Recent changes in building codes and standards have affected the way allowable loads on some connectors are determined when using Allowable Stress Design. Building codes and material standards have traditionally allowed designers and manufacturers to take a one-third stress increase on the calculated design capacities of some building materials and components when designing for forces generated from wind and seismic events. Specifically, the codes allowed Simpson Strong-Tie to take a one-third stress increase on the calculated tensile capacity of steel connectors.

Newer codes and standards only allow a steel stress increase when using an alternate set of load combinations. For example, the International Building Code and Uniform Building Code do not allow a stress increase when designing with the Basic load combinations of Allowable Stress Design. However, these codes do allow the use of a stress increase when using the Alternate (or Alternative) basic load combinations, as long as the referenced material standard/section permits it. The ASCE7 Standards do not have Alternate basic load combinations,so stress increases are not permitted.

Simpson Strong-Tie determines allowable loads in accordance with building codes and steel design standards (AISI and AISC). In the past, these steel standards have permitted the use of a one-third stress increase when designing for wind and seismic forces. However, the latest editions of these standards have discontinued that practice.

Because of these changes, Simpson Strong-Tie re-evaluated its entire productline to determine which loads were governed by calculated steel strength and included an allowable stress increase on the steel. The majority of Simpson Strong-Tie's products were not affected, since their allowable load is governed by a tested value, not a calculated value. Those products which were affected are shown with their revised capacities with no steel stress increase.

Additionally, it must be clarified that the load duration factors for wood permitted by the National Design Specification for Wood Construction areseparate and distinct from the one-third stress increase. Load duration factors for wood are permitted to be used in design calculations for all load combinations in the codes and standards covered by this catalog. For that reason there will be loads listed that include a load duration increase for wood or fasteners in wood, such as 1.33 or 1.6 for wind or seismic loading, with no further 1/3 stress increase permitted.

The table below summarizes when stress increases are permitted.