Steel Bridge System, Simple for Dead Load and
Continuous for Live Load
Nick Lampe1 and Atorod Azizinamini2
Introduction
A detailed market analysis was carried out to investigate the trends in bridge construction
in the mid-west region of the country. National Bridge Inspection Data (NBIS) compiled
by the Federal Highway Administration (FHWA) for Wyoming, Iowa, Colorado, Kansas,
Oklahoma, South Dakota, and Nebraska were analyzed. Study of the data resulted in
following major conclusions.
1) The use of timber as a bridge construction material, although basically limited to
lower span lengths, has significantly decreased.
2) In most states studied, reinforced concrete has remained a fairly consistent choice
for span lengths of 50 ft or less.
3) Prestressed concrete construction captured a large market in the 60 – 100ft span
range in the 1960’s and 70’s. The current trends indicate that prestressed concrete
has extended its presence as a construction material choice across all span lengths.
In the last two decades, steel bridge construction in all span lengths has remained
steady or decreased in number whereas there has been an increase in the number
of prestressed concrete bridges built in the longer span lengths.
One of the major conclusions was that in short span ranges (80 to 110 ft. span lengths),
prestressed concrete girder bridges were the dominating bridge type.
An investigation sponsored by the Nebraska Department of Roads was carried out to
develop a steel bridge system that would offer better economy in these span ranges.
Current State of Practice
Before developing a new steel bridge system, it was deemed necessary to evaluate the
factors that add to the cost of constructing steel bridges and issues that could enhance
steel bridge economy.
1
Former Graduate Student, University of Nebraska – Lincoln.
Currently Structural Engineer, HDR Inc, Omaha, NE.
2
Associate Professor, Civil Engineering Department, University of Nebraska-Lincoln.
In general, each girder in a two span continuous steel bridge consists of three pieces.
Over the pier is a middle piece which is field spliced to the adjacent segments using
bolted or welded type connections.
In a series of discussions held with designers, fabricators and contractors in the
preliminary stages of the project, two factors were identified to be essential in developing
a new system: a) elimination of field splices, and b) simplifying the type of details
currently used over the pier, which in general consists of various combinations of anchor
bolts, sole plate and often expensive bearing types.
Possible Alternatives
The decision was made to focus on developing a system that could be suitable for steel
bridges with two spans. For such situations, one alternative could be to use two simple
span girders and join them over the pier. The joining of the girders over the pier could be
achieved in various ways.
The reason for joining the girders over the pier is to provide continuity for imposed
gravity loads. Short and long term dead loads and traffic live loads are the main gravity
loads applied to bridge structures.
Two feasible possibilities would be to attach the two adjacent girders such that a) the
system would behave as simple for dead loads and continuous for live loads or b) the
system would behave continuous for both dead and live loads. In both scenarios super-
structure (girders) and sub-structure (pier) could be made continuous. This alternative
was believed to be suited mainly for seismic areas and therefore was not considered in
this study. Further study was carried out to assist in determining the type of continuity
that would be most suitable.
Trial bridge designs were carried out. Three two span bridges were considered. For each
bridge considered, each span length was 100, 120 or 150 ft. Two possibilities were
investigated for each span length. The girders were assumed to act as simple spans for
dead loads and continuous for live loads only or continuous for dead and live loads.
Figure 1 provides results of these trial designs in terms of maximum positive moments.
Figure 2 provides similar information in terms of maximum negative moments over the
pier.
From Figs. 1 and 2 the following conclusion could be drawn. When girders act as simple
spans for dead load and continuous for live load only, the maximum positive moment
increases while the maximum negative moment decreases when compared to cases where
girders are made continuous for both dead and live loads.
Making the girders continuous for live load only has several advantages. In such cases,
the decrease in negative moment coupled with an increase in positive moment allows use
of the same cross section for the entire length of the girder. This potentially could
eliminate the need for full penetration welds. This behavior makes use of rolled sections
an attractive alternative for short span bridges. Table 1 gives a summary of three designs
for two span continuous steel bridges with each span being 95, 100 or 105 ft. The girder
spacing for all three cases was 8 ft.-6 inches. Table 1 provides possible rolled shapes that
could be used for each span length.
