What Is Design Pressure?
Design pressure is a load value, expressed in pounds per square foot. Design pressure can define two things: the pressure of the wind on a building or the weight load capacity of a reinforcing member of various components of a building.
Frequently Asked Questions (FAQ)
The following information helps to demystify design pressure concerning Andersen products.
Q: How is the design pressure rating of a building determined?
A: The design pressure (DP) rating of a building is calculated using the wind speed of a specified location, the type of exposure the building has, the type of building, the average height of the roof (from eave to peak) and the zone of the wall and roof.
Our online Design Pressure Estimator may be used to estimate the design pressure requirements for windows and doors for a specific structure in a specific location. Once you know the design pressure requirements for your building, you will be able to use the performance grade (PG) rating of an Andersen® window or patio door to determine whether or not it meets these requirements.
Q: How do you determine the DP rating of a window or patio door?
A: Individual window and patio door units are no longer assigned DP ratings. Instead, our windows and patio doors are assigned a performance grade rating. The performance grade rating is a certification rating system that is standardized within the window and door industry.
Whereas the DP rating of a unit only reflects the rating of the product for uniform load deflection (how much load a window or patio door can withstand, expressed in pounds per square foot (PSF), without producing permanent deformation of the product and without impeding its operability), the PG rating takes several factors into account, including the design pressure. The performance grade rating is expressed in PSF, thus, a PG50 product has the structural capacity to withstand fifty (50) PSF of wind pressure.
For more information on PG ratings, please see Performance Grade and Air Infiltrations for Andersen® Products.
Q: How is the DP rating of a joined window or patio door determined?
A: When two (2) different window or patio door units are installed into the same rough opening of a wall, the units must be joined (or mulled) together. The materials used to join the two (2) products together have been engineered to provide specific wind load-carrying capacities expressed in PSF. Mullions are designed for wind load only and are not designed to support the building structure.
Q: Why are some joined or mulled units assigned a DP rating but not a performance grade rating?
A: While mulled or joined units are tested for their load-bearing abilities, many are not tested in any other category included for a performance grade. When you have a single window or patio door unit, it is assigned a PG rating, meaning that all components have undergone all performance grade tests. When you have multiple window or patio door units mulled or joined together, the overall joined unit is assigned a DP rating based on the wind load the joined material is engineered to bear.
The image below illustrates this, showing two units, each of which is assigned a PG rating, mulled together (mullion). Because the mullion components are rated for design pressure only, the resulting rating for this mulled unit is also for design pressure only.
Two Units Mulled Together, Rated for Design Pressure
Q: Do any Andersen® products that have been joined or mulled receive a PG rating?
A: The joining material for mullions can be calculated or tested. Mullions that have been tested are certified for air, water and structural deflection requirements and receive a PG rating. Mullions that are calculated are neither tested nor have required uniform load deflection limits and can only be assigned an estimated design pressure performance level stated in PSF.
Factory joined 100 Series products are currently our only products that have been tested and certified and can thus be listed as having a performance grade and deflection limits.
All other product line joining components have been engineered and calculated for specific loads, but have not been certified. In these cases, only a design pressure rating can be applied.
Q: How do I determine the design pressure rating of a specific combination of products?
A: Code requirements for design pressure and product compliance will vary by municipality. The Andersen Corporation is not authorized to determine whether specific products meet code requirements. Please check with your local building code official for specific code requirements prior to ordering your products. Only local building code officials are authorized to approve the use of products as meeting code requirements. Please see What Are Wind Load Requirements?
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