Peer-to-Peer Standards Exchange

There may be a better place to post this comment/QA but I had some difficulty finding an appropriate channel to start the discussion.

ASCE 7-22 (and ASCE 7-16) Chapter 30 - Components and Cladding - Section 30.9, addresses the wind forces on building canopies.  According to Figure 30.9-1B, the maximum net uplift pressure coefficient for a single-surfaced cladding component (say a connection from purlin to a WF beam) that supports 100 ft^2 of roof area, where hc/he is < 0.5, is slightly less than -0.5 (in magnitude). This seems quite low but for wind into the face of the wall it make sense that positive pressures above and below the canopy surface could develop when hc/he is small, but with some differential. When wind is parallel with the wall, it seems like it could be quite different. 

In contrast, a free-standing, mono-sloped flat roof (Section 30.5, Figure 30.5-1), has variable net pressure coefficient for design of cladding components, ranging from -1.1 to -1.2 for clear and obstructed wind flow.  At edges and corners of a fee-standing roof, the coefficients can be much greater.

In a situation where a building canopy is disproportionally large, (e.g., extends 80-ft out from the face of the building, and they do exist) and particularly when the wind direction is parallel to the side of the building wall that the canopy is attached to, couldn't the canopy (further from the building) possibly be subjected to pressures more like that of a free-standing roof?  I think the provisions of 30.9, as they have been written and without constraints on their use, opens the door for designing cladding for very large canopies for forces much lower than they might experience in design wind event.  Section 30.9 also makes no distinction between wind parallel to and normal to the wall the canopy is attached to.  I assume the figures are meant to envelope the demands for both directions? 

Has there been any discussion about adding some restrictions on the application of 30.9? For example, limiting the application to canopies roofs of a particular maximum dimension, beyond which some other provision (such as 30.5) would take over?

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David Webster Ph.D., P.E., S.E., M.ASCE
Senior Structural Engineer
Seattle WA
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The data in ASCE 7 are based on wind tunnel tests that were carried out at Concordia University and at FIU. The details of these studies can be found in the listed references in the commentary section in Chapter 30.

The maximum overhang that was tested was 6.5m and the results didn't vary much from those obtained from smaller overhang widths. There was discussion among the Chapter 30 task group about limiting the overhang width, to the 6.5m, but they felt it was unnecessarily restrictive, considering the weak dependance of the data on the overhang width (lack of variance in results between widths).

In your posting, you were comparing the overhang coefficients with those available for free-standing monoslope roofs (Section 30.5, Figure 30.5-1, as stated). This comparison is invalid because of the large aerodynamic differences between the two cases. In the case of the free-standing roof, the wind flow is free to pass under (unless there are obstructions such as merchandise, equipment, etc.). The overhang case is different because the walls of the building force the wind flow to pass around. These are aerodynamically different cases, inducing different wind pressures.

Lastly, as the Standard cannot address all cases - in the case of an 80' overhang, the structure may not be rigid, which is the fundamental assumption for the Chapter 30 coefficients - a wind tunnel study may be warranted for non-typical configurations. As always, we need to use engineering judgment. 

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Ronald Hamburger, SE
Consulting Principal
Simpson Gumpertz & Heger
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Thanks for your feedback Ron

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David Webster Ph.D., P.E., S.E., M.ASCE
Senior Structural Engineer
Seattle WA
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Original Message:
Sent: 03-16-2023 12:00 PM
From: Ronald Hamburger
Subject: Wind loads on building canopies ASCE 7-16 & 22

The data in ASCE 7 are based on wind tunnel tests that were carried out at Concordia University and at FIU. The details of these studies can be found in the listed references in the commentary section in Chapter 30.<o:p></o:p>

The maximum overhang that was tested was 6.5m and the results didn't vary much from those obtained from smaller overhang widths. There was discussion among the Chapter 30 task group about limiting the overhang width, to the 6.5m, but they felt it was unnecessarily restrictive, considering the weak dependance of the data on the overhang width (lack of variance in results between widths).<o:p></o:p>

In your posting, you were comparing the overhang coefficients with those available for free-standing monoslope roofs (Section 30.5, Figure 30.5-1, as stated). This comparison is invalid because of the large aerodynamic differences between the two cases. In the case of the free-standing roof, the wind flow is free to pass under (unless there are obstructions such as merchandise, equipment, etc.). The overhang case is different because the walls of the building force the wind flow to pass around. These are aerodynamically different cases, inducing different wind pressures.<o:p></o:p>

Lastly, as the Standard cannot address all cases - in the case of an 80' overhang, the structure may not be rigid, which is the fundamental assumption for the Chapter 30 coefficients - a wind tunnel study may be warranted for non-typical configurations. As always, we need to use engineering judgment. <o:p></o:p>

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Ronald Hamburger, SE
Consulting Principal
Simpson Gumpertz & Heger
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David,

I took a look at the provisions you mention and it seems you have a point.  I would take the approach that the canopy provisions are developed for cases where the wind behavior at the canopy is dictated by the building it belongs to, and shouldn't necessarily be applied if you anticipate part or all of the canopy being subjected to natural flow of wind, as for a long canopy with wind along the building facade.  I'm imagining the port cochere at a three-story Holiday Inn--it could easily be long enough to receive wind pressure independent of the building behind it.  At my firm, we sometimes run the wind loads a couple different ways for borderline cases and just envelope the pressures.

If you and I are correct, I would agree that section 30.9 should be revised to include some relative size limits and guidance on how to handle wind along the building facade.  The commentary does discuss some limitations of the testing, but does not address the length of the canopy.  I'm still pretty excited about the more robust canopy provisions in 7-22, but there may be room for refinement for 7-28.

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Christian Parker P.E., M.ASCE
Structural Project Engineer
Washington DC
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Thanks Christian.  I would be inclined to to the same (envelope cases).

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David Webster Ph.D., P.E., S.E., M.ASCE
Senior Structural Engineer
Seattle WA
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Original Message:
Sent: 03-16-2023 01:20 PM
From: Christian Parker
Subject: Wind loads on building canopies ASCE 7-16 & 22

David,

I took a look at the provisions you mention and it seems you have a point.  I would take the approach that the canopy provisions are developed for cases where the wind behavior at the canopy is dictated by the building it belongs to, and shouldn't necessarily be applied if you anticipate part or all of the canopy being subjected to natural flow of wind, as for a long canopy with wind along the building facade.  I'm imagining the port cochere at a three-story Holiday Inn--it could easily be long enough to receive wind pressure independent of the building behind it.  At my firm, we sometimes run the wind loads a couple different ways for borderline cases and just envelope the pressures.

If you and I are correct, I would agree that section 30.9 should be revised to include some relative size limits and guidance on how to handle wind along the building facade.  The commentary does discuss some limitations of the testing, but does not address the length of the canopy.  I'm still pretty excited about the more robust canopy provisions in 7-22, but there may be room for refinement for 7-28.

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Christian Parker P.E., M.ASCE
Structural Project Engineer
Washington DC
------------------------------