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Structural design of concrete and steel structures can be designed on the basis of results of load tests on model or prototypes. In this case, instead of theoretical structural analysis of complicated structural combinations, tests are done on models made of materials like Perspex or micro-concrete. Thus,
1. These tests can be used to give a very good physical idea of the action of these structures, or
2. The results of observations of deflections and strains interpreted by principles of model analysis can be directly used for design, or
3. The results of the experimental model tests can be used to determine the boundary conditions and form the basis for complex computer analysis of the whole structure.
The structural adequacy of reinforced concrete members which are factory made or precast in large quantities can also be tested for performance by means of laboratory tests on prototypes. These tests give only the strength but also the deflection and cracking performance of the structure under any given loading. Many factory made products like prestressed concrete sleepers have been developed by prototype testing.
In cases where the design and construction are to be finally passed on the basis of experimental load tests on prototypes, they should satisfy the necessary requirements of deflection and cracking, depending on conditions under which the product is likely to be used. Thus, prestressed concrete sleepers which will be subjected to a large number of repetitive loading during their life should be tested under millions of cyclic loads in addition to static tests.
IS 456, clause 18.3 gives the following recommendations for design based on experimental basis:
1. The structure should satisfy the specified requirements for deflection and cracking when subjected to a load of 1.33 times the factored design load for serviceability conditions for 24 hours. In addition, there should be 75 percent recovery of deflection after 24 hours of loading.
2. The structure should have sufficient strength to sustain 1.33 times the factored load for collapse for 24 hours.
Both these tests should be conducted by competent persons with reliable equipments. Testing of structures for acceptance should be carried out according to IS 456: clause 16.
