Load test protocols and acceptance criteria
Over the years, researchers and practitioners have investigated various methods for applying test loads and measuring structural-response parameters. To date, universally accepted (and very specific) load-test protocols are available for concrete structural systems.
Although less detailed, protocols for load testing steel buildings are also available. These protocols, published by ACI and the American Institute of Steel Construction (AISC), are currently adopted and enforced by International Building Code (IBC) and most of the local building codes.
ACI 318-14
The in-situ load test protocol adopted by ACI is outlined in Chapter 27 of ACI 318, Building Code Requirements for Structural Concrete. This form of load test is commonly referred to as the ‘24-hour load test.’ ACI 318 requires a uniform test load be applied in a manner that ensures uniform distribution of the load transmitted to the structure or portion of the structure being tested.
The test load magnitude (TLM), including the dead load already in place, is set equal to a defined weighted combination of the design loads (Figure 2). The test load is to be applied in at least four approximately equal increments, and the structure should be adequately instrumented to capture the maximum response values. An initial set of deflection measurements of each instrument shall be recorded not more than one hour before the application of the first load increment. After the removal of the test load, a final set of deflection measurements must be taken an additional 24 hours after the test load is removed.
The structural response can be considered satisfactorily when there are no signs of failure (i.e. spalling or crushing of compressed concrete and evidence of excessive deflections), when the measured maximum deflection (Ämax) does not exceed a pre-defined threshold that depends on span and overall depth of the test member, and when residual deflections (Är,max) do not exceed 25 percent of Ämax attained during the test. If the measured maximum and residual deflections do not satisfy the acceptance criteria, the load test can be repeated at least 72 hours after the removal of the first test load.
These criteria are aimed at identifying a structure (or part thereof) that does not behave linear-elastically when tested, and the maximum allowable deflections are based on the calculation of deflections of a simply supported concrete beam when subjected to allowable-stress-design load levels. While first introduced in the early versions of ACI (Figure 1), this criterion has ‘survived’ through the years and is still part of current documents. However, newer protocols and acceptance criteria have been developed, as discussed below.
ACI 437-12
The new ACI 562-14, Code Requirements for Evaluation, Repair, and Rehabilitation of Concrete Buildings, references the new ACI 437-12, Code Requirements for Load Testing of Existing Concrete Structures and Commentary, which contains provisions for the 24-hour load test procedure, as well as a new cyclic load test. The latter option introduces hydraulics as the means for the load application and removal.
Safety advantages are obvious in comparison with more traditional methods (e.g. sandbags and steel plates) because the load can be quickly removed once the tested structure exhibits the first signs of failure. Additionally, hydraulics provides more control on the load application, enabling the reduction of the duration (and cost) of the 24-hour load test, as well as the definition of more precise load cycles. The new method for the application of the load also allowed for the definition of new acceptance criteria designed to capture non-elastic behavior of the tested structure.
The cyclic load test protocol consists of applying concentrated loads in a quasi-static manner and in at least six cycles, with each individual cycle including four to six load steps to reach the TLM. The six cycles (A through F) are loading/unloading (Figure 3). The load applied in Cycles A and B is 50 percent of the TLM, while 75 percent of the TLM is applied in Cycles C and D, and 100 percent of the TLM is applied in Cycles E and F.
