The Building Codes in the US reference ASCE-7 for the design of the components and cladding of buildings (18.104.22.168 ASCE 7-(10)). The theory of wind design goes that the smaller the area in consideration, the greater the probability that a maximum burst of wind will occur in that area over any 3 second period. Wind doesn’t blow uniformly at all places at all times.
This variable is one of the many considered when converting wind velocity to wind pressure for use in analyzing a building component (GCp). Take for example ASCE 7 figure 30.4-1
which illustrates GCp varying from 10sqft to 500sqft of effective wind area for both interior zone 4 and exterior zone 5:
Very important here is to properly determine what the actual opening size is for the component being considered.
For a product like a 36” X 48” single hung window for example, the opening size to use with the above chart would be the width * height of the window, or 12sqft. For 2 single hung windows mulled together however (per the illustrations below), the ‘opening size’ is not the total opening size, but still only 12sqft because it’s separated by a structural break (the mullion). The mullion would also have its own separate design pressure to consider, which would be half the left + half to the right of the mullion (also 12sqft in this case), even though the actual opening is 24sqft.
On The Other Hand…
In the case of products like sliding glass doors where there is NOT a structural mullion… Many product approvals list sliding glass doors which have been tested per ASTM E1996/1886/E330, TAS 201/202/203, etc. at a certain size (i.e. 8′ tall x 12′ wide). Some may insist that the effective area of the opening (for determination of wind pressure) should be each single panel of the door, even though the product was tested as a multi-panel assembly.
It has been our position that for a tested, assembled system, the effective wind area of the ENTIRE PRODUCT shall be used for wind pressure determination since the entire product was tested to that wind load. The “single panel” theory breaks down when considering hinges, locks, double swing door systems, top and bottom sashes of single hung products, etc., which would be designed in an overly conservative manner if analyzed under the single-panel worst-case condition. This would effectively negate the usefulness of the GCpf wind load coefficients listed in ASCE 7-10 which permit coefficient adjustments based on the effective area (not tributary area) of the product. The effective area calculation takes into account thousands of data points from nationally certified testing agencies which correlate the fact that peak wind loading upon an area under analysis will converge with smaller area and diverge with larger area, essentially averaging out the total wind load requirement due to highs and lows within the wind load net force.
Let’s take a look at another example. For this metal frame building we see there are wind areas for a number of components – wall panels, fasteners, purlins, etc.
- For the wall panel (wall cladding) according to the definition of ASCE 7 would be the height and width of the panel, total area of the panel.
- For the fastener the effective area would be the tributary area that the fastener is securing.
- For a purlin or a long roof panel the effective area would be the length of the panel or purling by one third of the length of the panel; when the member gets too long the tributary width of the member no longer applies.
ASCE 7-10 Commentary (ASCE 7-10 Section C26.2, “EFFECTIVE WIND AREA, A”) has a clear definition for effective area: “For typical door and window systems supported on three or more sides, the effective wind area is the area of the door or window under consideration.” Therefore, the required demand upon a sliding glass door shall utilize GCpf coefficients from the total area of the door, and that demand value is utilized within a product approval for approved comparison. Product approvals (including those certified by this office) for sliding glass doors will frequently utilize approved wind load tables using the panel size. This is not to be interpreted to mean that the panel size should be used for the effective area; rather, the charts are listed that way for ease of lookup by the client as well as comparison to ASTM E1300 which outputs allowable tables in per-panel sizes. It follows, then, that a zone-interpolated wind load value should also utilize the entire size of the product for demand calculations as has been previously provided by this office.
Height * Height / 3
The logic goes like this… Wind is not likely to blow along a narrow surface for a long distance. For long narrow areas coming from components (roofing panels, wall studs roof trusses…) spaced closely together the effective wind area can be ’rounded out’ and may be taken as height X height / 3 (H*H/3) (height or length of the component multiply by one third of the height or length of the component).
Our conclusion is therefore that the entire product size shall be used for effective area determination, not only because this is the tested condition, but because it is approved by ASCE and correlated through numerous wind studies. BUT for products separated by structural breaks, the size of the product only shall be used. Only “specialty” systems need to be analyzed at each structural component. In other words, a hand calculation that is checking the product empirically would require a smaller effective area (i.e. the 10 square foot rule) since there is no other substantiating data about the component assembly. A tested product, such as that in question on this project, is exempted from this rule.
Please consult with a licensed professional engineer in order to determine the proper effective wind area for your building component and DO NOT try to use this information on your own to design a system that could cause harm to the public or property. We’ll be glad to discuss your need with you. Contact us by clicking here to send us your general question.
Also, our online calculators can help you exactly determine the wind pressure of a given opening for estimating and pre-design use, but only if you understand what each term means. Click Here to use our free Components and Cladding Wind Pressure Design Aid. Results from the design aid are only valid when reviewed and certified by a licensed professional. Use of the tool consents to your understanding of this, also listed in our terms and below.
Updated August 2019 Luis Orgega, EI