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All structures have been subjected to the thermal variation wich it's occurs during the day or the season .Result of this variation ,structures suffers from many consequences :-
1- In terms of the same material when one face of this material undergoes to more heating than the other face , as a result the relative movement between material layers will occurs and that leads to Generating an internal forces which effect on the material and crack it .
2- If we have structure with2 materials each one has coefficient of expansion differs about the other ,the results we gain from thermal variation will be more complex than the first case .
3- Expansion and contraction in the structures and their building units and connection between their differential components lead us to assign an expansion joints we choose their locations according to common specifications.
Consider that the concrete structure is not subject to the problem of differential settlement.
For contraction joints, it may be possible to design a concrete structure without any contraction joints. By using sufficient steel reinforcement to spread evenly the crack width over the span length of the structure, it may achieve the requirement of minimum crack width and cause no adverse impact to the aesthetics of the structure. However, it follows that the amount of reinforcement required is higher than that when with sufficient contraction joints.
there are many theories for design the first to design section to carry all forces including the forces resulted of thermal change and the second theory to reduce those forces by cracks control (expansion and contacting joint) that means you can make structure without joints.
For expansion joints, the consequence of not providing such joints may be difficult to cater for. For example, a concrete structure has the coefficient of thermal expansion of9x10-6 /oC and a Young’s modulus of34.5kN/mm2 . With an increase of temperature of20oC and it is restricted to free expansion, then the structure is subject to an axial stress of6.21MPa. If the structure is very slender (e.g. concrete carriageway), buckling may occur. Therefore, the structure has to be designed to take up these thermal stresses if expansion joints are not provided. However, for water retaining structures, most of them are not affected by weather conditions because they are insulated from the water they contain internally and soil backfill that surround them. Therefore, it is expected that a smaller amount of thermal movement will occur when compared with normal exposed concrete structure. Consequently, expansion joints may be omitted in this case with the view that the compressive stress induced by thermal expansion toughens the structure to limit the development of tensile stress.
I agree with the previous answers
There is another reason for joints, to limit the mass and inertia and therefore the eartquake - generated stress of a structure under eathquake loads.
It's able to be free of one of those, but not both of them in the same moment .. thanks
