Open Web Steel Joist Girders for Mechanical Units

[reverbz]

Hey Guys,

Designing a job with open web steel joists for the first time(as you may have seen from a previous post) and was wondering how you handle a couple things on mechanical unit support. Do you call out the joists supporting these as girders no matter what the weight of the mechanical unit is? Or do you have a limit you think a regular joist can handle? Will the manufacturer verify if a girder is needed if you give the loads on the drawings?

Also, do you call out your mechanical supports on a typical detail assuming the weights are similar and just use worst case? That's what I'm planning to do as of now.

Thanks again for the help!

 

[jerseyshore]

We always show the weights right on plan. That's what the joist manufacturer is going to look for.

Sometimes we'll show KCS if the unit might be moved. Old style was using double joists. Majority of the time we'll just do beams to make it easy.

But often I've found that no matter what you show on plan the manuf will do something different that's why we just show the loads and make sure it's picked up during shop drawings.

 

[gte447f]

I haven't designed these types of buildings in a while, but when I did, my preference was to show an RTU zone to allow for some flexibility in where the units were placed, and to specify KCS (constant shear) joists throughout this zone. You still need to know the weights of the units of course, so you can select the appropriate size KCS joists, and you still need to add some verbiage to the drawings specifying any limits and restrictions assumed in your design, like, for example, no more than one RTU can be supported by each joist, etc. I think its good to put the weights and maybe the max point load/weight distribution for the RTU's that were used as your design basis to make sure that the assumed design loads are not exceeded in practice.

The RTU zone approach was not always available, because some owners/architects/contractors wouldn't want to spend extra for KCS joists unless absolutely necessary.