on the other hand have fewer documented
failures than pot bearings; however, up until 1992 they were a patented system made
by a single manufacturer. In addition to the three bearings mentioned above, Caltrans
has used PTFE/elastomeric bearings on several structures with large longitudinal
displacements.

Supersedes Memo to Designers 7-1 dated November 1989

Br id g e Bea ri ng s Pag e 1
 M e mo t o D e s ig n e r s 7-1 « J u n e 1994

Bearing Selection
Bearing selection is influenced by many factors such as loads, geometry, mainte-
nance, available clearance, displacement, rotation, deflection, availability, policy,
designer preference, construction tolerances and cost. The designer must consider all
the applicable variables early in the design stage and design the structure and bearing
as a unified system. Too often bearings are selected at the last minute when forces and
available space are fixed. Such an approach increases the chances of future mainte-
nance problems.

The official policy of the Division of Structures is to avoid using an alternative

bearing system where a conventional reinforced elastomeric pad can provide the
required characteristics through shear deformation. When the practical limits of
elastomeric bearing pads are exceeded, designers should consider using PTFE/
spherical or PTFE/elastomeric bearings. The three bearing systems mentioned
should provide enough versatility to satisfy the design requirements of most struc-
tures designed by the Division of Structures. Other bearing systems may be appro-
priate for special circumstances; designers should consult with the Bearing Technical
Specialist for unique applications.

On widenings, designers are cautioned against mismatching bearing types. It has

been common practice to use elastomeric bearing pads, a yielding bearing, to widen
structures supported on steel rocker bearings, an unyielding bearing. While this
practice has worked satisfactorily on short to moderate length structures, it has
created problems when thick elastomeric bearing pads have been used on structures
with long spans . 6

Pag e 2______________________________ __________________________ Br id g e Bea ri ng s

Reinforced Elastomeric Bearing Pads
General
Reinforced elastomeric bearing pads, designed in accordance with Bridge Design
Specifications, Section 14, should be considered the preferred type of bearing for all
structures. The typical hinge or abutment configuration using many small elastomeric
bearing pads has proved highly reliable and redundant. In addition, these bearings are
extremely forgiving of loads and translations exceeding those considered in design.

Designing and Detailing Notes

The following data illustrates our current practice and provides practical information
about elastomeric bearings. In addition, design examples and various charts in this
memo provide background for the Bridge Design Specification.

Pad thickness is determined in increments of x/i inch. Minimum laminated pad

thickness is one inch. Maximum laminated pad thickness is 6 inches at abutments and
4 inches at hinges. Plan dimensions (length, width) are determined in two inch
increments. Maximum dimensions should not exceed 30 inches.

The minimum shape factor (5) for any reinforced bearing shall be 5.0. Unless shear
deformation is prevented, the average compressive stress, 5C, in any layer of any
reinforced bearing with an S > 5.0 shall not exceed 800 psi. The minimum average
compressive stress due to dead load will not be less than 200 psi. The Transportation
Laboratory has found that as overall pad thickness increases, the compressive
stiffness of the pad decreases although the shape factor is held constant . 3

Rotational stresses may be minimized by specifying the smallest pad width possible
within the limits of the application. Orient rectangular bearing pads so that the long side
is parallel to the axis about which the largest rotation occurs (see Figure 1, page 9).

All pads at a hinge or abutment should be the same size, and oriented similarly. Ensure
that the orientation is clearly detailed.

Bearing pads on skewed structures should be oriented parallel to the principal rotation
axis (see Figure 1). When insufficient seat width exists, the bearing pads may be
oriented normal to the support. The effects of skew and/or curvature must be
considered. This may result in varying the pad spacing to accommodate the increased
load at the obtuse comer. Minimum loads must be maintained to ensure that slippage
(movement) of the bearing does not occur.

Br id g e Bear ings Pag e 3

 Slippage will be prevented by maintaining a minimum compressive force five times
greater than the largest possible shear force under all service load conditions
including live load plus im pact

The effects of prestress shortening, creep, shrinkage, and thermal movements will be
included in bearing pad designs. The Bridge Design Specifications (Article 14.2.6)
state that the shear deformation shall be taken as the maximum possible deformation
caused by creep, shrinkage, post tensioning, and thermal effects unless a positive slip
apparatus is installed.

Testing at the Transportation Laboratory with positive slip apparatuses have shown
that prestress shortening may be partially accommodated by placing a greased
galvanized sheet metal plate (sliding bearing) above the pad. The plate should extend
a minimum of one inch in all directions beyond the calculated movement. (See Figure
2, page 10.) Long term tests have demonstrated that 50 percent of the total anticipated
prestress shortening may be relieved by this sliding bearing without any significant
shear deformation of the elastomeric bearing pad. The remaining prestress shorten-
ing, creep and shrinkage must be included in the bearing pad design. Note that
prestress shortening may continue beyond the calculated long term shrinkage,
particularly in the case of shallow structures with depth/span ratios less than 0.04.

The prestress shortening percentage (50 percent) used to design the bearing pad may
be reduced at hinges that have delayed hinge closure pours when the sliding bearing
detail is utilized. The reduction may be calculated by adding 20 weeks to the duration
of the closure pour waiting period and determining a new shortening value from the
prestress shortening curve (Attachment 1).

The designer needs to specify silicone grease on the plans when using the sliding
bearing to differentiate it from the previously used multipurpose, automotive and
industrial greases. Testing at the Transportation Laboratory has demonstrated that
multipurpose petroleum base greases do not provide the desired sliding effect and
may damage the elastomer because they are absorbed by the pad in a very short period
of time.

Minimum edge distance to any vertical face (backwall, face of abutment or hinge
seat) should be equal to “t" (pad design thickness), or 3 inches, whichever is greater.
For cast-in-place structures, surround the bearing pads with polystyrene of the same
thickness as the actual pad thickness. (See Figure 3, page 10.)

Plain pads are acceptable during stage construction of precast prestressed girder
superstructures that are continuous for live load where in the final condition the bent
cap becomes monolithic with the girders and slab.

Pag e 4 Br id g e Beari ng s
 Steel reinforced and fabric reinforced pads have different design criteria. Where
possible, the designer should prepare both designs (with one set of details) and allow
the contractor the choice as specified in Section 51-1.12H of the Standard Specifica-
tions. The following is an example of a note that should be shown on the plans:

“Fabric reinforced elastomeric bearing pads 22" x 28" x 2" or steel reinforced
elastomeric bearing pads 2 0 " x 26" x 2 " (elastomer only).”

In most cases the designer need not be aware of the increase in thickness due to the
steel reinforcing, since the design thickness relates to the thickness of elastomer, and
changes due to the actual thickness are taken care of in the specifications and during
construction. Exceptions may include retrofit projects where the actual thickness
should be shown on the plans. In these cases a substitution would not be allowed, and
the plans should state this clearly.

Properties of Elastomer
Durometer H ardness........................... 55 ±5*

Creep in Com pression........................25% of initial vertical deflection

Shear Modulus (Adjusted)

@ 70°F = 135 psi

@ 20°F 1.10 x 135 = 149 psi
@ 0°F 1.25 x 135 = 169 psi
@ -2 0 °F 1.90 x 135 = 257 psi

“Shear modulus ( G), is the most important material property for design, and it is,
therefore, the preferred means of specifying the elastomer. Hardness has been widely
used in the past because the test for it is quick and simple. However, the results
obtained from it are variable and correlate only loosely with shear modulus.” 1

The shear modulus of elastomer, obtained from testing, is approximately 100 psi at
70°F.3 The design value was increased to 135 psi at 70°F to include a safety factor
of 35 percent against horizontal overloads. For design calculations use the modulus
at 0°F, (169 psi), unless temperatures will be substantially lower.

* Railroads require 60 ±5 hardness on their structures. Specifications handles the change and there
is no change in design.

Bri dg e B ear in gs ________________________________________________________________ Pag e 5

 Elastomer properties are in accordance with the Research Report, “A Laboratory
Evaluation of Full Size Elastomeric Bridge Bearing Pads,” dated June, 1974, by the
Transportation Laboratory of the State of California, except as noted.

Design Criteria
1 . Temperature movement shall be calculated as per the examples in this memoran-
dum. In calculating movement, use 1V4 times the coefficients, as it is not possible
to always place the pad at a “mean” temperature. Rise and fall temperature values
are given in the Bridge Design Specifications, Article 3.16.

2 . Long term prestress shortening and shrinkage shall be included in the bearing
movement calculation. Prestress shortening per 100 feet of contributory length
will equal 0.10 feet for post tensioned concrete structures, and a minimum of 0.01
feet for pretensioned concrete structures. Shortening (shrinkage) of conventional
reinforced concrete structures will equal a minimum of 0.005 feet per 100 feet of
contributory length. Fifty percent of the prestress shortening may be discounted
when the sliding bearing is used. (See Figure 2, page 10.)

3. Pad thickness shall not exceed lA of the length or width, or be less than twice the
calculated horizontal movement. Maximum thickness for plain pads is xh inch.
Maximum thickness for laminated pads is 6 inches at abutments and 4 inches at
hinges. Minimum thickness for laminated pads is 1 inch (two 14-inch layers).
When design procedures require a pad thickness greater than the maximum
recommended thickness, investigate the use of F I FE bearings.

4. Average pressure on the pad shall not exceed 800 psi under a service load
combination of dead load plus live load not including impact. For steel rein-
forced bearing pads with a Shape Factor > 7.5, the average pressure shall not
exceed 1,000 psi. Minimum pressure on any pad due to dead load shall not be
less than 200 psi.

5. Initial vertical deflection (compressive strain) shall not exceed 7 percent (exclud-
ing the effect of rotation) of the uncompressed thickness of the pad.

Determine the initial compressive strain from Figure 4A or4B (page 11 and 12),
“Compressive Strain, Percentage”, using the compressive stress and shape factor.

Pag e 6______________________________________________________ ______ Br id g e Bear in g s

 Note:
^ one loaded surface area
Shape Factor = --------------------------------
total free - to - bulge area
_____________ W x L ___________
2( W + L) x (thickness per layer)

Where

L - length of bearing pad in direction of horizontal movement;

W = width of bearing pad normal to direction of horizontal movement.

For laminated pads, since the thickness per layer is always W , the formula
reduces to:
WL
Shape Factor = --------
W +L

6. If some combination of service loads (including live load plus impact) exists
which causes a shear force greater than Vs of the simultaneously occurring
compressive force, the bearing should be secured against horizontal movement.

No experimental determination has been made of the starting friction between

bearing pad and concrete or steel. But it has been observed in laboratory tests on
full size pads that slippage does not occur so long as the shear stress does not
exceed Vs the compressive stress acting on the interfaces. Therefore, the shear
force in bearing pad designs should be limited to Vs the minimum vertical load,
(usually Dead Load).

Elastomeric Bearing Pad Coefficient of Friction for Seismic Analysis

' " '
■
'
O’
.VVv?" ' \
.

Seismic analysis o f existing structures with elastomeric bearing pads often requires
a determination of the friction coefficient between the bearing pad and concrete
substructure.
,..v r;.* " ■ >• -- . ■
• • • : • .•' - - • V.' ■, : v; v• .
Bearings on new structures are normally designed to resist slippage by limiting the
shear force to 1/5 the minimum compressive stress acting on the neoprene/concrete
interface, i.e., coefficient of friction equals 0.20. This value is conservative and
ensures that bearings do not creep out o f position under service load conditions.
However, the value is unconservatively low for seismic analysis and should not be
used to determine substructure forces.

