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CVL342
Ch.3: Bolted Connections
PROF. SVETHA VENKATACHARI
DEPT. OF CIVIL ENGINEERING
IIT DELHI
Introduction
Connections –
◦ Flow of forces/moments from one member to another
◦ transfer of forces to the foundation
◦ Extending the length of the member
◦ Joining different parts of the structure during erection
A structure is only as strong as its weakest link – the design of
connections is critical
Connection failure is to be avoided –
◦ Can lead to the collapse of the whole structure.
◦ Connection failure is not ductile.
◦ To achieve an economical design, the connectors must develop full
(or a little extra) strength than the members.
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Types of Connections
1. Methods of fastening – rivets, bolts, and welds
2. Connection rigidity – simple, rigid, or semi-rigid
3. Joint resistance (bolted connections) – bearing
connections and friction connections
4. Fabrication location – shop or field fabricated Rivets
5. Joint location – beam-to-column, beam-to-beam, and
column-to-footing
6. Connection geometry – single web angle, single plate,
double web angle, top and seat angles, end plate, header
plate, etc.
7. Type of force transferred – shear connections, moment
connections, tension or compression, and tension or
compression with shear.
Ordinary bolts Welded girder
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Rigid Connections
Rigid connections develop the full moment capacity of the connecting members and retain the
original angle between the members under joint rotation
Eccentric bolted Bolted bracket Eccentric welded Flush end plate Extended end Column splice
bracket bracket plate
Beam splice Column base plate Moment connection in a portal frame
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Reality: All connections are semi-rigid. Some amount
Simple Connections of moment capacity exists.
Simple connections – no moment transfer is assumed between the connected parts and hence, as
hinged. Large rotational movement is present in these connections. Eccentricity less than 60 mm
is neglected.
Tie rod
Beam-to-column Beam-to-column Beam-to-beam
Beam-to-column Beam-to-column Beam-to-beam Column-to-footing
Bolted Connections
Types of bolts:
1. Black bolts (IS 1367) – least expensive, light structures, static loading, sometimes as temporary bolts during erection.
2. Turned bolts – similar to black bolts; shanks are made from a hexagonal rod; small tolerance – 0.15 to 0.5 mm; used
when no slip is permitted – special jobs; Grades 4.6 to 8.8 is used.
3. High-strength friction grip bolts (IS 3757) – An initial tension is induced which causes sufficient friction to eliminate
the slip in the joint under service loads. The induced tension is called the proof load, and the friction coefficient is
called the slip factor. Class 8.8 and 10.9 bolts are commonly used.
Grade of bolts:
As per the international standards organization, the bolts grade is x.y. x is 1/100th of the ultimate strength of the bolt in
N/mm2 and y is the yield-to-ultimate stress ratio.
For e.g., grade 4.6 bolts have an ultimate tensile strength of 400 N/mm2 and yield strength of 240 N/mm2.
Bolts of sizes 5 – 36 mm diameter are available and are designated as M5 to M36.
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Bolt Tightening Methods
Snug-tight: Defined as the tightness that exists when all the plies in a joint are in firm contact.
Black bolts are tightened to the ‘snug-tight’ condition.
Tightening methods for HSFG bolts (IS 4000):
1) Turn-of-the-nut tightening – Bolts are first made snug-tight and then the nut is turned by a specific amount
(half or three-fourth turns) to induce tension equal to the proof load.
2) Calibrated wrench tightening – The bolts are tightened using a wrench calibrated to produce the required
tension.
3) Direct tension indicator method – Special washers with protrusions are used. As the bolts is tightened, these
protrusions are compressed and the gap produced by them gets reduced in proportion to the load. The gap is
measured by a feeler gauge, having steel plates of varying thickness, which can be inserted into the gap.
Bolt Tightening Methods
High-strength bolted connection (b)
(c)
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Possible Failure Modes of Bolted Joints
• Shear failure of bolts/plate
• Bearing failure of bolts/plate
• Tensile failure of bolts
• Bending failure of bolts
• Tensile failure of plate
Specification for Bolted Joints
Bolt diameter – Few large-diameter bolts cost less. Larger diameter bolts are favorable in connection
where shear governs because the bolt capacity in shear varies as the square of the bolt dia.
Bolt holes (Table 19) –
◦ Usually drilled. Sometimes punched holes are used.
