Answer

What Is Design Pressure?

Design pressure is a load value, expressed in pounds per square foot, and can define two building-related things:

• The pressure of the wind on a building.
• The weight load capacity of a reinforcing member of various components of a building.

How is the design pressure of a building determined?

The design pressure of a building is calculated using the following:

• 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
• The zone of the wall and roof

Note: To estimate design wind pressures yourself, get the Andersen® Winde app. Once you know the design pressure requirements for your building, you can use the performance grade (PG) rating of an Andersen® window or patio door to determine whether it meets these requirements.

How do you determine the design pressure of a window or patio door?

What is the difference between design pressure and performance grade?

Design pressure reflects only the uniform load deflection, which is the product’s rating for how much load a window or patio door can withstand, expressed in pounds per square foot (PSF), without permanent product deformation and without impeding its operability.

Performance grade, on the other hand, takes other factors into account in addition to design pressure. Performance grade is expressed in PSF. A PG50 product has the structural capacity to withstand 50 PSF of wind pressure.

For more information on performance grade, please see Performance Grade and Air Infiltration for Andersen® Products. 

How is the design pressure of a joined window or patio door determined?

Do any Andersen® products, which have been joined or mulled, receive a performance grade?

The joining material for mullions can be calculated or tested.

Mullions that have been tested:

• Are certified for air, water, and structural deflection requirements.
• Receive a performance grade.

Mullions that have been calculated:

• Have not been tested.
• Do not have required uniform load deflection limits.
• Can only be assigned an estimated design pressure performance level, stated in PSF.

Note: Currently, factory-joined products from the Andersen 100 Series are our only product that is tested, certified, and is listed with a performance grade and deflection limits.

Joining components from all other Andersen product lines have been engineered and calculated for specific loads but have not been certified. In these cases, only a design pressure can be applied.

Why is a design pressure determined for some joined or mulled units, but not a performance grade?

It’s a matter of single vs. mulled (joined) window or patio units.

A single window or patio unit can undergo all performance grade tests and can be assigned a performance grade.

Because mulled windows or patio units are multiple windows or patio units that are joined, the overall unit can only be assigned a design pressure based on the wind load the joining material is engineered to bear.

The example below shows two window units joined by a mullion. Each window unit, separately, has been tested, certified, and assigned a performance grade (PG). Because mullion components have only a design pressure (DP) determined, the resulting mulled unit also only has a design pressure determined.

Window and Mullion Design Pressure vs. Performance Grade Relationship

How do I determine the design pressure of a specific combination of products?

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.

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. 

For more information, see What Are Wind Load Requirements?