ASCE 7-05 Wind Load Calculation

[PRIYOM]

Hi, everyone. I have manually calculated Wind Loads according to ASCE 7-05. I have also run a ETABS model to verify the wind loads I have calculated. But I can't intrepid the ETABS results. Help me, please.

 

[r13]

There is not enough detail for what kind of problems you have came up with. Does total wind forces agree between hand calculation and the analysis?

 

[PRIYOM]

The wind loads I calculate manually don't match with ETABS analysis. I think errors might be done in hand calculation. I need a complete workout example (for flat roof).

 

[r13]

This example may help, although it is for slopping roof, the procedure remains the same. Link

Or this one. Link

 

[PRIYOM]

In case of windward pressure, +CGpi value is used. And in case of Leeward side, -CGpi value is used. I didn'i get the point.

 

[PRIYOM]

You can see the attachment.

 

[r13]

The +/- CGpi is related to the direction towards, or away from the surface in concern, and is well explained in the charts provided in the code. You should review the code carefully.

 

[r13]

Note that, in the attachment, all direction shown on the figure with arrow head pointing to the building components are positive. The negative pressure in the table indicates it is away from the subject component (in reverse direction as shown).

 

[PRIYOM]

Positive internal pressure generally means towards to surface. I get that point. Let me clear about my confusion.

For Leeward Side,
Ext. Pre.= qh G Cp= 29.5 X 0.85 X (-0.5)= -12.5
Int. Pre.= qh CGpi= 29.5 X (-0.18)= -5.31 (When CGpi is -)
= 29.5 X (+0.18)= +5.31 (When CGpi is +)
Designed Pre.= -12.5 - (-5.31)= -7.2
= -12.5 - (+5.31)= -17.8
So, there are 2 value for leeward side. They have used -7.2.

For Windward Side,
Ext. Pre.= qz G Cp= 16.6
Int. Pre.= qh CGpi= 29.5 X (-0.18)= -5.31 (When CGpi is -)
= 29.5 X (+0.18)= +5.31 (When CGpi is +)
Designed Pre.= 16.6 - (-5.31)= 21.9
= 16.6 - (+5.31)= 11.3
They have used 21.9.

In both case -CGpi governs. So what is the function of +CGpi for enclosed structures? Or when I will use the values of +CGpi?

 

[r13]

You didn't follow through the suggestion to review the code Note that internal pressure produces a suction effect, thus it's always negative (away from the surface). So it causes higher pressure on the windward wall ext & int pressures in the same direction), at the meantime, it reduces the pressure on the leeward wall (ext & int pressures against each other). Hope this helps.