Tie off anchor attached to metal roof deck

[Robcat71]

Hi All,

I have a scenario where a contractor is proposing to utilize tie-off anchors that attached directly to the metal roof deck using several self tapping screws (approx. 25 locations). Per OSHA, the tie off design load is 5,000 lbs. and shall have a factor of safety of 2. My concerns are with the attachment of the deck itself to the roof structure, and the deck capacity. If I am understanding correctly, the base reaction of the tie off anchor will put the decking in combined bending and compression. Would the deck also have to resist the vertical shear/tension along the edge of a support that results from the base moment couple? I am not sure how you could rely on the deck in the weak direction (assuming it meets the shear, compression and flexural demand) with just a couple of side lap fasteners to transfer the load between panels. Does anyone have a good reference for the deck design under this type of loading?

Has anyone successfully designed this type of tie off anchor? I am not sure why such a product exists if its not feasible in most applications. I am working with 20 GA type B metal roof deck, fastened on a 36/4 pattern with PAF's.

The other option is to attached the tie-off directly to the roof steel, but results in torsion that the framing and connections were not designed for. The contractor is obviously trying to install these anchors without adding additional framing to brace the roof beams, but that is how I have typically designed and detailed roof tie offs in the past.

I appreciate all and any input / references.

Thanks!

 

[JStephen]

I was thinking the 5,000 lb requirement was typically treated as an ultimate load, not with a SF of 2, but would have to go back and check the wording.
The anchor is required to support that load, but not necessarily in an undamaged state; ie, if it put a dent or buckle in the decking but still held, that would meet the OSHA requirement per my understanding.

 

[jayrod12]

I'm with JStephen regarding the load, I believe the 5,000 lbs is inclusive of a SF=2.0. I.E. the actual expected load applied is 2,500 lbs maximum. Which in reality, would kill the user to be subjected to that strong of an arrest force.

I also agree that after a fall event, there can be permanent deformation, just not failure of the system. I believe however when proof loading any anchors, there must be no permanent set after testing to 2,500 lbs.

 

[Ingenuity]

I believe however when proof loading any anchors, there must be no permanent set after testing to 2,500 lbs.

And that is a subject that has been highly debated by WJE. They argue in many, many professional papers that 5k tie-offs etc should be TESTED to 5,000 lb.

HALF BAKED: HOW ASME A120 and IWCA I-14.1 HAVE ATTEMPTED TO UNDERMINE THE VALUE AND VALIDITY OF STRUCTURAL LOAD TESTING
G. R. Searer; J. E. Lewis; R. A. Dethlefs; and H. J. Hill​

A few other papers by WJE engineers that you can find via title search.

DAVITS AND LIFE LINE DESIGN AND TESTING

CERTIFYING THAT EXISTING SUSPENDED SCAFFOLD STRUCTURAL SUPPORT ELEMENTS AND LIFELINE ANCHORAGES IN CONFORMANCE WITH FEDERAL OSHA REQUIREMENTS

STRUCTURAL DESIGN AND LOAD TESTING OF FACADE ACCESS EQUIPMENT

DESIGN AND LOAD TESTING OF FACADE ACCESS EQUIPMENT
​

A position that is NOT supported by ASME, IWCA, SAIA and others.

 

[dik]

Why would there be that requirement?

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