Seismic Performance of Retrofitted Reinforced Concrete Bridge Pier

ACI Structural Journal, Nov/Dec 2005 by Schoettler, Matthew J, Restrepo, José I, Seible, Frieder, Matsuda, Ed

The experimental testing of a bridge pier for seismic performance is presented in this paper. A typical bent of Bay Area Rapid Transit's (BART's) West Oakland Aerial Guideway was investigated at half scale. Two pseudo-cyclic bidirectional tests were conducted on a single test specimen modeling an existing pier including proposed retrofit measures.

Twenty-four battered piles provided the foundation moment capacity during a system test. This test evaluated the likely failure mechanism of the pile-pile cap connection. The second test isolated the column-footing joint by restraining pile cap rotation and translation. In this test, retrofitted pile cap was evaluated for its joint shear response.

The system test results indicated a limited system ductility after pile failure, followed by a rocking response of the pier. Results from the joint shear test indicated the retrofit was sufficient to prevent a joint shear failure and helped establish the allowable column-1footing joint shear stress to use in retrofit design.

Keywords: foundation; joint; pile cap.

(ProQuest Information and Learning: ... denotes formula omitted.)

INTRODUCTION

Background assessment

Located in the San Francisco Bay area, the Bay Area Rapid Transit (BART) Aerial Guideway system is composed of 2000 bents. The Aerial Guideway is part of the 74 miles of original rail system run by the San Francisco BART Authority. Designed according to regulations that are now considered obsolete, the original system was built in the late 1960s. The BART system remains a significant means of transportation in the San Francisco Bay area and averages 300,000 passengers per day.1

Investigations of similar bridges built between the 1950s and 1970s suggested possible deficiencies in the design of structures similar to the Aerial Guideway. This resulted in catastrophic failures in the 1989 Loma Prieta earthquake and the 1994 Northridge earthquake. Seismic retrofits, however, have proven very successful in mitigating these deficiencies.2,3

The test specimen was a 1:2 scale model of BART's Aerial Guideway Bent P16. This bent was selected as it is representative of a standard bent used throughout the original BART system (Fig. 1(a)). Bent P16 was chosen due to its high column longitudinal reinforcement ratio of 6.2%, which is typical of the upper-bound reinforcement ratios used in the BART aerial structures. A column with a high reinforcement ratio delivers high joint shear stresses to the column-pile cap joint area. Bent P16 is shown in Fig. 1(b) and (c) and consists of a 1.524 m-diameter hexagonal column reinforced with No. 18 reinforcing bars that were not lap-spliced at the column-footing interface. A No. 5 spiral was used at a 76.2 mm pitch extending into the footing.

The areas of seismic vulnerability investigated in this test were the pile-pile cap connection, footing, and column-pile cap connection.4 A proposed retrofit was designed to mitigate each of these vulnerabilities. The allowable joint shear stress to be used in retrofit design calculations, however, was uncertain due to the retrofit detailing. The proposed retrofit consisted of dowels that are stopped above the existing bottom mat to prevent the possibility of damaging the bottom mat. Also, these dowels could not be headed or hooked at the bottom. A value of 1.0[radical][function of]^sub c^' MPa, as per Section 7.7.1.4 of the Caltrans Seismic Design Criteria Version 1.2,5 was considered to be too high. A lower value of principal tension was used in initial evaluations. This value, however, resulted in a thick concrete overlay and pedestals that were often higher than finished grade. The value used in the initial evaluations was felt to be conservative, but there was no justification for use of a higher value. The test results validated the use of an allowable stress that is much higher than that initially used and also verified that 1.0[radical][function of]^sub c^' MPa was too high. Phase II results allow a more economical retrofit design that has been validated through testing.

Test objectives

A two-phase test program was completed. The objective of Phase I was to analyze the pier response when largely influenced by the pile-footing connection. In this phase, the anticipated failure mode was pile dowel fracture. In Phase II, the joint behavior at the column-footing connection was examined by precluding failure at the pile-footing connection. The primary goal of Phase II was to assess the columnfooting joint performance after retrofitting to establish an appropriate allowable joint shear stress to be used during retrofit design of BART's Aerial Guideway. In summary, the test objectives were to examine the following:

* pile-footing connection behavior;

* retrofit overlay behavior; and

* joint shear response of the column-footing connection.

RESEARCH SIGNIFICANCE

This paper offers insight into the use of mechanical devices to incorporate foundation flexibility into a bridge pier test. An additional unique feature of this research was the bidirectional loading protocol used in the two phases of testing. While incorporating these unique features, this research identified an acceptable level of joint shear stress for retrofit and the mechanisms of response of a typical bridge pier in BART's Aerial Guideway system.