Research Activities: 2002

Experimental Study on Seismic Retrofit Techniques for Cap Beams, Columns and their Connections of Highway Bridges

 

Status

Complete

View Report:

 PDF
 

Sequential Number

R86
 

Matching Research Agency

Alaska Department of Transportation
 

Principal Investigator

Pedro F. Silva
Assistant Professor
University of Missouri-Rolla
1870 Miner Circle, Butler-Carlton Hall Office no. 331
Rolla, MO 65409-0030
p573-341-6280
f573-341-5419
silvap@mst.edu
 

Student Involvement

M.S. student
 

Project Objective

This study is aimed at: (1) to experimentally investigate the shear and flexural capacity of cap beams and their corresponding columns that are typical in the states of Missouri and Alaska, (2) to study the cyclic behavior of beam-column joints, and (3) to develop effective retrofit techniques for the seismic upgrading of the cap beams, columns, and their connections.
 

Project Abstract

Multiple column bridge bents are widely used in the construction of highway bridges. In many states including Missouri and Alaska, these substructures are often separated from the superstructure by steel bearings or rockers. Typically multiple column bridge bents are built with reinforced concrete materials and consist of one cap beam and several columns supported on spread footings or pile foundations. The current AASHTO Specifications adopt the modern seismic design concept that plastic hinges should form only at the top and bottom ends of the columns. Through the ductile response of the columns the earthquake energy is dissipated in the columns plastic hinge in the form of hysteretic damping. In addition, since the cap beam and joints are not conveniently used to provide energy dissipation, these members should be designed with sufficient overstrength factors to ensure that they are protected from any significant inelastic actions. This concept implies that the cap beams and the beam-column joints of a RC bridge bent must be stronger than the supporting columns and must be capable of transferring the seismic forces elastically. Contrary to this ideal design approach many bridge structures around the United States have been designed with major deficiencies that may lead to catastrophic failure of bridges in the event of an earthquake.

To effectively demonstrate the performance of various retrofitting techniques, large- or full-scale test specimens will be tested. The design of the specimens will follow the past practice of the Alaska Department of Transportation. To evaluate the performance of the test units under simulated seismic loading, each test unit will be monitored with LVDTs and strain gages. Testing of the test units will be conducted under quasi-static reversed cyclic loading to induce axial and shear forces as well as bending moments in the specimen, representing different loading conditions in field.

 

Task Description

To effectively demonstrate the performance of various retrofitting techniques, large- or full-scale test specimens will be tested. The design of the specimens will follow the past practice of the Alaska Department of Transportation. To evaluate the performance of the test units under simulated seismic loading, each test unit will be monitored with LVDTs and strain gages. Testing of the test units will be conducted under quasi-static reversed cyclic loading to induce axial and shear forces as well as bending moments in the specimen, representing different loading conditions in field. The complete investigation of this project includes the following tasks:
 
Task 1:Review the available retrofitting techniques and identify those that are applicable to Mid America and Alaska bridge types. In the tasks followed, we consider three diffetne methods, which consists of: (1) wrapping with composite materials, (2) pre-stressed tendons, and (3) corrugated steel shell as primary techniques.
Task 2:Design and cast 6 large- or full-scaled beam-column assemblies for laboratory development/demonstration of various retrofitting techniques. Three specimens represent half of the beam and half of the column of a bridge pier and typical reinforcement details in Missouri and the other three specimens represent those in Alaska.
Task 3:Wrap the beam-column joint of one or two specimen with composite materials for improved shear and flexural capacity.
Task 4:Retrofit the cap beam of one or two specimen with externally pre-stressed tendons along both sides of the cap beam for improved flexural capacity due to increase in axial compression force on the beam. Alternatively, the cap beam can be retrofitted with a pair of knee braces.
Task 5:Retrofit the beam-column joint of one or two specimen with corrugated steel tubes welded in field with non-shrink grout filled inside the tubes.
Task 6:Test each specimen (virgin and retrofitted) under a displacement-controlled progressive cyclic loading. Compare the load-displacement hysteresis loops among all specimens. The states of concrete cracking, reinforcing steel yielding, hardening, and failure as well as any premature failure associated with the retrofit materials are identified for future development of performance-based design equations.
Task 7:Provide a design example using one of the most effective retrofit techniques.
Task 8:Summarize the experimental results and design example in a report and recommend one or two techniques for application in bridge retrofit in Missouri and Alaska.
 

Schedule

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Anticipated Benefits

There is an urgent need to develop design guidelines for the retrofit of bridge beam-column joints. The application of the proposed experimental study will lead to retrofit schemes that will be both efficient and economically competitive. The proposed retrofit schemes will address the following requirements; (1) it is rather easy and simple to implement in the field, (2) traffic disruptions are maintained at minimum levels, and (3) it is durable.

The research results of this study can be used to effectively retrofit inadequate bridges along US60 and other emergency routes for enhanced performance. As a result, 76% of the bridges along US60 without seismic design can be upgraded to comply with the modern design codes; 24% of bridges will unlikely fail in a brittle manner during earthquakes. These efforts will ultimately reduce the potential loss of lives and property damage in Southeast Missouri in the event of a catastrophic earthquake event.

 

Modal Orientation

Bridge Assessment

After experiments are completed, the retrofit techniques used to retrofit the specimens will be compared and evaluated. The most effective technique will be used to retrofit an existing bridge in Missouri. UMR will provide practical design information and methodology that can be utilized by average practicing engineers or designers.

 

Milestones

Project Start Date:06/30/2002
Project End Date:07/31/2005
 

Relationship to other Research/Projects

This project is related to the FHWA project entitled Earthquake Mitigation Research Program of Highway Systems, which is part of the Natural Hazards Mitigation Institute.
 

Technology Transfer Activities

Workshops and meetings with DOT's officials.
 

Transportation Research Board Keywords

Seismic retrofit, FRP, capacity design, seismic evaluation