Wind Guides: Your Results May Vary

A famous Bob Dylan song points out, "You don't need a weatherman to know which way the wind blows."

However, in calculating wind loads for roofing systems, you do need to be very clear about what guide you're using and what building code requirements govern the specific locality in which you're working.

In researching data for an upcoming wind design seminar that SPRI is presenting with the Roof Consultants Institute (RCI) to members of both organizations, we noticed that different wind calculation guides could produce sometimes widely varying results.

And the imminent publication of yet another version of the wind design document from the American Society of Civil Engineers (ASCE) introduces an additional factor into the mix.

This third and newest revision, designated ASCE 7-98, "Minimum Design Loads for Buildings and Other Structures," is due out this summer and is reportedly easier to use. It follows ASCE 7-95 and ASCE 7-88. (While the last two digits do refer to the year of publication, the seven does not refer, coincidentally, to the month but rather to ASCE. s series designation for these design guides.)

Factory Mutual (FM) Research Corp.'s document 1- 28 is also commonly referenced for wind load calculations. That document lists system assemblies that meet specific guidelines as well.

Your choice among these reference documents for wind design calculations can significantly impact the outcome. These various results can be seen most clearly by applying them to sample wind load calculations. For example, we will consider two identical buildings located in different parts of the U.S. one will be in Columbus, OH with the other in Houston, TX. Each consists of a large, 21-ft.-tall warehouse with a truck door protected by an overhang canopy in a similar suburban location, a certain number of windows and doors, etc. (For the sake of simplicity, we will leave out various corner calculations.)

As you can see in the following table, the choice of wind design guide can have a significant impact on the final results.

		System Design Load
		FM	ASCE 7-88	ASCE 7-95	ASCE 7-98	Local Code Requirement
Houston	21-ft. Building	90 PSF	105 PSF	150 PSF	90 PSF	105 PSF
	Truck Canopy	105 PSF	105 PSF	225 PSF	150 PSF	105 PSF
Columbus	21-ft. Building	60 PSF	60 PSF	60 PSF	60 PSF	60 PSF
	Truck Canopy	60 PSF	60 PSF	120 PSF	90 PSF	60 PSF

The system design load builds in a safety factor of 2 and is for the field of the roof.

Each of the designs has different corner loading requirements and corner sizes. Furthermore, a large number of additional caveats go with this table, but it does accurately convey the fact that wind load calculations can vary, sometimes substantially, depending on what design guide you are referencing.

Of course, after the contractor has determined these loads, he must locate a roofing system that has been tested to the loads and then determine the securement for the insulation and membrane, based on the membrane chosen. How many fasteners you need can vary significantly depending on these design loads and considerably impact your bid on a project. One industry expert estimates that in fact, it could make up to a 20% difference in the cost of a roof.

For years, people were used to the idea that 90 PSF (not miles per hour, as is sometimes erroneously thought) was the benchmark. However, with Factory Mutual's increase of the upper limit to 240 PSF, the bar has been raised.

Plus, with ASCE, more attention is paid to other areas of loading such as drainage and snow loading, but in the last decade, roofing has attracted substantially more notice.

Furthermore, there presently appears to be some confusion for many within the design community on the best approach for meeting wind loading criteria for the roof. In geographic areas designated for low wind speed design, many roofs are actually over-designed, raising the installed cost of a roof system; conversely (and perhaps more seriously), in high wind speed areas, there may be roofs which are not adequately designed to meet actual loads.

Proper wind design calls for a thorough analysis of the building itself, including physical characteristics and actual geographic location. Everything must be carefully evaluated from determining the actual loads on specific zones of the roof, to ensuring the new roofing components when assembled as a system will adequately meet those loads.

Sufficiently anchoring the perimeter components is critical to a ensuring that the complete roof system will resist the calculated wind loads. A defect in either the design or installation of these elements may result in a breach of what was otherwise a well-adhered roof.

(To order ASCE documents, call 800-548-2723.)

This article appeared in the July 1999 Issue of RSI Magazine.