Br id g e Bear in g s ________________________________________________________________Pag e 7
 M e mo t o D e s ig n e r s 7-1 • M a r c h 1996

sesrr:^ • '%
r • > 7Kxr?zr~y«s-Ttr ■—~r~.-~.-~'-*,-— ■••-.•:~,r:, , ,«rrr \^s^r:~*.r-:- '
--
Friction determination is an inexact science dependent on many variables which are
not easily quantified by the Bridge Engineer. Therefore, a con servali ve/reasonable
value must be used to ensure that substructure forces are not underestimated. A
review o f several test reports indicates that friction coefficients equal to 0.40 for
concrete to neoprene and 0.35 for steel to neoprene interfaces are more realistic values
for seismic analysis.
.
Designers should also investigate the maximum force exerted by the bearing pad
through shear translation, prior to slippage, and determine which case controls. It is
...... . . ..estimated that elastomeric bearing pads will resist a maximum shear strain o f ±150
percent prior to failure. Laboratory tests reviewed show negligible damage to
elastom enc bearings translated ± 1 0 0 percent o f their design thickness (± 1 0 0 percent
shear strain).

Equations

By Definition:

Shear Force
OL w , , Stress Pad Area
Shear Modulus = ------- = ———— :-------
Strain Deflecuon
Pad Thickness
Fs
G - f - E -
A, AAS
T
FST GAAS
Rearranging terms, As = ----- and Fs = ---------
° ° GA T

Substituting G = 169 psi @ 0°F and Fsmax = Vs DL

169x Ax Aj
Fs = ------- ---------^ Fsnax

Pag e 8 B r id g e Bear in g s
 M e mo t o De s ig n e r s 7-1 « J un e 1994

Figure 1. Bearing Pad Orientation

Br id g e B eari ngs ________________________________________________________________ Pag e 9

 M e mo t o De s ig n e r s 7-1 - J u n e 1994

Figure 2. Sliding Bearing Detail

Figure 3. Detail

N ote: Abutment shown - hinges similar.

Pag e 10___________ ____________________________________________________ B ri dg e Bear in g s

 M e mo t o De s ig n e r s 7-1 » J une 1994

Figure 4A. Recommended Compressive Stress vs. Strain Curve

for Fiberglass Reinforced Bearing Pads

Br id g e B ea ri ng s ________ _________________________________ Pag e 11

 M e mo t o D e s ig n e r s 7-1 • J u n e 1994

Figure 4B. Recommended Compressive Stress vs. Strain Curves

for Steel Reinforced Bearing Pads

Pag e 12________________________________________________________________Br id g e Bear in g s

Design Procedures
Example 1. Fabric Reinforced Pad, Steel Girder Structure
Given:

Span Length - one end fixed = 112 feet

Contributory Length - structural steel = 112 feet
DL Reaction - Service Load = 69 kips
LL Reaction - Service Load = 56 kips
Bottom Flange Width = 16 inches
Moderate Temperature Zone - Rise and Fall = 50°F.

Step 1. Temperature Movement = 1.5 x 0.0000065 x 50° x 112 x 12 = 0.66"

Step 2. Concrete Shortening— Not Applicable (Steel Girder)

Step 3. Minimum Thickness of Pad = 2 x horizontal movement = 2 x 0.66 = 1.32”.

Try 7 = 1.5".

W = flange width = 16"

D L+ LL
Maximum Pressure = ------------= 800 psi
W xL

L = ^ + — = —L—— = 9 g minimum. Use 10".

0 .8 x W 0 .8 x 1 6

Trial Pad: 10" x 16" x 1.5"

Maximum Thickness of Pad: lA of length = — = 3.33” > 1.5"

7 = 1.5" Okay

Step 4. Compressive Stress

D L+ LL 125,000 . OAA
------------= ----- 1----- =781 psi< 800
W xL 16x10

Compressive Stress (Dead Load Only)

DL 69,000 . AAA
-------- = ---------- = 431 psi > 200
W xL 16x10

Br id g e B e a r in g s ______________________ ________________________________________ Pag e 13

 Step 5. Initial Vertical Deflection

cu _ W xL 16x10 , CA
Shape Factor = -------- = ------------ = 6.15 > 5.0
F W + L (16 + 10)

From Figure 4A, for compressive stress = 781 psi and Shape Factor = 6.1,
initial compressive strain is 5.1 percent of pad thickness < 7 percent (by
extrapolation).

Initial Thickness = 1.50 - (1.5 x 0.051) = 1.42".

Final Thickness after creep = 1.42 - (1.5 x 0.051) x 0.25 = 1.40".

Step 6 . Calculate the maximum shear force at slippage (minimum compressive

force = DL in this example).

Frrruu ~ ~ ~

Actual Design Shear Force Fs = ------- - =

T
Modulus x Area x Movement 169 x 160 x 0.66 , , . , .
---------------------------------------= ---------------------- = 11.9 kips
Pad Thickness x 1,000 1.5x1,000

11.9 kips < 13.8 kips

Therefore T = VA." Okay.

Specify “Fabric Reinforced Elastomeric Bearing Pads 10" x 16" x 1.5".”

To reduce rotational (compressive) stresses, orient rectangular bearing pads so that

the long side is parallel to the axis about which the largest rotation occurs.

To complete the design, repeat steps 3 through 6 using the design criteria for steel
reinforced pads (if applicable) and include this design on the contract plans.

Pag e 14_______________________________________________________________ Br id g e Bea ri ng s

Steel Laminated Elastomeric Bearing Pads
Background
Our policy has been to standardize on V4" layers of elastomer. Until recently, we used
very thin steel plates and a minimal elastomer cover at the top and bottom for the steel
reinforced pads. The minimal thickness of cover and of steel was ignored and the
bearing thickness shown on the plans was the sum of the Vi -inch thick layers. This
resulted in a simple, standard Caltrans procedure for the design and manufacture of
both the fabric reinforced and steel reinforced bearing pads.

Steel reinforcement option was removed from the 1981 Standard Specifications because
the bearing manufactures could not properly mold the bearing with the thin steel plates.

Current Policy
The current specifications for elastomeric bearings permit the use of the steel
reinforced bearing as an option. However, the proper design of the steel reinforced
bearings requires 14 gauge (0.075 inch) steel plates full '/6 -inch elastomer layers
between the plates and a '/4-inch cover top and bottom. These two '/4-inch layers are
considered one '/6 -inch layer for design purposes. Therefore, because of the steel plate
thickness, the steel reinforced bearing will always be slightly thicker than the
corresponding fabric reinforced (fiberglass) bearing pad.

Design
In permitting the use of the steel reinforced bearing as an option, the specifications
require that the contractor notify the Resident Engineer of their choice. If the steel
reinforced bearing is selected, the bearing seat elevation will be adjusted (lowered)
by the Resident Engineer to allow for the increased thickness. The minor increase in
compression on the steel plates due to the W side cover may be ignored.

For most cast-in-place concrete, precast concrete and steel superstructures, there
should be no difficulty in adjusting the bearing seat elevation at the time the contractor
selects the bearing type. In general there is no need for the designer to be concerned
with the choice.

For some applications, the designer may want to limit the bearings to only one of the
two types. If this is the case, the designer should send a memo to the specification
writer who will denote the specific type of pad in the Standard Special Provisions.

B r id g e Beari ngs _______________________________________________________________ Pag e 15

 M e mo t o D e s ig n e r s 7-1 • Jun e 1994

“The maximum size of steel reinforced bearings is governed by the fabricators ability
to vulcanize a large volume of elastomer uniformly and completely” . 4 Since
elastomers are poor conductors of heat, achieving a full cure in the center of the
bearing without overcuring the outside becomes increasingly difficult as the bearing
size increases . 4 Steel reinforced elastomeric bearings should be limited in size to
approximately 500 kips based on an allowable stress of 1,000 psi to ensure proper
vulcanization of the elastomer.

The new LRFD Bridge Design Specifications will allow a maximum allowable
compressive stress of up to 1,600 psi in the absence of rotation (service limit). To
utilize the higher stress limits, the designer will have to use a more complex design
procedure and specify more rigorous testing. The current Bridge Design Specifica-
tions allow alternative design procedures as outlined in NCHRP Report 298.

The various modes of failure for steel reinforced pads are: debonding, fracture of steel
plates and instability . 4

Steel reinforced bearings have a greater overload capacity before failure than fabric
reinforced pads. The Transportation Laboratory reported that the ultimate compres-
sive stress of steel reinforced pads was approximately 6,000 psi before the 14 gage
steel yielded. In comparison, the ultimate compressi ve stress of fabric reinforced pads
was approximately 1,800 psi . 2 Therefore, steel reinforced elastomeric bearings
provide a greater factor of safety against overloads than do fabric reinforced.

“Holes are strongly discouraged in steel reinforced bearings. However, if holes are
used, their effect should be accounted for when calculating the shape factor because
they reduce the loaded area and increase the area free to bulge.” 1 Suitable shape
formulae are:

L W - Z — d2
for rectangular heanngs: S, =

for circular bearings: S, = - 7 ^-7—— ——

4 hri{D +Ld)

where:

L = length of a rectangular bearing (parallel to the longitudinal bridge axis) (in.)

W = width of bearing in transverse direction (in.)
hr, = thickness of the elastomeric layer (in.)
d = diameter of the hole or holes in the bearing (in.)

Pag e 1 6 ____________________________________________________________ Br id g e Bearing s

 M e mo t o D e s ig n e r s 7-1 »J u n e 1994

D = diameter of the projection of the loaded surface of the bearing in the

horizontal plane (in.)

To assist the designer, the total thickness for steel reinforced bearings is tabulated below.

Design Number of Number of Actual Thickness

Thickness Vi" Layers* Steel Plates
Minimum Maximum**

1.0" 2 2 1.15 1.29

15" 3 3 1.73 1.89
2.0" 4 4 230 2.48
25" 5 5 2.88 3.08
3.0" 6 6 3.45 3.67
35" 7 7 4.03 4.27
4.0" 8 8 4.60 4.86
45" 9 9 5.18 5.46
5.0" 10 10 5.75 6.05
55" 11 11 633 6.65
6.0" 12 12 6.90 7.24

Steel Laminated Elastomeric Bearing Pad

1.5", 3-Layer Pad Shown

* The Vi" layer top and bottom is considered one Vi" layer for design purposes.
** Includes consideration for allowable tolerances for steel plates and elastomer thickness.

