Thermal Analysis

Thermal loads are forces or stresses induced in a structure due to changes in temperature. When a structure experiences temperature variations, different parts of the structure expand or contract at different rates, leading to internal forces, deformations, and potentially, structural issues if not properly accounted for in the design. Here’s a detailed overview of thermal loads and how they impact concrete structures:

Understanding Thermal Loads:

• Thermal Expansion and Contraction:

Expansion: When temperature increases, materials generally expand. In concrete, this means the length and volume of structural elements increase.

Contraction: Conversely, when temperatures decrease, materials contract, leading to a reduction in length and volume.

• Effects on Concrete Structures:

Cracking: Uneven expansion or contraction can lead to thermal cracking, especially if the structure is restrained or if there are significant temperature gradients.

Deformation: Differential thermal expansion between different materials or between different parts of the same material can cause warping or bending.

Stress: Thermal loads can introduce additional stresses in the structure, potentially affecting its load-carrying capacity and stability.

The guidelines provided below outline the design considerations for concrete structures under two different conditions: the operational phase and immediately after construction. Here's a breakdown and explanation of these considerations:

1. Concrete Structures - Occupied Operational Phase:

Design Considerations:

External Structural Elements:

• These include roof floors, external beams, and columns at the boundary of the building.
• Thermal Variation: Design should account for a thermal variation temperature ΔT of at least 25 degrees Celsius. This means the structure must accommodate temperature changes of this magnitude without experiencing undue stress or deformation.

Basement Slabs and Internal Structural Elements:

• These include typical slabs, internal beams, and internal columns.
• Thermal Variation: Design should consider a thermal variation temperature ΔT of no more than 10 degrees Celsius. This is because internal elements are generally less exposed to temperature fluctuations compared to external elements.
• Axial Stiffness Modifier: In ETABS (a structural analysis software), the axial stiffness modifier should be set to 0.5 to account for the reduced modulus of elasticity due to thermal effects. This modifier adjusts the stiffness of the structure to reflect the decrease in the modulus of elasticity at higher temperatures.

2. Concrete Structures - Just After Construction Case:

Design Considerations:

Full Structure:

• This includes all elements of the concrete frame—external and internal components like the roof, typical floors, internal and external columns, and basement slabs.
• Thermal Variation: Design should consider a thermal variation temperature ΔT of at least 25 degrees Celsius. This is necessary to ensure that the structure can handle thermal changes even before it starts to be occupied and subjected to operational loads.
• Live Load: During this phase, Reduction in live load may be considered. This reflects the lower occupancy or load conditions immediately after construction.
• Modulus of Elasticity: The modulus of elasticity should be considered as 1.0 EC (where EC is the standard modulus of elasticity). No reduction in the modulus of elasticity is allowed in this phase, as the concrete has not yet undergone significant thermal aging or degradation that would affect its stiffness.

Summary:

Operational Phase:

• External elements designed for ΔT of at least 25°C.
• Internal elements designed for ΔT of no more than 10°C.
• Axial stiffness modifier of 0.5 in ETABS to account for reduced modulus of elasticity.

Post-Construction:

• Full structure designed for ΔT of 25°C.
• Reduction in Live load to be considered.
• Modulus of elasticity is 1.0 EC (no reduction).

These guidelines ensure that the concrete structure can accommodate both thermal variations and loading conditions specific to its operational state and immediately after construction.