StructuralWarrior
Structural
- Jun 28, 2011
- 3
Fellow Structural Engineers,
I am designing an enclosed big box store in Miami, FL. While designing it for MWFRS I noticed some significant difference between various wind load analysis methods listed in ASCE7-05 code. My mean roof height is 21ft and store rectangle dimensions are 373’ x 213’ without any expansion joints, basic wind speed is 150 mph, building class III and exposure category is C. Roof is a flexible diaphragm and all exterior store walls are shear walls. When I analyze this building per simplified method as per article 6.4.1.1 and figure 6-2, I get 53.5 psf corner wall pressure (Zone A) and 35.5psf rest of wall pressure (Zone C, which is significantly less than base wind pressure, qh = 51.3psf). I am getting exact same results for MWFRS Method 2 for h <= 60ft per article 6.5.12.2.2 and figure 6-10 (Zone A = Zone 1E + 4E and Zone C = Zone 1 + 4). But, when I analyze this building per analytical all height method as per article 6.5.12.2.1 and figure 6-6, I get significantly different results. For wind blowing normal to longer dimension I get 54.3 psf uniform pressure up to 15ft and then it changes to 56.7psf at h = 21ft. And these pressures are quite higher than what I get from the other two methods. And not to mention that I will have to run case 1, 2, 3, 4 as per figure 6-9 for this method when I use all height MWFRS, and this may end up being lot more pressure than other two methods.
My question is, can I design my building from simplified method and ignore all height method numbers?
Also one more question is for the parapet MWFRS pressures. I have an average of 8ft parapet everywhere on this store, and at some places it is as high as 14ft high. I understand that I should follow article 6.5.12.2.4 to calculate these pressures and it has +1.5 GCp factor for windward parapet and -1.0 for leeward parapet and this has to be multiplied with qp evaluated at top of parapet. By doing so I am getting too much wind pressure in my diaphragm. By comparing this with all height method, it seems like this factors are obtained by adding windward wall pressures with negative roof pressure Cp which is consistent with components and cladding parapet pressures in article 6.5.12.4.4.
My question is do I really have to take +1.5 windward GCp factor for MWFRS when my parapets are 14ft high? I can see the wall pressure part acting on it and I can see the roof uplift acting up to certain height of parapet, but do I have to take it for full 14ft height? And when I consider my total diaphragm shear, does +1.5 and -1.0 factor work at the same time when my length of structure is 373ft?
Thank you for your time,
Structural Warrior
I am designing an enclosed big box store in Miami, FL. While designing it for MWFRS I noticed some significant difference between various wind load analysis methods listed in ASCE7-05 code. My mean roof height is 21ft and store rectangle dimensions are 373’ x 213’ without any expansion joints, basic wind speed is 150 mph, building class III and exposure category is C. Roof is a flexible diaphragm and all exterior store walls are shear walls. When I analyze this building per simplified method as per article 6.4.1.1 and figure 6-2, I get 53.5 psf corner wall pressure (Zone A) and 35.5psf rest of wall pressure (Zone C, which is significantly less than base wind pressure, qh = 51.3psf). I am getting exact same results for MWFRS Method 2 for h <= 60ft per article 6.5.12.2.2 and figure 6-10 (Zone A = Zone 1E + 4E and Zone C = Zone 1 + 4). But, when I analyze this building per analytical all height method as per article 6.5.12.2.1 and figure 6-6, I get significantly different results. For wind blowing normal to longer dimension I get 54.3 psf uniform pressure up to 15ft and then it changes to 56.7psf at h = 21ft. And these pressures are quite higher than what I get from the other two methods. And not to mention that I will have to run case 1, 2, 3, 4 as per figure 6-9 for this method when I use all height MWFRS, and this may end up being lot more pressure than other two methods.
My question is, can I design my building from simplified method and ignore all height method numbers?
Also one more question is for the parapet MWFRS pressures. I have an average of 8ft parapet everywhere on this store, and at some places it is as high as 14ft high. I understand that I should follow article 6.5.12.2.4 to calculate these pressures and it has +1.5 GCp factor for windward parapet and -1.0 for leeward parapet and this has to be multiplied with qp evaluated at top of parapet. By doing so I am getting too much wind pressure in my diaphragm. By comparing this with all height method, it seems like this factors are obtained by adding windward wall pressures with negative roof pressure Cp which is consistent with components and cladding parapet pressures in article 6.5.12.4.4.
My question is do I really have to take +1.5 windward GCp factor for MWFRS when my parapets are 14ft high? I can see the wall pressure part acting on it and I can see the roof uplift acting up to certain height of parapet, but do I have to take it for full 14ft height? And when I consider my total diaphragm shear, does +1.5 and -1.0 factor work at the same time when my length of structure is 373ft?
Thank you for your time,
Structural Warrior
