An Analysis of Wind Load Design—Load Side

By James R Kirby AIA 11-30-2017
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In the roofing industry, one of the most important aspects of roof system design is wind resistance. Roofs need to resist the uplift loads acting on the roof. The uplift loads acting on a roof are influenced by a number of factors—geographic location, the local building code, dimensions, exposure, use and occupancy (aka risk factor), and type. It would appear that the factors for wind design are all fixed and not debatable, but there are presumptions made during the wind design of every roof. Location and applicable code are fixed, as are the buildings dimensions—length, width, and height. Exposure should be easily determinable because it is based on a building's relative location to adjacent surroundings. However, some of these factors can be "in the eye of the beholder," and a designer's level of conservatism may come into play. And who says complying with the bare minimum is the correct choice?

Let's discuss each of the six factors in more detail

The location of a building determines the wind speed that is used to determine the uplift loads. Figure 1 shows a map taken from IBC 2015. IBC 2015 considers four risk categories (RC) ranging from lowest to highest hazard to human health. Risk Category takes into account occupancy and use—we'll discuss that more in a bit. RC 2 covers what are generally regarded as "ordinary" structures. The map shown here provides Ultimate Design Wind Speeds for risk category 2 buildings. It should be noted that IBC 2015 has one map for category 1, one map for category 2, and one map for categories 3 and 4—so three maps to choose from based on use and occupancy. Wind Speed

Source: International Code Council.

As building codes are updated, so are the references to the design standards within them. The IBC—depending on the year (e.g., 2009 IBC, 2012 IBC, 2015 IBC)—references different versions of ASCE 7, the American Society of Civil Engineers document that is used to determine the myriad loads acting on a buildings, including wind loads. Subsequently, these loads are used to determine how strong a building's components need to be (e.g., structure, roof, cladding). If a building is in a location that uses IBC 2015, then ASCE 7-10 is the version to be used to determine the wind loads acting on a roof. The three wind maps included in the 2015 IBC are exact versions of the three wind maps in ASCE 7-10.

It's important to remember that building codes are the minimum requirements. Buildings can be, and often should be, designed to better-than-code requirements. That said, the location of a building determines the minimum wind speed to be used to calculate the wind loads. However, a conservative approach for wind design is to use the next higher wind speed (a 5 or 10 mph increase will matter!). Undoubtedly, this will increase the wind loads acting on the roof and, subsequently, the roof system selected to meet those higher wind loads will have a greater resistance to potential damage during a high-wind event.

Within ASCE 7, the most conservative method to determine wind loads is to use the Strength Design method, which uses Ultimate Wind Speeds (the previously referenced wind-speed maps). However, ASCE also includes a method called Allowable Stress Design (ASD). Using ASD reduces the wind loads relative to the Strength Design/Ultimate Wind Speed method. The end result of choosing the ASD method is a reduction of the wind loads acting on roofs by 40%. Understanding the different methods of design is critically important!

Obviously the dimensions of a building are set by the size of the building! The dimensions influence a couple of specific things. Taller buildings are exposed to faster winds. The wind maps are based on wind speed at 33 feet above the ground. Most roofs less than 60 feet above grade use the values shown on the maps, but once a roof is taller than 60 feet, the wind speeds are increased based on height. Also, the perimeter and corner regions of a roof have increased wind loads (relative the field of the roof). The size of the corner and perimeter regions are based on the dimensions of the building.

Exposure of a building takes into account the relative effect of a building's immediate surroundings, or surface roughness. Surface roughness adjacent to a building partially determines how wind affects the building.

  • Exposure/Surface Roughness B is for buildings in urban and suburban areas, wooded areas or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger.
  • Exposure/Surface Roughness C is for buildings in open terrain with scattered obstructions having heights generally less than 30 feet. This category includes flat open country, and grasslands.
  • Exposure/Surface Roughness D is for buildings next to flat, unobstructed areas and water surfaces. This category includes smooth mud flats, salt flats and unbroken ice.

Use and occupancy is, of course, based on the activities that occur in the building, and different occupancies carry a different risk.

  • Risk Category 1 includes buildings that are a low hazard to human life in event of failure, such as agricultural facilities.
  • Risk Category 3 includes buildings that are a significant hazard to human life, such as public assembly buildings that can handle more than 300 people as well as power-generating stations and water treatment facilities.
  • Risk Category 4 includes essential buildings, such as hospitals, and fire, rescue ambulance and police stations.
  • Riske Category 2 includes all other buildings that don't fit into categories 1, 3, or 4.

