Expert: Arshad Khan - 11/16/2008

Question
QUESTION: I would be thankful to you, if you answer this few questions.
1. what is the maximum distance for expansion joints in RCC  framed structures having a zone of  least earthquake possibilities?
2. should I provide an expansion joint for a flat slab having a area of 23m x 50m.The structure is fully RCC, and it is in a least earthquake possible zone.

ANSWER: Hi Mohammed,

The distance for expansion joints is mainly determined by temperature changes and less so by seismic considerations, unless there is a big change in shape or height of adjacent parts of a building. Generally speaking, expansion joints between 30-50m apart can easily be catered for within design/detailing of RCC structures but in some cases (with proper thermal analysis and rebar detailing), this can be increased to much more.

Also, for underground or buried structures where daily or seasonal temperature variations are low, you normally can get away without any expansion joints (but you should incorporate temporary shrinkage/movement strips during construction).

All the above assumes that differential foundation settlement is not an issue.

For your 23x50m flat slab you do not need an expansion joint but you should work out the maximum movement and design the supporting columns for the additional moments caused by lateral deflection due to slab movement.

---------- FOLLOW-UP ----------

QUESTION: Please answer these questions,
1.If in case the RCC retaining wall is at the edge of the property line and toe slab of retaining wall cannot be provided, but according to analysis the toe has to be provided, is there any alternate solution for this. The height of retaining wall is around 2.5m to 3m and there is no differential settlement of the foundation and soil has good bearing capacity.
2.Nowadays for residential projects most of the structural design is done by uneducated contractors having some good experience.They provide some of the concealed beams having a depth of 0.15m and width of 0.45m  to carry the slab load.Is it OK to provide concealed beams to carry the slab load.

ANSWER: Hi Mohammed,

1. For retaining wall at edge of property, you can make it work by incresing the base width (length) in direction away from the property line. For your case, a base length of around 1-1.5m should be more than enough to ensure stability, especially if you make the base thicker than the wall by a ration of say 1.5.

2. By concealed beam, if you mean a beam flush with slab soffit, then it is likely that these slabs may have a deflection beyond code limits unless the slab is designed as a flat slab. However, unless the span is very small 0.15m deep flush beams are unlikely to work due to the reduced effective depth after the cover to rebar is taken into account. Beam depth should be around span/20 plus cover to rebar plus half diameter of rebar.

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QUESTION: 1.why should we provide plinth beams.
2.what is the function of a plinth beam?


Answer
Hi Mohammed,

If what you mean by a  plinth beam is a down-standing beam, then this is a more economical solution than a flush beam because the efficiency of a beam is increased by a factor equal to square of the effective depth. In other words, a beam 4 units deep is about 4 times more efficient than a beam 2 units deep, instead of just twice.