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

INSTRUMENTATION

Instrumentation was nearly the same for all of the specimens. Full details of the transducer layouts are presented in Reference 3. The specimens were extensively instrumented with strain gauges, displacement transducers, load cells, and accelerometers. The data were recorded at a rate of 160 samples per second by the use of a data acquisition system.

Strain gauges for the longitudinal steel were placed at the two critical sections, the column to pedestal, and the pedestal to footing connections. Gauges were also placed at 127 mm (5 in.) above and below these sections to track bar pullout and strain variation. In addition, strain gauges were placed on the column ties at 140 mm (5.5 in.) levels from the top of the pedestal to approximately one column depth above the pedestal.

Column curvature, pedestal uplift, and horizontal pedestal slippage were measured with displacement transducers. For pedestal slippage and uplift, transducers were located on the long and short sides of the pedestal, respectively. For column curvature measurement, transducers were located in pairs at five 127 mm (5 in.) intervals from the base of the column on either side of the column.

Displacement transducers were used to measure column movement in the loading and transverse directions. Load cells were used to measure the column axial loads and the link force between the column and the inertial mass on the mass rig. Acceleration was measured by an accelerometer at the top of the stub head and by the internal table accelerometer.

TEST PROCEDURE

The shaketable test setup is shown in Fig. 3. Scaled masses of the tributary dead load on the actual structure were applied through an inertial mass rig and vertical hydraulic jacks. The mass rig design is presented in Reference 4. The mass rig consists of a steel frame pinned at the base with a 89 kN (20 kip) reinforced concrete block bolted to the mass rig deck. The effective horizontal mass of the steel frame is 9072 kg (20 kip-mass).4 The total effective inertial mass was 18,144 kg (40 kip-mass). Hydraulic jacks supplied the axial load to the specimen through a steel spreader beam bolted to the top of the column. The hydraulic jack system consisted of two jacks connected in series to an accumulator and pump. The accumulator helped prevent large variations in axial load during the tests. The axial load rods went through the footing and were bolted into the table. An axial load of 218 kN (49 kip) was applied to the specimen with the jacks, which produced the same level of axial stress as that of the prototype.

The acceleration record used to drive the shaketable was the 90-degree component of the 1994 Northridge earthquake as recorded at the Sylmar Hospital parking lot. The Sylmar record was chosen because it was established through preliminary analysis that the Sylmar record would place the largest displacement ductility demand on the columns. It is also representative of an earthquake in the western United States, and is widely used in structural research. The record was applied to the columns in 0.25× acceleration increments until failure. To scale the acceleration record for use on the specimen, the timescale of the record was compressed by the square root of the scale factor and the square root of ratio of inertial load and axial load. The timescale adjustment for the difference between the inertial and axial load was required because the mass rig blocks could only be added in 89 kN (20 kip) increments. The total effective mass from the mass rig, block, and link assembly was 20,100 kg (44.3 kip-mass). The change of timescale accounted for the difference between 197 kN (44.3 kip) inertial loading and the 218 kN (49.0 kip) target loading. The timescale used for testing was 0.476.