◦ Bolt holes are made larger than the bolt diameter to facilitate erection and allow for inaccuracies in fabrication.
Spacing of bolts –
◦ Pitch (p) – C/C distance between two consecutive bolts measured along the direction of load. If the bolts are placed in a zig-
zag pattern, then the pitch is referred to as a staggered pitch.
◦ Min. pitch: 2.5 times the nominal diameter (Clause 10.2.2)
◦ Max. pitch (Clause 10.2.3) – to reduce the length of the connection, to have uniform stress in bolts, avoid failure of plates in
built-up tension/compression members.
◦ Gauge (g) – C/C distance between adjacent bolt lines or the distance between the back of a rolled section and the first bolt
line, or the c/c distance between two consecutive bolts along the width of the member (refer to SP 6(1))
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Specification for Bolted Joints
Edge and end distances –
◦ Edge distance – Distance at a right angle to the direction of load/stress from the center of the bolt hole to the
adjacent edge of the member
◦ End distance – Distance in the direction of load/stress from the center of the bolt hole to the end of the member
◦ Min. distances are specified to avoid the failure of the plate in tension.
◦ Max. distances are specified to avoid moisture getting between the parts.
◦ Edge and end distances are specified in Clause 10.2.4.
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Shear Connections with Bearing Type Bolts
Force transfer of bearing type bolts:
The tension in one plate is equilibrated by the bearing stress between the bolt and the hole in
the plate.
The bearing stress is mobilized only after the plates slip relative to one another and start bearing
on the bolt.
Section x-x in the bolt is the critical section for shear.
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Shear Strength of Bolts
• The bolt shank shear along the plane of slip, i.e., the interface.
• The no. of planes along which the bolts can be sheared indicates the number of shears. For e.g.,
single shear, double shear,…
• The nominal capacity of the bolt in shear (Vnsb) is given by
𝑓
𝑉𝑛𝑠𝑏 = 𝑢𝑏 𝑛𝑛 𝐴𝑛𝑏 + 𝑛𝑠 𝐴𝑠𝑏
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where, Anb – net shear area of the bolt at the threads ≃ 78% of gross area (IS 1367)
Asb – nominal shank area
fub – ultimate tensile stress of the bolt
Threads passing through the shear plane
nn – no. of shear planes with threads intercepting the shear plane
ns – no. of shear planes without threads intercepting the shear plane
𝑉
The design strength of the bolt in shear, 𝑉𝑑𝑠𝑏 = 𝑛𝑠𝑏
𝛾𝑚𝑏
𝛾𝑚𝑏 = partial safety factor for the material of bolt = 1.25
Threads not passing through the shear plane
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Shear Strength of Bolts
For long joints, long grip length, and with packing plates, the shear capacity will be lesser and is given
by
𝑓𝑢𝑏
𝑉𝑛𝑠𝑏 = 𝑛𝑛 𝐴𝑛𝑏 + 𝑛𝑛 𝐴𝑛𝑏 𝛽𝑙𝑗 𝛽𝑙𝑔 𝛽𝑝𝑘𝑔
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𝑙𝑗
Reduction factor for long joints (𝑙𝑗 > 15d) – 𝛽𝑙𝑗 = 1.075 − for 0.75 ≤ 𝛽𝑙𝑗 ≤ 1.0
200𝑑
lj is the length of the joint measured from the first to the last row of bolts in the direction of load.
8𝑑
Reduction factor for large grip lengths (𝑙𝑔 > 5d) – 𝛽𝑙𝑔 = where lg is the grip length
3𝑑+𝑙𝑔
◦ 𝛽𝑙𝑔 should not be more than 𝛽𝑙𝑗
◦ 𝑙𝑔 ≯ 8𝑑
Reduction factor for packing plates (> 6mm) – 𝛽𝑝𝑘𝑔 = 1 − 0.0125𝑡𝑝𝑘𝑔
◦ 𝑡𝑝𝑘𝑔 is the thickness of the thicker packing plate in mm.
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Bearing Strength of Bolts
The bearing limit state relates to the deformation around a bolt hole. A shear tear-out failure may
also occur when the end distance is small.
