CROSS Safety Report
Failures of cold formed steel floor systems during construction
This report describes numerous collapses during construction of cold formed steel floor systems caused by lack of bracing/fastening and overloading of an incomplete structure.
Key Learning Outcomes
For structural design engineers:
It is good practice to include in the project design documents the requirement that cold formed steel joists be properly braced before loading.
It is good practice to not load cold formed steel joist systems until properly braced.
For code and standards writers:
Consider the addition of prescriptive requirements for bracing cold formed steel floor systems, similar to those adopted in New York City.
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The correspondent reports that in recent years there have been collapses during construction of cold formed steel floors. This is not just a recent trend, as several accidents of this type have been recorded since 2005. Although not very numerous, these accidents have led to serious injuries and even fatalities. Almost all had a common cause: floor overload during construction. Manufacturers’ instructions clearly define the allowable floor loads, but these refer to a system where the joists are properly attached to decking and braced. In some construction operations such bracing and fastening does not immediately follow the installation of the cold formed joists, leaving them in a potentially unstable condition. Of course, construction loads should not exceed manufacturer’s specifications, but several incidents have occurred when a lower load of construction material was deposited, and collapse occurred because the floor was unbraced. Collapses have occurred under a variety of loading, including depositing CMU, studs bunched together, or concrete being cast. In some cases, the depositing of the construction material occurred when the material was delivered by a lumber yard. Commonly the load might not have been extreme, but it was concentrated over a small area at mid-span.
Almost all had a common cause: floor overload during construction
The potential of local instability of cold formed floor joists is such that one should consider a cold steel formed floor as a one way spanning system. Despite this the IBC 2015 does not include specific requirements for special inspection of bridging.
The New York City Buildings Department issued a recent bulletin, with numerous and specific instructions, with the intent to curb these types of accidents. Through this bulletin, the following provisions related to special inspection of cold-formed steel were added to the New York City Building Code.
From Table 1704.3.4, Cold Formed Steel Light-Frame Construction, Bracing:
‘5.a. Verify that temporary bracing, shoring, jacks, etc., are installed, and not removed until no longer necessary, in accordance with the approved construction documents and approved erection drawings.
5.b. Verify that permanent bracing, web stiffeners, bridging, blocking, wind bracing, etc., are installed in accordance with the approved construction documents and approved erection drawings.’
Experience has shown that because this special inspection might occur at a late date, the requirements are not capable of detecting missing or improper bracing before construction operations take place and allow accidents to happen.
Expert Panel Comments
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Cold formed steel joists are increasingly employed nationwide, especially in multifamily residential and hotel construction. The stability of traditional wood framing is less critical because of the dimensions (material thickness) of typical wood joists. Wood joists are inherently more stable.
OSHA 1926.750 contains extensive provisions for steel erection, which include cold formed steel framing. 1926.754(a) requires that ‘Structural stability shall be maintained at all times during the erection process.’ While OSHA 1926.750 contains specific provisions for metal decking, column anchorage, beams and columns, open web joists, and system engineered metal buildings, there are no similar specific provisions for cold formed steel framing.
The American Iron and Steel Institute Code of Standard Practice for Cold-formed Steel Structural Framing 2020 Edition (AISI S202-20), Section F6 Temporary Bracing, requires, ‘The installer shall determine, furnish, and install all temporary bracing for the cold-formed steel structural framing. This temporary bracing shall secure the framing against loads that are expected to be encountered during installation.’
Given the increasing use of cold formed steel joists in floor construction, the incidence of problems, and the evolving nature of codes and standards, it may be prudent for structural design specifications and drawings to make special note of the need to properly brace such floor systems before loading.
It may be prudent for structural design specifications and drawings to make special note of the need to properly brace such floor systems before loading
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