ASCE Figure 6-10 Wind Loads
ASCE Figure 6-10 Wind Loads
(OP)
Note 6 in ASCE 7-02 Figure 6-10 states, " Except for moment resisting frames, the total horizontal shear shall not be less than that determined by neglecting wind forces on roof surfaces.
The question is what does it mean by roof surfaces For 3:12 and 4:12 roof slopes the net horizontal forces of roof zones 2 and 3 will be negative while the net horizontal force on the walls is positive.
I've always interpreted note 6 to mean that when figuring the loading on my diaphragm I am to totally neglect the roof forces (make them zero) when they are net negative. In truth I usually flip the negative sign to a positive to be safe cause I've never felt comfortable with just making them zero.
I've had an engineering crisis of faith so I started researching. The wind load calc guide is vague just like ASCE 7-98 and ASCE 7-02. In fact ASCE 7-02 is more clear. Still I'm not sure. So I compare some wind loads using the different methods. When I use the all heights method the roof forces turn positive in some of the load cases always (usually anyway) resulting in a higher diaphragm load than the anaylitical less than 60 ft method. However when I used the simplified method less than 60ft (here ASCE states very clearly to zero out the negative loads) I got pretty much the same results as the less than 60ft analytical method.
1) So is my interpretation right?
2) If so why is the all heights method the only method that has load cases where Cp on the windward roof is not negative (remember I'm talking about low slope roofs)?
3) If I'm wrong should I just ignore the windward roof loads or what?
Any help would be appreciated
The question is what does it mean by roof surfaces For 3:12 and 4:12 roof slopes the net horizontal forces of roof zones 2 and 3 will be negative while the net horizontal force on the walls is positive.
I've always interpreted note 6 to mean that when figuring the loading on my diaphragm I am to totally neglect the roof forces (make them zero) when they are net negative. In truth I usually flip the negative sign to a positive to be safe cause I've never felt comfortable with just making them zero.
I've had an engineering crisis of faith so I started researching. The wind load calc guide is vague just like ASCE 7-98 and ASCE 7-02. In fact ASCE 7-02 is more clear. Still I'm not sure. So I compare some wind loads using the different methods. When I use the all heights method the roof forces turn positive in some of the load cases always (usually anyway) resulting in a higher diaphragm load than the anaylitical less than 60 ft method. However when I used the simplified method less than 60ft (here ASCE states very clearly to zero out the negative loads) I got pretty much the same results as the less than 60ft analytical method.
1) So is my interpretation right?
2) If so why is the all heights method the only method that has load cases where Cp on the windward roof is not negative (remember I'm talking about low slope roofs)?
3) If I'm wrong should I just ignore the windward roof loads or what?
Any help would be appreciated






RE: ASCE Figure 6-10 Wind Loads
RE: ASCE Figure 6-10 Wind Loads
My question is this. When the horizontal shear component of the roof loads in sum are in the opposite direction as the walls do I ignore all of the roof surfaces or do I pick and chose which roof surfaces to ignore so I can get the most conservative result.
In the past I took note 6 to mean that I should ignore all roof surfaces when it is unconservative to include them and to include all roof surfaces when it is conservative to include them.
Thanks for your responses
RE: ASCE Figure 6-10 Wind Loads
DaveAtkins
RE: ASCE Figure 6-10 Wind Loads
DaveAtkins
RE: ASCE Figure 6-10 Wind Loads
RE: ASCE Figure 6-10 Wind Loads
In either case I'm based in florida and none of my buildings have ever had any problems....so I must be doing something right. Especially when one considers the poor state of construction out there.
Factors of safety...I love em.
Thanks again everyone for your replies.
RE: ASCE Figure 6-10 Wind Loads
RE: ASCE Figure 6-10 Wind Loads
The 10 psf minimum loading is for comparision to the total windward and leeward wall and windward and leeward roof pressures not just the windward roof. So, unless you have a low wind speed and Exposure B, the 10 psf on the windward projection shouldn't ever control because the combined windward and leeward wall pressures should already be more than that value (See ASCE 7-02 Section 6.1.4.1). The reason for requiring this was to make sure the design didn't come up with the weird case of a building going upstream against the wind. (This is more clear with ASCE 7 Method 2, all-heights design method.)
You were using the Method 2, low-rise design. You may not be aware that Method one is a simplification of that method, where the net coefficients have been algebraically added and the roof is proportioned for the net negative or positive cases depending on roof slope. It is "easier" than Method 2 and because it is based on more modern wind tunnel tests, and more calibrated than the Method 2 All-Heights case, resulting in net pressures will be a little lower than Method 2, all heights.
So, you may just want to use Method 1 if you only have the enclosed building cases. (partially enclosed and open structures can't use it.) You will note that ASCE Section 6.4.2.1.1 requires the projections of the wall pressures, A, B, C, & D and ignores the roof pressures (E, F, G, & H) and then requires that each of A, B, C, or D equal 10 psf minimum when determining the minimum net horizontal pressures for the design.
You mentioned that ASCE Wind Load Calculation Guide is vague and I completely agree with you. You may want to check out the SEAW's "Wind Load Commentary" and SEAW's Handbook to a Rapid-Solutions-Methodology of Wind Design", which discusses the IBC and ASCE 7 and other standards and has excellent, if very detailed & thoroughly comprehensive, problem solutions for a myriad of practical problems.
For information, go to the bottom right of the page: http://www.seaw.org/resources_products.cfm
or go to:
http://www.iccsafe.org/e/category.html
then click on: "Engineering"; then click "structural engineering"; and finally click on: "Wind". They are shown as Items 1 and 4.