The nominal bearing strength of the bolt is given by, 𝑉𝑛𝑝𝑏 = 2.5𝑘𝑏 𝑑𝑡𝑓𝑢
𝑒 𝑝 𝑓𝑢𝑏
◦ 𝑘𝑏 = smaller of , − 0.25, , 𝑎𝑛𝑑 1.0
3𝑑0 3𝑑0 𝑓𝑢
◦ 𝑑0 is the diameter of the hole
◦ e, p are the end and pitch distances of the fastener along bearing direction
◦ 𝑓𝑢𝑏 is the ultimate tensile stress of the bolt
◦ 𝑓𝑢 is the ultimate tensile stress of the plate
◦ d is the nominal diameter of the bolt in mm
◦ t is the aggregate thickness of the connected plates experiencing bearing stress in the same direction. In the
case of countersunk bolts, then is should be taken as the thickness of the plate minus half of the depth of
countersinking.
The design strength of the bolt in bearing is given by 𝑉𝑑𝑝𝑏 = 𝑉𝑛𝑝𝑏 /𝛾𝑚𝑏
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Example 5.1
Design a lap joint between two plates as shown in the figure so as to transmit a factored load of
70 kN using M16 bolts of grade 4.6 and Fe 410 grade plates. Assume the width of the plates as 80
mm.
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Example 5.2
Two plates 10 mm and 18 mm thick are to be jointed by a double-cover butt joint. Assuming
cover plates of 8 mm thickness, design the joint to resist a factored load of 500 kN. Assume Fe
410 grade steel plate and grade 4.6 bolt.
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Shear Connections with HSFG Bolts
Force transfer of slip-critical connections:
At service loads, these bolts do not slip and the joint is called a slip-resistant connection.
At ultimate loads, the bolts do slip and the joints behave like bearing-type connections.
The connections must have sufficient shear and bearing strength in the event of an overload
that may cause slip to occur.
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Slip Resistance of HSFG Bolts
The frictional resistance to slip between the plate surfaces 𝐹𝑜 = minimum bolt tension (proof load) at installation = 𝐴𝑛𝑏 𝑓𝑜
subjected to clamping force opposes the slip due to externally
applied shear. 𝐴𝑛𝑏 is the net area of the bolt at the root of the threads
The nominal slip resistance or nominal shear capacity of the bolt is 𝑓𝑜 = 𝑝𝑟𝑜𝑜𝑓 𝑠𝑡𝑟𝑒𝑠𝑠 = 0.7𝑓𝑢𝑏
given by
𝑓𝑢𝑏 is the ultimate tensile stress of bolt
𝑉𝑛𝑠𝑓 = 𝜇𝑓 𝑛𝑒 𝐾ℎ 𝐹𝑜
where 𝜇𝑓 is the slip factor (Table 20)
𝑛𝑒 is the number of interfaces offering frictional resistance to slip The design shear strength is given by 𝑉𝑑𝑠𝑓 = 𝑉𝑛𝑠𝑓 /𝛾𝑚𝑓
𝐾ℎ = 1 for fasteners in clearance holes 𝛾𝑚𝑓 = 1.1 for slip resistance designed at service load
𝐾ℎ = 0.85 for fasteners in oversized and short slotted holes and
𝛾𝑚𝑓 = 1.25 for slip resistance designed at ultimate load
for fasteners in long slotted holes loaded perpendicular to the slot
𝐾ℎ = 0.7 for fasteners in long slotted holes loaded parallel to slot
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Modified Slip Resistance for Long Joints
For long joints the nominal shear capacity (Vnsf) of the bolt is reduced by multiplying it with βlj.
𝑙𝑗
𝛽𝑙𝑗 = 1.075 − for 0.75 ≤ 𝛽𝑙𝑗 ≤ 1.0
200𝑑
d = nominal shank diameter of the bolt
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Bearing Resistance
As the parallel shank friction grip bolts slip into bearing at the ultimate limit state when
subjected to shear forces, the bearing stresses between the bolt and the plate need to be
checked.
The bearing strength of the HSFG bolts will be greater than that of the plate; hence, bearing
strength of the bolt need not be checked.
The factored shear force (Vsf) should satisfy 𝑉𝑠𝑓 ≤ 𝑉𝑛𝑝𝑏 /𝛾𝑚𝑏
𝑉𝑛𝑝𝑏 can be determined as that of bearing-type bolts
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Example 5.3
The connection shown in the figure uses 20 mm diameter 10.9S grade bolts with threads in the
shear plane to connect an ISF 150 x 12 mm with a gusset plate. Determine the strength of the
joint if
a) Slip is not permitted
b) Slip is permitted
Block shear strength need not be checked.
