asce figure 6-10 wind loads

huangyhg

asce figure 6-10 wind loads
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
i think asce 7-02's intention is that shear forces due to wind loads cannot reduce the total horizontal shear.  if you have a case where the shear force due to winds increases the total horizontal shear, then that needs to be considered.
when the net horizontal shear from the roof loads is in the same direction as the net horizontal shear on the walls loads i understand that the roof loads must be included.
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
also make sure that you use at least 10 psf on the projected area of the roof and wall--this is also covered in asce 7.
daveatkins
i believe your original interpretation is correct (and that is how i do it).  if the sum of the horizontal components of the wind loads on the windward and leeward roof surfaces results in a force which is in the opposite direction as the wind load on the wall, then you should ignore this "negative" wind load on the roof surfaces.  again, as i mentioned above, asce requires that the wind load on a structure be at least 10 psf times the projected area of the wall and roof.
daveatkins
i agree with daveatkins, and that's how i've done it as well.  turning those loads to positive and applying them would of course be quite conservative.
thanks guys for your help.  i'm at least more confident now that at least two people have been applying asce the way i have.  i think it is the right way but every now and then i have one of those sleepless sweating bullets nights.
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.
when it comes to wind pressures on a gable roof the negative is a zero and not deducted. asce "bounds" both negative and positive roof pressures for some roof angles. thru lab testing both situations can actually occur on a gable roof on its windward side at the same time in a windstorm. nice thread . good luck.
you are correct in your assumption that it seems wrong to have negative windward roof pressures that subtract from the net base shear (but is real because the internal pressure is larger than the exterior wind pressure on the windward roof, usually in the case of a "ballooning" partially enclosed building with the walls and openings on the windward side.)the net pressures on the building walls are in the direction of the wind because the net internal pressures algebraically add out. because the roof is at a critical low angle where the turbulance causes flucuations, there could still be inward (positive) and outward (negative) pressures on the windward side roof.
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.
or go to:
then click on: "engineering"; then click "structural engineering"; and finally click on: "wind". they are shown as items 1 and 4.