Wind Loads on Canopy

[EIT2]

I frequently design drive-through and entrance canopies for churches, banks and other structures.

I intend on designing these as open structures. I see no other ASCE 7-02 provision for wind loads on canopies.

In ASCE 7-02, Figure 6-18, why does this portion of the load and wind pressure determination process start with Monoslope roofs in lieu of Gable and Hip roofs? Are there no force coefficients for open structures with Gable or Hip roofs?

What am I not considering?

Thank you!

 

[ERV]

There is no internal pressure coefficient on an open structure. GCpi = 0.00 from figure 6-5 (ASCE 7-02). For open structures with gable or hip roofs use the force coefficients (Cf) fromm figure 6-18. Design as per 6.5.13, each slope = Af. F = qz*G*Cf*Af (lb).

 

[EIT2]

ERV,

I did just as you and have suggested, as per 6.5.13. However, the resultant pressure=qz*G*Cf, before applying the roof area, is very low (14 psf) in comparison to what I would expect for a canopy, of which, in the absence of any other provision, I want to compare to an overhang, at the very least.

I realize, I can design at my discretion. I'm just amazed at the great range of difference between the extremely high pressures of the Simplified Procedure and the extremely low pressures of the Analytical Procedure.

By the way, I have downloaded the demo for Wind Loads on Structures, that you have recommended. Do you use this exclusively for all of your structure designs? Do you find it friendly and convenient?

Thank you, very much, for all of your guidance.

 

[ERV]

The simplified procedure procedure is generally used for enclosed buildings as outlined in 6.4.1.1. A building must meet ALL of the conditions as outlined in 6.4.1.1 (ASCE 7-02) for use as the simplified procedure. The simplified procedure has some built-in safety factors (coefficients) that will often value wind pressures higher than the analytical procedure. An enclosed building will have higher wind pressures than an open building.

If I'm analyzing a simple structure I generally use the wind program alone, however, I generally check hand calcs against the ones from the program on more complex buildings. If they are reasonably close I leave well enough alone. The reports make a very good presentation and include them in with all my other calcs for submittal. I've found that the program is fairly accurate and can save an enormous amount of time (allways check to be sure, a program is only as good as the input).

 

[ERV]

EIT2,

I failed to mention that in the simplified procedure mean roof height is taken at 30'. Pressure on buildings with a mean roof height of less than that will come in higher than in the analytical procedure.

 

[EIT2]

ERV,

Thank you for your time and consideration!

 

[fbaxley]

I came upon the same question regarding Cf for an open structure with a hip roof today. This helps answer my question.

ERV,
Where can I find the demo for Wind Loads on Structures to be downloaded?

Thanks!

 

[ChuckerD]

ASCE 7-05 addresses wind loads on open buildings - our company has already incorporated these provisions to be used with all IBC jobs.

I do not know where we obtained the draft copy however.

 

[Ron]

ChuckerD...don't jump too quickly! Municipalities, Counties, and States are not quick to adopt new codes, so the latest and greatest is not always the applicable one. As an example, the 2003 version of IBC references ASCE 7-02, yet the 2003 version of the Florida Building Code references ASCE 7-98.

Governing bodies are generally free to adopt any guidance code they wish, and any version of such, and to modify or supplement such codes to "fit local conditions".

 

[ChuckerD]

I always abide by Random County's decision on what the governing building code shall be. However....

When the design reference - in this case any ASCE wind provision that you mention - does not address a condition, how then do we proceed? Make something up? Use the provisions defined in an older code such as SBC 1997?

Why not use the newer, unadopted version of the reference that actually defines some provisions for the condition being considered? This is certainly better engineering practice. And for the throngs out there afraid of "Lawsuit", this approach is likely more defensible than the alternatives.