Seismic Retrofit of Octagonal Columns with Pedestal and One-Way Hinge at Base

ACI Structural Journal, Sep/Oct 2005 by Johnson, Nathan, Saiidi, M Saiid, Itani, Ahmad, Ladkany, Samaan

This study focused on the development of retrofit methods for octagonal single column bents with a pedestal and a one-way hinge detail at the pedestal base that are commonly found in bridge construction. Three identical quarter-scale column specimens were built for shaketable testing. One was tested as-built, the others were enhanced with seismic retrofits. The columns were subjected to the 1994 Northridge Sylmar earthquake in the strong direction until failure. Lateral steel of the as-built pedestal was highly deficient, resulting in an undesired pedestal failure. The first of the retrofitted columns was tested with a pedestal extension and a glass fiber-reinforced polymer (GFRP) pedestal jacket. The second was tested with the same retrofit as the first but several of the column bars were severed at the base of the column to reduce plastic shear demand. It was determined that the pedestal retrofit was successful to strengthen the pedestal sufficiently to shift the plastic hinging into the column. The severed column bars lowered shear demand and increased the plastic deformation. Analytical models to determine the shear strength and push-over behavior were evaluated using the test data.

Keywords: bridges; columns; reinforced concrete; retrofit; shear.

INTRODUCTION

Recent earthquakes in California (1994 Northridge earthquake) and Japan (1995 Kobe earthquake) have generally demonstrated that the response of retrofitted bridges is satisfactory. Many seismically-deficient bridges, however, still remain. Because of the high strength and stiffness of bridge superstructures, the substructure is expected to be the primary outlet for dissipating seismic energy in concrete bridges. Many existing bridges, however, are supported on piers that lack proper detailing to resist strong earthquakes. Pier deficiencies for concrete bridges include the column shear strength, confinement, and structural detailing. These shortcomings usually cause nonductile and unexpected modes of failure. Although retrofit measures to address the deficiencies in common circular and rectangular columns have been developed, the applicability of these methods to irregular columns with varying stiffness along the height is questionable. The complications arising from irregularities warrant further investigation and retrofit development for columns with shapes and stiffness properties that differ from those of regular columns.

This paper addresses the evaluation and retrofit of bridge columns that are irregular both in the cross section and along the height, with the latter caused by the presence of a pedestal and a one-way hinge connection between the footing and the pedestal.

RESEARCH SIGNIFICANCE

Column in large number of existing bridges do not meet the current seismic detailing requirements. Many of these columns are considered to be irregular and their performance and retrofit needs cannot necessarily be addressed using the available data for regular columns. The study presented in this article led to information on the characteristics of the shaketable response of as-built models and the effectiveness of two retrofit measures, both of which differ substantially from standard retrofit techniques. The results of this study led to retrofit recommendations for the columns of three ramp structures for a multispan highway bridge.

PROTOTYPE SELECTION

The primary objective of this study was to identify seismic vulnerability and develop retrofit strategies for single-column piers in three ramp structures that connect to the Las Vegas Downtown Viaduct, a 24-span freeway bridge in Las Vegas, Nevada. The structure was built during 1968-1969 with no seismic detailing. Based on an initial evaluation of the ramp structures, the columns in Ramp DW were found to be most critical based on the shear demand. This structure consists of a reinforced concrete box girder superstructure supported on six bents, each consisting of an octagonal column. Four of the six columns incorporated a pedestal with a one-way hinge at the pedestal base. Others are directly connected to the footing with a one-way hinge. The mean unconfined 28-day compressive strength of the concrete based on cylinder tests conducted during construction was 33.8 MPa (4900 psi). Accounting for concrete strength gain over time, a compressive strength of 41.3 MPa (6000 psi) was assumed. The reinforcing steel hadan average yield and ultimate strength of 303 and 524 MPa (44 and 76 ksi), respectively.

The pedestals and columns were found to be highly deficient with respect to lateral steel. Selection of the prototype column for testing was based on susceptibility to shear failure in the strong direction. The shortest column with a pedestal at the base (Pier 8DW) was selected as the prototype. The total height from top of footing to superstructure centerline is 7.04 m (23.1 ft). The column height above the pedestal is 5.21 m (17.1 ft) providing an aspect ratio in the strong direction of approximately 2. Based on the current American Association of State Highway and Transportation Officials (AASHTO) code,1 the development lengths for all the reinforcement are adequate. According to the current California Department of Transportation (Caltrans)2 and AASHTO specifications, the column confinement inside the plastic hinge region is deficient by 87 and 74%, respectively.