Suspending weight from roof trusses

[cruzinbear]

I am working on a project to build a 50'x50' walk-in-cooler inside a commercial warehouse structure. The warehouse structure was designed in the late 1980's. The ceiling panels can only span 25 ft and thus I need to provide a support in the middle for the two joining panels. Not having the plans to work off of, I thought one option would be to build a frame inside the cooler to support the ceiling panels. However, the client wants to avoid this and the contractor on the job is suggesting to suspend from the roof trusses above.

This is a viable option if I had structural drawings and/or roof calculations of the original building. The landlord doesn't have any drawings and city is looking in their archives but we have not been lucky yet to locating these drawings and/or calculations. The contractor is keep pushing to suspend from the trusses above. While I don't doubt that it can handle the weight, since the they are foam panels with sheet metal, approx. 2.5 psf, however, I just can't call it out on the plans without understanding the structural system of the wooden roof trusses above and verifying the loads it originally was designed for.

One option I was thinking is to take use a lift and go measure the dimensions of the roof truss elements then conduct an analysis to determine whether loads can be supported from them. Does anyone else have a better idea? Any advice/ recommendations would be greatly appreciated it.

 

[XR250]

Is it a metal plate connected truss? If so, you would have to know the shear capacity per square inch of the plates - which is usually proprietary (that is why the manufacturer's of the plates typically supply the truss software). For a 24" O.C. truss, you are only adding 125 lbs of load - which is probably not that significant.

 

[ztengguy]

Are you going to support the panels at each truss, or just at the end of the 25 foot, thus 62.5 plf along the length of the truss? Like Excel said, if your doing at each truss, its 5plf, not a big deal.

 

[SteelPE]

While others are saying that it's not a big deal I will take the other side of the argument.

The IBC only allows you to increase the loading on any existing member by 5% before the member needs to comply with today's loading criteria. The key is the 5% limit which will not be much on a wood trussed roof. You may be able to squeeze 2.5 psf out of it, depending on the framing situation and location of the building, but you may need to support the panels at every wood truss (I bet the contractor and owner wouldn't like that at all).

If you are over the limit or the contractor doesn't want to support the panel system frequently, you may need to analyze the existing truss to see if it can handle the new loads (62.5plf at 25' spacing). I have somebody I work with who helps me out with these problems and as stated above, the gussets end up being the key to the whole system.

 

[XR250]

SteelPE,

I would assume that it would be supported at each truss. If not, a strongback could be added to distribute the load to all of the trusses.
I agree on the 5% limit, but typically, wood roof trusses are designed for more load than they actually see. Of course, there is no easy way to verify that.

 

[SteelPE]

From the way I understand it, if you can prove that the loads are less than 5% no analysis is needed. However, if they are greater than 5% then you will need to run an analysis to make sure the truss is OK. This is when you will find out how much your wood truss is "over designed". As stated earlier, I have someone who helps me with wood trusses, however, if they are anything like open web steel joists, there is not much extra capacity at all. These companies are not in the business of over designing structures.

 

[hawkaz]

I don't agrere that trusses are usually designed for extra capasity. Look over some truss calcs, the chords usually have some extra capasity, but you will see that in many cases, they design the press plates to unities of .97-.99.
Unless the original plans instructed them to design for future load, it probably was not done.

As noted above, field measurements of prefab wood trusses isn't worth much without the press plate data.

 

[cruzinbear]

The trusses are spaced 8 ft apart so that's 500 lbs + 300 lbs live load for maintenance worker = 800 lbs. I can choose to locate the joint of the panels so that one truss supports all the weight or to distribute it amongst 6 trusses (ztengguy) - the latter is best I think.

Excel, I believe the they are steel connections which I will verify. Thanks for bringing the plates to my attention.

SteelPE, where in the IBC do I get an increase of 5% - thanks. I can distribute the weight so it's not supported by one truss. The client and contractor I believe would still prefer this over building the frame inside.

Hawkaz, I am waiting to hear back from the city. Thanks for highlighting the issue with the plates, I will consider this.

One more thing, what about IBC table 4-1, I see justification for 300 lbs concentrated load for worker but not the minimum distributed load since there will be no storage and inaccessible? I suppose I can treat it for 10 psf - inaccessible attics (residential) - but still this would make the 5% proof difficult. Furthermore, I think it's irrelevant since they are not storing anything up on top. Any comments on this?

Thanks for everyone's responses.

 

[charliealphabravo]

The trusses were almost certainly designed for a bottom chord load much greater than 2.5psf. Does the warehouse have a ceiling currently or extensive lighting or mechanical systems in the trusses? If not, you should be fine to support the cooler ceiling uniformly. By uniformly I mean that I would not space the hangars any farther apart than the truss spacing or the panel point spacing.

If the warehouse already has a ceiling then check the total weight of the warehouse plus cooler ceiling. I suspect that it will also be less than the design ceiling load. If I still have reservations I have always been able to work with a truss design technician or perhaps an engineer at Mitek or Alpine to help me with a load analysis. They will recognize a photograph of their own plates.

 

[XR250]

@Hawkaz;

I agree the members are designed pretty much to the max, but the design loads are usually high (i.e top and bottom chord dead loads)

 

[hawkaz]

Be careful- Sounds like this was a panelized wood roof. These could have been designed for as little as 12 psf DL and they would have used every bit of the live load reductions

 

[charliealphabravo]

I'm also not sure I agree that the 5% rule should be applied to require "strengthening or replacing" when you can make the case that the new construction is within the scope of the original design criteria and design capacity.

 

[ztengguy]

If its only one truss, just sister or place a steel beam or gluelam next to the truss and support the load. Leave the truss alone. Surely the span is not that far.

 

[dhengr]

Cruzinbear:
Interesting design problem... I assume the vert. wall panels will support the 25' ceiling panels where they bear on the walls. And, all of these panels lock together at their mating edges to provide air tight seals. There are a number of these systems available. The bigger problem that I see, is that your existing light framed storage building (pole bldg.?, wood trusses 8' o/c) probably deflects an inch or two under max. lateral loads (wind or EQ), and for vert. loads too, and your insulated panel system will get ripped apart by these lateral movements, if it is hung from the roof trusses. The panels are very stiff as individual shear panels, so when they are caused to rack, they each rack independently about their lower compression corner, and tear the joints apart, in fairly short order. I’ve gotten to explain this problem to juries when we showed photos of icicles 3'x2'x15 or 20' high hanging on the wall panels at failed joints. If this is a freezer you also have foundation freezing issues to deal with.

I would put the ceiling panel joint parallel to the existing roof trusses. Then provide a couple new 50' long trusses, making up a bit of a box truss system of sorts. This could be assembled on the existing floor slab and lifted into position. Provide four small columns and new footings, by cutting out the existing slab. This structure can all be outside the freezer/cooler, and can have tie rods from the column tops/trusses to the floor slab adjacent to the exterior of the two walls.