Making an opening in an existing pan joist roof

[faromic]

I'm working on a project with a pan joist roof. It's a 60'x40' building. I took the measurements and it's 20" form with 5" ribs 25" o.c.. The depth of the ribs is 8" with a 3" topping for a total depth of 11". I don't know the amount of reinforcement so I assumed the minimum. According to the CRSI tables the joists have a factored allowable uniform load of 112 psf. they want to install a 900 pound RTU but I don't think it works. The joists have a 60 psf dead load and a 25 psf snow live load. The uniform load from the footprint of the RTU is 33 psf. This is a factored load of 164 psf, about 50 psf over the allowable. The two openings for the air ducts in the pan joists are no more than 21" wide, so they won't interfere with longitudinal joists.
First question is how to reinforce the existing ribs of the pan joists? I'm thinking of bolting an angle to the underside of the ribs under the load of the RTU. How would I provide the required resistance at the supports for the applied shear?
Secondly, When I frame out the openings in the pan joists for the ducts, I'll probably run some C3x4.1 normal to the pan joists and connect them to the longitudinal joists with L2x2 angles to transfer the loads. Should I also bolt a C3x4.1 longitudinally to the joists for reinforcement or is it not necessary.
The CRSI manual says that for small openings (ones that don't affect any joist, i.e. narrower than the width of the rib) no reinforcement is required, but for a 21" wide opening (1" wider than the pan joist form) some of the joist may get cut, so I'm debating on whether or not to use the channels?
Your input would be appreciated.
Thanks

 

[msquared48]

Not sure you have much flexibility in the location of the cut from your post, but, if possible, I would try to place any cut close to a support wall so that if some of the purlin grid happens to be cut, the effect on the positive reinforcing steel will be less. Hopefully, there is minimal negative steel at the outside wall.

The shear reinforcing you describe is a different matter. Could some of the voids on the underside of the slab be filled in to increase the shear capacity locally as needed?

Are there any existing cuts in the slab that might aide you in telling the size of the existing reinforcing? Have you taken any core samples to test the current f'c of the concrete? Might be higher than you think.

Just a few thoughts.

Mike McCann
McCann Engineering

 

[hokie66]

I believe you can distribute the load of the unit beyond the actual footprint, and it should work strengthwise. If you compare the 900 pound load with the 60000 pounds of snow the whole 60x40 roof has to carry, it is a small load. I would distribute the load to probably 4 joists and see how that works. If no good, I would try 6 joists, then work out how to distribute the load. Distributing the load to more joists rather than trying to reinforce individual joists is the way to go.

I would try to avoid cutting into the ribs for the ducts. The mechanical engineer should be able to resize the ducts to rectangular. Stand up for yourself and make him do some work.

As long as you only cut the hole in the slab rather than in the ribs, the CRSI Manual is right in stating that no reinforcement is necessary. I don't think the angle reinforcements you described would do anything except go along for the ride.

 

[JAE]

I agree with hokie66 - don't cut any ribs. The ME should be able to arrange for irregular duct openings.

Also, when you cut the slab (flanges) between ribs for the duct openings, it is much better to do so near the end span regions of the joists as this reduces the effect of the "T" beam action of the slab with the ribs.

If strengthening is required, using steel wide flanges between the joists or under the joists, tied into the supporting girders or columns is the easiest way to support new loading.