MCEER HIGHWAY PROJECT
FHWA CONTRACT DTFH61-98-C-00094
Seismic Vulnerability of the Highway System

Task C3-2: Seismic Performance of Bridges with Steel Superstructures

Subject Area: Special Bridges - Substructures and Superstructures 
Research Year 2

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.

Background

It is often assumed in seismic design that bridge superstructures remain elastic during strong ground shaking and do not require explicit seismic design. This assumption appears to be well justified for concrete superstructures, but the situation is not so clear for steel superstructures. Their inherent flexibility 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. Specific design therefore appears warranted but little is known about the in-plane behavior of these superstructures to guide the engineer through the design process.

Approach

It is intended to 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 steel bridges with steel columns as to those with concrete columns. Studies 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

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:

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. 
   
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.
   
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.
   
4. Correlate experimental performance with the results of numerical modeling, interpret the data and refine the models as necessary.
   
5. Develop design recommendations for the seismic design of short span, steel superstructures.

Major steps for the long-span structures program are 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 is to be fabricated which 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. 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 and other response modification devices, for improving the performance of long-span trusses. This work will build on the work already completed by Professor Bruneau in this regard. This second phase will not commence until the first phase on the Bay Bridge is complete, and funds in the current year will be used to perform preliminary studies and develop this part of the research plan in greater detail.

Research Plan (First Year)

The first year of this project will accomplish the following:

Short Span Bridge Program

• 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
• Conduct bearing study and investigate performance various bearing systems.
• Design/construct passive energy dissipation systems.

It is noted that Caltrans has agreed to co-sponsor this work and provide the funding to construct and instrument this model .

Long Span Bridge Program

• Construct, instrument and test shear links for the tower of the East Bay crossing
• Preliminary studies of response modification devices and systems for long span trusses.

It is noted that cost of construction and initial testing of this model is also being met by Caltrans through a contract at the University of California at San Diego.

Products

By the end of Research Year 2, 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, at the end of the project period, will include design guidelines for the seismic design of steel superstructures, with and without response modification devices (isolation bearings and embedded energy dissipators).

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.

8/14/01