Load Test of a Continuous Post-Tensioned Parking Garage Beam

Substantial discrepancies were found between plans and as-built conditions at the E. D. Jones parking structure in St. Louis, Missouri. One such discrepancy was a beam width of 12 in. instead of the 18 in. shown on plans. The observed deviations raised concerns regarding the amount of prestressing in these beams. Based on the results of some initial investigations, the structural system had been found to be deficient in a number of areas.

CTLGroup was engaged to load test a post-tensioned beam supporting the second level of the parking structure. Testing was planned to run continuously through a weekend with two CTLGroup crews, each consisting of an engineer and technician, to continuously control and monitor the load test. The objective was to evaluate the strength of a post-tensioned beam through load testing in accordance with Chapter 20 of ACI 318-89, "Building Code Requirements for Reinforced Concrete." The test was limited to the post-tensioned beam along the center longitudinal column line.

Eleven strain gages and eight displacement transducers (potentiometers) were installed on the test beam. These were positioned at approximate locations of maximum deflection and flexure in each span, respectively. A computer-controlled data acquisition system was used to monitor and record the sensor readings.

Because of the amount of loading required and the need to limit it to the beams, temporary highway barriers were used to apply the load. CTLGroup's loading plan was based on an assumed weight of 4,000 lbs/barrier and a length of 10 ft. The teams realized that the actual barrier weights might vary; so during loading, they weighed a total of 30 barriers at random to obtain a more accurate representation of the average weight and standard deviation.