B r id g e B ea ri ng s _______________________________________________________________ Pag e 17
 M e mo t o D e s ig n e r s 7-1 * J un e 1994

Example 2. Steel Reinforced Pad - CIP P/S Structure

Given:

Span 5 Hinge, (See Attachment 2)

Structure Length = 740 feet

Contributory Length - 100' (CIP P/S), 156' (PCC) = 256 feet
DL Reaction/Girder - Service Load = 148 kips
LL Reaction/Girder - Service Load, No Impact = 41 kips
Moderate Temperature Zone - Rise and Fall = 35°F
Sliding Bearing Used

Step 1. Temperature Movement = 1.5 x 0.0000060 x 35 x 256 x 12 = 0.97"

Step 2. Concrete Shortening (sliding bearing accounts for 50 percent of shortening)

P/S Shortening 0.5 x (0.107100’) x 100 x 12" = 0.60"

Concrete Shrinkage (0.0057100') x 156' x 12" = 0.09"

Step 3. Minimum Thickness of Pad = 2 x horizontal movement = 2 x (0.97 + 0.60

+ 0.09) = 3.32". Try 7 = 3.5" (elastomer only)

D L+ LL
Maximum Pressure = ----------- = 1,000 psi
W xL

148 + 41
W x L = -----------= 189 in .2 = 13.7" x 13.7"
1.0

Trial Pad: 14" x 16" x 3.5"

Note: 14 x 16 pad was selected because 14 x 14 pad has a shape factor of

only 7.0, which results in a maximum compressive stress of 800 psi.

14
Maximum Thickness of Pad: lA of length or width = — = 4 .7
3
7 = 3 .5 " Okay

Pag e 18_______________________________________________________________ B r id g e B ea ri ng s
 M e mo t o D e s ig n e r s 7-1 « Jun e 1994

Step 4. Compressive Stress

DL+ LL _ 148 + 41 _ < i q OO Okay (ShapeFactor>7.5

W xL 14x16 required)
Compressive Stress (Dead Load Only)

= 660 psi > 200 Okay

W xL 14x 1 6

Step 5. Initial Vertical Deflection

OL ^ W xL 14x16 „
Shape Factor = -------- = ---------- = 7.47 - 7.5
F W + L 14 + 16
From Figure 4B, for compressive stress = 843 and shape factor = 7.5, initial
compressivestrainis4.5 percent of pad thickness <7 percent (by extrapolation).

Initial Thickness = 3.5 - (3.5 x 0.045) = 3.34"

Final Thickness after creep = 3.34 - (3.5 x 0.045) x 0.25 = 3.30"

Step 6 . Calculate the maximum shear force at slippage (minimum compressive

force = DL in this example)

_ DL 148 „ , . . .4 .
Fjmai = — = —— = 29.6 kips (Allowable)

Actual Design Shear Force =

Modulus x Area x Movement 169 x (16 x 14) x 1.66
---------------------------------------= -------- J--------- ----------= 18.0 kips
Pad Thickness x 1,000 3 .5x1,000

29.6 kips > 18.0 kips Okay

Therefore T = 3.5” Okay

Specify “Steel Reinforced Elastomeric Bearing Pads 14” x 16" x 3.5" (elastomer
only).”

To reduce rotational (compressive ) stresses, orient rectangular bearing pads so that

the long side is parallel to the axis about which the largest rotation occurs.

To complete the design, repeat steps 3 through 6 using the design criteria for fabric
reinforced pads (if applicable) and include this design on the contract plans.

Bri dg e Bea ri ng s _______________________________________________________________ Pag e 19

 M e mo t o D e s ig n e r s 7-1 * J u n e 1994

Example 3. Steel Reinforced Pad - CIP P/S Structure

Given:

Span 5 Hinge, (See Attachment 2). Due to construction staging spans 5 and 6 will
not be completed for 1 year.

Span Length = 740 feet

Contributory Length - 100' CIP P/S (Stage 1), 156’ PCC (Stage 2) = 256 feet
DL Reaction/Girder - Service Load = 148 kips
LL Reaction/Girder - Service Load, No Impact = 41 kips
Moderate Temperature Zone - Rise and Fall = 35°F
Sliding Bearing Used

Step 1. Temperature Movement = 1.5 x 0.0000060 x 35 x 256 x 12 = 0.97"

Step 2. Concrete Shortening

P/S Shortening (refer to Attachment 1). Enter graph with 72 weeks after
stressing (52 week waiting period + 20 weeks for sliding bearing).

Shortening/100' = 0.0257100' (Stage 1 after 72 weeks)

(0.0257100') x 100' x 12" = 0.30"

Concrete Shrinkage (0.0057100') x 156' x 12" = 0.09"

Step 3. Minimum Thickness of Pad = 2 x (0.97" + 0.30" + 0.09") = 2.72".

Try T = 3.0" (elastomer only)

DL+ LL
Maximum Pressure = ----------- = 1,000 psi
WxL p

148 + 41
W x L = ----------- = 189 in.2 = 13.7" x 13.7"
1.0

Trial Pad: 14" x 16" x 3.0"

Note: 14 x 16 pad was selected because 14 x 14 pad has a shape factor of

only 7.0, which results in a maximum compressive stress of 800 psi.

Pag e 20________________________________ _______________________________Bri dg e B eari ng s

 M e mo t o D e s ig n e r s 7-1 • Jun e 1994

Chltrans

14
Maximum Thickness of Pad: lA of length or width = — = 4.7
3
7 = 3 .0 " Okay

Step 4. Compressive Stress

D L+ LL _ 148 + 41
= 843 psi < 1,000 Okay (ShapeFactor>7.5
W x L ~ 1 4 x16 required)

Compressive Stress (Dead Load only)

DL 148
= 660 psi > 200 Okay
W xL 14x16

Step 5. Initial Vertical Deflection

ni_ _ W x L 14x 1 6 „
Shape Factor = -------- = ---------- = 7.47 - 7.5
F W + L 14 + 16

From Figure 4B, for compressive stress = 843 and Shape Factor = 7.5, initial
compressive strain is 4.5 percent of pad thickness < 7 percent (by extrapo-
lation).

Initial Thickness = 3.0 - (3.0 x 0.045) = 2.87”

Final Thickness after creep = 2.87 - (3.0 x 0.045) x 0.25 = 2.84"

Br id g e Bea rin gs Pag e 21

 M e mo t o D e s ig n e r s 7-1 - J u n e 1994

Chlt/ans

Step 6 . Calculate the maximum shear force at slippage (minimum compressive

force equals DL in this example)

Fsnux = = 29.6 kips (Allowable)

Actual Design Shear Force =

Modulus x Area x Movement 169 x (16 x 14) x 1.36
--------------------------------------- = ------------------- ----------= 17.2 kips
Pad Thickness x 1,000 3.0x1,000

17.2 kips < 29.6 kips Okay

Therefore T = 3.0" Okay

Specify “Steel Reinforced Elastomeric Bearing Pads 14" x 16" x 3.0" (elastomer
only).”

To reduce rotational (compressive) stresses, orient rectangular bearing pads so that

the long side is parallel to the axis about which the largest rotation occurs.

To complete the design, repeat steps 3 through 6 using the design criteria for fabric
reinforced pads (if applicable) and include this design on the contract plans.

Page 22 B r id g e Bea rin gs

 M e mo t o D e s ig n e r s 7-1 - J u n e 1994

Ga/trans

PTFE Sliding Surfaces

General
Polytetrafluoroethylene (PTFE) was first used in bridge bearings in the early 1960's
because of its low frictional characteristics, chemical inertness and resistance to
weathering. 4 Many modem bearing systems such as pot, spherical, disk, etc., use
PTFE in contact with stainless steel as a sliding surface.

PTFE is usually used in the form of sheet resin (filled or unfilled) and woven fabric made
fromPTFE fibers. 4 Although the actual chemical formulation of PTFE is of little interest
to the designer, the physical properties and performance characteristics are.

The two most important design aspects of PTFE are the coefficient of friction and the
wear rate . 4 The coefficient of friction controls the forces transmitted to other parts of
the bearing device and the substructure. The wear rate affects the design life and
maintenance of the bearing.

Extensive research and testing by Stanton, Roeder and Campbell has demonstrated
that the coefficient of friction and the wear rate are affected by several variables in
addition to the type of PTFE and mating surface.

PTFE Types

Unfilled PTFE Sheet Resin (Dimpled Lubricated)

Dimpled lubricated PTFE manufactured from unfilled sheet resin has the lowest
coefficient of friction under all load conditions . 4 The dimples function as reservoirs
that store the grease for continuous lubrication. Testing by Stanton, Roeder and
Campbell has shown that the absence of dimples will result in a higher coefficient of
friction as the grease is dissipated. While dimpled lubricated PTFE has the lowest
coefficient of friction, it is also the most sensitive to problems and deficiencies.4 '

Woven Fabric PTFE

Woven fabric PTFE made from PTFE fibers has a higher coefficient of friction than
dimpled lubricated PTFE, however, it has a greater resistance to creep. Woven fabric
can take up to 30 times the compressive stress without cold flow as compared to
PTFE resin . 4

Br id g e Beari ngs Pag e 23

 M e mo t o De s ig n e r s 7-1 • Ju n e 1994

Caltrans

Filled PTFE Sheet Resin

Filled PTFE sheet resin has the highest coefficient of friction as compared to unfilled
PTFE sheet resin and woven fabric PTFE. Fillers such as glass fiber, graphite and
ceramics etc., are incorporated in PTFE resins to alter properties, such as cold flow,
compressive strength etc . 4

Factors that affect PTFE Performance4

Friction

• Friction increases with decreasing contact pressure, decreasing temperature,

increasing sliding speed and increasing number of cycles.
• Static friction is usually higher than dynamic friction.
• Maximum coefficient of friction (spike) occurs at the onset of movement.
• Type of surface finish on mating surface affects friction.
• Direction of surface finish on PTFE mating surface affects coefficient of friction.
Surface finishes parallel to the direction of sliding give lower friction values.
• Friction increases to near static value if PTFE surface remains loaded for a period
of time without movement. This generally is not a problem on highway structures.
• Lubrication reduces friction.
• Friction values could be 5 to 10 times higher at seismic speeds as compared to
thermal expansion speeds.

Wear and Creep (Cold Flow)

• Sliding speed is the most significant factor affecting wear. Increased sliding speed
increases wear.
• Decreased temperature increases wear.
• Lubricant reduces the wear rate of sheet resin.
• Wear and creep increase with increasing loads.
• Sheet resin disks should be recessed half their thickness into their backing plate
to reduce creep.

Pag e 24 Br id g e Beari ngs

 M e mo t o D e s ig n e r s 7-1 • Ju n e 1994

CtUtrans

Suggested Values for Coefficient of Friction

The minimum coefficient of friction used for design shall be as specified by the
bearing manufacturer or as given in Article 15.2.6 of the Bridge Design Specifica-
tions. Designers are cautioned against using design friction values that are too low.
Low friction values are sometimes difficult to obtain in manufactured bearings and
may result in transfer of larger forces to the bearing and substructure than calculated.

Suggested Design Bearing Pressures for PTFE Surfaces

The average bearing pressure on PTFE surfaces due to all loads shall not exceed:

Filled P T F E ...................................................... 3,500 psi

Unfilled PTFE (Recessed)...............................3,500 psi

Woven Fabric PTFE.........................................3,500 psi

Additional values may be found in Article 15.2.7 of the Bridge Design Specifications.
Bearing pressures below 2,000 psi are not recommended as they produce high
coefficients of friction and poor bearing performance. Unfilled PTFE (not recessed)
should not be used on Division of Structures designs.

Br id g e B ea ri ng s Pag e 25
 M e mo t o De s ig n e r s 7-1 • Ju n e 1994

Caltrans

PTFE/Spherical Bearings
General
PTFE/Spherical bearings designed in accordance with Bridge Design Specifications,
Section 15 should be considered for use only when the practical limits of reinforced
elastomeric bearing pads have been exceeded.

