Building Under Negative Pressure 3

[SteelPE]

I am currently looking at a project where the building will be put under negative pressure. The reason for this is so the client can make sure the air that leaves the facility is properly filtered/cleaned. I am wondering how this would be handled in regards to building loading? I am guessing that you would just need to manipulate GCpi in some way, but I am wondering what the proper approach is. Are there any recommendations on how to handle this situation within ASCE-7?

 

[klaus.]

well just add another load case called internal (negative) pressure

 

[SteelPE]

While I understand what you are saying, I don't believe it's that straight forward. I believe this negative pressure would need to be added to dead/live/snow loads of the envelope as the intent to always keep the pressure below that of the outside.

 

[jayrod12]

Correct, you would need to run the appropriate load combinations.

Where I practice, I tend to assume that the buildings see both internal pressure and suction for my basis of design unless there are other mitigating circumstances. So for the design of your windbearing components you could possibly have a controlling load case that is external pressure, internal suction, plus snow/live/dead as appropriate.

 

[EngineeringEric]

How much pressure are you talking about? usually when we talk about negative pressures for rooms, like the mail room (to keep the anthrax there) or storage rooms (chemicals & smells) we are talking minor pressures. nothing that has any significant load effect on the structure. would you care about 0.1 psf? probably not. May be worth seeing what the pressure differential is, and if minor you can neglect it.

As for wind, i doubt the pressure differential will be there during design events. no way the pressure difference is comparable to a 115mph leeward suction. At least not if a habitable space.

I think the pressure differential will be acting on all walls and thus canceling out, ie normal MWFRS on enclosed building. If the space has that much pressure then you won't see a change during wind, it is airtight. Plus suction just means it will help hold the roof down during a hurricane! Less Uplift!!!

 

[Ron]

Ignore it. Negative pressure from an HVAC standpoint has no significant effect on the structure. I would be much more concerned about the waterproofing than the structure.

 

[klaus.]

@ SteelPe

Well of course you need to combine the internal (negativ) pressure with other existing load as required...

 

[Graham Knapp]

Hi @SteelPe ,
I would tend to agree with @EngineeringEric in most cases, the exception being if you are talking about very high internal pressures. Negative external pressure governs most cladding load calculations so it is conservative to assume that the internal depressurisation is inactive.
It raises the question of what happens when the (wind-generated) external pressure is more negative than the internal pressure - you are likely to end up with some wind situations where the chain of internal pressures leads to flow inside the building despite your system. For critical facilities you can used CFD (for the average situation) or wind tunnel testing (for peak gust situations) to examine the external pressure field and internal flows. With some desk study work you can estimate the number of hours per year when these situations will arise from the regional MET data.

Wind Engineer
CSTB Nantes

http://recherche.cstb.fr/en/services/specialist-area/major-structures/

 

[SteelPE]

This building is a little interesting as it is used to grow an unusual product. The project is located near some residences who have raised some concern about odor that may come from the facility. I am being told that an odor specialist is recommending the building be put into negative pressure to handle the odor objections which raises the question about the loading requirements for the building.

At this point everything is hypothetical and I don't know what the pressure will be inside. I am guessing the difference in pressure will be so slight that it won't matter in regards to the design of the building structurally. But then that raises a question, considering what we know about leeward wall suction (if ASCE-7 is remotely correct), how "tight" will this structure really be. Seems like some leakage would be expected unless they are really serious about keeping objectionable odors inside.

 

[BUGGAR]

I've seen negative pressures applied to sewage treatment buildings for odor control. It was treated as another design loading condition. It was insignificant because other building loads dominated. They expected rain leakage and designed internal gutters and drain pipes to carry it away.

 

[TehMightyEngineer]

as it is used to grow an unusual product

I agree with Ron and Eric; for just a typical negative pressure HVAC system you're going to have small pressures. Normal buildings struggle to maintain even a small amount of pressure as there is too many leakage paths. Plus, as soon as you get to something like 0.05 psi doors stop being able to be opened if they swing out, or they hurt people if they swing in.

When you make your millions off of this "farming" operation and go to get into your Gulfstream jet make sure the pilot didn't accidentally pressurize the plane on the ground. Pressures and aircraft doors have definitely killed people. This is actually one reason that large commercial jets use plug type doors that swing inward.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[canwesteng]

0.05 psi is about 7 psf, not quite negligible over the structure. It doesn't affect the lateral loads seen by the foundation, but does affect girt, cladding, maybe even column design. I think the proper approach is just to treat it like another wind load that can act concurrently or independently from external wind on the structure.

 

[TehMightyEngineer]

Oh my point wasn't that it's negligible; simply that the owner likely wont want the building seeing such high negative pressures. Thus, there's no reason to design it that high unless the building is actually sealed well enough that it might accidentally hold such a negative pressure.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[KootK]

I've seen internal walls collapse in a negatively pressurized hospital operation room. While exterior wall and MWFRS loads may be negligible, it may be worth keeping an eye on things in the interior that don't normally get much of our attention.

 

[TehMightyEngineer]

Huh, I would have thought hospital rooms would be positively pressurized to force germ-laden air out of a clean room.

Either way; great point about internal partitions.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[KootK]

Huh, I would have thought hospital rooms would be positively pressurized to force germ-laden air out of a clean room.

Positive pressurization is correct. As you can imagine, I didn't much care about the direction of the load, only that it knocked the CFM wall down.

 

[TehMightyEngineer]

Yikes! Undoubtedly that was quite expensive as I imagine just the lost OR time alone would be extreme.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[KootK]

Fortunately for me, the wall came down during the commissioning of the mechanical systems. Otherwise, I might be practicing under a false name in Paraguay.

 

[TehMightyEngineer]

Oh! It was your design, I assumed it was a repair after the fact. YIKES!!

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[azcats]

Don't hold out on us KootK....what's the story?