Development of Reinforcement Schemes for Cold-Formed Steel Joists with Large Web Openings

The floor joists of cold-formed steel (CFS) structures may use large web openings and a cost-effective way to alleviate the detrimental effects of a large web opening is to affix appropriate reinforcements around the opening regions, to restore the original strength and stiffness of the member. The primary aim of this paper is to define the “Flexural Zone” and the “Shear Zone” and establish construction guidelines for the reinforcement schemes for cold-formed steeljoists with large web openings in these corresponding zones. To that end, a total of twenty-three laterally braced CFSjoists were simply supported and subjected to uniformly distributed loads until failure for flexural tests, which considered solid sections, circular and square web openings (65% of web depth) and sections with reinforced web openings. The reduction in the flexural strength of a cold formed steel joist section due to a large web opening is lessthan 15%. Twenty-seven joist sections were subjected to short span, mid-span point load, shear tests to establish the shear resistances of sections having a large web opening and a reinforced web opening. The reduction in the shear strength of a CFS section with a web opening may be as high as 60%. Thus, the residual shear strength of a joist witha large opening may be as low as 40%. A Virendeel type reinforcement system can restore the original shear strength of a cold-formed steel joist section. Based on these studies it is established in this paper that; The mid 40% region of a joist (0.30L and 0.70L) can be defined as “Flexural Zone” and will need flexural reinforcements. The regions outsidethe mid 40% region of a joist can be defined as “Shear Zone” and will need the shear reinforcements.