PHYSICAL LOAD TEST ON STRUCTURES
The procedure of load test presented here depends on ACI -2008 Chapter 20. In case there is doubt about the safety requirements of a structure, licensed design professional or building official can ask for a strength evaluation. In the start, methods simpler than the load test are considered and load test can be avoided if all involved parties are satisfied with the result of such evaluation. A load test is required to determine the serviceability of the structure when the presence / effect of the strength deficiency and its remedial measures are not fully known or when the required dimensions and material properties for analysis are not available. A load test is usually not made until the portion of the structure to be subjected to load is at least 56 days old. The test can only be performed at an earlier age if the owner of the structure, the contractor, and all involved parties agree. Load test is more suitable to clarify the doubts about the shear or bond strength but it can also be used to check deficiencies related with flexure or axial capacity. It is preferable to compare the results of the load test with the results of the analysis.
If a load test is decided as a means of the strength evaluation process for a particular project, the first step is that all the involved parties decide and agree upon the region to be loaded, the magnitude of the load, the load test procedure, and acceptance criteria. For a structure with significant deterioration, periodic reevaluations are recommended to be conducted even if the structure passes a load test. If the doubt about safety of a part or all of a structure involves deterioration, and if the observed response during the load test satisfies the acceptance criteria, the structure or part of the structure is permitted to remain in service for a specified time period. Periodic reevaluations are usually conducted at the end of each specified period. Another option for maintaining the structure in service is to limit the live load to a level determined to be appropriate.
The time period between successive inspections is based on the nature of the problem, environmental effects, nature of loading, and service history of the structure, repair and maintenance program and scope and extent of the inspection. After each evaluation, the building is declared serviceable for a specified period only.
Sometimes a concrete structure believed to be deficient passes a load test. This confusion or misunderstanding is due to the conservative design of concrete structures, extra reinforcing steel to control shrinkage, cracking and thermal effects, conservative design theories, over-estimation of the loads, extra concrete strength and multi-directional sharing of the loads not considered in ordinary designs.
1. Load arrangement
a) The spans and panels having more doubt during survey are considered.
b) The number and arrangement of spans or panels loaded are selected to maximize the deflection and stresses in the critical regions of the structural elements to be tested.
c) More than one test load arrangement is used if a single arrangement does not simultaneously produce maximum values of the force effects required to be studied for the adequacy of the structure.
d) The load is applied at locations where its effect on the suspected defect is a maximum. However, it is better to apply same type of load (point load or uniformly distributed load) as is expected on the structure under examination.
e) The pattern loading expected for the structure must also be considered in deciding the loading to produce maximum load effect in the area of the structure being tested. This includes use of checkerboard or similar type pattern loads.
2. Load intensity
The total test load is taken larger of the following three values:
(a) 1.15D + 1.5L + 0.4(Lr or S or R) – De
(b) 1.15D + 0.9L + 1.5(Lr or S or R) – De
(c) 1.3D – De
Where, D is the total dead load, L is the live load on floors, Lr is the roof live load, S is the snow load, R is the rain load and De is the dead load already in place. The live load L can be reduced as allowed by the building code. The load factor on the live load L in (b) is allowed to be reduced to 0.45 except for garages, areas occupied as places of public assembly, and all areas where L is greater than 4.8 kN/m2.