ABSTRACT
Load-displacement behavior for single beams may be modeled to ultimate failure with the cracked beam-column finite element program FEBRF. Results of the single beam analysis allow application of a model that simulates failure of laminated beams all the way to collapse. Time dependence of strength is important, with reduction of strength by half in 10 years. But, the very strong dependence of subcritical crack propagation velocity on stress allows even very long periods of lesser stress to be ignored when analyzing a roof recently loaded by a surcharge or drop in the water table. The modeling suggests that a drop in the water table will not precipitate immediate failure, but will cause failure over tens or hundreds of years.
