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Temperature load

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Temperature load shall be considered by the designers as it can excert significant stresses on structurs. Unlike dead and live load, the temperature load is not generated due to material or people weight. It is generated due to temperature changes that will cause structure elongation and contraction. 

Temperature load is the stress generated in the structure due to change in structure temperature. This effects occurs due to daily weather variation or seasonal variations or due to direct exposure to sun or fire or due to the temperature difference between the construction and operation stages. 

No stress will be generated for structure element that can move or on other word accommodate the structure expansion and contraction due to temperature effects. But if the structure elements movement is restricted, then the structure element will resist the changes in size which will generate stresses in the structure. 

Example, a very good example of the effects of temperature load is bridge structure, bridge structure will usually have a long span, so temperature changes will result in a considerably changes in the structure lenght, if the structure has no ability to accommodate the movement then stresses will be generated due to structure expansion or contraction. 

Type of temperature effects:

The temperature load affects the structures by different ways:

1-uniform temperature change: when the structure temperature increases or decreases uniformly across the structure element and this will cause unform and equal expansion or contraction across the element. This usually occurs in long bridges, steel girders and concrete slabs. 

2-Temperature gradient occurs when part of the element has a higher temperature than the remaining part. An example of this, is the upper part of a bridge which may gains temperature quicker than the lower part, this unequal distribution of temperature with the structure element can cause structure curvature or bending. 

Thermal Expansion Formula

Thermal movement of a structure element can be calculated using the following equation:

∆L = α × L × ∆T

Where:

· ∆L: change in length 

· α: thermal expansion coefficient 

· L: original member length

· ∆T: change in temperature 

From this equation we can notice that as the member length increases the generated movement will be more. Furthermore, it can be noticed that different materials will have different thermal expansion coefficient which resulted in different movement changes depending on the materials. 

Concrete structure significantly affected by temperature variation. Especially during the early age of concrete or during the curing. In mass concrete the dehydration temperature will significantly increase the internal temperature while the surface cool faster which results in temperature variation between concrete layers and this can cause cracks. 

In bridges, retaining walls, slabs and long structure elements, the temperature effects should be studied carefully. Usually, the expansion joints and appropriate reinforcement is used to control the cracks and accommodate the movements. 

Steel has a considerably high thermal expansion coefficient which means that the steel structures will have a high movement due to temperature changes. The temperature changes can generate internal stresses on the structure if not sufficiently accommodated and considered during the design and construction stage. 

Expansion joints are a very common solution to accommodate the temperature movement as it allow the structure elements to move and accommodate the movement without generating high internal stresses. 

In bridges, usually the expansion joints are placed in a specified locations to allow the bridge structure to accommodate and contract safely. Structure can damage if the expansion joint eliminated or if the expansion joint get blocked and can't accommodate movement. 

During the design stage, the designer should consider minimum and maximum temperature, element length, material properties, elements degree of restrain and the type of support. Furthermore, the designer should check if the temperature change is uniform or uneven as the unform cause axial movement while uneven cause curvature or bending in structure. 


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