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Adding load to bottom chord of steel truss

Adding load to bottom chord of steel truss

Adding load to bottom chord of steel truss

(OP)
I have been asked if a rack system can be hung from the bottom chord of a steel truss. The existing steel trusses are space 5' o.c. and the rack would attached to two of them. The trusses span 50' and are supported by two 24" steel I-beams. How would I determine how much load can be supported by the steel trusses? Is it even practical to add loads to the bottom chord of steel trusses? I have attached a picture and the red lines are where they would like to install the supports and the rack would hang from these supports.

RE: Adding load to bottom chord of steel truss

It's quite feasible, particularly if your support points can be close to the truss panel points. Start by figuring out how much the new load is as a percentage of the load that the trusses were originally designed for.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Adding load to bottom chord of steel truss

Yes, but normally you indicate this to the truss manufacturer, you'd say the top chord has a DL of 20 and LL of 20 psf and the bottom chord has a DL and LL of 5 psf. So it would be designed to carry a small load connected to the bottom chord. It's not the fact that it's practical or not, it's the fact that architects just do it. The other thing is that could model the truss using FEA and see if it deflects more, but again this would depend on the load of the rack.

RE: Adding load to bottom chord of steel truss

(OP)
The building is existing so we can't indicate what we want to the truss manufacturer and the owner also does not have any plans so we have no idea what kind of loads were used to design the trusses.

RE: Adding load to bottom chord of steel truss

What is the magnitude of the load they wish to add? If we are talking hundreds of pounds, it is likely quite possible, but if it is 1,000's it can be time consuming to track down the information for an analysis.

RE: Adding load to bottom chord of steel truss

(OP)
The rack would hold skis and they would like the rack to go almost the entire 50' length. The rack would be about 5' tall. I could probably get the owner to figure out how many sets of skis will be there and how much a set of skis weighs.

RE: Adding load to bottom chord of steel truss

That sounds like hundreds of pounds and should be possible. If they are buying a standard rack from someone I would use the rated capacity of the rack. Start with a simple analysis using some assumptions and estimate the magnitude of the load increase at your chords and webs. I would doubt this will increase it beyond 2 - 5% given the joist span and depth. Once you know that you will have a much better idea of the next steps. You might want to stiffen the bottom chord for local bending, but that might not be necessary either.

RE: Adding load to bottom chord of steel truss

Word to the wise: if significant truss reinforcement is required, make sure that the owner understands the costs before you get too far into it. I reinforced some joists to hold up an enormous liquor cabinet last year. There was so much reinforcement that the existing truss was pretty much just scaffolding for a new truss. Once the owner got the costs in hand, he was perfectly happy to go with a floor mounted unit.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Adding load to bottom chord of steel truss

Must calculate maximum forces (shear, moment) for design loads (all load cases) and compare to design capacity ........so that you have to identify the roof joist designation (Steel Joist Institute (SJI) provides service with data supplied by yourself)........and (very important) date of fabrication (to get grade of steel).

You may want or need to add vertical members (welded between chords).

For light loads......should not be any problem unless original design was deficient.

Also check deflection.

Especially for low-slope ("flat") roof and heavier loads (but even for any loads considering liability these days).........be sure to check potential for ponding failure. You may find that original design is deficient for ponding.......in which case, you should inform owner at the very least and perhaps recommend additional ("secondary" or "overflow") drains if necessary.

John F Mann, PE
www.structural101.com

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