ASCE 7-05 Wind Load GCN Values - Monoslope Roof

[HOWDOO]

Hello,

with regard to determining the pressures acting on a monoslope roof of pitch = 0°, Components and Cladding provisions direct me to Figure 6-19A to which I am given two distinct gust coefficient (GCN) values for each zone, one positive and one negative. I am having difficulty in understanding the following note given on the page:

Plus and minus values signify pressures acting toward and away from the top roof surface, respectively.

Does this mean I have two possible load cases in which the wind force is either positive or negative?

OR

Does this mean I apply both negative and positive forces concurrently and the resulting force is the sum of the two?

I apologize in advance if this is blatantly obvious to a seasoned structural engineer, my background is mainly mechanical.

Alex

 

[Jerehmy]

Separate load cases. And this is for an open building, are you sure that's what you want?

 

[HOWDOO]

Yes, the structure is open all sides.

Thank you for the clarification. I must confess though I did make an error in posting the wrong note as that one was from Figure 6-17a, the note I should have typed is:

1. CN denotes net pressures (contributions from top and bottom surfaces)

This one implies to me that they act concurently, or am I missunderstanding?

Alex

 

[structSU10]

No, the note means that there is a pressure on both sides of the open roof, the top surface and the bottom surface, and the Cn value in the table corresponds to both of those loads acting simultaneously. There are two separate cases for this within the table, but those are not acting concurrently.

 

[HOWDOO]

Sorry I meant concurrently as in simultaneously (I tend to find those two words interchangeable).

But that clears things up, thank you.

So in essence do these two pressures cancel each other out to a degree (as in they are opposing each other) or would the load be a summation of them acting in the same direction? From the code it suggests that positive means acting toward the surface and negative means acting away. I know I may have just answered my own question there but if they do combine the resulting load does seem overly onerous in my particular application.

Alex

 

[Ron]

They do not cancel each other. Check them separately and design for the worst case. I have designed over 600 open structures and have followed this conundrum through ASCE 7 from 1988 to present.