Conventional Lateral Bracing of Pipe Racks hung from Steel Joist supported roofs

[jochav52802]

Good Morning,

I frequently see mechanical details showing pipe support racks hanging off of steel joists, which concerns me as lateral loads such as seismic will induce torsion, weak/strong axis moment/shear in the joist were not considered in design. While the loads can be light, I've not seen much due diligence as far as confirming that there is a complete load path that won't fail the joists. In high seismic areas, the additional stresses induced in the joist could be significant.

With that said, here are my questions:

1) Can anyone help me understand if there's a good conventional practice for bracing pipe racks to joist supported roofs and how to determine when lateral loads become a concern? My thought is to eliminate these additional moments by adding kicker braces both parallel and perpendicular to the joists that are supported by an angle that spans between the top chord of the joist, which seems to be the quickest and least eccentric load path to the roof diaphragm. Is this a good approach? It adds a lot of steel, so this approach is drawing criticism; I'd like to be able to say that I'm following standard practice.

2) Is it okay practice to give the joist supplier these additional loads to design for? This can definitely be tough as the location of pipe racks doesn't seem to be known definitely up front; how are others addressing the loads induced in joists when the pipe racks aren't necessarily known?

3) Any good practice on determining when these loads can be ignored?

Thank you!

 

[BridgeSmith]

I would expect that if you follow the load combinations, you don't combine anything close to the maximum vertical loading with the seismic load. Generally, the chance of the design vertical load occurring at the same time as an earthquake is so low, it's not considered. The flexibility of both the pipes themselves and the connections to the joists, I think, would greatly reduce the actual seismic forces transferred to the joists. For the most part, it takes rigid connections to transfer significant seismic forces.

 

[skeletron]

There is a section of the building code in my area that deals with "Elements of Structures, Non-structural Components, and Equipment". The equation provides you with the estimated lateral force (due to seismic) for the component. The component table does list "pipes, ducts (including contents)" as one of the options. I mention this because you could calculate the seismic load and then determine whether they will govern versus the load in the pipe (I've seen a hydrotest load as the comparison).

To answer #1 and #2 (without knowing the type of structure): Yes. Provide a load path and a reasoning for it. Bridging or bracing seems reasonable. Provide the specific loads to the joist supplier (issuing envelope loads could be overly conservative). I wouldn't allow someone to be freewheelin' with where they are tying piping into a joist bottom chord. At least specify the panel points within a certain area for the joist supplier to design to.