Limit States Design Concepts for Timber Engineering

The concepts and definitions required for limit states (LSD) using the safety index are developed using glued-laminated beams in flexure as an example. The advantages of LSD include unification of structural codes, consistent safety measures, and improved resource allocation. The disadvantages include changes in design practice, changes in relative economy of some systems of construction and, perhaps, additional work on the part of the designer. Beam strength, based on 56 full-size glued-laminated beams tested by others, and snow load information from 6 Canadian cities were used to determine safety index variation over a range of design variables for current Canadian design specifications. The safety index for small beams and high dead-snow-load ratios was about 3.2, while for large beams and low dead-snow-load ratios it was as low as 2.0. Based on this preliminary study, a value of about 2.5 appeared to be representative of current design practice in the normal range of variables. This value was used for illustrative purposes only and should not be interpreted as a recommended target value. Safety factors were chosen to provide consistent safety at the target value under the proposed LSD rules.

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