Duct bracing requirements 2

[WARose]

Sometime back, I got some excellent advice on duct support spacing here:

https://www.eng-tips.com/viewthread.cfm?qid=472932

A few follow up question(s) here:

1. What if you have a short (dead end) branch that is less than 30 or 60 feet? (In fact, say it's just 10' or less.) Any lateral/longitudinal support required there?

2. Any min. spacing requirement for vertical supports. IIRC, I read somewhere it was 10' some years back. But I think it depends on duct size.

Thanks in advnce.

 

[EnergyProfessional]

SMACNA Duct Construction standards will tell you exactly what you need for the size and shape of duct. You often have the option between heavier gage duct and wider support spacing vs. thinner gage and closer spacing of supports.

I just followed that link to the thread and you asked the same question and I gave you the same answer (again) and others also mentioned sources for seismic restraints. Ultimately you have to do the sizing/design yourself. We don't get paid and don't know anything about your system or location. I bet San Francisco has different rules than Poland.

 

[WARose]

I just followed that link to the thread and you asked the same question....

Actually no: I asked nothing about vertical support in that thread, nor did I ask anything in that thread about a short run.

...and I gave you the same answer (again)...

And again: it is useless as I don't have that reference. I just need a short answer for the early part of a project where the HVAC engineers aren't on board yet. Hopefully the same people who gave me good answers in that thread will show up here again.

 

[EnergyProfessional]

I agree - our advice is useless since you refuse to look into the references we mentioned.

Maybe someone better than I stops by to provide free design service.

 

[WARose]

I agree - our advice is useless....

Actually no: just your advice. (Since two others replied in that thread with very useful info.)

We are talking just some quick info for the very early portion of a project. No design will be performed based on it.

 

[willard3]

>>>We are talking just some quick info for the very early portion of a project. No design will be performed based on it.

I can't tell you how many times I have heard this bad advice and wind up trying to fix something a structural engineer did not understand. Hire an HVAC Engineer

 

[WARose]

I can't tell you how many times I have heard this bad advice and wind up trying to fix something a structural engineer did not understand. Hire an HVAC Engineer

He/she will be on board soon. I'm a independent structural consultant. This was a question that came up in a walk-through as part of a FEL on a project. Again: nothing final will be based on it....the client was just curious about any duct supports to be added. He asked me....and I said ask the HVAC guy/gal when he comes on....he then said: "well for now, ask any HVAC person you know"......and here I am.

These sorts of guesses are common in early parts of a project. (And yes, a lot of times they are wrong.)

 

[ProcessHVAC]

SMACNA standards are designed to simplify structural design work for HVAC engineers. As a structural engineer, you should be qualified to design this over and beyond what SMANCA does in a prescriptive manner. You are asking for free design assistance on a discipline that should be your domain.

We go the other way about this, we use SMACNA for basic/simple ductwork support design. When we are dealing with complex work (e.g. 3 m diameter high pressure blower line overhead in an industrial plant in a high seismic load area) we work with an structural engineer to design the supports and bracing.

 

[WARose]

As a structural engineer, you should be qualified to design this over and beyond what SMANCA does in a prescriptive manner. You are asking for free design assistance on a discipline that should be your domain.

Not really. As as structural guy, I don't design duct. Ergo, I have no idea how far it can span. The rule I have seen over the years is: 10-30-60 (i.e. vertical support every 10', transverse every 30', and longitudinal every 60'). But the fact is: that's controlled by the HVAC requirements quite simply because a lot of times (in low seismic zones, sometimes even in the high ones) those brace loads are next to nothing. In other words, if I wanted a brace (say) every 500'....that would be great for me...not so much for the duct. Get it?

This is no different than the times I have asked piping how far their pipe can span (without support) or electrical how far their conduit can span. And have typically gotten prompt, polite responses. And I have always gone out of my way to help them not just in the workplace.....but on the structural board here as well.

 

[chicopee]

If you research the web under the title of HVAC duct construction you'll notice many articles. the duct supports that some articles will have analysis detailing calculations for deflection and stresses. Further more there will be an added component for seismic events to the standard classical formulae that you learned in statics for simply supported uniformly loaded continuous beams. So start your research as there is an abundant amount of information.

 

[WARose]

If you research the web under the title of HVAC duct construction you'll notice many articles. the duct supports that some articles will have analysis detailing calculations for deflection and stresses. Further more there will be an added component for seismic events to the standard classical formulae that you learned in statics for simply supported uniformly loaded continuous beams. So start your research as there is an abundant amount of information.

Sounds great but these are codes I don't normally deal with and they aren't structural. Bottom line: I don't stamp HVAC drawings so I'm not about to say what support spacing should be (beyond my capabilities to provide support). It's the same thing with (to go back to my previous analogy) process piping: you are talking a lot of governing codes and materials I don't know.

In any case, a HVAC guy is coming on the job this week and I may post his answer here. (So that a structural engineer (in the future) won't have to waste much time.)