| Principal Investigator and Institution
Ian G. Buckle and Ahmad Itani, University of Nevada, Reno
Michel Bruneau, University at Buffalo
Objective
Research will be undertaken to improve the in-plane performance of
flexible steel superstructures and to develop guidelines for their safe
and economic design. A new class of seismically resistant steel bridges
will be explored in which energy dissipators and intelligent bearings are
embedded in the superstructures of these bridges so as to reduce seismic
loads on substructures and improve seismic performance.
Approach
It is often assumed in seismic design that bridge superstructures
remain elastic during strong ground shaking and do not therefore require
explicit seismic design. This assumption appears to be well justified for
concrete superstructures, but the situation is not so clear for steel
superstructures. The inherent flexibility of steel superstructures may
lead to unexpected load paths and the overstress of basic components, such
as cross frames, floor beams, unbraced compression members, and bearings.
Damage sustained by steel bridges in recent earthquakes has highlighted
this vulnerability. The development of specific design criteria therefore
appears warranted, but little is known about the in-plane behavior of
these superstructures to guide the engineer through the design process.
Furthermore, the use of energy dissipators in large bridges is becoming
more common and the behavior of these devices (such as inelastic shear
links) using new and largely untried materials (such as high strength
steels) is of particular interest.
This task will therefore undertake a detailed numerical and
experimental research program on both short-span, slab-and-girder bridges,
and long-span truss bridges, subject to lateral (in-plane) loads. Results
will be equally applicable to steel bridges with steel columns and to
those with concrete columns or substructures. Studies will include:
- Identification of load paths through slab-and-girder decks; relative
roles of the slab, girders, and crossframes; effectiveness of end and
intermediate crossframes; influence of bearing type on load
distribution; influence of bi-directional loading; and influence of
spatial variation in ground motion.
- Improvements to performance using conventional strengthening of
critical load paths, including bearings and other connections to
substructure elements.
- Improvements to performance using innovative technologies, such as
embedded energy dissipators and intelligent bearings, using new
configurations and materials.
This multi-year project is divided into two parallel streams -
short-span structures and long-span structures. Major steps in the
short-span program are as follows:
Step 1. Construct and instrument a test bed model that
represents typical slab-and-girder bridge construction, for the purpose of
conducting a range of experiments to determine seismic performance of
as-built construction and as-modified with various response modification
devices. The target bridge is a 0.4-scale model, two-girder bridge with
multiple cross-frames. The steel girders and cross frames support a
concrete slab that is composite with the girders. The model is 60 ft long
and is currently under construction on the strong floor at UNR.
Step 2. Construct numerical models to simulate expected
performance under both cyclic lateral loads applied to the bridge deck and
dynamic loads using servo-controlled actuators and large capacity shake
tables, for as-built conditions and as-modified.
Step 3. Conduct a series of experiments to determine as-built
performance and optimum ways to improve performance through possible
strengthening of load paths, replacement of bearings, use of ductile end
diaphragms, embedment of smart devices in the cross frames, use of base
isolation, or any combination of the above. The influence of biaxial
motions and differential motions on response will be studied using the
multiple shake table facility at UNR.
Step 4. Correlate experimental performance with the results of
numerical modeling, interpret the data, and refine the models as
necessary.
Step 5. Develop design recommendations for the seismic design of
short span, steel superstructures.
Additional work will be conducted at UB to investigate whether the
bi-axial energy-dissipating end-diaphragm systems, developed in
collaboration with UNR, could be expanded for implementation into skewed
bridges (i.e., bridges that have their supports oriented at a skew instead
of at the usual 90 degree from the bridge longitudinal axis). Year 3
research will focus on the development or modification of a bi-directional
ductile diaphragm that could be effective up to the largest possible skew
angle, and analytical validation of the concept. Effectiveness of the
proposed schemes will be assessed through comparison with the seismic
response of traditional reference skewed bridges. However, the approach in
this first phase of the project will be to use simple analytical models
(as opposed to complex finite element ones) to capture as much as possible
the possible modifications of seismic behavior introduced by the ductile
devices. The most promising scheme will then be re-analyzed considering
more refined models to enhance confidence in the effectiveness of the
proposed scheme. This analytical work will provide valuable information
that will then be experimentally validated at UNR in Research Years 4 or
5.
Research under the long-span structures program will be conducted in
two phases. The first phase is an experimental study of the effectiveness
of shear links as energy dissipators in the towers of suspension bridges.
The case study being used for this work is the new East Bay crossing of
the San Francisco-Oakland Bay Bridge, which is currently under design. A
half-scale model of a segment of the tower has been fabricated and
includes the four shafts that comprise the tower and the shear links that
interconnect the shafts. Both single and double links are to be
investigated. The shear links are constructed from A709 grade 50 steel.
Loads will be cyclic but essentially static, and applied through a
purpose-built test assemblage on the strong floor at UNR.
The second phase will investigate the use of shear links for improving
the performance of other major bridges such as long-span trusses, and will
study the use of HPS70 steel for these links. This work will build on the
work already completed at UB under this task. This second phase will not
commence until the first phase on the Bay Bridge is complete.
This project will accomplish the following:
Short Span Bridge Program (Research Year 2)
- Construct and instrument the bridge model and perform
"as-built" experiments
- Construct numerical model and perform predictive analyses
- Design and install shear load cells for load path determination
- Retrofit model and repeat "as-built" experiments (angle
cross-frames) with load cells
- Design hysteretic cross-frames, fabricate and install
- Repeat experiments but with hysteretic cross-frames (and load cells)
- Compare performance and report conclusions / recommendations
Short Span Bridge Program (Research Years 3 and 4)
- Conduct bearing study and select bearing types for
fabrication/purchase
- Perform multiple shake table study on 2-span continuous girder
bridge model to study load distribution to bearings
- Conduct isolator study and select isolators for fabrication/purchase
- Perform multiple shake table study on 2-span continuous girder
bridge model to study isolator performance under differential ground
motion and bi-directional motions.
- Report conclusions/recommendations
Note - Caltrans has agreed to co-sponsor this work and provide the
funding to construct and instrument this model.
Long Span Bridge Program (Research Year 2)
- Construct and instrument half-scale segment of tower and shear links
for new the East Bay crossing (SFOBB)
- Conduct experiments on single and double shear links (A709-50 steel)
and load distribution within tower shafts
- Report conclusions/ recommendations
Long Span Bridge Program (Research Years 3 and 4)
This will be determined in greater detail once preliminary results are
available from Research Year 2. However, among the task accomplishments
will be:
- Investigation of shear links constructed from A709 HPS70 steel
(cyclic inelastic performance)
- Numerical modeling and optimization of shear link performance
- Investigation of shear link applications to long-span trusses
- Reporting conclusions and recommendations
Note - The cost of construction and initial testing of this model is
also being provided by Caltrans through a larger and more extensive
contract awarded to the University of California at San Diego.
Further, the American Iron and Steel Institute has been asked to co-fund
the study on HPS70 steel.
Products
By the end of 2002, preliminary reports are expected on the as-built
behavior of slab-and-girder bridges under lateral loads, and the
performance of shear links as energy dissipators in the tower of the East
Bay suspension bridge. The final product, which will be at the end of the
task in Research Year 4, will include design guidelines for the seismic
design of steel superstructures, with and without response modification
devices (isolation bearings and embedded energy dissipators), and the use
of shear links as energy dissipators in long span structures.
Technical Challenges
The main challenge at the present time is working simultaneously with
two physically large specimens, and the consequential competition for
space and resources. |