wind load estimation

huangyhg

wind load estimation
im not a civil eng but i have to estimate the wind load of a roof.so i dont have confidence about my estimation.
i used australia standard as 1170.2
my problem is calculating the net basic wind pressure.the book says:
         +the external pressure for windward section is -0.95kpa to 0.
         +the external pressure for leeward section is -0.55kpa.
         +the internal pressure is -0.7kpa to +0.75kpa
so,i think the net basic wind pressure for the leeward section is 1.25kpa(downward) to -0.75kpa (upward).
is it correct,reasonable?
and do i need to concern about both case of loads (1.25kpa,-0.75kpa) in my design?
   thank you very mucn for yout help.

hi, totti.
not good news, i'm afraid.
yes, i read the 'simplified procedure' in section 2 of as 1170.2 exactly as you have posted.  in fact, i have never used section 2, since section 3, (which is a more detailed process), generally permits lower loads.
but your arithmetic conclusions are incorrect.  you also appear to have only considered one wind direction.
the upward net pressure on your leeward section is the sum of 0.75 kpa internal pressure and 0.55 kpa external suction, giving you a total upward net pressure of 1.3 kpa.  but the upward pressure on the same section of roof increases to (.7+.95)=1.65 kpa for wind in the opposite direction.
the maximum downward pressure load on any section of roof is (0+0.7) = 0.7 kpa.
and yes, if you are trying to design the building (or even just its roof) you have to consider a number of load combinations. you need to consider at least two wind directions (parallel and perpendicular to the ridge), combined with positive or negative internal pressures.
you also need to look at increased local pressures near the edges of the roof.
and of course, you also need  to consider normasl dead and live loading (ref as 1170.1).
i have to ask the question - should you really be doing this?  (what would you think if a bridge designer wanted to design a vehicle?)  you could be breaking the law (depending on your home state, you may be legally required to be registered).  without the appropriate form of professional indemnity cover, you could be exposing yourself to future claims.  even if it is your own building, such claims could follow you if you ever sell it.
were you to ask me to do it, i would have to decline, even though i have been doing structural design in various forms since 1957. (the form of my own pi cover does not permit me to design buildings).