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Tension Strength of Bolts
The nominal capacity of a bolt in tension Tnb is given by
𝛾𝑚1
𝑇𝑛𝑏 = 0.9𝑓𝑢𝑏 𝐴𝑛 < 𝑓𝑦𝑏 𝐴𝑠𝑏
𝛾𝑚0
where 𝐴𝑛 is the net tensile area taken at the bottom of the threads
𝐴𝑠𝑏 is the shank area of the bolt
𝑓𝑦𝑏 is the yield stress of the bolt
𝑓𝑢𝑏 is the ultimate tensile stress of bolt
𝛾𝑚1 = 1.25 (ultimate stress) and 𝛾𝑚0 = 1.1 (yielding)
The factored tensile load Tb should satisfy
𝑇𝑏 ≤ 𝑇𝑛𝑏 /𝛾𝑚𝑏 where 𝛾𝑚𝑏 = 1.25 (partial safety factor for bolt material)
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Prying Forces
If the connecting plates in tension are flexible, then additional prying forces are induced in the
bolt.
When thin plates are used, either bolt failure can occur first, or plastic hinges may form in the
plates due to the prying forces developed.
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Prying Forces
The additional force Q in the bolt due to prying action is given by
𝛽𝛾𝑓0 𝑏𝑒 𝑡 4
𝑄 = 𝑙𝑣 Τ2𝑙𝑒 𝑇𝑒 −
27𝑙𝑒 𝑙𝑣2
where 𝑙𝑣 is the distance from the bolt center line to the toe of the fillet weld or to half the root radius of
the rolled section (mm)
𝑙𝑒 is the distance between the prying force and the bolt center line (mm)
𝛽𝑓0
𝑙𝑒 = 1.1𝑡
𝑓𝑦
𝛽 = 2 for non-tensioned bolts and 𝛽 = 1 for tensioned bolts
𝛾 = 1.5
𝑏𝑒 is the effective width of the flange per pair of bolts (mm)
𝑓0 = 0.7𝑓𝑢𝑏 is the proof stress in consistent units
𝑡 is the thickness of the end plate (mm)
The second term in the formula can be neglected if the plastic hinges are assumed at the bolt line and
Elastic stage
the root, when min. flange thickness is used in design
𝑄 = 𝑇𝑒 𝑙𝑣 Τ2𝑙𝑒
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Prying Forces
The maximum thickness of the end plate to avoid yielding of the plate is obtained by equating the
moment in the plate at the bolt centerline and a distance 𝑙𝑣 from it to the plastic moment capacity of
the plate 𝑀𝑝 .
𝑀𝐴 = 𝑄𝑙𝑒 and 𝑀𝐶 = 𝑇𝑙𝑣 − 𝑄𝑙𝑒 ⇒ 𝑀𝐴 + 𝑀𝐶 = 𝑇𝑙𝑣 (1)
Let plastic hinges form at A and C,
𝑀𝐴 = 𝑀𝐶 = 𝑀𝑝 (2)
From (1) and (2),
𝑀𝐴 = 𝑀𝐶 = 𝑇𝑙𝑣 /2
𝑓𝑦 𝑏𝑒 𝑡 2
Taking 𝑀𝑝 =
1.1 4
The minimum thickness of the end plate can be obtained as
4.4𝑀𝑝
𝑡𝑚𝑖𝑛 =
𝑓𝑦 𝑏𝑒
The corresponding prying force can be obtained as 𝑄 = 𝑀𝑝 /𝑙𝑒 Elastic stage
The total force in the bolt (T + Q) must be checked against the tension capacity of the bolt.
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Combined Shear and Tension
When bolts are subjected to both shear and tension, their
combined effect can be assessed from an interaction diagram.
IS 800 assumes a circular interaction curve as follows:
𝑉 Τ𝑉𝑠𝑑 2 + 𝑇Τ𝑇𝑛𝑑 2 < 1.0
where 𝑉 is the factored shear force
𝑉𝑠𝑑 is the design shear capacity
𝑇 is the externally applied factored tension
𝑇𝑛𝑑 is the design tension capacity
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Efficiency of a Joint
Holes are drilled in plates for bolted connections; hence the original strength of the fill section is
reduced.
The joint that causes the minimum reduction in strength is said to be the most efficient.
Efficiency = (Strength of joint pitch length / Strength of solid plate per pitch
length) x 100
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