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

Extension of the pedestal was designed in two phases. The first was to design overlays to level out the pedestal stubs with the top of the pedestal. The second was to design a semicircular extension of the pedestal (Fig. 7 and 8). Because the ends of the as-built pedestal are only 2/3 of the height of the pedestal at the column-pedestal interface where cracking occurred, wedge-shaped overlays were designed to level the pedestal ends with the top of the pedestal. The design force for the pedestal overlay was 130 kN (29.2 kip). This is the top force in the strut-and-tie model multiplied by a factor of 1.5. The shear friction method was used to design the reinforcement for the overlays.

The purpose of the extension was to provide sufficient capacity in the strong direction of the one-way hinge to reduce or eliminate plastic hinging at the pedestal base. The extension also needed to preserve the weak direction hinge response. A semicircular extension of the pedestal ends with a diameter equivalent to the pedestal width was determined to be the best solution. A bar configuration with four No. 3 bars on each side of the pedestal was selected. With the additional bars, the yield moment capacity of the pedestal base became 35% larger than the demand associated with plastic hinging of the column pedestal interface. It was felt that with this margin, yielding of the bars at the pedestal base would be limited. The additional hinge bars increased the moment capacity in the weak direction by 20%, but kept the weak direction moment capacity of the hinge at a small value. The lateral hoops connecting each side of the pedestal extension were designed using the shear friction method.7 The design force was the vertical shear at the interface between the existing pedestal and the extension.

For the pedestal jacket, a GFRP fabric with unidirectional fibers was used. The sheets were applied to the pedestal with fibers oriented in the horizontal direction. The design was based purely on a tensile force applied to the layers on each face of the long direction of the pedestal. The force used was derived from the strut-and-tie model discussed previously. Design values were based on Caltrans Memo to Designers2 and Federal Highway Specifications.8 The design maximum strain was 0.6%, and the design modulus of elasticity was taken as 75% of the specified value. A permissible strength was calculated from these values and used to determine the required number of layers. The design ultimate strain, however, was reduced to 0.4% to prevent excessive expansion and to reduce the width of the potential cracks at the column-pedestal interface. The contribution of the existing horizontal steel in the pedestal was ignored. The required number of GFRP layers was 2.85, leading to three layers in the actual retrofit (Fig. 8).

Test results

No damage was observed in the specimen until 0.75xSylmar when flexural cracking began to appear on the south side of the column. Flexural cracking steadily increased throughout the lower 2/3 of the column until 1.25xSylmar. During 1.5xSylmar, shear cracks began to appear on both sides of the column. From 1.75xSylmar to 2.25xSylmar, the shear and flexural cracks propagated and widened steadily. At 2.25xSylmar, spalling was observed at the base of the north side of the column. The number and length of shear cracks continued to increase during 2.5xSylmar and the cracks extended to the base of the column. During 2.75xSylmar the column failed in shear/flexure mode (Fig. 9).