What impact does the linear expansion coefficient have on steel structure design and construction?

Document: The Impact of Coefficient of Linear Thermal Expansion (α) on Steel Structure Design and Construction

In the structural design of large-scale buildings, bridges, pipelines, and mechanical equipment, the coefficient of linear thermal expansion (α) is a key indicator for assessing the impact of temperature changes on structural stability. It is directly related to the safety and durability of the structure.

Formula for Coefficient of Linear Thermal Expansion (α):

α = ΔL / (L₀ × ΔT)

• α: Coefficient of linear thermal expansion

• ΔL: Change in length

• L₀: Initial length

• ΔT: Change in temperature

How does the coefficient of linear thermal expansion affect steel structure design and construction?

I. Core Impacts on Structural Design

1. Thermal Deformation Control and Expansion Joint Design

   Steel structures undergo linear expansion and contraction (ΔL = α × L₀ × ΔT) due to temperature changes (e.g., diurnal temperature variations, seasonal changes, or environmental heat sources). Failure to account for α in design can lead to:

   Therefore, design must calculate the expansion/contraction amount based on α, releasing deformation through the use of expansion joints, sliding supports, or flexible connections (e.g., the spacing of bridge expansion joints must consider the steel's α value and local temperature differentials).

• Localized Compression or Cracking: For example, in long-span structures like bridge girders or industrial building roof trusses, unreleased expansion can generate excessive stress at supports and joints, leading to bolt fracture or weld cracking.

• Risk of Overall Instability: In structures with strong constraints (e.g., rigidly connected frames), internal stresses from thermal expansion/contraction under significant temperature differences can cause buckling instability.

2. Thermal Stress Checking and Material Selection

   The coefficient α directly affects the calculation of thermal stress under constrained conditions (σ = E × α × ΔT, where E is the modulus of elasticity). Design must ensure thermal stress does not exceed the allowable stress of the steel:

• For high-temperature environments (e.g., steel structures near industrial furnaces, boiler supports), α may increase with temperature (e.g., austenitic stainless steel's α at 500°C can be 20%~30% higher than at room temperature). Stress must be recalculated using the high-temperature α value to avoid material yielding from overstress.

• When connecting dissimilar materials (e.g., steel-concrete composite beams, steel and glass curtain wall connections), the difference in their α values (steel α ≈ 12×10⁻⁶/°C, concrete α ≈ 10×10⁻⁶/°C) must be accommodated. This is done using elastic connectors or transition layers to reduce the risk of interface debonding.

3. Structural Stiffness and Precision Control

   For precision steel structures (e.g., machine tool guides, large inspection platforms), even small differences in α can lead to accuracy deviations:

• If steel with a higher α (e.g., austenitic stainless steel) is used, fluctuations in ambient temperature will cause component length changes, affecting equipment operation accuracy. Therefore, materials with low α (e.g., ferritic stainless steel, Invar alloy) should be prioritized, or active temperature control systems should be used to counteract thermal deformation.

II. Key Impacts on the Construction Process

1. Temperature Compensation Measures During Installation

   When the temperature during construction differs from the design reference temperature (usually 20°C), installation accuracy must be adjusted based on α:

• Pre-deformation Setting: For example, when hoisting large-span steel trusses, if the construction temperature is lower than the design temperature, the cold contraction amount must be calculated and pre-camber must be reserved to avoid excessive deflection after subsequent temperature increase.

• Welding Timing Selection: When welding long seams in high or low-temperature environments, thermal expansion and contraction due to α can exacerbate welding residual stress. Preheating, layered welding, or working during periods with smaller temperature differentials are necessary to reduce the risk of welding cracks.

2. On-site Connection and Joint Treatment

   The construction quality of steel structure joints directly affects whether deformation caused by α can be effectively released:

• Bolted connections require appropriate clearance to avoid excessive bolt preload from thermal expansion leading to fracture. Welded joints require controlled weld stiffness, using elastic weld design if necessary (e.g., stiffener weakening).

• For structures designed as "jointless" or with limited joints (e.g., continuously welded rails, long-span connected buildings), construction simulation is needed to calculate temperature stress distribution, ensuring the on-site welding sequence aligns with the stress release path.

3. Accuracy Review During Construction Acceptance

   Measured data must be corrected using α during the acceptance stage:

• For example, measuring the verticality of steel columns or the spacing of steel members requires recording the ambient temperature. If the temperature differs significantly from the design reference temperature, the actual deviation at the reference temperature must be calculated using the α value.

• For steel structures serving at high temperatures (e.g., chimneys, waste heat boiler supports), deformation allowances for high temperatures must be预留 (reserved) during construction. Acceptance should verify that the joint displacement capacity meets design requirements.

III. Potential Impacts on Long-Term Operation and Maintenance

The cumulative effect of α can influence the lifespan of steel structures:

• Under repeated temperature cycles, periodic expansion and contraction due to α can cause joint fatigue (e.g., bolt loosening, weld fatigue cracks). Design should select materials with high fatigue strength and optimize joint details to distribute stress.

• Drastic environmental temperature changes (e.g., fire, extreme cold) can cause abnormal changes in α (e.g., carbon steel's α increases to over 14×10⁻⁶/°C at high temperatures). The structure might collapse due to excessive instantaneous thermal deformation. Therefore, high-temperature α characteristics must be considered in fire-resistant design, including setting up fire protection layers or emergency cooling measures.

Differences in Coefficient of Linear Thermal Expansion for Different Types of Steel

Reference Table of Coefficient of Linear Thermal Expansion for Common Steels (10⁻⁶/K)