Duct/Utility Openings in Open Web Metal Trusses

[Trillers]

We have been asked by a client to utilize an open web metal truss system to support a concrete roof slab. Because of architectural considerations the ceiling space is fairly crowded so it appears some A/C ductwork will pass through the trusses.

Does anyone have a good reference of how to reinforce these openings? One of my colleagues has recommended I just call out that the truss manufacturer should provide their standard reinforced opening detail for the contractor to reference.

Is this something normally done or should I actually design the reinforcement around the opening (thus the request for reference in Para 2 above)?

Would appreciate your thoughts and recommendations.

 

[dik]

What I've done is use the maximum unbalanced loading to determine the shear at the middle of the opening and then design top and bottom chords for the regular axial loading (as a regular truss) plus the additional moment caused by the shear force * half the opening width and reinforce the opening appropriately.

This can be modeled in any number of 2D or 3D frame programs.

Dik

 

[hokie66]

Just tell the ME to use circular ducts. Or better still, triangular ducts.

 

[dik]

and if the circular duct is too large, they can flatten the sides... and, go back to your rectangular opening.

Dik

 

[XR250]

Why not throw this back on the supplier? (I am assuming you are talking about bar joists)

 

[gravityandinertia]

I use to work at a steel joist manufacturer. You simply should request they design for this, giving them the size and location of it. They will handle the rest, and ask for any clarification/changes that need to be made to make it work.

 

[berkshire]

As a ducting designer we used to handle this by fabricating Bifuricated ducts.
B.E

You are judged not by what you know, but by what you can do.

 

[Trillers]

Thanks All:


Great recommendations and ideas.

We will go with throwing this back on the supplier as suggested by XR250 and GRAVITY AND INERTIA.

Hokie 66 - We tried your approach with our ME - he considered circular ducts and our meeting ended earlier than anticipated when we suggested triangular ducts!!! Some people have no sense of humor!

Anyway thanks much everyone - all helpful and insightful.

 

[JLNJ]

Page 15 of Newmill's joist catalog has some approximate max duct sizes through joists. I presume this is without special design or reinforcement.

http://www.newmill.com/pdfs/steeljoists-sec1.pdf

 

[Trillers]

JLNJ - Our openings are larger than the standard sizes so we will need special design or reinforcement where the duct openings occur.

 

[hokie66]

Just keep your joists close enough together to make installation difficult. Maybe the next time, the architect and ME will use some common sense.

 

[Tim_B]

As someone who works for the supplier that you are likely going to "throw it back on" I can tell you that it totally fine. If it is an SJI open web steel joist, we can evaluate and fabricate that. It may be possible to get it to fit without a rectangular ("vierendeel") opening by increasing the panel size. That is the first option because it is a lot cheaper. If it will still not fit, we will design the vierendeel opening and fabricate everything in our shop. There will be no extra work for the contractor in the field. Another bit of info that may help determine how hard to push back on the ME to go with bifurcated ducts is how close the opening will have to be to the end of the joist. The closer to the end the higher the shear and the bigger chord members it takes to span that distance with no webs. If it is w/in 10' of the end they might have to bifurcate.

Thanks!

Tim_B

 

[Trillers]

TIM B - The ME has already agreed to bifurcate at critical locations. The duct which is giving us conniptions is 8" x 24" and 100' long passing through 32" deep joists. The problem we are faced with is that the duct opening can be located no further than 4 1/2' away from the end of the joist. We have no problem with bigger chord members but the joist depth cannot exceed 32". It would seem to me this type of fabrication should be fairly routine?

On another project I recall seeing a vierendeel section of truss installed close to the joist ends in a similar situation. The vierendeel then switched back to an open face web truss. This is fairly routine on the part of the manufacturers I would assume?

Thanks.

 

[hokie66]

So the structure has to bear all the cost of the mechanical engineer not wanting to revise his ductwork? He could easily split the ductwork into two which would fit through the joists.

I think it is far from routine to make the joists like that.

 

[Tim_B]

Trillers,

To be clear when I am referring to a vierendeel opening I am referring to a single rectangular opening in the middle of what is otherwise a regular open web steel joist with warren or modified warren panels.

With that said, putting a vierendeel opening in a joist happens occasionally but is not the norm. It is even less normal for them to be so close to the end of the joist for the reasons I describe above. I don't know how long your joists are and how many will be impacted but I can tell you it will likely more than double the weight of the joist and add 50%+ more labor. If it is only a couple joists it may be worth it. If it affects almost all the joists on the entire building that is a different story. If you have 100' a long duct I assume it is affecting in the ball park of 20 joists...? I could see that going either way. It may depend on the extra cost from the ME and HVAC installer to implement other solutions (if other solutions exist).

I will also note that that size duct should fit through a standard 32LH joist without a vierendeel opening if the duct is closer to the middle so long as the loading and deflection requirements are not excessive. However, you probably cannot get it through at 4'-6" from the end without a vierendeel opening.

Thanks!

Tim_B