MWFRS Wind loads on a monosloped building 1

[PostFrameSE]

When designing a monoslope roof building for wind hitting the sidewalls of the building (parallel with the roof slope) I'm curious how some of you would do this and so I have several questions. The building I am referring to would have the high side completely open and the low side closed with a agricultural roll-up fabric curtain. Height on the high-side is 22' and on the low side perhaps 14'. The roof width is 60'

1. When analyzing the MWFRS loads on enclosed buildings, the internal pressure components cancel out. On a building as "open" as this, I assume that is NOT the case.

2. With the low side curtain wall being the windward wall, using the middle elevation of Fig 6-6 in ASCE 7-05 I would apply windward forces to the low wall AND internal suction forces to the same wall and nothing on the high side as is there is no surface to apply load to. Of course I would apply forces to the roof as well. I'm comfortable with this.........unless somebody disagrees.

3. My biggest question then is how I analyze wind coming from the high side. Like a venturi, wind is funneled towards the low side wall. (See the right elevation of the mono-slope building in Fig 6-6 without a windward surface to take loads at the high side) I know I need to apply leeward forces to the outside of the low wall as is shown in addition to the forces on the roof. In addition to that, do I apply windward pressures to the inside of the low wall using a Cp of .8? Do I add to that the Cpi of .55 to the .8? Would I just use an internal pressure only with a Cpi of .55? I'm unsure what is right here.

4. Lastly.........a fabric curtain as a wall. Is it commonplace to determine what the allowable wind loading is on the curtain and at pressures above that, assume that the curtain "takes flight" and now the internal pressures drop as there is no longer a low-side wall to take lateral wind pressure?

Thanks for your thoughts!!

 

[spats]

Without getting into a lot of detail, you need to determine for each wind direction whether or not it is classified as an open or partially enclosed building, and take it from there.

 

[TXStructural]

When directed at the open wall, the internal pressure would be most accurately represented by a force equal to the pressure at the open side times the area of the open side, for MWFRS. The wind pressure is what it is, and it is pushing on the air trapped inside the building. Absent a better representation in the code, the shed is partially open. An open structure implies that air is free to enter and leave the building, but does not achieve full stagnation under wind pressure.
I would not use less than 0.8 for internal. A key failure mode for wind events is interior pressurization through a breach in the windward wall. This is the situation you have described, so it is reasonable to use full wind pressure on all interior surfaces for C&C. .

 

[PostFrameSE]

Thanks.

Spats.......I believe the building is partially enclosed regardless of which way the wind is blowing isn't it?

TXStructural..........thanks for the clarity of your answer. That makes for some tremendous wind pressures!! Absent the curtain being down or off, I think a case could be made for an open structure with wind blowing on the low side, but due to the venturi effect, the wind hitting the high side still would seem to be acting on a partially enclosed building. Please help me understand one thing. You mentioned that you wouldn't use less than .8 for internal. .8 is for external, so for the high side "wall", are you suggesting that you wouldn't go less than .8q for that imagined surface area...........but since this is truly partially enclosed, wouldn't it be (.8+.55)q, plus the contribution from the roof, plus the contribution from the low sidewall?

Thanks.

 

[TXStructural]

Yes, you would add the two sides of an intact wall. For MWFRS, it is the profile in the wind, and how the wind circulates around it that makes the difference.

Sorry for the confusion, I was reading at lunch on an iPhone and didn't get the story straight. (I read it as a solid wall on the low side and an open or fabric-covered opening on the high side, so I got it completely wrong.) Because all sides are open aside from the fabric, the building is open (or maybe partially enclosed if the fabric is strongly supported on all edges), since the fabric would not contain the pressure as a solid wall would.

If the design allows the wind pressure to rapidly bleed away though loose edges and openings, the pressures would be somewhere between open (80% open area on the wall) and partially enclosed (less open on leeward than open on windward.) I would not worry about a "venturi effect" per se, but would use the tables for open buildings for the roof. These account for internal and external pressures which occur on monoslope roofs. Unless there is a safety issue, I would also design the fabric wall(s) to pull open at some relatively low pressure, unless you desire to use it to block strong winds. Your design wall pressure would then be only the pressure/force the fabric can withstand. The wind speed on the maps is a 3-second gust, so if you rely on breakaway fabric or panels, they have to yield immediately, not after a few seconds (when the damage is already done.)

So I would use GCpi=0.55 up to the fabric strength, then 0 once it is open. BUT, consider that you would have what is essentially a monoslope free roof, with those very high values (which are realistic, in my experience.)

 

[PostFrameSE]

OK. Great. Thanks.

 

[Ron]

PFSE...let me see if I understand your building as I'm more confused than TX was...

You have a building with two solid walls, no openings, running parallel to the slope. Perpendicular to those sidewalls, the building is completely open on the high side and has a fabric "wall" on the low side. Do I have that right?

If so, your building meets neither the definition of an open building (ALL walls have to be at least 80% open) nor partially enclosed (meets neither condition 1 nor 2), so according to ASCE 7's definition it is an "enclosed" building. THAT's ABSOLUTELY ABSURD!

I agree with TX's assessment. I would treat it as open and apply the pressure to each exposure to get the worst case. I would treat the fabric as sacrificial.

 

[kcprofessor]

Just to add to your worries, if the fabric "wall" is secured at both the top and bottom edges, it will generate an uplift at the bottom and a corresponding downward force at the top due to catenary action. This effect is frequently overlooked when dealing with fabric covered buildings and requires a detailed analysis which considers the tensile stiffness of the material!