Wind Zone analysis B and D zones?

[jtwright4216]

Just had a couple of questions.

It is referring to the main wind force resisting system analysis.

My questions are:

1.) When analyzing a structure per MWFRS some vales for the roof loads (Zones B and D) are negative values stating that it is pulling out on the roof structure instead of flowing into it. So how do you deal with those values? I have seen designers instead of using the negative values they simply just switch the sign to + and call it conservative. I have also seen designers assume if zones B and D <0 than assume =0. Does the ASCE7-05 Address this issue (where)? And if it does and you can either assume 0 or use a positive value then why have does the ASCE7-05 even have the negative values showing, wouldn't it make sense to set them to zero or positive in the first place.

I know that the negative values are stating that they are coming out of plane therefore create a circular motion and a possible moment. But when doing the analysis calculations I have never subtracted the negative B and D regions from the A and C when finding the shear reactions.

2.) This question is referring to another code provision. It is specifically ASCE7-05 6.4.2.1.1 about minimum pressures. What is the take on this? I see it two different ways:
1. That you must have 10 psf minimum on all zones A,B,C and D. Implying that if B and D were negative or < than 10 then they would then become positive 10.??
2. The minimum design pressures on any structure have to exceed a minimum of 10 psf for all zones based on the area Af? And if the pressures are less than that when analyzed then 10 psf's have to be used for all the zones

thanks for everyones future help.

 

[civilperson]

Negative values of wind pressure on a roof are uplift forces and can control the design of various components, especially connections of the roof to the frame.