MWFRS vs C&C

[dfanfan]

I have a weather/sound enclosure for a piece of rooftop equipment that I am running a finite element model for 150 mph winds (Miami-Dade). The enclosure is 25 ft by 9 ft in plan and stands about 10 ft high. (Attached is a picture of my NASTRAN model) It is made of 5052 H32 aluminum and I am using formed angle 8 gage steel to brace the panels. I originally applied the wind loads as a MWFRS where we have positive pressures on the windward and suction (negative) pressures on the leeward walls. Sideward and roof uplift pressures were also applied. There has been debate regarding this methodologyand may not be accepted by local jurisdictions because this enclosure is considered a component. Therefore, the design methodology should be done for Component and cladding which are at higher pressures than MWFRS.



ASCE 7-05 tells us to apply one wind load in one direction on one surface of the enclosure (Sec. 6.5.12.4 through 6.5.15.1) I believe that this enclosure is big enough to qualify as a MWFRS. I do not have a problem with using the higher external pressure coefficients prescribed for components and cladding, however, I feel that realistically there would be suction forces on the leeward side of the enclosure. I feel that applying this elevated pressure is grossly conservative. Also, the code does not make mention of roof uplift for components and cladding.



Using the MWFRS is ideal because it would provide a more realistic internal force distribution in my finite element model. Please let me know what you think of this methodology. Should I use MWFRS with the elevated pressures prescribed in the Components and Cladding sections.

 

[JAE]

I think ASCE 7 has a provision that if your "element" has over 700 sf of surface area you can use the MWFRS loads instead of C&C.

 

[ash060]

I would probably call this a rooftop stucture and use the provisions for that in ASCE 7
Check Figure 6-21 in ASCE-7 05

 

[UcfSE]

I woudl call it an "other structure" based on ASCE 7-05 6.5.15.1 and use that section for the wind loads. Since the FBC does not yet reference the 7-05, I would still use that line of thought and call the RTU a C&C or an "other structure", but not MW whatever the case.

 

[dfanfan]

JAE: Figure 6.5.12.1.3 says "Tributary" Areas Greater than 700 ft^2. I thought tributary area referred to only the first surface to hit normal to the wind direction.

Others: How do we account for the suction elements on the leeward face? A Ph. D at Berkeley agrees with me that MWFRS is the way to go to account for these suction elements.

I am thinking of applying the "other structure" loads on both the windward and leeward sides. This is grossly conservative and will require me to add more bracing but it will minimize debate. Let me know. And thank you all.

 

[JAE]

I thought tributary area referred to only the first surface to hit normal to the wind direction

The concept of whether the wind hits the first, or nearly the first surface is ONLY a statement made in the commentary and is used mostly to try to clarify the intent of the code. In my opinion, it muddies the water too much.

Wind loads (C&C vs. MWFRS) should be primarily based on tributary area and not on whether the wind hits the surface first or not. They are simply trying to suggest that as you go deeper into the wind resisting load path, the tributary area goes up and thus the high localized peak wind pressures from C&C wind (for small At) go away.

 

[UcfSE]

When you ask "How do we account for the suction elements on the leeward face?" do you mean to ignore the negative C&C pressures?

The intent of MW is to provide pressures for members that provide stability to the structure as a whole. Roof top units don't tend to do that. These are the averaged pressures across the entire structure area at a given point in time. It would not be appropriate then to apply those same pressures to an appendage of your building, typically.

For elements with very large tributary areas the localized effects wash out across that area and the average overall time-varying pressure drops, for that area. Given that ASCE 7-05 does specifically address roof top units and you are already referencing 7-05 in your post, I would go with that.

The 7-05 does address roof top structures specifically, though not in the MW or C&C sections. It is in the "other structures" section.