Thermal load is the stress induced in a structure by temperature changes — expansion when warm, contraction when cool. Per IS 875 Part 5:1987 + IS 456:2000 Cl. 27, thermal loads are significant for long structures (bridges, buildings >40 m), buildings exposed to large daily temperature swings, and structures with heat-generating equipment. Coefficient of thermal expansion: concrete 10×10⁻⁶/°C, steel 12×10⁻⁶/°C — close enough to allow composite RCC behaviour without major thermal incompatibility.
For a 100 m long Indian building exposed to ΔT = 60°C (summer to winter range): thermal expansion = 0.10 × 100 × 60 × 10⁻⁵ = 6 mm. If the building cannot expand (rigidly tied to foundation), the resulting compressive stress = E × α × ΔT = 25,000 × 10×10⁻⁶ × 60 = 15 MPa — significant fraction of M25's design strength. Mitigation: expansion joints (IS 456 Cl. 27) at 40-60 m intervals to absorb thermal movement; flexible foundation connections; controlled placement of concrete to limit early-age temperature differentials.
Where used
Long buildings (>40 m) — expansion joints per IS 456 Cl. 27
Bridges — thermal expansion of girders, deck slab
Pre-stressed concrete — thermal effect on prestress
Pavement design — thermal cracking in concrete pavements
Acceptance / threshold
Per IS 875 Part 5 + IS 456 Cl. 27: design temperature range per location and structure; stress = E × α × ΔT for restrained members; expansion joints at appropriate intervals (40-60 m typical for buildings).
Site example
Site reality: a 90 m Pune commercial complex was designed without expansion joints. Within 4 years, cracks appeared at predictable locations (40 m from each end + middle). Remediation: cracks repaired + expansion joints retrofitted (₹50 lakh). Original design with expansion joints would have added ₹8 lakh — 8× cheaper than retrofit and remediation.
Frequently asked
What is thermal load?
Thermal load is the stress induced in a structure by temperature changes — expansion when warm, contraction when cool. Significant for long structures (bridges, buildings >40 m). Coefficient of thermal expansion: concrete 10×10⁻⁶/°C, steel 12×10⁻⁶/°C. For 100 m building with 60°C ΔT: expansion ≈ 6 mm; if restrained, stress 15 MPa.
How is thermal load designed for in buildings?
Two main approaches: (1) Allow movement — expansion joints at 40-60 m intervals (IS 456 Cl. 27) absorbing thermal movement. (2) Resist movement — design for thermal stress E × α × ΔT in restrained members. Most Indian buildings use approach (1) — expansion joints prevent thermal cracking and structural distress.
What is the coefficient of thermal expansion of concrete?
10 × 10⁻⁶ /°C (also written as 10 με/°C). For comparison: steel 12 × 10⁻⁶ /°C, aluminium 23 × 10⁻⁶ /°C. The closeness of concrete and steel coefficients allows composite RCC behaviour without major thermal incompatibility — concrete and steel expand and contract nearly equally, preserving bond.