C-5.45.4 evaluation by load tests

5.4.1 General Requirements

Generally, structures that can be designed according to the provisions of this Specification need no confirmation of calculated results by testing. However, special situations may arise where it is desirable to confirm by tests the results of calculations. Minimal test procedures are provided to determine the live load rating of a structure.

It is essential that the engineer of record take all necessary precautions to ascertain that the structure does not fail during testing. A careful assessment of structural conditions before testing is a fundamental necessity. This includes accurate measurement and characterization of the size and strength of members, connections, and details. All safety regulations of OSHA and other pertinent bodies must be strictly followed. Shoring and scaffolding should be used as required in the proximity of the test area to mitigate unexpected circumstances. During the load test, deformations are to be carefully monitored and structural conditions are to be continually evaluated. In some cases, it may be desirable to monitor strains as well.

5.4.2 Determination of Load Rating by Testing

The live load rating established by testing presumes $\phi=1.0$ for all failure modes.

The engineer of record uses judgment to determine when deflections are becoming excessive and to safely terminate the tests even if the desired loading has not been achieved. Incremental loading is specified so that deformations can be accurately monitored, and the performance of the structure carefully observed. Load increments should be small enough initially so that the onset of significant yielding can be determined. The load increment can be reduced as the level of inelastic behavior increases, and the structural behavior at each load increment carefully evaluated to determine when to safely terminate the test. Periodic unloading, after the onset of inelastic behavior, will help the engineer of record determine when to terminate the test to avoid excessive permanent deformation or catastrophic failure.

It must be recognized that the margin of safety at the maximum load level used in the load test may be very small, depending on such factors as the original design, the purpose of the tests, and the condition of the structure. Thus, it is imperative that all appropriate safety measures be adopted. It is recommended that the maximum live load used for load tests be selected conservatively. It should be noted that experience in testing more than one bay of a structure is limited.

The provision limiting increases in deformations for a period of one hour is given so as to have positive means to confirm that the structure is stable at the loads evaluated.

5.4.3 Serviceability Evaluation

In certain cases, serviceability performance must be determined by load testing. It should be recognized that complete recovery (in other words, return to the pre-tested deflected shape) after removal of maximum load is unlikely because of phenomena such as local yielding, slip at the slab interface in composite construction, creep in concrete slabs, localized crushing or deformation at shear connections in slabs, slip in bolted connections, and effects of continuity. Because most structures exhibit some slack when load is first applied, it is appropriate to project the load-deformation curve back to zero load to determine the slack and exclude it from the recorded deformations. Where desirable, the applied load sequence can be repeated to demonstrate that the structure is essentially elastic under service loads and that the permanent set is not detrimental.

Because the amount of acceptable permanent deformation depends on the specific structure, no limit is specified for permanent deformation at maximum loading.