Structural Systems
Jack Beams: Removing Columns in Steel Buildings
A jack beam is a primary beam that spans between two interior columns of a multi-span building and supports the rafter of a frame whose interior column has been removed. It creates a wider column-free bay for loading, vehicle movement or equipment without changing the whole building to a clear span.
Key facts
- Jack beams run along the building length, parallel to the ridge.
- They carry the rafter reaction of the frame whose column is omitted.
- Neighboring columns and foundations carry increased loads.
- Beam depth may reduce clear height at that location.
- They are a lower-cost alternative to making the whole building clear span.
How does a jack beam work?
In a multi-span building, each frame has interior columns under the rafter. When one interior column is removed, the rafter at that point needs another support. A jack beam spans between the interior columns on the frames either side and picks up the rafter at the omitted column position.
The load from the supported rafter is passed through the jack beam to the neighboring columns, which then carry more than a typical interior column.
When are jack beams used?
Jack beams are useful in warehouses and factories where a few areas need extra open space, such as truck turning areas, production lines, large machines or loading docks. They allow the rest of the building to stay economical with regular interior columns.
Several adjacent columns can be removed with longer or multiple jack beams, though the beams become heavier as their span grows.
Design considerations
The jack beam must be deep and stiff enough to support the rafter without excessive deflection, which could affect roof drainage and the frame. Its depth may lower the clear height under it, so check equipment and vehicle clearances.
The connections between the jack beam, rafter and supporting columns are designed for the transferred forces, and the foundations of the supporting columns are enlarged accordingly. The project engineer confirms all of this to the governing code.
Planning column-free bays
Identify the areas that need wider spacing at the concept stage, together with their required clear width and height. Adding jack beams after design is complete can change frame, column and foundation sizes.
If large column-free zones are needed in many places, compare a jack beam layout with a full clear-span frame or a wider module, as one may be more economical overall.
Points to confirm with your engineer
- Locations and number of interior columns to be omitted.
- Required clear width and clear height in each open bay.
- Deflection limits for the jack beam and roof drainage.
- Increased foundation loads at supporting columns.
- Vehicle protection for the supporting columns.
Frequently Asked Questions
Does a jack beam reduce clear height?
It can, because the beam sits below the rafter. The beam depth and resulting clear height should be checked against vehicles and equipment.
Can a jack beam be added to an existing building?
Sometimes. A qualified engineer must check the existing rafter, columns and foundations for the new load path, and temporary support is needed while the column is removed.
Is a jack beam the same as a header beam?
No. A header usually spans over a door opening in a wall, while a jack beam supports a main frame rafter inside the building.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
