Effect of conduits for electrical cables on strength of structures


I.S.GILL (Structural Consultant)

It is a normal practice to lay p.v.c or steel pipes for electrical cables in R.C.C slabs.As per structural design a opening can be created at centre of the slab thickness in direction of the shorter span after proper checks and calculations.In case these are given haphazardly in curves and diagonal or other directions it can cause weakness in the slab which can be fatal at times of earthquakes.

During the laying of p.v.c conduits no precaution is taken with respect to direction or position of the conduit which normally lies on the bottom tension reinforcement and thus not at mid depth.

Secondly at places near main boards these come in from various directions with negligible gaps between them causing reduction in concrete mass and effective depth thus reduction in strength of slab.

In case conduits are to be provided in R.C.C slab these should only be provided along the shorter span of the slab and care should be taken to provide spacers below it so that it remains in the centre of the slab thickness. Pipes should not be grouped together in the slab.

In the case of walls chases ar cut indescriminately in all directions ie. horizontally, vertically and diagonally in both 4.5" and 9" thick walls.This causes weakness in these walls, firstly due to the hammering the mortar joint is weakened and secondly from where ever the chase passes it creates a plane of weakness.These weaknesses can become fatal at the time of severe earthquakes.

In case conduiting is to be embedded in walls then it should be provided in wall portions which are oversized for the load coming on them or in portions which are not load bearing. Chases when cut should be done with electrical chase cutting machines or with small cutters used for cutting stones thus avoiding hammering of walls with chisels and hammers. Diagonal andr horizontal chases should be avoided and generally should be vertical only. These chases should be repaired properly with cement mortar before plastering or applieng P.O.P.

Thus it is very much necessary to take approval of the structural designer w.r.t the postion of conduits in slabs and the cutting of chases in load bearing brick walls.

During the early years of the last century all cable work used to be provided on the surface of walls and below slabs this used to help maintain the strength of supporting structures.Where ever possible it should be tried to do surface conduiting or other non-invasive arcitectural possibilities should be looked into.