Torsion in purlins

[BAGW]

Hi,

I have a condition where a pipe is hung from the bottom flange of the purlin as shown below. Normally, I would evaluate such situations with AISC DG for torsion. BUt in this case there is a 3" deck welded to top of the purlin and the purlin cannot technically go into torsion. How to evaluate such situations?

 

[Lomarandil]

I don't know that the purlin "technically can't go into torsion". Depending on your span lengths (and purlin end connections), the deck will some take some proportion of the applied torsion. But you'll almost certainly have at least some torsion at the ends of the purlin, even if the deck is stiff enough to carry the torsion and the purlin flexible enough to go along for the ride along most of the span.

 

[1503-44]

There are common pipe hangers available that can apply a near equal load to both flanges thereby avoiding any torsional loads being applied to the beam. If you cannot find one, why not consider welding a clip to the beam flange that aligns its load with the web.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[XR250]

I would not fully count on the deck as it may only be welded on one flange or not at all lol. Aren't these situations self limiting anyway? It can only rotate so far before equilibrium is reached.

 

[phamENG]

While deck is typically sufficient for flexural stability, it doesn't necessarily have the strength and/or stiffness to resist directly applied, torsional loads. Draw a free body diagram over your section and see what it takes to balance the forces about the centroid of the beam. Chances are you'll need the beam to take it.

While XR is right that such a load would be self limiting, that is primarily used for LTB checks with the load below the NA. For a torsional load such as this, the self limiting nature would still allow the beam to rotate far enough that the load would be directly under the centroid. That much rotation is sure to cause issues at the deck to beam connection to say nothing of the beam to support connection, or the beam itself.

 

[1503-44]

Not necessarily self-limiting if the lower flange rotates vertically. Stiffeners could also be installed to prevent bottom chord rotation. That's what I would probably do here. Just a small triangle on the opposite side might work. If not, a full stiffener plate.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."