The basic spherical bearing design is comprised of a convex base with a mating
concave element for rotation. On expansion bearings, an upper sliding plate is
added for translation. All contact surfaces are polytetrafluoroethylene (PTFE) to
stainless steel.

PTFE/Spherical Bearing Types

PTFE/Spherical bearings are available in three different forms, each designed to meet
different functional requirements. The three standard types of PTFE/Spherical bearings
in use today are: (1) expansion (non-guided), (2) expansion (guided), and (3) fixed.

Expansion non-guided bearings allow horizontal movement and rotation in all

directions, (see Figure 5, page 29). Expansion guided bearings allow horizontal
movement along only one axis and rotation in all directions, (see Figure 6 , page 30).
Fixed bearings are restrained from horizontal movement in all directions while
allowing rotation in all directions, (see Figure 7, page 31). Due to previous problems
experienced with guided bearings, they will not be considered for use by the Division
of Structures.

PTFE/Spherical expansion bearings (non-guided) are comprised of four basic com-

ponents: (1) sole plate, (2) concave plate, (3) convex plate, and (4) masonry plate.
PTFE/Spherical fixed bearings are similar except that a nonsliding sole plate may or
may not be required. The function of the four components mentioned above and the
typical manufacturing materials are as follows, (see Figure 8 , pages 32 and 33): ••

• Sole Plate - Transfers superstructure loads to the bearing and provides a stainless
steel sliding surface for superstructure translation. The sole plate is fabricated
from A36/A36M steel and has a stainless steel surfacing.
• Concave Plate - Provides PTFE sliding surface for sole plate and PTFE concave
surface for rotation. The concave plate is fabricated from A36A/36M steel. A
woven PTFE pad is epoxy bonded and mechanically fastened to the flat and
concave surfaces. Dimpled lubricated PTFE has been used by some manufac-

Pag e 26 Br id g e B ear in g s
 M e mo t o D e s ig n e r s 7-1 • Jun e 1994

C a J tfa n s

tures. However, woven PTFE fabric is preferred for this type of bearing and
should be used for Division of Structures designs.
• Convex Plate - Provides stainless steel mating surface for rotation of concave
plate and transfers load to masonry plate. The convex plate is usually made from
solid stainless steel, or A36/A36M with a stainless steel weld overlay.
• Masonry Plate - Transfers load from convex plate to bearing seat. The masonry
plate is fabricated from A36/A36M steel.

Design Requirements
PTFE/Spherical bearings are designed in accordance with Section 15 of the Bridge
Design Specifications. All loads are service loads. Minimum vertical loads are for
dead loads and superimposed dead loads. Maximum vertical loads are for dead loads,
superimposed dead loads and live loads plus impact. PTFE fabric stresses are limited
to 3,500 psi maximum. The coefficient of friction for fabric containing PTFE fibers
varies from 0.08 to 0.04 at bearing pressures of 500 psi and 3,500 psi respectively. A
design coefficient of friction of 0.06 is recommended for designs with bearing
pressures from 2,000 psi to 3,500 psi. Bearing pressures below 2,000 psi (DL only),
should not be used.

Design Guidelines
The nucleus of all spherical bearings is the concave/convex plate interface (spherical
surface). All loads, vertical and horizontal are transmitted through the interface. Since
the spherical interface slides on low friction materials (PTFE to stainless steel), all
stresses that pass through the interface are assumed to be radially transmitted through
the geometric center of the sphere (see Figure 9, page 34). The low friction interface
is assumed to provide no frictional resistance to horizontal loads. Due to the
complexity of the analysis required to accurately determine the stresses at the
concave/convex plate interface, simplified design guidelines were developed by
bearing committees and adopted by AASHTO. The procedure to design the interface,
complete with formula derivations, and other PTFE/spherical bearing components is
outlined below. Refer to Figure 9 for bearing geometries.

Br id g e Bearing s Pag e 27
 M e mo t o D e s ig n e r s 7-1 • J un e 1994

QUt/ans

PTFE/Spherical Bearing Notations

Ah = total assembly height (inches)
^ pt f e = PTFE area of flat sliding surface (in.2)
c = minimum vertical clearance between rotating and non-rotating bearing parts
Cm = minimum convex chord length (inches)
DB3Cl = minimum concave bearing pad diameter (arc length)
Dm = diameter of minimum allowable projected bearing area, concave plate (inches)
H = height of convex spherical surface (inches)
/ / act = overall height of convex plate (inches)
Lcp = length and width of concave plate (inches)
= maximum longitudinal movement (inches)
L0 = sole plate safety overhang (inches)
Lsp = longitudinal length of sole plate (inches)
Mm = minimum metal depth of concave surface (inches)
P/Zmax = maximum horizontal load (kips)
•Ptmax = maximum longitudinal load (kips)
/Vmax = maximum transverse load (kips)
/Vmax = maximum vertical load (kips)
/Vnun = minimum vertical load (kips)
Racl = actual radius of concave bearing sliding surface (inches)
/?max = maximum allowable radius of concave bearing sliding surface (inches)
Tm = maximum transverse movement (inches)
//max = maximum allowable unit pressure for PTFE (3.5 ksi)
Wsp = transverse width of sole plate (inches)
Q?injn = minimum angle to prevent uplift (degrees)
P = minimum design rotation capacity of bearing, (/?=/£+$;), 2 degrees minimum
pc = maximum rotation resulting from construction tolerances (0.02 radians)
ps = minimum design rotation capacity of structure from: DL, LL , camber
changes, and construction/erection sequences
7min = minimum angle of convex surface (degrees)
VOmn = minimum angle of concave bearing surface (degrees)

Pag e 28 Br id g e Beari ng s
I L __ ______,;;;;_ _
MEMO TO DESIGNERS 7-1 • JUNE 1994
____;____~

liz/trtl/16

r-------------------------------

r - - - - - - -_-____ -----_- - - - - - - ~

I
.,
,,, .,
- .,- ...,.-----
......... ,,,,,. -
',
.........'
' '
'
I

,, + I
,;
I
,, ' ' , ,
I I ' \ I
I / / \ \ I
I / / \ I I

It I
11,
\ II
II I I
11
I I 11
11 I I I I
I I \ I / I
I \ \ / /
I \ \ / / I
\. \. I I
' ' / I
' ','....
' / ,
.,,.,
I ','..,., _ _ _ _ , : . , . , I

·----- -========------~
I
-------------------------------

Plan
Anchorage Typical

Sole Plate

Stainless Steel
Woven PTFE Pad Sliding Surface
bondedtoflatand - - - - ~ " " " =,.~j------- Concave Plate·
concave surfaces
,..--r~~:--14-~~-#-~L~-..<,~$...~...;'4~9...1----~-,-=---,--- Convex Plate
Stainless ~......,...__....,.,_~__,.,.__...,.,._~__,.,.__..,,,._..,.__........__......,_~---- Masonry Plate
Steel Convex
Surface
Section X-X

Figure 5. PTFE/Spherical Expansion Bearing (Non-Guided)

BRIDGE BEARINGS PAGE 29 !

'6 - - - MEMO
- -
TO DESIGNERS 7-1 • JUNE
- 1994

tidt,ans

I
I
I
I
I
I
I
I
I
I I

'r - - - - - - - - - - - - - - - - _,
I - -- ---
,,
11 ,,,, -
.............. ; , ,
- - - - - ...... .....
........' '

,
, +
1• , , ' '
,, ,',' ', ',
II / / ' \
11 I I \ \

~--
11 / / I \

111 f
I

: I
I I
\ I
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I

I I
f I
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--~
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' I I
'' ' ' , ,, /
,',
','....
_- __
.... .... ' -- ..,,,.-".,"'
_,,,,
IL - - - - - - - - - - =: - - - - - - - -
I
I
I
I
I
I
I
I
I
I

Plan

Stainless Steel
Woven PTFE Pad ~-+--- Sliding Surface
bonded to flat and Concave Plate·
concave surfaces
~"-,(,t--'\'!~~"-,(,t-~~~'--,(,t-~
//,~,c,t~=;;====.--- Convex Plate

Stainless =--~.,,._~-=-_:,,,.__...=_~_:,,,.__...=_ ~.......,.__~_,..._,i - - - - Masonry Plate

Steel Convex
Surface
Section X-X

Figure 6. PTFE/Spherical Expansion Bearing (Guided)

PAGE 30 BRIDGE BEARINGS

' a__
lizl'lran6
_____,;.;;. _ __
MEMO TO DESIGNERS 7-1 • JUNE 1994

---------- - - - -----------
- - --
., ., - ---
-- --- -...-......' ' '
I
I
I
I
I
I ,, .,,, ,, I
✓ I
I
✓ ✓
I
''' I

''
I I I \ I
I I I \ \ I
I I I \ I I

+
1, I
,,
J
I I I

ct-- I/ I

••
1' I
I
'.
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/ I
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., ,, ,,
,,
✓
I

' '.... ...

-- -- --...
, I

- - -- ~--
I ., I
I .... .....
"-------- ------- --- I

Plan

Woven PTFE Pad - - - -- - Concave Plate

bonded to
concave surface
- - - Convex Plate
.,,,..,<'<""""~_..1.....4~~-4L$..~L,#..~z~-i:,,1,..,~""'-=-+-.[:---.:.-~--:--_7
Stainless ~~::.._~__::...._~..__;:.:,,..~::.._~~~-=-~~.__~ - - -- Masonry Plate
Steel Convex
Surface
Section X-X

Figure 7. PTFE/Spherical Fixed Bearing

BRIDGE BEARINGS PAGE 3 1

' . , , _ _ _ _ _ _ _ _ _ __ __ ...:M=EM:.:.:.O.::.......:.T-=0--=O::.:E=..:S:;.;.IG=.;N~E=R..:..:S:....7.:...-...:1_•....;J:.:U:..;.N.:.=E:....1.:..:9:.:9~4

lb/trans

0 0 0

---Anchorage Typical

Stainless Steel
Sliding Surface
Elevation

Transfers superstructure load to the bearing and

provides stainless steel sliding surface for
superstructure translation.

Plan

Sole Plate

., .,
----- ...
" '"
I
I ' \
I \
I

Woven PTFE Pad. I

Epoxy bonded and \

Elevation mechanically '' I

fastened to f lat and

" ... ____ _.,. ., ., "
concave surfaces
'

Plan
Provides PTFE sliding surface for sole plate
and PTFE concave surface for rotation.

Concave Plate

Figure 8. PTFE/Spherical Expansion Bearing Components

PAGE 32 BRIDGE BEARINGS

' II, - - - - - - MEMO TO DESIGNERS 7-1 • JUNE 1994

/2zltrans

Stainless
- - - - Steel Convex
~ Surface

r:flll~
Elevation

Plan

Provides stainless steel mating surface for rotation of

concave plate and transfers load to masonry plate.

Convex Plate

..·-·

Elevation

Transfers bearing load to bearing seat

Plan

Masonry Plate

Figure 8. PTFE/Spherical Expansion Bearing Components (continued)

BRIDGE B EARINGS PAGE 33

 ' I) _____________ ....:M..:..:..=.
E= MO....:
T--=0:....:0=-.:E
=S::.;.IG
.:::.N:..:;E=R..:..:S::...7..:...-.....:1~•....:J::....:U::.;.N.:.=E=--1..:..;9::...:9:.....:.
4

lb/trans

Dm12 Dm12

Concave Plate-~---•--'<-

Hact
Convex Plate ---•~<.

