Using a mechanical duct to support live load 1

[tdstructural]

??? from a structural engineer:

I have to design gravity and seismic support for a small platform approximately 4’ wide x 17’ long. The platform is along the side of a boiler and will be used by employees to maintain some small mechanical units.

We are having problems fitting the platform into the structure because of the numerous ducting/insulation/mech units around it. We are considering using a large steel duct to transfer the platform forces to the structural steel columns. The steel duct will be fabricated out of ASTM A36 flat plate and it is about 3’ deep and 5.5’ wide.

I know I can design the duct to be able to carry the loads (it might need stiffeners) but is there anything in the code that does NOT allow this. The duct will be transferring heated air at about 0.25 psi pressure. My concern is carrying live load on a mechanical duct. I’ve never heard of anyone doing this.

Thanks

 

[MiketheEngineer]

Sounds a bit nuts - but most anything can be done. My concern is that the duct can carry all the loads. Ducts tend to be pretty fragile - although I have seen some that look like you could hang a car from.

What if it is insulated. What if it need replacement.

Look up - You might be able to bring a brace down from above to solve your problems.

BTW - how hot is the heated air. Can guys even work that close to it??

 

[BigInch]

Ask this question in the Structural forum.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[racookpe1978]

Yeah - Good idea. Ask in structural section & include what temps you expect. (Expansion, contraction will be controlled by boiler movement through its thermal cycle most likely.

 

[tdstructural]

From Original Poster:

I did post this question in both the mechanical and the structural forums. Funny thing - I received a response in the Mech forum from a Struct Eng and a response in the Struct forum from a Mech Eng. I'm glad to hear that we try to help each other.

The duct will be tied hard to the surrounding structural steel brace frames/gravity columns. There is an expansion joint just above the duct where it connects to the multiple small ducts which are tied hard to the boiler. To summarize - the big duct I will design with structural steel plates and most likely stiffeners will not expand with the boiler. But - it will expand slightly with it's own thermal conditions. I've taken that into consideration with my design.

I didn't think about vibration but I have thought about insulation and this could very well be a problem for me.

I appreciate everyone's input. Again - if you know of anywhere in any code that I am NOT ALLOWED to do this (similar to not supporting gravity on seismic brace frame diagonals) please let me know. I might be stuck with this crazy concept.

Thanks - Tracy

 

[BigInch]

You should also ONLY post to ONE forum.

As far as I see it is totally a structural problem and should only go in a structural forum. It could only go here, just barely, in the widest context possible. You might have gotten an answer, but being in the wrong forum, and nothing against Miketheengineer, perhaps you should be concerned with the relevance and quality of any answer you get. Did the answer you received get you any farther along to where you wanted to go.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[tdstructural]

To: BigInch

I didn't realize there were rules against posting the same question in two different forums.

The reason I posted in the mechanical forum is that I thought a Mechanical Engineer might have run across this problem sometime during their career and they would have some information for me.

I am sorry if I offended you.

Tracy Dudgeon, PE

 

[BigInch]

You didn't offent me. Its a site rule and it wastes less time. If you only post once, we only have to read once and we only have to answer once.

This is totally a structural question. Your "duct" is actually a beam, a fabricated box girder or box section, commomly used in bridge constructions and other long span locations, so wouldn't you think the structural forum just might be a good place to post. You didn't even post your question there did you. OK. well just do better the next time, now that you know the rules.



We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer