Long Span Joist - Camber

[cgstrucg]

Hello Everyone,

I have a question related to camber for the long span joists. I have a huge roof which has like 60-70 long-span joists and they support a bunch of roof equipments. I know that we calculate camber by considering dead load/superimposed dead load deflection and specifying that deflection plus-minus tolerance as camber. Vulcaft have a recommendation table which shows camber proportional to joist length so we can compare how much approximate we need.

My question is - As mentioned I have bunch of roof equipments and hence concentrated dead load at a lot of points. Do we take deflection because of that concentrated load also into account for camber or is it just uniform load. The reason I am asking this is because of these additional point loads, some joists will be at higher camber and the joists adjacent to them will be at a lower level as they don't have those point loads.

If my approach is wrong, can someone please explain the way we shall specify camber for this type of situation. I am located in New York.

Thanks

 

[KootK]

1) In general, I'd be inclined to design all of the joists for the same camber. Differential camber from one joist to the next, as you suggested, can be a real hassle.

2) A good bridging system that can help with load sharing between joists might be worthwhile. Bridging is notably easier to install when camber is uniform-ish.

3) I'd take a critical look at just how heavy those rooftop units are relative to everything else and then make your decision based on that to ensure that you don't compromise drainage etc.

4) If the mechanical weight is minor relative to the overall roof weights, I'd be inclined to just design the camber for the uniform loads.

 

[cgstrucg]

Mechanical units are high in some cases around 18,000 lbs that's why I am confused as to how to decide camber as at the post points, the deflection will be significantly higher.

 

[dold]

That's a big unit to be on a 70ft span. At some point it might be best to give the joist designers the loads and give them some guidance on what you're trying to achieve. They're usually pretty good about interpreting the load tables I use for special joists. Let them know that after all dead loads applied, adjacent joists should have compatible deflections (or some wording to that effect).

 

[cgstrucg]

Yes, I have a very comprehensive load table that shows all dead/live/seismic/wind loads along with in-plane end moments caused by live/seismic/wind loads. This camber thing is tricky though. So shall I not mention camber at all and mention joist manufacturer to maintain standard camber for all joists and camber shall be modified for these special joists by joist manufacturer itself? Do they do that?

 

[Agent666]

I think I'd tend to put in a heavier member/joist and set the camber to be the same for many of the reasons kootk noted. Heavier member means less differential dead load deflections, and also give more scope for replacement units in future being heavier/larger (I feel this is something designers often neglect to factor into initial designs for roof mounted plant), etc.

 

[KootK]

Further to some of Agent's comments, doubling up the joists in the vicinity of the rooftop mechanical might be a viable option to even things out a bit.