The 3 Types Of Window & Glazing Supports On A Structure & How They Work

There's more than one way to support glass...

Each of these window support systems comes with a unique set of design & installation considerations.

Here’s a breakdown of what each is & how they are designed & used.

Click any image on this page to expand.

This project incorporates all three types of window supports discussed below.

1 - Supporting Mullion

To connect or ‘mull’ two or more windows/doors together, an intermediate support is required.

The purpose of this support is to provide an equivalent supporting jamb to fasten the glazing product to.

It’s important that this support be able to withstand the highest design lateral forces (usually wind, but could be snow/gravity if used for vertical or inclined surfaces) acting on the mullion.

In addition to stress, deflection plays an essential role in determining the mullion size. End reactions are also an important consideration.

Deflection when holding glass is never less than L/180, with L/175 and even L/240 being possible considerations. 

The loading on the glass can either follow a rectangular or trapezoidal load pattern as shown below.

For further study, a referenced standard used for the design of glass & supporting surfaces is ASTM 1300

PGT Industries publishes a design tool that helps illustrate this design. It can be used in estimating PGT and other mullions of similar size & strength. Use the tool by clicking the button below. 

An example of trapezoidal loading. The mullion is the sum of Area 1 + Area 2. In rectangular loading, the sum is simply half the distance to the left plus half the distance to the right.

+1 - Supporting Mullion, Alternate

Also technically a mullion, windows (such as single & double hung or continuous storefront glazing) and doors (such as sliding glass doors) also have mullions integrated into the system.

The difference with these mullions are that they are tested and approved within the product, and therefore do not need additional engineering design.

They would typically be held to an L/175 or L/180 standard and undergo testing with safety factors to achieve a rated overall design rating.

2 - Structural Frame

Connecting Multiple Windows, Part 2

For larger and more complex openings that an (aluminum) mullion can handle, steel (or wood) framing is typically used to create structural frames to support the glazing products.

Notice that one mullion is dependent on the other for support, creating a more complex design that needs to handle concentrated loads as well as distributed wind forces.  

Gravity loads also become an issue, as well as thermal expansion, waterproofing, anchorage, and corrosion/rust prevention.

The deflection on these supports is typically L/240, with larger deflection limitations also considered for more precise designs & structural finishes.

Reactions at the ends of these structural supports also get quite large, exhibiting a large concentrated force on the host structure which must be designed for, and larger anchor plates needed to provide sufficient anchorage.

3 - Structural Supporting Column

In this situation, columns supporting the glazing products also support gravity loads from the structure above.

These must be designed for maximum deflection while feeling the maximum compression (or tension in the case of uplift) forces from a critical design, while maintaining critical deflection, stress, and connection limitations.

In this example, the column is also on a building corner, so the design must accommodate maximum loads from both directions. 

Notice the top plate over the steel column – it’s designed to accommodate anchors while maintaining necessary anchor edge distances and sizing & waterproofing of windows. This makes for a complex design that typically requires a glazing specialist to design for.

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