Amax, Ract

Figure 9. Spherical Bearin.g Geometrics

PAGE 34 BRIDGE BEARINGS

 M e mo t o D e s ig n e r s 7-1 - J u n e 1994

Caltrans

Design Procedure

Concave Plate
• D ia m e te r (Dm) o f M in im u m A llo w ab le P ro jected B ea rin g A rea

The minimum diameter (D m) of the concave spherical plate must be large enough to
ensure that the maximum bearing stress ( a u) on the horizontal projected area of the
plate does not exceed the maximum allowable stress on the PTFE fiber (3,500 psi).
Therefore, the minimum diameter (Dm) may be determined from the maximum
vertical load (/Vmax) and the PTFE maximum allowable unit pressure (£/„**).

,, .. '
- ... --+.------­ Pvtmx- Maximum Vertical Load

I
I \
I Umax = M axim um allow able unit
\ I pressure (PTFE = 3.500 psi)
'-
' ,, I
..... - "--4-_,__------

f ■p.,
* v max \

^ L = Area = ^ / . Z ) m =
u„ Dm = 2
V max

fAnax n 71 U„

• P T F E A rea (A pt f e ) o f F la t S lid in g S urface

The flat PTFE sliding surface area on the concave plate should be size to the nearest
0.25 inch using the maximum allowable stress on the PTFE fiber (3,500 psi).

—— Concave
Plate
/V r
----- Flat PTFE Apt f e -
Sliding Surface U„

Br id g e B ear in g s Pag e 35
 M e mo t o D e s ig n e r s 7-1 • J u n e 1994

Ch/trans

• M in im u m A ngle ( c ^ n ) R eq u ired to P re v e n t U plift

The minimum angle (a ^ n ) is used to calculate the spherical radius required to resist
the greatest ratio of horizontal to vertical load without unseating of the concave plate.
On expansion (non-guided) bearings, the maximum horizontal load cannot be
transferred from the sole plate to the concave plate because the sole plate is free to
slide horizontally. Therefore, on expansion non-guided bearings, it is recommended
that a horizontal load equal to 10 percent of the maximum vertical load and a
minimum dead load of 50 percent of the maximum vertical load or the actual
minimum dead load, whichever is smaller, be used to determine c x ^ .

For fixed bearings, the horizontal to vertical load ratio should not exceed 40 percent
when using simplified design procedures. Using the simplified procedure on load
cases above the 40 percent level will result in over stressing the PTFE fabric at the
spherical interface. Hence, external shear devices are required to fix bearings during
seismic activity exceeding the 40 percent ratio.

PHmax= Max horizontal load

Pvmjn= Min vertical load (including uplift)

It is evident from the formula above that spherical bearings virtually have no
resistance to horizontal loads unless a vertical load is present. Without the presence
of a vertical load, the concave plate will ride up and off the convex plate.

Pag e 36 B r id g e B eari ng s
 Memo to Designers 7-1 • June 1994

GUtrans

• Minimum Angle (v^iin) o f Concave Bearing Surface

The minimum angle (vVm) of the concave bearing surface determines the combined
rotation and horizontal load capacity of the bearing. The minimum design rotation
capacity (/J) for spherical bearings is usually 2 degrees and should include rotations
from DL , LL, camber changes, construction tolerances and erection sequences.

V'min = ^min + P P = minimum design rotation capacity of bearing.

(P= Ps +Pc), 2 degrees minimum.

• Maximum Allowable Radius (RmiX) o f Concave Bearing Sliding Surface

The maximum allowable radius (Rma) defines the spherical curvature which resists
the applied horizontal forces and provides rotation. R ^a is calculated from the
minimum projected diameter (D m) and the minimum angle (^min)- Due to manufac-
turing limitations, R„a, should not exceed 36 inches. Hence, the actual radius (Raa)
of the concave bearing surface may be less than Rmu- This limitation is usually not
a problem because the smaller radius will increase the allowable rotation and/or
increase the lateral load capacity.

Derivation:

. _ D m/ 2
Sin I[f nun
UX

R =
2sin(l^min)

If Rma > 36", use R m„ = 36".

Bridge Bearings Page 37

 M emo to D esigners 7-1 » J une 1994

• Minimum Concave Bearing Pad Diameter (DBact)

The minimum concave bearing pad diamter (DB iCl) is used to calculate the minimum
metal depth (M m) of the cancave surface, and the minimum angle (/„un) of the convex
surface.

DBiCt is the arc length along the concave bearing sliding surface.

Derivation:

Arc length = radius x angle in radians

( D /2"\
Vm = sin -1 —^— (degrees)
Rid j

tc/ 180 converts to radians, multiply by 2 to get diameter.

p,D _ Jn f * ( ■ , A»/2V)'
P “[180r JJ.
• Minimum Metal Depth (Mm) of Concave Surface

Derivation:

Arc length = radius x angle in radians

2 " ^ '^ ^ (lS o )

_Z >2W 18(>Ì
- 2R m I TT J
Y „ n
COSl/r min— , Y — Raa COS1/^min
Aaci

fi DB V l8 0 V
Y = R¿a cos R3Ci cos — — ----- Mm - Rac - Y + PTFE Thickness
V,V2Rad / \ TT / y

Page 38 Bridge Bearings

 Memo to Designers 7-1 • June 1994

Galtrans

Mm = R*a - R«* C0Sf ( ' f ^ EL] ( ~ ) + P ™ Thickness

Use 0.09375" thick PTFE for design. Actual values vary form to Vs” as
specified in the Standard Special Provisions.

Note: Some manufacturers attach the PTFE fabric to a substratum which is

attached to the concave surface.

• Minimum Metal Thickness at Center Line (T ^ )

Tmo = 0.75" minimum

• Maximum Metal Thickness (Tmax)

Tma = Tmn + Mm+ 0.125" (Vs” allows for maximum PTFE thickness and
and substratum

• Length and Width (L^) of Concave Plate

Lcp = Projected diameter (Dm) + 1.125"

Note: 1.125" allows for edge distance,

substratum, and PTFE.

Convex Spherical Plate

• Minimum Angle (y^,) of Convex Surface

The minimum angle (/min) of the convex surface allows the maximum rotation of the
concave plate without loss of contact area.

Tmin Vmin P
y min = ymin + b

Bridge B earings Page 39

I• - - - MEMO
- -
TO DESIGNERS -
7-1 • JUNE 1994

tizltrans

• Minimum Convex Chord (Cm) Length

Derviation:

. Cm/2
SlflYmiD =--
!?act
• Height of Convex Spherical Surface (H)
Derivation:

H= Rad-z

2
Cm 2
+z 2 =Rae1
( 2 )

• Overall Height of Convex Plate (HacJ

Rael= H + 0.75"

Hae1 includes thickness from stainless steel surfacing and ¼" recessed into masonry
plate. The 0.75 inch vertical sides may be increased to provide minimum clearance,
or to provide minimum fillet weld height

PAGE 40 BRIDGE BEARINGS

' • _ _ _ _ _ _ _ _ _ _ _ _ _....;.M=EM;.;,.;.O~
T-=0--=0:::..;E=.,;S::..:.IG=N
:....:.;E=..:R..;.;S:...7.;_·--=
1-•--=J::....::U::..:.N~E:......1..;..;9::....::9;....:.4

l1zltJ'ang

• Minimum Vertical Clearance ( c)

The minimum vertical clearance (c) ensures that the concave plate does ·not contact
the base plate during maximum rotation.

Spherical bearings square in plan:

c=0.7 Lcp/3+0.125"

Spherical bearings round in plan:

c = 0.5 Dmf3 + 0.125"

V ~
.._______ _j_c
__,l-f

-~r::
: :=-=-:=--:=~~~--~--~._j_T
1.-----------'I
. . 1/s" e= minimum at
maximum rotation

Sole Plate
The sole plate must be sized so that it remains in full contact with the concave plate
under all loading conditions.

The safety overhang (L0 ) provides a minimum edge distance and allows for additional
sliding surface beyond the calculated movement The value reflects the certainty or
uncertainty of the total movement calculation.

Lcp = length and width of concave plate

Lmax = maximum longitudinal movement (including creep, shrinkage, post-
tensioning, thermal effects and seismic)
L0 = safety overhang
Tm = maximum transverse movement

• Longitudinal Length of Plate (Lsp)

Lsp = Lcp + Lmax (total) + LoL

BRIDGE BEARINGS PAGE 41

I• MEMO TO DESIGNERS 7-1 • JUNE 1994

t1zltl'an6

• Tranverse Width of Sole Plate (Wsp)

W.sp = Lcp + Tm+ Lor

• Plate Thickness (TJP)

Design in accordance with AISC design procedure for column base plates when mounted
on concrete.

Recommended minimum thickness: 0.75 inch.

• Anchorage
Anchorage may be accomplished with shear studs, bolts or welding depending on the
structure type. Studs smaller in diameter than ¾ inch are not recommended.

• Bevel
Bevel sole plate to provide a level bearing plate on steel and precast concrete girder
structures.

Masonry Plate
Design in accordance with AISC design procedure for column base plates when
mounted on concrete.

Convert convex plate area to equivalent square area to design plate thickness.

Recommended minimum thickness: 0.75 inch

Length and width to accomodate the seating of the convex plate.

Anchorage may be accomplished with shear studs, sleeved anchor bolts or welding
depending on the structure type. Studs smaller in diameter than ¾ inch are not
recommended.

PAGE 42 BRIDGE BEARINGS

I• ---~ ~
MEMO TO DESIGNERS 7-1~
• JUNE 1994

tiz/tran,

Example 4. PTFE/Spherical Expansion Bearing (Non-Guided)

CIP PIS Structure
Given:

Span 3 Hinge (See Attachment 3)

Structure Length = 785 feet

Contributory Length - 176' + 255' (CIP PIS) = 431 feet
DL Reaction/Girder- Service Load = 271 kips
LL+ I Reaction/Girder- Service Load = 50 kips
Moderate Temperature Zone - Rise and Fall = 35°F
f'c = 4 ksi
Fy = 36 ksi

Concave Plate
• Diameter (Dm) of Minimum Allowable Projected Bearing Area
Pvmax (maximum venical load)= 271 + 50 = 321 kips

Um.ix (maximum allowable PTFE pressure) = 3.500 psi

• PTFE Area (APTFE) of Flat Sliding Surface

- -]\,=
APTFE- -- - 321,000 -- 91 . 7.m.2 ( mm
. )
Urn:i.x 3,500
(91.7) 112 = 9.582 use 9.752 :. APTFE = 95 in.2

• Minimum Angle ( CXuun) Required to Prevent Uplift

PHrnv. (maximum horizontal load)= (0.10)(321) = 32.1 kips

Pvnun(minimum venicalload)=(0.50)(321)= Hi0.5kips<27lkips :. use 160.Skips

. _ _1(PHmax)-
aauo - tan -l( 160.5
- - - tan
Pvmio
32.1 )-ll 3lo - .

