Mitigating Wood-Framed Egress Stair Stringer Failures

Details in construction drawings prepared by design professionals for multi-story, wood-frame egress stair construction are generally limited to stair opening size and code-prescribed tread, and riser dimensions. A flight of stairs constructed to meet code tread and riser dimensions can conflict with several wood engineering design principles and other building code sections. This disregard of wood design principles has caused performance issues, wood stringer failure, and in some cases personal injury from a partial or complete stair collapse. The primary reason is typical site-built or prefabricated wood-framed stair assemblies are constructed for a 40 pound per square foot (psf) live load application. This stair construction becomes problematic for egress stairs that are required by the building code to meet 100 psf live load. Forensic evaluation of several in-service stairs in these type buildings confirm insufficient stair stringer design capacity, fabrication deficiencies, and/or improper wood-framed stair construction. These shortcomings can be traced to three sources: the absence of code prescriptive guidelines for each stair component, the rarity of building design professionals to understand, analyze, and specify project specific stair stringers, and stringer support/connection details. This paper reviews available wood-framed stair stringer prescriptive information for 40 psf residential live loads to form a rational approach to egress stringer design and evaluation for use in higher live load applications.