What is a Hollow Metal Frame?
Openings in commercial buildings that require heavy duty durability and security are where you will typically find hollow metal frames. Metal or wood doors are most commonly installed in hollow metal frames. The frames are considered “hollow” due to the open throat that allows the frame to anchor to the wall around it. Here we will discuss several key terms when talking about hollow metal frames.
Hollow metal frames are made from steel sheets of metal that are bent and shaped into various profiles. The metal sheets are cold-rolled to a certain thickness or “gauge”. The lower the gauge the thicker the steel sheet.
The gauge of hollow metal is specified in increments of 2, starting with 20 gauge all the way up to 12 gauge steel. 14 and 16 gauge frames are the most common where 14 gauge is used at exterior and 16 gauge is used at interior.
Cold-Rolled vs. Galvannealed Steel
Cold-rolled steel is the most common metal used for hollow metal frames. First the steel is hot-rolled and then processed further and reduced to the required gauge using the cold-rolled method. Galvannealed steel is cold-rolled steel that has been treated with a protective zinc coating which resists corrosion. Cold-rolled steel is used at interior openings while galvannealed steel is frequently installed at exterior openings.
There are several terms that are unique to the framing industry but are critical to specifying and supplying the correct frame. Here is the quick list, refer to the diagram below:
- Face – Most hollow metal frames have a face dimension of 2″ wide. They can be as narrow as 1″ or as large as 10″ wide.
- Jamb Depth – The dimension measured from the outside of one face to the outside of the other. This term is often confused with the throat dimension.
- Backbend – Bent at a 90 degree angle from the face of the frame toward the wall material. This provides rigidity to the frame faces.
- Throat – The dimension between the backbends of the frame. This is where the frame anchors engage the wall material.
- Soffit – The area of the frame that is between the rabbets and parallel to the jamb depth. The soffit size will vary as the width of the jamb depth changes.
- Rabbets – Areas on either side of the soffit. The door will be installed in one of the rabbets. Frame profiles can be single rabbet or *double rabbet.
- Stops – This is the area of the frame where the door will stop against it.
Additional terms and components to be familiar with include: hinge jamb, strike jamb, and head. The jambs are the vertical pieces of the frame. The head is the horizontal top piece. A typical 3-sided frame for a single door will have one hinge jamb, one strike jamb, and one head piece. A frame for double doors will have two hinge jambs and one head piece. These frames can be supplied knock-down or set up and welded.
Most openings will have a standard 3-sided frame (as pictured above) but sometimes a more custom design is wanted (see below). Here are a few more terms to be aware of when looking at these more detailed frames:
- Mullion – Closed frame sections that are installed vertically or horizontally and that separate two areas of the opening in the frame.
- Sill or Base – The bottom frame section of a sidelight or borrowed light.
- Sidelight – These are areas of the frame that have glass installed in them and are separate from the doors.
- Transom – Transoms are the areas located above the door and can have glass or panels. The transom glass or panel is usually separated from the door below by a horizontal mullion.
- Borrowed Light – This is essentially just another term for a window frame where no door is installed in the frame.
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