BRIDGE BEARINGS PAGE 43

, i) _____________ ....:M:..:.=E~M:.=O....:T~0:.....:0=:.:E=..:S::.:.IG.:::.N:..:.E=.:R..:.:S::....:..7-_1:__•....:J::.:U::.:.N~E=-1..:..;9::.:9:...;.4

l1zltran.s

• Minimum Angle ( 1/fmm) of Concave Bearing Surface

lXuuo = angle required to resist uplift = 11.31 °

f3s = Structure rotation, 0.003 radians = 0.17°

f3c = Construction rotation, assume 0.02 radians= 1.15°

/3 = f3s + f3c = 1.32° use 2° min = 2.0°

Note: Bridge Design Specifications, Article 15.2.2 requires a minimum rotation
capacity of 0.015 radians (0.86°).

l/lmio = lXmio + /3 = 11.3 1° + 2° = 13.31°

• Maximum Allowable Radius (RmvJ of Concave Surface

Rmu = . Dm = l LOO = 23.89" (Okay to round)

2sm (lJlm.in) 2sin (13.3 1°)

Use Rad= 23.75" < 36" Okay

• Minimum Concave Bearing Pad Diameter (DBacJ

2
= 2[23.75[_!:_(sin- 11 1.00/ ) ]] = 11.10" (Do not round)
180 23.75

• Minimum Metal Depth (Mm) of Concave Surface

Mm = Raci - Mm = Rac.1 - [ Raa. x 7

cos (( DBaa l80 ))] + PTFE Thickness
.
2

M. = 23.75 -[23 75cos((

2
g/~ )(1!
5
)
0
))] + O09375"= 0.739" (Do not round)

PAGE 44 BRIDGE BEARINGS

i I, - - - - - - - - -MEMO
' -TO=DESIGNERS
'~ 7-1~
• JUNE~
1994

tiz/trll/1$

• Minimum Metal Thickness at Centerline, (Tmin)

T rruo = 0.75"

• Maximum Metal Thickness, (Tmax)

Tmv. = Tmin+ Mm+ 0.125 = 0.75 + 0.739 + 0.125 = 1.61 - 1.625" (Round up)

• Length and Width (Lcp) of Concave Plate

Lcp = Dm + 1.125 = 1 LOO"+ 1.125 = 12.125" (Okay to round)

Convex Plate
• Minimum Angle ( Yaun) of Convex Surface

-(DBac.1
- - + f3
Ynun- - -)(180)
2Rac1 1r

. . = ( 11.10 )(180)+ 20 = 15_390

Ynuo 2(23.75) 7r

• Minimum Convex Chord ( Cm) Length

Cm= 2[23.75(sin(l5.39))] = 12.61" Say 12.625" (Okay to round)

• Height of Convex Spherical Surface (H)

H=23.75 - 23.752 -(12·i25)2 =0.854"-0.85"

BRIDGE BEARINGS PAGE 45

I I) ______________ M:. .:. ;.:; E.;. :.M;.; ;o;._T;. : o:. . ;D=-=Es:: . :1.;: .G;_;,NE;: ; R. . :.;s:. . ;_7-;._1;._•;._J::. . ;u:;.;,N.;.;:E:. . ;1~9::. . ;9; __;_4

lblt,tll16

• Overall Height of Convex Plate (HacJ

Hact. = H + 0.15

Hact. = 0.85 + 0.75 = 1.60"

• Minimum Vertical Clearance (c)

C =0.7 lcp/3 + 0. 125 (/3 is in radians)

C =0.7 (12.125{2°(i;0 ))+0.125 =0.42" minimum required

Actual vertical clearance provided is greater than 0.50" (0.75 inch vertical sides
on the convex plate are recessed 0.25 inch into masonry plate).

Sole Plate
• Longitudinal Length of Plate (Lsp)

Lsp = Lcp + Lrnu. (total) + LoL

Lcp = 12.125 (Concave Plate)

Temperature movement= (1.5) (2) (0.0000060) (35°F) (431') ( 12 in/ft) = ± 3.26"

P/S shortening= (0.70)(0.10 ft/100 ft)(431 ')(12 in/ft)= 3.62"

Seismic Movement = ±3.0"

Lrnu = 3.26 + 3.62 + 2 (3.0) = 12.88"

L0 L = 2 (1.0) = 2.0" (Edge distance)

Lsp = 12.125 + 12.88 + 2.0 = 27.00"

PAGE 46 BRIDGE BEARINGS

' t) _____________ ...:M=E;;.;.;M.;:;.O...:T...:::;0...:0:;_E::.:S::.:.IG.=..:.NE::.:R....:.;S~7-...:1;.....•;.....J:;_;U::.:.N.:.::E:;_1...:9::....;9:._:_4

Qz/t:rllll$

Notes: 1) Thermal movement was multiplied by 2 because 35°F is rise or fall

temperature.
2) Thermal movement was multiplied by 1.5 because it is not always
possible to place the sole plate at a "mean" temperature.
3) Position sole plate to account for one directional movement of P/S
shortening.
4) Approximately 70% of PIS shortening remains at time of hinge
closure pour (see Attachment 1).

• Transverse Width of Sole Plate (Wsp)

Lcp = 12.125"

Tm= ±l.0" (Seismic)

L07 = 2 (1.0) = 2.0" (Edge·distance)

Wsp =12.125 + (2)(1.0) + 2.0 = 16.125" Say 16.00"

• Plate Thickness (Tsp)

Design in accordance with AISC design procedure for column base plates.

Lsp = 27 .00"

Wsp = 16.00"

J! = 4 ksi F1 = 36 ksi

(Ref. BDS, Article 8.15.2.1.3)

Assume for this example that A2/A 1 = 1.5

Maximum bearing pressure (fb) on loaded area

/b = 0.30 X 4,000 X .JLl = 1,470 psi

BRIDGE BEARINGS PAGE 47

I • _____________ ....;M:.=E.;.;.;M:.:::.o....;T...;:o;_;D=-E
=-s:..:.1..:::G~NE=-=R...;..:s:;__;_7-_1~·-J::....:u:..:.N..:.::E~19::....:9::.....:.4

tiz/tranG

Determine Required Plate Area

321 000
• = 218.37 in.2
1,470

Since the length of the sole plate was determined for sliding purposes, determine
the required length to distribute the load to the concrete.

218.37 = 13.65" 2.125·· I

16.0
PT FE
Sliding
r;t~~~3:::;:==j="t
Use plate size of 16" x 14" to design Surface in
It)

t--: .....
C\I
.....
thickness. C\i
O>
.....
Actual bearing pressure (fb) on loaded area
9.75"
12.125"
Jb = 321•000 = 1,433 psi< 1,470 Okay
14x16

Note: Model shows 1" transverse movment (Tm)- Design Model

Determine Plate Thickness

(AISC 3-106, 9th Edition)

1 433
T;p =2(4.125) • =1.65 use 1.75" > 0.75 minimum
36,000

• Anchorage
Since structure is cast-in-place PIS, use shear studs.

• Bevel
Bevel is not required for cast-in-place concrete.

PAGE 48 BRIDGE BEARINGS

• II _____________ ....:.M::.:;E::.:M.:.:.o=---=-T~o-=D::...::E::.::s:.:..;1G::.;,N..;,;:E::..;_R:.:.s;_7;_-....:.1_•...:J:..::u;.;..N:=E;_1:....:9:..::9:.....:.4

lb/t,1111$

Masonry Plate ·
Design in accordance with AISC design procedure for column base plates.

Cm= 12.625"

lb= 1,470 psi (A2/A1F 1.5)

J; = 4,000 psi

• Determine Required Area

Area= Wmp X Lmp

321 000
• = 218.37 in.2 = 14.78" X 14.78"
1,470

Use 15" x 15" plate

Wmp= 15"

Lmp = 15"
n
Actual Bearing Pressure (fb) on Loaded Area

321 000 15"

lb= • = 1,427 psi< 1,470 Okay
15 2

Equivalent
Square Area
15"

• Determine Plate Thickness

Convert convex plate area to equivalent square to determine (n)

2 625
Area Convex Plate= n{l - )2 = 125.19 in.2
4

BRIDGE BEARINGS PAGE 49

' • _ _ _ _ _ _ _ _ _ _ _ _ _....;M=-=E~M:.=O....;T:...:O:..;O=ES:::.:l:.=G::...:N=E:..:.RS::....:..7_-1.:__•_J::.:U~N-=E::._:_19:::.;9:::....:.4

t1zltrans

125.19 in.2 = 11.19" X 11.19"

n = (15" - 11.19)/2 = 1.91"

1,427
Tmp = (2)(1.91) 1- - = 0.76"+ recess depth
36,000

Tmp= 0.76 + 0.25 = 1.01" use 1.00" plate

• Anchorage
Since structure is cast-in-place, use shear studs

• Determine Total Bearing Height (Ah)

Ah= Tmp + Hae, - 0.25 + Trrun +Tsp+ 2(PTFE thickness)+ (sole plate stainless
thickness)

= 1.00 + 1.60 - 0.25 + 0.75 + 1.75 + 2(0.09375) + 0.060

=5.10"
Notes: Actual bearing thickness may vary slightly depending on the thickness of
the PTFE, substratum, and stainless steel used. Minimum and maximum
values are given in the Standard Special Provisions.
Use Bridge Standard Detail Sheet XS 12-80 if possible (see Attachment 4).

PAGE 50 BRIDGE BEARINGS

I• MEMO TO DESIGNERS 7-1 • JUNE 1994

Cb/trans

P'fF'E/Elastomeric Bearings

General
P'IFE/elastomeric bearings designed in accordance with Bridge Design Specifica-
tions, Sections 14 and 15 should be considered for use only when the practical limits
of reinforced elastomeric bearing pads have been exceeded.

The P1FE/elastomeric bearing concept and design procedure covered in this section
was adopted with few exceptions from Ted Jensen' s paper titled "ElastomericrrFE
Bearings", (October 1987).

The basic PTFE/elastomeric bearing design is simply comprised of PTFE disks

sliding on stainless steel surfaces to accommodate the longitudinal movements and
elastomeric bearing pads to accomodate the rotational movements (see Figure 10.
page 58).

P1FE/elastomeric bearings are suitable for structures with moderate to large longi-
tudinal translations, and relatively small rotations. This non proprietary bearing is
simple to design and fabricate. Good performance can be attained with careful
attention to loading, rotation and the physical properties/limitations of the manufac-
turing materials.

PTFE/Elastomeric Bearing Components

PTFE/elastomeric bearings are comprised of five basic components: (1) sole plate,
(2) PTFE disk, (3) intermediate plate, (4) elastomeric bearing pad and (5) masonry
plate. The function of the five components mentioned above, and the typical
manufacturing materials are as follows, (see Figure 10, page 58):

• Sole Plate- Transfers superstructure loads to the bearing and provides a stainless
steel sliding surface for super structure translation. The sole plate is fabricated
from A36/ A36M steel and has a stainless steel surfacing. The stainless steel
surface is bonded to the sole plate with epoxy resin and stainless steel cap screws,
or by perimeter welding.
• PTFE Disk- Provides a low friction sliding surface for the sole plate. The PTFE
disk is manufactured from 100 percent pure virgin unfilled dimpled sheet resin.
The PTFE disk must be recessed one-half its thickness to control cold flow.
• Intermediate Plate - Transfers loads from PTFE disk to elastomeric pad. The
intermediate plate is manufactured from A36/A36M steel.

