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

Measured strains

The measured strains at the two critical sections (the pedestal-footing interface and column-pedestal interface) for the specimens were compared for select earthquake runs. For 1.5xSylmar, the run during which all of the specimens had effectively yielded, the maximum strains in both retrofitted columns at the footing-pedestal interface were similar and close to the yield strain throughout the section. Strains in the as-built were much larger and were concentrated in the middle 760 mm (30 in.) of the section, which is directly below the column. The pedestal bars outside this area, although yielded, did not reach high strains in OLVA. These relatively low strains were a consequence of the pedestal separation from the column after vertical cracks in the pedestal developed. At failure, retrofitted columns OLVR-1 and OLVR-2 both reached maximum strains at the pedestal-footing interface that were beyond their yield point but well below strains reached in OLVA.

Strains at the column-pedestal interface were compared for the failure run of each column. Rather than comparing the extreme bars at the column ends that are not representative of the section, strains were compared at a location 200 mm (8 in.) into the depth of the section. The maximum longitudinal bar strains in OLVA, OLVR-1, and OLVR-2 were 1900, 13,300, and 17,300 microstrains, respectively. All of these strains are above the measured steel yield strain of 1440 microstrains. The strains show success in the pedestal retrofit given that both of the retrofitted columns OLVR-1 and OLVR-2 underwent much more extensive yielding at the column base than OLVA. Due to the reduced section retrofit of OLVR-2, the maximum strain of OLVR-2 at this location was 30% higher than strain of OLVR-1.

ANALYTICAL INVESTIGATION

Shear capacity

Due to the brittle nature of shear failure, it is important to avoid pure shear failure in seismic design. The margin against shear failure was one of the main considerations in all of the specimens that were tested in this study. For this reason, the shear capacity of the columns was compared with that predicted by equations from Caltrans2 and the Federal Highway Administration (FHWA).8 These equations were chosen because they both include ductility as a parameter, which is a realistic consideration. Specimens OLVR-1 and OLVR-2 were both included in this comparison because they failed in shear. OLVA was not included because the premature pedestal separation prevented column shear failure.

The FHWA and Caltrans provisions recommend a shear crack angle of 30 and 45 degrees, respectively. For consistency of comparison, the FHWA values were calculated for both angles. A comparison of capacities predicted by Caltrans and FHWA with the experimental results from the retrofitted columns is shown in Fig. 17(a). The plots in the figure show force for a given displacement ductility. The yield stress of the steel used in the equations was modified to account for the measured average strain rate effect in OLVR-1 and OLVR-2.9 Shear capacity from both equations is constant for small displacement ductilities and drops linearly until it reaches a constant value at larger displacement ductilities. The variation of shear capacity with ductility comes from the calculated contribution of concrete strength. The mark at the end of each load-deflection diagram is the point where the columns failed in shear. The intersections of the measured and calculated curves indicate that for OLVR-1 the Caltrans and FHWA methods predict shear failure at displacement ductility of 2.1 and 3.0 or 3.7 (depending on the crack angle), respectively. The measured ductility at shear/flexure failure was 4.6. For OLVR-2, the Caltrans and FHWA methods predict shear failure for a 45-degree shear crack at displacement ductility of 2.7 and 3.7, respectively. For a 30-degree shear crack, FHWA predicts that the shear capacity would not be reached. The measured ductility at shear/flexure failure in OLVR-2 was 5.9. For a 45-degree shear crack, the Caltrans and FHWA methods are both conservative, a feature that is highly desirable with respect to shear capacity. The FHWA equation with a crack angle of 30 degrees indicated no shear failure and was unconservative.