Thread: Steel Structural tube going through supply plenum

Hi All,

I would like to know the technical limitations of a structural steel tube passing through a supply plenum. (Please see attached pdf drawing)

This is necessary as there is no space between the facade & staircase sill to install the plenum. Hence we are thinking of including it inside the plenum & sealing the ends.

Kindly help me.

Thanks,
Sameem

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You really need to get an Engineer who is approved by the Saudi authority who is over this project to sign off on this. A concern I could see would be the possibility of enhanced corrosion to the steel tube in the plenum, this could be addressed with additional protective corrosive coatings on the steel.

This is way over my pay grade.......

I agree with Scott, this should go back to the Architect or Engineer, to many variables, just a couple off the top of my head.

- Could have condensation issues on the steel being in the supply.
- You might have to oversize the supply to compensate for loss of air flow from the steel passing through.
- Vibration/movement noise between the surfaces.
- Movement allowance for the steel to flex.

Whole list of issues, as Scott said, over my pay grade…

Hey, Thanks guys...

The condensation, I think is the one I should be watching out for. Ill make a quick calc for it.
Unfortunately, Im the engineer who will be approving it!!!

Well, they will seal the sides of plenum in contact with the steel with mastic, so, I think leakage should not be a problem. The steel shall be galvanized to avoid rusting Or shall be painted. However, noise can be a point which is worth looking out for.

I am suggesting to use normal bar grill to avoid the resulting pressure drop, which also shows a throw of 3.5m @ the chosen airflow. Supply temperature is 18 Deg C.

Any comments / suggestions please!!!!!

Thanks again.
Sameem

As the others have said - the designer of record (engineer or architect of record) needs to address this.

In my opinion the main concern would be protecting that structural steel tube from fire, as in a fire in this commercial space should not bring the tower down because it caused that structural steel tube to fail.

Condensation could be another cause of such failure due to slowly rusting away, but if you solved the fire protection issue then you have also removed the condensation issue at the same time, but this does not necessarily work in the opposite by solving the condensation issue - the fire protection issue may remain.

'Engineer' a new facade? It seems to me the ductwork can be made to split and go around the pipe, without compromising the structure in any way.

Originally Posted by John Kogel

'Engineer' a new facade? It seems to me the ductwork can be made to split and go around the pipe, without compromising the structure in any way.

Not sure if you missed this part or not, seems like you may have:

Originally Posted by Sameem

This is necessary as there is no space between the facade & staircase sill to install the plenum.

That means (when taken as it was stated) "This is necessary as there is no space" ... "to install the plenum".

I'm taking him at his word, are you calling him a liar?

Originally Posted by Jerry Peck

Not sure if you missed this part or not, seems like you may have:



That means (when taken as it was stated) "This is necessary as there is no space" ... "to install the plenum".

No, I saw that. They designed the place and now find they forgot to allow for ductwork. Change the design, simple. Customize the ducts to go around, can do.

Originally Posted by John Kogel

Change the design, simple. Customize the ducts to go around, can do.

Cant redesign, cause its already being installed. They will finish the steel structure in a couple of weeks...

Originally Posted by Sameem

Hi All,

I would like to know the technical limitations of a structural steel tube passing through a supply plenum. (Please see attached pdf drawing)

This is necessary as there is no space between the facade & staircase sill to install the plenum. Hence we are thinking of including it inside the plenum & sealing the ends.

Kindly help me.

Thanks,
Sameem

I would post this at the Eng-TIPs forum, either in the Structural Engineering or Mechanical Engineering (HVAC) forums.

Why not sleeve it with a larger diameter pipe, just as we do with gas lines and plumbing that pass through foundation walls in residential?

If there is no room between the facade and staircase, where is the room for the beam?

I would have to see a print to consider the solution. You seem to have doubt about the steel in plenum design. If you as the engineer cannot affirm the effect of the plenum on the steel (or the steel on the plenum), you should consult someone who is. Perhaps it would be diligent to evaluate the entire building regarding the air quality.

If you feel that humidity may be a concern, then consider how the application is going to function as a system. You should consider the materials and how moisture will effect them, absolute humidity, relative humidity, and climate. Since it is moving air, there is air exchange.

If the plenum is cooler than the surrounding air; considering that moisture moves from warm to cold (if you observe a glass of ice water, you will notice the condensation is on the outside of the glass), so theoretically if there is condensation it will be on the outside of the [cooling] supply plenum. For the sake of the building, I would want air exchange (ventilation) on the outside of the plenum (too). But this has nothing to do with the steel beam, and is a building science issue.

Are the rest of the beams fire protected? If not, what is the purpose of fireproofing this one? Is it because you are considering the inside of the plenum as an open fire penetration? Isn't the entire area open?

Is there a damper behind the linear grille? If the plenum serves more than one area I would hope so.

