A Standard Approach Recommended for the Prototype Testing of Timber Trusses

There is-a need for a universally acceptable approach to prototype testing, based on rational criteria with a minimum of arbitrary limits so that tests can be compared. The approach must be impartial in its assessment of truss designs using different jointing media and dead load/live load combinations, and it must be adaptable to varying durations of test and to the data-recording methods used. The tests procedure which best befits these criteria is as follows: Handling: each truss should undergo a handling test suited to local conditions before being subjected to load and deflection tests. Test loads: the design load should be reduced to an equivalent load relative to the duration of the test; the strength/load duration relationship used should be the best currently available. Loading rate: loads should be applied to a truss either where they will be applied in practice or where they will have the worst possible effects; after the preload test, loading should proceed evenly or in steps at a rate that will strain the most important joints at about the same rate at which single joints are tested. For nondestructive tests, loading should proceed to 150 percent of the equivalent total tested sign load and be maintained for a minimum length of time to obtain sufficient relative creep for comparisons with rheological data on the joints. The load should be decreased at the same rate at which it was applied and the truss allowed to recover. Deflection limits: deflection limits under the test load should consist of an over-riding limit which takes into consideration the appearance and effect of the deflection of the truss on the entire structure, and a limit such that the relative creep of the truss is equal to that of a single joint loaded to the same proportion of its design load. Load factor: the load factor used in deciding the acceptance of a truss must be that multiple of the equivalent test design load which ensures the minimum probability of unserviceability based on prescribed live loads and the inherent variability of the timber and joints. This factor should be modified to account for the number of units of a particular design subjected to the test. These procedures could do much to enhance the impartiality and inter-comparability of tests on prototype trusses, and thus facilitate the effective structural utilization of wood.

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