Load Models and Load Combinations

For many structures and for the development of codes load combinations can be studied as linear combinations of scalar stationary random processes. A review of proposed load models suggests that the class of random processes consisting of stationary Gaussian and renewal pulse processes provides a good description of the macro-time variation of most loads acting on buildings. The extreme value distribution of linear combinations of these processes can be bounded in terms of the mean upcrossing rate of a constant threshold level. Exact results and bounds on the mean upcrossing rate for linear combinations of independent processes can be calculated by Rice's formula. A convenient and close upper bound on the mean upcrossing rate involves only convolutions of mean upcrossing rates and random-point-in-time distributions of the individual load processes. This upper bound is often also the exact result.

The results for the mean upcrossing rate imply a format for checking equations in a level 1 code. The format is the same as the present CEB-code format based on the application of Turkstra's rule.