After evaluating the advantages and disadvantages of various alternatives, as evident
from information presented in Figs. 1 and 2, Table 1 and discussions with fabricators and
contractors, it was decided to make the system continuous for live loads only.
The details of the selected system
The selected system consists of placing simple span girders over the supports and makes
them continuous for live loads only. This will require use of a detail over the pier that
will allow the girders to act as simple spans for dead loads consisting of weight of
girders, cross frames, wet concrete and formwork. The detail should then allow the girder
to behave as continuous for superimposed dead loads (weight of railing system and
overlays) and traffic live loads.
Based on the input from fabricators and contractors, the detail selected to connect the
girders over the pier is as shown in Figures 3 and 4.
The bottom flange of the girders is made continuous over the pier and welded using
partial penetration welds prior to placement of the wet concrete. The girders are placed
over bearing pads. The detail over the pier also consists of a reinforced concrete
diaphragm. A series of embedded small bars are used to connect the concrete diaphragms
to pier cap beams. Embedded small bars prevent the longitudinal movement of the girders
with respect to the pier, allowing the pier connection to be defined as a fixed point. Prior
to casting the concrete diaphragms, a thin layer of foam is placed over the pier, separating
the concrete diaphragms from the pier cap beam. This allows rotation of the girders over
the pier when subjected to live loads.
Experimental Test
A full-scale specimen representing a portion of a two span continuous steel bridge system
utilizing the system described above was constructed and a series of tests were conducted
to evaluate its behavior. Figure 5 shows the dimensions of the beams and connection
details used for testing. Figure 6 shows the test specimen prepared for fatigue test. The
specimen was subjected to 2,000,000 cycles of cyclic loads simulating 75 years of heavy
truck traffic loads. The specimen was then loaded to collapse to evaluate its ultimate
capacity.
The test specimen survived the cyclic test without any deterioration in stiffness or
strength. The ultimate capacity of the specimen was well above the predicted values
using AASHTO provisions.
A report providing a complete summary of the project is in preparation and should be
made available soon.
Acknowledgment:
This project is sponsored by the Nebraska Department of Roads, for which authors are
grateful. The authors would like to thank Mr. Lyman Freemon and Mr. Moe Jamshidi of
NDOR for their vision and support of this project. Lincoln Steel provided the test
specimen. The authors gratefully acknowledge this contribution.
Maximum Positive Moment
16000
14000
12000
10000
Moment (k*ft)
Solid = Continuous
8000 Dashed = Simple Span
6000
4000
2000
0
90 100 110 120 130 140 150 160
Span Length (ft)
Figure 1. Maximum Positive Moment Comparison
Maximum Negative Moment
18000
16000
14000
12000
Moment (k*ft)
10000
Solid = Continuous
Dashed = Simple Span
8000
6000
4000
2000
0
90 100 110 120 130 140 150 160
Span Length (ft)
Figure 2. Maximum Negative Moment Comparison
Span Length 95' 100' 105'
W - Section 40 X 215 40 X 249 40 X 277
D. L. Defl. (int/ext) 4.4" / 4.2" 4.7" / 4.5" 5.3" / 5.1"
L. L. Defl. (cal/all) 99.4 95.9 98.8
Pos. Flex. (Mu / Mn) 92.6 88.8 88.5
Neg. Flex. (Mu / Mn) 83 78.2 78.3
Pos. Perm. Defl. (all = 47.5) 96.5 48.1 91.8 48.3 92.0 50.7
Neg. Perm. Defl. (all = 47.5) 62.6 80.9 60.5 75.7 82.5 75.2
Table 1. 95, 100, 105ft Design Summary
Concrete Diaphragm Reinforcing Bar for Live load
Continuity
Partial Pen Weld No. 6 Embedded Bars
End Plate
Pier Cap Beam
Elastomeric Pad
Figure 3. Connection Detail at Pier
Figure 4. One Girder in Place Prior to Placement of Partial Penetration Weld
Figure 5. Dimensions of Test Specimen
Figure 6. Prior to Fatigue Loading
