steel-structure-thermal-stress
Steel Structure Thermal Stress: Joints, Temperature Zones & Bearings
Engineering line drawing of a steel workshop expansion joint detail—dual columns at the joint, a PTFE sliding bearing on the free line, and a roof slide cover bridging the gap, with red arrows marking the direction of thermal movement, clean white drafting style.
Steel moves in the sun. A 60 m (200 ft) frame that is hot at noon and cold at dawn can grow and shrink by several centimeters—movement that is harmless until something holds it tight. Steel structure thermal stress appears not from the expansion itself, but from restraint: when columns, bracing, or a fixed bearing refuse to let the frame slide, that locked movement turns into tension and compression that can crack walls, buckle members, and snap bolts.
This guide walks through how much a steel frame actually moves, how to split long buildings into code-limited temperature zones, how to calculate the locked-up stress when movement is restrained, how to select fixed-versus-sliding bearings, and how crane rails and cladding must be detailed to follow the frame instead of fighting it. Overall frame stability and deflection limits are covered elsewhere; see steel structure stability design and steel structure deflection control. This article is about temperature—the slow, seasonal expansion that quietly loads your connections. That locked-up force is precisely what steel structure thermal stress design exists to release before it cracks a wall.
Why Steel Expands & How Much It Moves
Steel is a predictable material when it heats. Its coefficient of linear thermal expansion is about α = 12×10⁻⁶ per °C (6.5×10⁻⁶ per °F)—a standard value from AISC and steel material handbooks. That number means a steel member grows by 0.012 mm per meter of length for every degree Celsius it warms. The free thermal movement is simply ΔL = α·L·ΔT, where L is the member length and ΔT is the temperature change from the installation temperature.
Worked example: a 60 m (197 ft) single-bay roof girder, welded in place at a 15 °C (59 °F) morning temperature, can reach 55 °C (131 °F) in summer sun and drop to −20 °C (−4 °F) in winter. The total seasonal swing is about 75 °C (135 °F). From the formula, ΔL = 12×10⁻⁶ × 60,000 mm × 75 = 54 mm (2.1 in). Spread across both ends of the frame, that is 27 mm (about 1 in) of free movement at each free end.
Where does that temperature change come from? Three sources matter in practice:
- Seasonal ambient swing between the steel's installation temperature and its hottest/coldest in-service temperature—this is the big one for most frames.
- Solar temperature difference between the sun-facing and shaded sides of a member, which can bow a long roof girder sideways rather than just stretch it.
- Indoor–outdoor difference in heated or cooled buildings, which shifts the neutral line the frame actually sits on.
The key concept is free vs. restrained movement. If the frame can slide, ΔL happens with zero stress. The moment a column base, a stiff cross-brace, or a bolted-down bearing prevents that slide, the free movement is converted into force. That is the whole story of steel structure thermal stress—and why every design decision below is really a decision about what gets to move.
| Length (m) | Length (ft) | ΔL at ΔT = 30 °C (54 °F) | ΔL at ΔT = 50 °C (90 °F) |
|---|---|---|---|
| 30 m | 98 ft | 10.8 mm (0.43 in) | 18.0 mm (0.71 in) |
| 60 m | 197 ft | 21.6 mm (0.85 in) | 36.0 mm (1.42 in) |
| 100 m | 328 ft | 36.0 mm (1.42 in) | 60.0 mm (2.36 in) |
| 150 m | 492 ft | 54.0 mm (2.13 in) | 90.0 mm (3.54 in) |
Free thermal movement ΔL = α·L·ΔT with α = 12×10⁻⁶/°C. Movement alone is harmless; restraint is the problem. For load combination definitions, see ASCE 7 Minimum Design Loads.
Temperature Zones & Expansion Joints
No steel frame is allowed to grow forever between restraints. Codes limit the maximum length a frame can reach without an expansion joint—a temperature zone. Exceed that length and the accumulated thermal movement across the frame becomes large enough that the reactions at the restraints, and the distortion of attached cladding, are unacceptable.
Allowable temperature-zone lengths depend on the code, the structure type, and how well the roof and walls are insulated. Well-insulated buildings see a smaller through-section temperature change, so they are permitted longer zones; uninsulated, exposed portals heat up and cool down through their whole depth and get shorter allowable zones. Typical values fall on the order of 150–200 m (500–650 ft) for portal frames and similar braced bays—always verify the exact limit against your governing code and AISC guidance, because these values are not universal. For the exact code maximum spacing limits per AISC and MBMA, plus double-column versus sliding-bearing joint types and roof/wall flashing details, our steel building expansion joint spacing guide covers placement rules and joint detailing.
When a building is longer than the allowable zone, the standard solution is a steel building expansion joint: split the frame with a transverse double-column, double-girder line. The two halves are physically separated across the joint; the joint width is sized to absorb the sum of thermal movement from both adjacent zones, plus a safety margin. Wall and roof cladding bridge the joint with sliding cover plates or flexible closures—they are never welded or screwed rigidly across, because then the cladding itself becomes the restraint.