BRIDGE BEARINGS PAGE 51

' • ------=---__ , ;TO, DESIGNERS
MEMO _ , , ; 7-1
, ,• _ _
JUNE -
1994

tizltrans

• Elastomeric Bearing Pad-Allows rotation of the superstructure while maintain-

ing 100 percent contact between the PTFE disk and the sole plate. The steel
reinforced elastomeric bearing is fully vulcanized to the steel plates. Fabric
reinforced pads are not allowed.
• Masonry Plate-The masonry plate transfers load from the elastomeric bearing
pad and anchors the bearing to the seat. The masonry plate is fabricated from
A3/A36M steel.

Design Requirements
P1FE/elastomeric bearings are designed in accordance with Bridge Design Specifi-
cations, Sections 14 and 15. All loads are ser:vice loads. Minimum vertical loads are
for dead loads and superimposed dead loads. Maximum vertical loads are for dead
loads, superimposed dead loads and live loads (no impact).

Unfilled PTFE sheet resin stresses are limited to 3,500 psi maximum. The design
coefficient of friction varies from 0.08 to 0.04 at bearing pressures of 500 psi and
3,500 psi respectively.

Steel reinforced elastomeric bearing pads with shape factors 7.5 may be loaded to
a maximum stress of 1,000 psi. The shear modulus (G) used for design is 100 psi.

Design Guidelines
PTFE surfaces should be loaded to a minimum of 2,000 psi (DL only) for optimum
performance. A design coefficient of friction of 0.06 is recommended for designs
with bearing pressures from 2,000 psi to 3,500 psi. Actual lubricated friction values
are lower, however, they should not be used for design because the long term effects
of the grease are unknown.

A minimum and maximum PTFE thickness of 3/26 inch and ¼ inch respectively
should be shown on the contract plans; as the limits are not specified in the Standarfi
Special Provisions.

To reduce rotational stresses, orient rectangular bearings so the long side is parallel
to the axis about which the largest rotation occurs.

A bearing pad with a low shape factor accomodates rotation most readily, and a
bearing pad with a high shape factor is best for resisting compresssion. 1 Therefore,
the best choice represents a compromise between the two. A minimum shape factor
of 7.5 is recommended.

PAGE 52 BRIDGE BEARINGS

f 6 _____________ ....:M=EM=o...:.T..;::o-=D::..:E::..;s::;.;,.1G;::;N;,.:..:E::..;R...;.;s:.....7~-_;1_•....:J::..:u::;.;..N.:.::E:.....1.:....:9::..:9~4

liz/trans

Design Procedure

Elastomeric Bearing Pad

1. Determine the width (W) and length (L) of the elastomeric pad for the applied
vertical load (DL + LL not including impact) using an allowable unit stress of
1,000 psi. 5 To use 1,000 psi the shape factor (S) 7.5.

DL+U =WxL
1,000 psi

2. Check the compressive strain of the elastomer due to dead load and live load from
the stress/strain curves for various shape factors shown in Figure 11 (page 59).
These curves, developed by the California Transportation Laboratory,2 are based
on tests of pads constructed with ½ inch layers of elastomers between steel plates
meeting California specifications. To account for compressive creep of the
elastomer under sustained dead load, the initial deflection from dead load is
increased by 25 percent. The total deflection from dead load (DL) and live load
(LL) shall not exceed 0.07 times the thickness of the elastomeric bearing.

3. Determine the initial thickness of the elastomer required for structure rotation.
The structure rotation should include rotations from DL, U, camber changes,
construction tolerances and erection sequences.

The relative rotation between top and bottom surfaces of the bearing shall be
limited by:

Lal + Waw 26.c for rectangular bearings

D~a.1 + a.a, : :; 26.c for circular bearings

6.,=r.ecit; :. t.,=e,O(T
i

Therefore, the elastomer thickness (7) may be determined from:

T~ LaL+Waw (rectangular bearings)

2EToc

(circular bearings)

BRIDGE BEARINGS PAGE 53

' 6 _____________ ....:M..::.:.=.E.:.;.;M;=:O....:T..::0....:0:::;,.E==-S=..:l..;;;;G;..;;.N=ER~S=----=--7--1:__•_J;;;..;U=..:N..:.:E=----=--19=..:9~4

lb/trans

L = gross length of rectangular bearing parallel to the longitudinal

axis of the bridge (in.)
W = gross width of rectangular bearing perpendicular to longitudinal
axis of the bridge (in.)
D = gross diameter of circular bearing (in.)
a L, (aw) = relative rotation of top and bottom swfaces of bearing about an axis
perpendicular, (parallel) to the longitudinal axis of the bridge (radi-
ans)
l::.c = instantaneous compressive deflection of bearing (in.)
Ee; = compressive strain of ith elastomer layer (change in thickness
divided by unstressed thickness)
£Tot = total compressive strain of elastomer
t; = thickness of ith elastomer layer (in.)
T = total elastomer thickness of bearing (in.)
= I:.t;

4. Determine the Maximum Allowable Shear Force (Fs) in the Elastomer

A
F's= G-1::.s
T
~s = Shear deflection of bearing (in.)
G = Shear modulus of elastomer (psi) at 73°F
A = plan area of bearing (in.2)

The maximum allowable shear force in the elastomer must be greater than the
maximum lateral force required to slip the P1FE disk under dead load (see
Figure 12, page 60).

Note that the shear modulus (G) decreases with increasing temperature and
increases with decreasing temperature. A value of 100 psi is recommended for
this calculation.

The maxim um shear deflection (~s) in the elastomer shall be limited by:

PAGE 54 BRIDGE BEARINGS

i• MEMO TO DESIGNERS 7-1 • JUNE 1994

tizlutln6

If the maximum allowable shear force is exceeded, the area of the elastomeric pad
may be increased to provide greater shear capacity. It is evident from the above
formulas that the elastomer design is sensitive to both the shear modulus and the
friction force transmitted through the stainless steel sliding surface.

PTFE Disk
1. Determine the area of the PTFE disk required to support vertical loads, (DL + U,
no impact), using a 3,500 psi maximum compressive stress. Note that the
allowable compressive stress for the PTFE is 3.5 times the allowable stress for
the elastomer. To minimize the thickness of the intermediate plate in which the
PTFE is recessed. the length, width or radius of the PTFE should be such that the
edge distance is held to a minimum. A 2,000 psi to 2,500 psi (DL only) design
compressive stress on the PTFE should provide a reasonable intermediate top
plate thickness.

PTFE disks are recommended to facilitate fabrication of the recess in the steel
intermediate plate.

2. Calculate the Lateral Force (F1) required to slip the P1FE disk under Dead Load.

F1 =µN
µ = friction coefficient
N=Dead Load

The friction values given in the "PTFE Sliding Surfaces" section and Article
15.2.7 of the Bridge Design Specifications should be used for this calculation.
Note that the actual coefficient of friction will probably be less because the
stainless steel slider plate will be coated with silicone grease. Initial coefficients
of friction as low as I to 2 percent were observed by the Transportation
Laboratory on greased samples loaded to 3,170 psi. However, these low friction
values should not be used for design because the long term affects of the silicone
grease are not known.

3. Compare the Maximum Allowable Shear Force (F1 ) in the elastomer with the
Lateral Force (F1) required to slip the PTFE under Dead Load.

BRIDGE BEARINGS PAGE 55

I & _____________
l

...:M=EM:.:.:o.::::....:T--=o...:D::.:E:=.:s::.:.1G-=-N:...:.:E::.:.R..:.:s=--7.:....·...:1_•...:J:.::u::.:.N-=E=-1.:....:9:.::9:.....:.4

tillatl/16

Intermediate Plate
1. Size the intermediate plate, length and width to match the dimensions of the
elastomeric bearing pad.

2. Determine required plate thickness. Design in accordance with AISC design

procedure for column base plates. Suggestion-convert PTFE disk area to
equivalent square area to design plate thickness.

Sole Plate
The sole plate must be sized so that it remains in full contact with the P1FE disk under
all loading conditions.

The safety overhang (L0 ) provides a minimum edge distance and allows for additional
sliding surface beyond the calculated movement. The value reflects the certainty or
uncertainty of the total movement calculation.

DD = Diameter of PTFE disk

Lron =
Maximum longitudinal movement, (including: creep, shrinkage, post
tensioning, thermal effects and seismic).
Tm = maximum transverse movement
L0 = safety overhang

1. Longitudinal Length (Lsp) and Transverse Width (Wsp) of Sole Plate

Lsp = DD + Lf'CIU (total) + LoL

Single Disk

2. Plate Thickness (Tsp)

Design in accordance with AISC design procedure for column base plates when
mounted on concrete. Recommended minimum thickness: 0.75 inch.

3. Anchorage may be accomplished with shear studs, bolts or welding depending on

the structure type. Studs smaller in diameter than ¾ inch are not recommended.

PAGE 56 BRIDGE BEARINGS

I• MEMO TO DESIGNERS 7• 1 • JUNE 1994

t:altrans

Masonry Plate
1. Size the masonry plate, length and width to match the dimensions of the
elastomeric bearing pad unless a larger plate is required for anchorage.

2. A plate thickness of 0.75 inches is recommended.

3. Anchorage may be accomplished with shear studs, sleeved anchor bolts or

welding depending on the structure type. Studs smaller in diameter than ¾ inch
are not recommended.

Testing
Until recently our policy was to test scale bearings fabricated in the same manner as
the full size bearings. The test bearings were detailed in the contract plans and tested
at the Transportation Laboratory. This practice was abandoned June 1994 after it was
determined that some test bearings were not representative of the actual bearings
delivered to the job site.

The current specifications required that full sized P1FE bearings be proof tested and
evaluated for compression and coefficient of initial static friction in the presence of
the Engineer. The specifications also require that the manufacturer furnish one
sample of elastomeric bearing to the Transportation I,,aboratory for testing. Test
bearings should not be detailed in the contract plans.

BRIDGE BEARINGS PAGE 57

I & _____________ ....:M.:..;.=.
EM ;.;_;,o~T..;;;
o ....:D
::..;E::..;s;_1G.;::;.N:..;;E::.;R..;.;s;;_7.:...·....:1....:•....:J::..;u;_N,;,::E;_1_;_;9
:;_;:9~4

11z/trans

Longitudinal
Sole//?_ Movement

Intermediate Fe. + ~ ;....-,.;:;....,..:;;;:;;;:;;;i~..L~ ~""-l.,L~~~,.i;..~:....~ - - Rotation

"?t Movement
Elastomeric tff.:
t>:O
Bearing Pad :-,:;' Q '
P"i)
o<
:s·~
-~·o
Pi

Figure 10. PTFE/Elastomeric Bearing

PAGE 58 BRIDGE BEARINGS

I • _____________ .....;M..;..;.;;;;.E;.;.;M..:;;.O.....;T....;;:o.....;D:;_E:;_s=1..:;;.G~N=ER-'s~7--'1_•.....;J::..;:u~N.:.::E;.._1,;_;9::..;:9:......:.4

til/atl/l6

1,200 r----r--..,..----,-""'T""--,---y--,-----,---rr----r---,.---,----,----...---,-,

Shape Factor 12

1,000

Shape Factor 6

800

Shape Factor 3
ui
0..
vi
...
(/)
Cl)

ci5
Cl) 600
>
·u;
...
(/)
0)
0..
E
0
0
Steel Reinforced
Bearing Pad
400

200

0 _ __.___.....__ _.__ __.__ __.._ _.___......__...,___......__ __,__ __.__ __,_ __.

