ASCE 7-05 WInd Loads Fig 6-9

[asp1111]

Hello

I have following questions:
ASCE 7-05 page 52, MWFRS Method 2 Fig 6-9 Design Wind Load Cases,

1. What happens to the Roof design pressure calculated per section 6.5.12.2.1 equation 6-17 ?
Fig 6-9 does not mention anything about Roof pressure Loading?
I may not be interpreting it correctly,
please provide with any ideas as to how to account for Roof Pressure along with these load cases of fig 6-9.

2. Also I assume that fig 6-9 accounts for side wall pressure loading due to windward pressure load acting in principal axis. So no need to separately include side wall pressure calculated per section 6.5.12.2.1 equation 6-17.
Kindly correct if wrong.

Thank you.

 

[Stillerz]

I have always included the associated roof pressures.
For example, for Case 1, I would include whatever roof pressures are associated with PWX & PLX
What actual effect this has on the MWFRS is probably not very much if anything. I have also always inluded the associated sidewall pressures. Again, when designing the MWFRS in the X direction, the associated side wall pressures would be actig act 90º to the X direction 9or in the Y direction) and the sidewalls would be acting opposite to one another and would probably have little effect on the design of the MWFRS in X direction.

 

[Geometrico]

Further to Stillerz's comment,

1)
Section 6.5.12.3 states "The MWFRS of buildings of all heights whose wind loads have been determined under the provisions of Sections 6.5.12.2.1 and 6.5.12.2.3 shall be designed for the wind load cases as defined in fig. 6-9."

It seems pretty clear that the cases in figure 6-9 are simply different ways to combine the loads that you determined for the roof and walls under 6.5.12.2.1, so yes, you should include the roof pressures.


2)
I believe you still need to consider the side wall pressures from 6.5.12.2.1. The combinations in figure 6-9 are to account for simultaneous wind action in two directions and to incorporate overall moments. The moments could introduce some governing conditions for your bracing.

 

[asp1111]

Thanks to both of you for your comments,

However what I intend to know or how I interpret the whole situation is as follows,

As there is no clear mention of roof and side wall pressures in Fig 6-9, it can be included as follows

1. Apply FULL roof pressure calculated from equation 6-17 along with each load combination of Fig 6-9.
(There isn't any factor given, similar to that given for Pwx and PLy and Pwy and PLy etc. 0.75, 0.563 etc.)

2. For side wall pressures per equation 6-17 --> include it with load combination of Fig 6-9,
(ie. sum it, add or subtract with the load combination of fig 6-9. based on the positive or negative sign obtained from equation 6-17)

Let me know if this is the way to do it.

Thank you.

 

[BAretired]

You should learn to phrase your question so that it is not code specific. You may find that Eng-Tips members from other parts of the world may be able to contribute to your knowledge and understanding of the issues which concern you.

As it is, I cannot contribute as I do not know the code to which you refer.

BA

 

[cntw1953]

I think there is a forum for disscussing code issues. These days it is imperative for one to learn the coding languages, which tend to get more specific on locations and applications, and more complicated.

 

[Ron]

You compute all of your load values for both walls and roofs and then apply those values, directionally, as noted in Fig. 6-9.

 

[CTSeng]

From the often misunderstood ASCE7-05 Figure 6-9: "wind pressure acting on the projected area perpendicular to each principal axis".

Side wall and generally roof pressures do not act perpendicular to a principal axis and are therefore not required. However, for a steep slope roof structure there will be projected areas that must be included.

 

[bnickeson]

I'm a little late to this, but I'll try and answer your question.

As to part 1, yes, roof pressures must be included if they have a net horizontal force in the direction of the wind. For instance, gable roofs over 10 degrees in slope have a net pressure that acts horizontally over the projected area of the roof. The same goes for monoslope roofs whether they be leeward or windward slopes (depending on h/L limit). The roof pressure in the direction of the wind is then multiplied by the projected height and you get a force in pounds per linear foot on the diaphragm. You treat this force the same as you would on a floor diaphragm with respect to Figure 6-9.

As to part 2, I have never included side wall pressures in calcs unless you have a building with an expansion joint. In that case, even though you have side wall pressures that are equal and opposite in direction, the MWFRS will only see one of the side wall pressures and have a force exerted on it. Now whether or not this case will ever control is something you would have to determine.