Internal pressure on a pole building 1

[iainuts]

Hi folks. I'm looking for qualitative answers on this one, nothing quantitative. This regards typical warehouse type buildings made of steel beams with sheet metal walls. Assume the building is uninsulated such as a metal pole building, roughly 30 feet high with a flat cement floor 40 x 60 feet. For 'typical' structures built and installed in the south east US:
[ol 1]
[li]Can they withstand higher external pressure (ie: from wind) than internal pressure? I'm thinking they can, simply because the metal walls must press against the steel structure when external pressure or wind is creating the loading but when internal pressure is put to the walls, then only the strength of the fasteners are providing the restraint to hold the skin on. Assume this building is built to code to handle typical wind loading.[/li]
[li]What internal pressure can such a typical building be expected to withstand? For the sake of clarity, please consider that there is no external wind, only a pressure internal to the building caused for example, by a very large volume of air being vented into the inside of this building.[/li]
[/ol]
There's no need to provide guidance on analysis or anything qualitative. I'm just looking for some insight into how typical structures like this are built so I can get a rough idea of the dangers in having a building that might be subjected to relatively high internal pressure. I'm concerned that even relatively low internal pressure (equivalent to low velocity wind) might cause damage to the walls, even though the building might be built to withstand hurricane force winds. Is that a reasonable concern?

 

[Ron]

Correctly or incorrectly, these buildings (pre-engineered metal buildings...PEMB's) are usually designed as enclosed buildings thus internal pressures are lower. I have disagreed on several occasions when I've reviewed the design of such buildings and argued that they were partially enclosed buildings thus having higher internal pressures.

I'm a bit confused by your description...on one hand you talk about pressurization from wind and on another you discuss internal pressure from venting pressurization. Which is it?

Is your concern reasonable? Not if I understand your premise. Damage is not likely in your scenario.

 

[hokie66]

There is no good answer to this question. External pressure can be positive or suction, and the same is true for internal pressure. The general goal in designing metal buildings (just as for any building type) is to make them capable of resisting all loading conditions. With that in mind, the sheeting, girts and purlins, and structural framing all have to be designed accordingly.

 

[Ron]

Actually hokie66, there is a good answer to his question and you gave it.

 

[iainuts]

Ron, hokie, thanks for the responses. Sorry for being vague but I can't get too specific. My concern regards a project that may result in there being a very large volume of air being vented into a building as described above. The building hasn't been built but will be shortly.

Wouldn't an external pressure due to wind of 3" water on one side of the structure be considered 'not unusual' and wouldn't that be covered by typical building practice?

If the internal pressure were 3" water on all sides including the roof due to air being vented into the building, do you think the building would need to be specially designed to handle that amount of internal pressure? If so, why wouldn't a building designed to handle significant external pressure not be capable of handling this low of an internal pressure?

 

[Ron]

iainuts....yes, an additional differential pressure of 3" would need to be considered in the design, taking into account the external wind considerations and the internal and external pressures resulting from those winds. A 3-inch differential pressure is almost 16 psf, so that would certainly need to be considered.

 

[iainuts]

Thanks Ron. Just to clarify, please disregard internal pressure for a moment. Doesn't wind loading produce a pressure on the side of a building greater than 3" of water? I'm thinking a typical building is built to handle considerably more force than that due to wind.

 

[wannabeSE]

iainuts,
Your assumption is incorrect. The building may be designed for a wind load less than 15 psf. To give you a frame of reference, 10 psf is the minimum wind load per ASCE 7-05, Minimum Design loads for Buildings and Other Structures. The design wind pressure might be higher than 15 psf, but it depends on many factors including building location (based on expected wind speed in the region), exposure (surrounding terrain with closely spaced buildings or wooded areas have less exposure), height above the ground (pressure increases with height), and topography (top of a hill may have higher pressure). Also, certain building elements are subject to higher loads (corners, roof overhangs, rooftop equipment, etcetera).

 

[iainuts]

Thank you wannabe.

 

[hokie66]

You can't rely on external pressure counterracting internal pressure, as the two may be in the same direction. A typical situation is suction on the leeward roof and walls, combined with positive pressure inside. So the pressures inside and outside act together, in the same direction. Wind actions on structures is quite a complex subject, depending on many variables.