Long buildings are therefore cut into shorter temperature zones along their length. A 120 m (394 ft) workshop, for example, is split by one transverse expansion joint into two 60 m zones; each zone then behaves as a short, manageable frame. Bracing at the joint needs special attention: lateral bracing must not be duplicated across the joint in a way that locks the two halves together—see our steel building bracing system guide for how bracing lines are arranged at expansion joints.
| Structure Type | Typical Max. Temperature Zone | Imperial Equivalent | Notes |
|---|---|---|---|
| Portal frame, insulated roof | ~180–200 m | ~590–650 ft | Smaller through-section ΔT |
| Portal frame, uninsulated | ~120–150 m | ~395–490 ft | Larger solar heating |
| Braced multi-story frame | ~150–180 m | ~490–590 ft | Depends on floor stiffness |
| Crane runway / long girder | Per code, often shorter | Per code | Rail joint demands dominate |
Typical ranges only; verify against your local steel structure code and AISC Steel Construction guidance.
Restrained Thermal Stress
Free movement needs no design. Restrained movement does. When the calculated ΔL is forcibly prevented—by a both-ends-fixed frame, a rigid floor, or heavy masonry walls that the steel frame bears against—the displacement is eliminated and stress appears. For full, 100% restraint, the stress is simply σ = E·α·ΔT, where E is the modulus of elasticity (about 200 GPa / 29,000 ksi for steel).
Plugging in numbers: E = 200,000 MPa, α = 12×10⁻⁶/°C, and a seasonal ΔT of 40 °C gives σ = 200,000 × 12×10⁻⁶ × 40 = 96 MPa (about 14 ksi). That is a steady, year-round stress—comparable to a moderate live-load effect—and it stacks on top of dead load, live load, wind, and crane loads in the governing combination. It also changes sign: tension in summer as the frame tries to expand and is held back, compression in winter as it contracts. Partial restraint (from stiff bracing, composite slabs, or tight cladding) gives a fraction of the full value, but not zero.
The damage signatures are familiar in the field: wall and curtain-wall panels pulled off their fasteners, roof panels buckled or popped between seams as the frame pushed sideways, diagonal bracing buckled in compression, and column bases carrying unexpected moment. None of these show up in a strength-only check that ignores temperature—that is why temperature reactions must be included in load combinations, and why steel structure thermal stress is a design input, not an afterthought.
The relief principle is simple: let the structure slide in one direction while it resists lateral loads in the other. Never anchor a long rigid frame at both ends at once. When temperature and geometry nonlinearity interact, a second-order analysis is the right tool—see steel structure second-order analysis and the fastener behavior in steel structure connection design.
| ΔT (°C) | ΔT (°F) | Fully Restrained Stress | Stress (ksi) | Notes |
|---|---|---|---|---|
| 20 °C | 36 °F | 48 MPa | ~7 ksi | Mild; often tolerable |
| 40 °C | 72 °F | 96 MPa | ~14 ksi | Typical design value |
| 60 °C | 108 °F | 144 MPa | ~21 ksi | Large; must be released |
| 80 °C | 144 °F | 192 MPa | ~28 ksi | Critical; near yield for mild steel |
σ = E·α·ΔT at full restraint with E = 200 GPa. Partial restraint gives a fraction of these values. Verify load combinations per ASCE 7.
Designing a Long Frame That Won't Fight Its Own Temperature?
A frame that is fixed at both ends will convert every degree of seasonal change into locked stress—cracked walls, popped bolts, buckled bracing. Tell us your building length and climate range, and our engineers will size the temperature zones and bearings before fabrication.
Sliding Bearings & Support Strategy
The classic support strategy for a steel frame is one fixed end and one sliding end. One column line—the fixed (pin) line—carries vertical load and anchors the frame laterally against wind and seismic drift, but allows rotation. The opposite line—the sliding line—carries vertical load and lets the frame expand and contract along its length while still resisting sideways load.
Practically, the sliding line uses a low-friction bearing surface: a PTFE (polytetrafluoroethylene) pad sliding against a polished stainless-steel plate, or a roller bearing. These surfaces release the longitudinal thermal displacement while preserving vertical capacity and lateral fixity. The column base plate on the sliding line tells the same story with fasteners: the fixed line gets anchor bolts in standard round holes that lock the base; the sliding line gets slotted (long) anchor holes oriented along the direction of movement, so the base can travel while the bolts stay captured. Get the slot direction wrong and you have re-created the full restraint you were trying to avoid.