Each risk category carries an importance factor. The higher the risk category, the higher the importance factor, and therefore higher wind loads. Each of the wind speed maps is based on a risk category with the importance factor already built in. For example, the wind speed for central US for Risk Category 2 is 115mph, for Risk Category 3 and 4 is 120mph, and for Risk Category 1 is 105mph. A designer needs to know the risk category for the building before selecting the wind speed map.

The type of building—the building configuration—is based on the possibility of a building becoming internally pressurized. The two common types are "enclosed" and "partially enclosed." The general premise is that an enclosed building will not be subjected to significant internal pressurization from winds and a partially enclosed building will see much greater internal pressures. The type has to do with the amount of doors and windows relative to the wall area. Choosing between the two types includes the underlying assumption that windows and doors will or will not remain in place during a storm. It is more conservative to design for wind loads assuming the building envelope will be breached to some extent, and therefore, using the "partially enclosed" type. When using FM Global's Loss Prevention Data Sheet 1-28, Wind Design, going from enclosed to partially enclosed adds approximately 30% to the wind loads.

Sources to determine wind loads

There are three common methods in the roofing industry used to determine wind loads. They are:

  • FM Global Loss Prevention Data Sheet 1-28, Wind Loads. (FM LPDS 1-28)
  • Roof Wind Designer from MRCA, NERCA, and NRCA. (RWD)
  • ANSI/SPRI WD-1, Wind Design Standard Practice for Roofing Assemblies. (WD-1)

FM LPDS 1-28 provides a number of look-up charts and tables based on exposure, wind speed, and roof height which provide design wind loads for the field of the roof. A separate table includes multipliers to go from an enclosed building (the base assumption) to a partially enclosed building. FM uses their own wind speed maps.

The Roof Wind Designer online software only performs calculations for "enclosed" buildings, and provides Strength Design and Allowable Stress Design results. It is left to the user to determine which output to use. It is not possible to use Roof Wind Designer to design your roof system by assuming "partially enclosed."

The 2014 version of the ANSI/SPRI WD-1 document provides no basis for determining wind loads acting on a roof, but says to follow ASCE 7. WD-1 2014 does provide good information about designing the roof system based on the loads the designer chooses to use. However, the 2012 version of WD-1 provides charts based on Allowable Stress Design, risk category 2, and assumes "enclosed" buildings. Determining wind loads from the 2012 version of WD-1 assumes less conservative factors. It matters which version of WD-1 is used.

Real world example

Let's take a look at a real-world example. Here are the factors: Des Moines, Iowa; ASCE 7-10; 28' x 75' x 200'; Exposure C; Risk Category 2, and Partially Enclosed. Note that wind loads are expressed as uplift pressures.

Field of the roof uplift pressures:

FM: 101 psf (Partially enclosed)

RWD: 67 psf (Enclosed and Ultimate Wind Speed)

WD-1: 47 psf (Enclosed and Allowable Stress Design)

What a difference a few conservative choices makes!

So let's talk about GAF for a moment. If a designer calls GAF to ask us for information regarding wind design of a roof system, we will assume Partially Enclosed, Risk Category 2, Exposure C, and will use the Strength Design/Ultimate Wind Speed method. AND very importantly, we will increase the wind speed by one zone! If you use GAF's method, you're using a very conservative approach to wind design.

Back to the example. GAF's uplift pressure for the field of the roof would be 114psf. And so to recap:

GAF: 114 psf

FM: 101 psf (Partially enclosed)

RWD: 67 psf (Enclosed and Ultimate Wind Speed)

WD-1: 47 psf (Enclosed and Allowable Stress Design)

Final thoughts

I don't know about you, but I'm not seeing wind events reducing in number, nor are they lessening in strength. Perhaps the flurry of this year's hurricanes are a "once every ten years" event, or even "once every 20 years" event. The conservative approach to wind design is to believe it is "not if, but when" these incredible wind storms will be encountered. Certainly, hurricanes are predominately located in the Gulf and East coast regions, but there are significant un-named high wind events in all other regions of the US. There are "special" wind zones across the front range of the Rockies, and there are Santa Ana winds in southern California, for example. The point is that high winds can occur everywhere and proper roof design can help protect against them! So let's design for them!

You can design your roofs to the lowest code-allowable wind loads and take your chances, but perhaps it makes more sense to use conservative factors when designing your roofs so they resist those "once in a lifetime" wind loads that seem to be occurring all too regularly.

NOTE: This blog is for information purposes only. GAF does not provide professional design services. You should always consult with a design professional to determine whether the roofing system to be installed is suitable for the particular needs of your building. GAF's system guarantees cover leaks resulting from winds up to 55 mph and do not cover leaks or other damage caused by hurricanes. Jim Kirby is GAF's Building and Roofing Science Architect for the east coast. Note: This is part two of GAF's blog series about wind design.

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