I avoid ANY penetration of a supply duct. Seal it with anything and it will leak someday. I am an old guy, I know. I am not opposed to penetrating a return duct or plenum, return air has a tendancy to find it's way home, but supply air is different. I want it to go where I want it to go.

I had a project recently where an immovable conduit with fiber optics that controlled an entire plant could not be relocated. I did what I had to do and provided an "avoidence" fitting. I am still concerned about two different systems contracting and expanding at different rates.

conduit avoidence fitting small size - Copy.jpg

Originally Posted by Rod Butler

Is there a damper behind the linear grille? If the plenum serves more than one area I would hope so.

I avoid ANY penetration of a supply duct. Seal it with anything and it will leak someday. I am an old guy, I know. I am not opposed to penetrating a return duct or plenum, return air has a tendancy to find it's way home, but supply air is different. I want it to go where I want it to go.

I had a project recently where an immovable conduit with fiber optics that controlled an entire plant could not be relocated. I did what I had to do and provided an "avoidence" fitting. I am still concerned about two different systems contracting and expanding at different rates.

conduit avoidence fitting small size - Copy.jpg

Looks a lot like a sleeve!

Originally Posted by Sameem

Cant redesign, cause its already being installed. They will finish the steel structure in a couple of weeks...

Never say never... . (Where did I see that?) They might not like it, but I have seen where the redesign was done after installation or the installation was stopped because a design error was called out at a weekly construction meeting. Yes, the poor design was corrected, lot of yelling back and forth, and yes that did delay the project---but the idea was to leave the completed project with no future problems (as best they could), such as has been questioned.
Would liked to see a slice or side view though.

Penetration of a plenum with non-combustible material does not appear to be a problem according to the 2009 International Mechanical Code, Section 602.2, as long as the material you use to seal the penetration meets a flame spread index of 25 or less and a smoke developed index of 50 or less according to NFPA 255, ASTM E 84, or UL 723. If the pipe is part of the structural frame system and needs to be fire-rated, which means coated with fire-proofing, I would be concerned that the constant flow of air may cause the coating to be blown off. In that case not only will the building inhabitants be breathing particles of the fire coating, but the fire resistance of the building will be reduced.

Originally Posted by Thom Huggett

If the pipe is part of the structural frame system and needs to be fire-rated, which means coated with fire-proofing, I would be concerned that the constant flow of air may cause the coating to be blown off. In that case not only will the building inhabitants be breathing particles of the fire coating, but the fire resistance of the building will be reduced.

Depends entirely on the type of fire-resistance rating one chooses to use in attaining the required rating.

The best solution would be to box around the pipe, in which case there would be nothing to be blown off. If one choose the cheap way and to apply a sprayed on coating, yes, that could be a problem - but I would have to wonder about the person who would chose a sprayed on coating when it was in an air stream ... unless it was also encapsulated in a barrier which would resist such damage, but then that would likely not be any cheaper than boxing around the pipe.

Originally Posted by Jerry Peck

Depends entirely on the type of fire-resistance rating one chooses to use in attaining the required rating.

The best solution would be to box around the pipe, in which case there would be nothing to be blown off. If one choose the cheap way and to apply a sprayed on coating, yes, that could be a problem - but I would have to wonder about the person who would chose a sprayed on coating when it was in an air stream ... unless it was also encapsulated in a barrier which would resist such damage, but then that would likely not be any cheaper than boxing around the pipe.

Sprayed fire-resistant material is the most common system for protecting steel frames of commercial buildings, which is why I singled out specifically the problems associated with that method in this case. There is, of course, no indication on the plan as to what method is to be used to protect the columns. They might be boxing the columns, but that is not shown on the plans, and is probably cost prohibitive as there appear to be columns every 2-1/2 meters in both directions. They may also be using concrete filled columns, as they appear to extend exposed through the usable space. Only Sameem can tell us for sure.

Originally Posted by Thom Huggett

Sprayed fire-resistant material is the most common system for protecting steel frames of commercial buildings, which is why I singled out specifically the problems associated with that method in this case. There is, of course, no indication on the plan as to what method is to be used to protect the columns. They might be boxing the columns, but that is not shown on the plans, and is probably cost prohibitive as there appear to be columns every 2-1/2 meters in both directions. They may also be using concrete filled columns, as they appear to extend exposed through the usable space. Only Sameem can tell us for sure.

While all of the above is true, there may not be any fire-proofing shown on that drawing because: a) none was specified; b) it is shown on the FP (Fire Protection) drawing sheets address fire-proofing, firewalls, fire-dampers, fire-stopping, etc.

I was pointing out that just commenting on sprayed on fire-proofing is not necessarily jumping on the right thing as there are other choices, including 'no choice' because nothing was specified (either intentionally or unintentionally).