Refer to ASCE 7 for application. In general, you will have to apply factors to the wind load depending upon wind direction and shape of structure. You will also have to check at least two directions, perhaps more depending upon configuration of the structure.
My concern is that if you are asking this question in a forum, you have no one to check and mentor you through this process. This can be a bit tricky depending on many factors and should be done by a structural engineer. If you are not a structural engineer, get some help with it.
I know that,but the thing is I'm not a very good english speaker
I don't remember the formulas and I'm from other country.
I gonna try to find some place else thank you anyway
Ron is correct. ASCE 7 (preferably 7-95) is essential to your answer.
The load on a lattice truss is heavily dependent on the shape of the truss members and their projected areas to the plane normal to the wind.
The ASCE Standard has empirical formulas for computing the shape coefficient to be used based on those and other parameters. I have seen those coefficients be as high as 3.5 times the projected area, so obviously, this is a wind load consideration that is not to be ignored.
Check your local building code. Some (such as NY State and Massachusetts) have wind load pressures already listed. How high up is your truss, pull the design wind loading off the chart, apply it to your vertical projected area, and badda boom - badda bing, your off to the races.
However, let us work a simple example.
Assume the following:
Gable Roof, mean height (h=15'), exposure c, category 2.
Base on ASCE 98
Qz=0.00256xKzxKztxKdx(V)^2XI
Kz= 0.85 Table 6-5
Kzt=1 (No topographic effect)fig 6-2
Kd= 0.85 table 6-6
I=1 Table 6-1
V= 120
Therefore Qz = 27psf
For Main Force Resisting System (MFRS) 6.5.12.2 use this equation
P= Q.G.Cp-Qi(GCpi)
For Component and gladding (c&c) use this equation
P= Qh[GCp-GCpi]
G is the gust effect factor 6.5.8.1. in the standard
Cp is the external pressure coefficient in figure 6-3 for MFRS
GCp is the external pressure coefficient for C&C obtained from the figure 6-5 in the standard.
GCpi is the internal pressure coefficient obtained from table 6-7 in the standard
Using the above equations only for the following condition:
Rigid Building for all heights.
For different type of buildings ASCE 98 has different equations supported with figures and tables to calculate pressure.
that is why it was not easy for Ron to answer your question. I hope this information will give a good start.