0 1 2 3 4 5 6 7 8 9 10 11 12 13
Compressive Strain, percentage

Figure 11. Recommended Compressive Stress vs. Strain Curves

for Steel Reinforced Bearing Pads 2

BRIDGE BEARINGS PAGE 59

I • _____________ ....:M=EM=o....:T...;;;o....:D::..:E
:=.;s:;.;,.1G=N;..:.;E=..:.R..:.;s::...7..:...-.....:1_ •.....:J::..:u:;.;,.N.;;:E::...1..:...9::..:9:....:..4

Ozltrtzn6

A
Fs = G-fls
T
T 2fls
F1 =µN
F1 SFsmu
Fs = shear force in elastomer
G = shear modulus (100 psi@ 70°F)
T = total elastomer thickness
fls = shear deflection of bearing
F1 = force required to slip P1FE disk under DL
N =DL
µ = coefficient of friction

Figure 12. Shear Force in Elastomer

PAGE 60 BRIDGE BEARINGS

' • - - - - - . : . :MEMO
. . TO=
DESIGNERS
~ 7-1~
• JUNE~
1994

ti,Jt,ang

Example 5. PTFE/Elastomeric Bearing CIP PIS Structure

Given:

Span 3 Hinge (See Attachment 3)

Structure Length = 785 feet

Contributory Length - 176' + 255' (CIP PIS) = 431 feet
DL Reaction/Girder - Service Load = 271 kips
U Reaction/Girder - Service Load, No Impact = 43 kips
Moderate Temperature Zone - Rise and Fall = 35°F
le' = 4 ksi
Fy = 36 ksi

Elastomeric Bearing Pad

1. Determine Width (W) and Length (L)

DL+U=WxL 271 kips+ 43 kips= 3 14 in.2

1,000 psi 1,000 psi

Try 12" x 28" Area = 336 in.2 > 314 Okay

Note: Slender bearing pad selected to maximize rotation capacity.

12x28 .
Shape Factor (S) = - - = 8.4 > 7.5 : . 1,000 psi Okay
12+28

Actual load on elastomer:

271 kips+ 43 kips = si

934 (DL+ LL)
12x 28 P

271 kips =807 si (DL)

12x28 P

BRIDGE BEARINGS PAGE 61

I• MEMO TO DESIGNERS 7-1 • JUNE 1994

tizltrans

2. Check Compressive Strain

S = 8.4, obtain strain values from the curves shown on Figure 11 (page 59).

Pn= 934 psi en=4.4%

PDL = 807 psi

ETotal = 4.4 + (4.0) (0.25) = 5.4% < 7.0% Okay

3. Determine Initial Thickness (7) for Rotation

assume Waw negligible

:. T LaL = (l 2)(0.0I 5 ) = 1.67" say 2" (elastomer only)

2ET01 2(0.054)

Structure Rotation f3s = 0.003 radians

Construction Rotation f3c = 0.01 radians

Bridge Design Specifications require 0.015 radians (minimum)

4. Determine the Maximum Allowable Shear Force (Fs) in the Elastomer

A 12x28 .
F's= G-lls = 100 x - - - x 1 = 16.8 kips
T 2

I:!,. T 2 l"
=-=-=
smax 2 2

PAGE 62 BRIDGE BEARINGS

' t- _____________ ....:.M.:.:=.E:.:.:M.::.O....:.
T..;:O:_:D:::.:E::.:S::.:.l.=G:...:.N:.:ER~S::....:..7-_:1~•--=J:..:U::.:..N:.=E:_1.:..:9:..:9~4

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PTFE Disk
1. Determine Area of PTFE Disks

Use Two Disk Design:

. 271 kips n DD 2
DL Area Required = - ---=--
2 X 2,500 psi
=-
4
- Dv =8.3"0

Try Dv = 8.5"0
271
DL Stress on PTFE = ~-~ 2 = 2,388 psi> 2,000 psi Okay
2xnx-
4

271
DL + U on PTFE = + :~52 = 2,766 psi< 3,500 psi Okay
2xnx-
4

Use two V4" x 8½"0 PTFE disks.

2. Calculate the Lateral Force (F1) Required to Slip PTFE

µ =0.06
N = 271 kips (DL)

F1= 0.06 X 271,000 = 16.2 kips

3. Compare Allowable Shear Force (Fs) with Slip Force (FF)

Fs Fi 16.8 > 16.2 kips Okay

BRIDGE BEARINGS PAGE 63

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- 1;_•_J::....:u:;..;.N..;;:E~19::....:9::;...;.4

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Intermediate Plate
1. Size Intermediate Plate Length (L) and Width (W), to match Elastomeric Pad
Dimensions

L= 12"
W=28"

2. Determine Plate Thickness (Tp)

14"

2.75" - 1-- ~.....:::;::22-'-'--.5"- - - - i

5.5"
• 1 -- n 7 5" n

-- - -,
I
I
7.5•
I
I
I

'- - - _,
I

PTFE Disks lntennediate

Plate
Actual Bearing Design Model

f,, = Actual bearing pressure on elastomer

Fy = Steel yield strength
Fb=0.75 Fy

Model - Convert Disk to Equivalent Square to Determine (n)

+ = 271 k+43k = 934 si

JP 12x28 p

n= 14-7.5 =3.25"
2

Total Plate Thickness (Tp) including ¼" Recess

Tp = 1.05 + 0.125" = 1.17'' say 1.25" thick

PAGE 64 BRIDGE BEARINGS

I• - - - MEMO
- - -
TO DESIGNERS 7-1 • JUNE 1994

liz/aans

Sole Plate
1. Longitudinal Length (Lsp) and Width (Wsp)

Temperature movement= (l.5)(2)(0.0000060)(35°F)(431')(12 in./ft) = ±3.26"

PIS Shortening= (0.70)(0.10 ft/100 ft)(431')(12 in./ft) = 3.62"
Seismic Movement = ±3.0

4nv. = 3.26 + 3.62 + 2 (3.0) = 12.88"

L0 L = 2(1.0) (Edge distance) =2.0"
DD =8.5"
Tm = (Seismic) =±1.0"
Lsp = DD + 4nv. (total) + Lol
Lsp = 8.5 + 12.88 + 2 = 23.38" say 24.0"
Wsp =DD+ Tm+Lo
LOT = 2(1.0) (Edge distance) =2.0"
Wsp = [(2)(8.5) + 5.5) + 2 (1.0) + 2.0 =26.5"

Notes: 1) Thermal movement was multiplied by 2 because 35°F is rise or fall

temperature.
2) Thermal movement was multiplied by 1.5 because it is not always
possible to place the sole plate at a "mean temperature".
3) Position sole plate to account for one directional movement of P/S
shortening.
4) 70% of PIS shortening remains at time of hinge closure pour (see
Attachment 1).

2. Plate Thickness (Tsp)

Design in accordance with AISC design procedure for column base plates
mounted on concrete.

Lsp = 24.0" Wsp = 26.5"

J! = 4 ksi , - - - F1 = 36 ksi
/b = 0.30/!.JAi I A1 :s; 0.60/; (Ref. BDS, Article 8.15.2.1.3)

Assume for this example thatA 2 /A 1 = 1.5

Maximum bearingpressure(fb)onloadedarea: /b = (0.30)(4,000)ill = 1,470 psi

BRIDGE BEARINGS PAGE 65

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3. Determine Required Plate Area

271 43
+ = 213.6 in.2
1,470

Since the length of the sole plate was determined for sliding purposes, determine
the required length to distribute the load to the concrete.

213 ·6 in·2 =8.1" usediskdiameter + edgedistance(8.5+2)=10.5"

26.5

Design Thickness for 26.5" x 10.5'' Plate

n 13.2s·
1.5"

10.S-
--- -,
I
I
I
I
1·
26.5"
5.5" 8.5"
0

p:
'----·
I I I
1_ - - - I _ ___._ I
, ', I
, ' ,

..
I I I I

... _
\ I \
' ,

Design Model
Sole Plate

Actual Bearing

Model lnformation

• Convert disks to equivalent square

• 1" transverse movement (Tm) shown

fh = Actual bearing pressure

Fy = Steel yield strength (36 ksi)
Fb = 0.75 Fy

PAGE 66 BRIDGE BEARINGS

' I, - - - MEMO
- -
TO DESIGNERS -
7-1 •JUNE 1994

tilllYanG

Ji = 271 k + 43 k =1 128 si
b 2(10.5 X 13.25) ' p

1 128
Tsp= 2 X 4.25 X • = 1.50"
36,000

Masonry Plate
1. Size Plate Area to Match Elastomer Area

Use 12" x 28" plate

2. Plate Thickness

A plate thickness of¾" is adequate since the masonry plate has the same area and
load as the elastomeric pad.

934 psi< 1,470 psi.

3. Anchorage

Since structure is cast in place, use shear studs.

BRIDGE BEARINGS PAGE 67

,. I, - - _ _ _ _ _ _ ; ; ; . ~ ~ ~
MEMO TO DESIGNERS 7-1 • JUNE 1994

lb/trans

Summary of Calculations
Plan Bearings

Elastomeric Bearing Pad 2" X 1'-0" X 2'- 4"

P1FE Disk (2) ¼"x8½"0
Intennediate plate 1¼" X 1'-0" X 2'-4"
Sole Plate 1½" X 2'-0" X 2'-2½"
Masonry Plate ¾" X l '-0" X 2'-4"

T otal Bearing Height

Sole Plate 1.50

Stainless Steel 0.060
P1FE Disk (¼" thick recessed 118'') 0.125
lntennediate Plate 1.25
Elastomer 2 + (4)(0.075) · 2.30
Masonry Plate 0.75

Total 5.99" - 6.0"

RL:jgf
Attachments

PAGE 68 BRIDGE BEARINGS

' j!, _____________ ....;M~EM=O....;T..::::O....;D=.,;E=S~l.:::G.:...:.N=ER~S::......:..7_-1=-•-J:::..;:U~N~E=--1..:....:9:::..:9~4

lb/tranG

References
1. AASHTO LRFD Design Specification (Version 4.02) Section 14, Joints and
Bearings.

2. ..An Evaluation of Fiberglass and Steel Reinforced Elastorneric Bridge Bearing

Pads," dated January 1982 by the Transportation Laboratory of the State of
California.

3. ''A Laboratory Evaluation of Full Size Elastomeric Bridge Bearing Pads," dated
June 1974 by the Transportation Laboratory of the State of California.

4. NCHRP Report 10-20A "High Load Multi-Rotational Bridge Bearings," Final

Report -John F. Stanton, Charles W, Roeder, T. Ivan Campbell.

5. Caltrans "Elastomeric/fFE Bearings," by Ted Jensen, P.E., dated October 1987.

6. Caltrans QAI Newsletter No. 3, "Suggestions to Avoid Future Bridge Problems,"

August 3, 1992.

BRIDGE BEARINGS PAGE 69