Limit States Design—A Probabilistic Study

Abstract

Canadian structural standards for buildings are moving toward a unified limit states philosophy with common safety and serviceability criteria for all materials and types of construction. Structural steel and cold formed steel will have limit states design rules by 1975 and concrete, masonry, and wood will follow later.This paper compares the new rules with existing NBC/CSA requirements on the basis of probability of failure calculated by simplified theory. The main emphasis is on load combinations of dead, floor, and wind loads for office and residential buildings where failure occurs by yielding of steel. Other aspects of the new limit states design rules—column formula for structural steel, performance factors for composite structures, the importance factor which reflects the seriousness of failure, and safety factors during construction, are also considered.The results indicate that the new rules provide more consistent safety than existing rules for different combinations of loads and materials; and that simple rules are sufficiently accurate, keeping in mind the predominating influence of human error on failures and the simplifications used in analyzing complex building structures.