About the Author
Mr. Burr has been a structural engineer since 1984. His practice is concentrated on seismic evaluation and design for new and retrofit structures.
When a building undergoes a change of use or occupancy, sometimes the design superimposed load on a suspended floor is increased to meet the new occupancy. When this happens, we are often asked to verify that the suspended floor can support the increased design load. This can be achieved by back-analysis of the floor framing or by load testing. To back analyze a floor, details of the existing floor framing must be known, which can be determined from original structural drawings or from field investigation.
In one example of this at a multi-family apartment building in Burlingame we were asked to verify the load capacity of a second floor post-tensioned lightweight concrete floor slab which was to house a new social and exercise room. The new design superimposed load for the assembly-occupancy spaces was 100 pounds per square foot. The original design superimposed load was not known for certain, but was likely in the range of 50 to 70 psf.
Although we were provided with copies of the original structural drawings we did not have the detailed post-tensioned reinforcement drawings, and therefore were not able to back analyze the floor slab with any accuracy. Also the geometry of the suspended slab was irregular, making it difficult to determine the slab reinforcement and impractical to field verify the size, location and number of reinforcement bars for all the areas of slab and each supporting beam. Therefore the floor slab was load tested to verify its capacity.
The load testing was performed in accordance with ACI 318-05, Chapter 20, which required the slab to be tested to 85% of the full ultimate design load of the slab and the deflection of the slab to be monitored and compared to deflection criteria in the ACI document. To meet the total required load, the slab was tested with an added load of 203 pounds per square foot, using 32 gallon barrels filled with water and the load held for 24 hours. The load was applied incrementally and the slab deflection measured and compared against the acceptance criteria. Shoring was installed below the slab as a precaution against excessive deflection, with a small gap to allow the slab to deflect within reasonable parameters.
The testing was successful. The actual maximum slab deflection was 0.04” which was well within the allowable deflection of 0.26”. The test showed that the suspended floor slab is adequate to support the increased design load of 100 pound per square foot, which is approximately 40 to 100% higher than its original design load.
Load testing can be a cost effective and good alternative to analysis to determining the capacity of existing structural members, especially where details of the existing structure are not available or where determining existing conditions is impractical, destructive or cost prohibited.