A real-world illustration makes this concrete. A 120 m (394 ft) single-bay workshop is split by one transverse expansion joint into two 60 m zones, each zone sitting on one fixed bearing line and one PTFE sliding line. With a 40 °C seasonal range, each free end slides about 29 mm (1.1 in). The joint gap, the cladding slide cover, and the slotted base anchors are all detailed to that number before steel is ordered—never guessed at site.
| Bearing Type | Fixity | Allowed Movement | Typical Use |
|---|---|---|---|
| Pin / fixed bearing | Full lateral + vertical | Rotation only | Fixed column line |
| PTFE sliding bearing | Lateral fixed, vertical load | Longitudinal slide | Free expansion end |
| Roller bearing | Lateral fixed, vertical load | Longitudinal slide (older design) | Long girders / bridges |
| Slotted-base anchor | Controlled slide along slot | Movement along slot direction | Sliding column lines |
Bearing types per supplier technical data; confirm slide capacity and coefficient of friction with the bearing manufacturer. For base plate detail, see steel column base plate design.
Crane Rails, Cladding & Secondary Effects
The main frame is only half the problem. Everything attached to it inherits the same thermal movement and must be detailed to follow.
Crane rails grow with the crane girders they ride on. Rail joints must leave expansion clearance or use purpose-made rail expansion joints; without it, the rail pushes against itself in summer, buckles, and derails the crane wheel. The longitudinal movement of the crane girder also has to be coordinated with the crane's braking and travel system so the runway does not bind. This is why the crane rail expansion gap is sized from the same ΔL calculation as the frame itself.
Cladding and secondary members need the same treatment. Wall and roof panels across an expansion joint must lap and slide—cover pans with a loose fastener, never a weld or a self-drilling screw locked tight at both sides. On long purlin runs, purlin-to-frame connections use slotted holes so the purlin can thermally slide relative to the rafter; otherwise the purlin becomes a trussed restraint that transfers force into the frame. Purlin spacing and lapping logic is covered in steel purlin system design, and the waterproofing of joints sits in the drainage picture—see steel building gutter & drainage design.
The rule across all of it is the same: the frame moves; everything attached must be allowed to move with it. A single rigid weld or a single tight-bolted connection across a joint is enough to turn a well-designed sliding system back into a fully restrained one.
Conclusion
Steel structure thermal stress is simply thermal expansion converted into force by restraint. Free movement (ΔL = α·L·ΔT) is harmless; locked movement (σ = E·α·ΔT) is not. Release it the way every long steel frame does: split the building into code-limited temperature zones with expansion joints, use the one-fixed/one-sliding bearing strategy, slot the anchor holes on the free line, and detail rails, purlins, and cladding to slide across every joint. The maximum temperature zone, the joint width, the sliding-bearing direction, and the slotted base holes must be locked at the drawing stage—thermal reinforcement after the frame is up is slow, expensive, and often impossible in an occupied building. Resolving steel structure thermal stress at the drawing stage is far cheaper than fixing locked stress in service.
Sizing Expansion Joints and Bearings for a Long Steel Frame?
We calculate real thermal movement from your climate range, split the frame into code-compliant temperature zones, and detail fixed-vs-sliding bearings so seasonal expansion never becomes locked stress.
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Reference Links
- AISC 360 Specification for Structural Steel Buildings
- ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures
- ISO 12944 Corrosion protection of steel structures by protective paint systems
About the Author
Senior Structural Engineer
With over 20 years of hands-on experience in steel structure design and prefabricated building engineering, our in-house senior structural engineer has personally contributed to more than 500 steel building projects—including warehouses, industrial factories, aircraft hangars, agricultural buildings, and commercial structures. The focus is on translating design codes such as AISC 360, ASCE 7, and Eurocode 3 into buildable, cost-effective steel solutions that balance structural performance, fabrication efficiency, and total project cost.
Learn more about our engineering team
Frequently Asked Questions
How much does steel expand with temperature?
Steel has a coefficient of thermal expansion of about 12×10⁻⁶/°C (6.5×10⁻⁶/°F). Free movement is ΔL = α·L·ΔT: a 60 m (200 ft) frame with a 50 °C (90 °F) seasonal range moves about 36 mm (1.4 in). Free movement is harmless; restraint is the problem.
What is a temperature zone in a steel building?
A temperature zone is the maximum length a frame can reach without an expansion joint before locked thermal stress becomes too large. Allowable lengths vary by code and insulation (often on the order of 150–200 m / 500–650 ft)—verify against your local code and AISC guidance.
What is restrained thermal stress?
When expansion is prevented—by fixed columns, stiff bracing, or a both-ends-fixed bearing—the free movement is forced back and stress builds. For full restraint, σ = E·α·ΔT: with E = 200 GPa (29,000 ksi) and ΔT = 40 °C (72 °F), that is roughly 96 MPa (14 ksi), added on top of dead and live loads.
How do sliding bearings control thermal stress?
The classic strategy is one fixed (pin) end and one sliding (PTFE/roller) end. The sliding bearing lets the frame expand along its length while still resisting lateral loads; the sliding column base uses slotted anchor holes instead of fixed bolts.
Do crane rails and cladding need thermal joints?
Yes. Crane rails need expansion joints or gap compensation to avoid binding and derailment, and wall/roof panels must slide (not weld) across building expansion joints—otherwise the cladding itself becomes the restraint that cracks, buckles, or pops fasteners.
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