Thermal Expansion Allowance in Long Steel Frame Runs

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Long steel frame runs grow and shrink with temperature, so the design must include a thermal expansion allowance, usually as planned movement joints or sliding connections at set intervals. The allowance depends on the run length, the temperature swing the frame will see, and the way each section is fixed to the building. Exact movement values and joint sizes should be calculated by the structural engineer for your specific project.

Why long steel frames move with temperature

Steel expands and contracts as its temperature changes, and a long run accumulates that movement along its length. A frame that sits in direct sun on a south or west façade can reach a noticeably higher temperature than the same frame in shade, and the difference adds up over many meters. If the frame is rigidly fixed at every point, that movement has nowhere to go and can cause distortion, cracked glazing seals, strained anchors or noise.

The goal of a thermal expansion allowance is to let the frame move freely in a controlled way, so the stress stays within what the structure and fixings can handle.

Where to place movement joints

Movement joints are normally placed at intervals along the run, with the spacing set by the engineer based on the material, the exposure and the expected temperature range. Joint positions should be planned early, not added after installation, because they affect how panels, glass and infill are divided.

Locate joints near natural breaks such as corners or changes in direction where possible.
Keep joint lines consistent with the architectural grid.
Avoid placing a joint directly where a heavy load point is anchored.

Sliding connections and fixing details

At each joint, one side of the frame is typically fixed and the other is allowed to slide, using slotted holes, sliding brackets or a telescopic overlap. The fixing point that anchors the run should be the only fully restrained point, so that movement radiates outward toward the free ends.

Points to confirm on the drawing

The slot length, clearance and fixing type should be written explicitly in the specification, so that the fabricator and installer work from the same requirement.

Materials, finishes and joint sealing

Different steel sections and protective finishes respond to movement in similar ways, but coatings such as hot-dip galvanizing or powder coating must not be damaged where sliding parts rub. Sealants at movement joints need to accommodate repeated cycles of opening and closing, so their type and width should be selected for the expected movement rather than for appearance alone.

Coordinating with glass, infill and the building

Glass panels, infill sheets and the supporting building structure all have their own movement behavior. A frame allowance that ignores these can still cause problems at the glazing edge or where the frame meets the building. Share the expected temperature range and the frame layout with the glazing supplier and the architect at an early stage, so that gaps and tolerances match across trades.

Information to prepare before design

To make the allowance calculation practical, gather the basic facts before the design is finalized. Clear input makes it easier for the engineer to propose joint positions and sliding details that suit the site.

Total length of each frame run and its orientation to the sun.
Expected temperature range for the location.
Fixing points available on the building.
Type of infill or glazing to be installed.

Applicable railing requirements such as the Israeli standard SI 1142 should be reviewed with the engineer, who will confirm the exact values for your project.

Frequently asked questions

How often should movement joints be placed on a long steel frame?

The spacing depends on the material, exposure and expected temperature swing, so it is set by the structural engineer for each project. Joint positions should be decided at the design stage, not added later.

Can a long steel frame be fixed rigidly at both ends?

Rigid fixing at both ends leaves no room for movement and can lead to distortion, stress on anchors and damaged seals. Usually one point is fixed and the rest is allowed to slide or telescope.

Does sun exposure change the thermal expansion allowance?

Yes, a frame in direct sun can reach a much higher temperature than one in shade, so the expected movement is greater. Orientation should be included in the engineer's calculation.

Who should calculate the allowance for my frame?

A structural engineer should calculate the allowance and joint details for your specific project. Share your drawings and site conditions with them early, and we can discuss the fabrication side with you.

Common mistakes to avoid

Designing a long run as fully rigid, or placing movement joints without checking how they align with glazing and anchors, is a common cause of later problems. Leaving the sliding details undefined on the drawing also leads to field improvisation.

Our recommendation

Mark the fixed point, joint positions and slot details on the shop drawing before fabrication starts, and have the engineer approve them. This avoids costly rework once the frame is on site.

Summary

A thermal expansion allowance lets long steel frames move safely with temperature, through planned joints and sliding connections set by the engineer. Plan it early and coordinate it with glazing and building fixings.

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