Torsion on W-Beam due to fall arrest anchor installation

[Ipetu]

I have to install a fall arrest anchor on top a W18x40 simply supported beam with a span of 30 feet; the beam is part of a one storey steel frame building. OWSJ's frame into the W18x40 beam at 6 foot spacings. There is also a roof deck. connected to the OWSJ's. The factored force to be resisted by the fall arrest anchor is 5000 lbs or 22 kN acting approx. 18.5" above the shear center of the beam. Intuitively I feel that the beam (with local strengthening of the beam as shown on attached sketch) and roof system can take the loading. Any ideas on how this can be shown with detailed calculations? Any help or guidance or reference material is greatly appreciated.

 

[Lion06]

Look at AISC's Design Guide for Torsion.

 

[Ron]

I don't see the beam as being your issue. If your sketch is to scale, the welds holding the pipe to its mounting plate a small. Make sure you have enough weld for the loading.

Is this a standard Thaler device or a "knock-off"? Check the load rating if it is a Thaler device.

The next problem I see is a roofing problem. You have a built-up roof on the left and an inverted membrane roof on the right and a single spun flashing shown at two different base elevations. That won't work for a rigid flashing.

This indicates you have a step in the flashing at the centerline and unless you have a two-piece flashing welded together at that point, you can't get what you've shown as a standard spun accessory.

Also, you have your deck spanning 6 feet. That's a long span for decking....check your deflection under full load. That's a potential ponding problem.

 

[Lion06]

I didn't look at the sketch when I made my first post. I would also NOT extend the bottom chord of both joists to the W18. This creates continuity of the joist across that joint. The joist was most likely NOT designed for that continuity and the resulting end moments (and bottom chord compression). I understand that the DL is already applied so any continuity would only affect LL or SL, but I would not do it.

 

[Ipetu]

Ron,

Thank you for your post. The roof detail is not accurate as shown. The roof deck is continuous over the beam. The fall arrest device is a Thaler unit.

 

[Teguci]

Consider the W18 as a top flange plate and a bottom flange plate that bend laterally over the 6' due to torsion. Add that stress to the vertical bending stress and check against allowable.

 

[Ron]

ADeRaj...Thaler products are good quality.

While I don't think you'll run into significant issues with the joist tails as SEIT does, I'm just not sure you need it. The bottom flange is going to be in tension, so lateral bracing of a relatively stiff section is probably not necessary.

 

[BAretired]

I suggest that you extend the bottom chord of one joist only to avoid the issue that SEIT has raised (which I agree is a valid issue).

BA