“Feel the structure” MSA
“This blog will acts as a precautionary tool for any structural engineer to allow customer/client to put any heavy point load on existing slab of structure or not “
Precaution is better than cure.
This blog will guide you whether you will permit additional point load on any building on which that much load was not considered during design. There are two vital points that will be discussed here.
One is failure due to punching due to gravity loading and second one is failure occurred due to change in mode of vibration of building during earthquake
1. PUNCHING FAILURE:
AS per clause -126.96.36.199 of Indian standard, IS:456-2000 (Indian Standard code for plain and reinforced concrete – code of practice, edition-2000), existing slab is not able to respond huge punching stress subjected by a point load of 1 ton or more which was earlier not considered in design.This happens due to inadequate thickness of slab. Actually during design of slab, that much unexpected point load was not supposed to occur throughout it’s design life & when unexpected heavy point load is put on slab, failure takes place. This kind of punching failure is brittle in nature and slab will fail without any alarm. Generally,dead load and imposed live load is applied as KN/m2 or Ton/m2 unless & until any provision of equipment or pointed object is there on slab.
Another reason for not permitting any point/concentrated load on existing slab at slab level is that concrete lose its strength when it becomes old due to its exposure to environment having deleterious chemicals which corrodes reinforcement.
Secondly, any building adjust on its own by prolonged creep and shrinkage action when structural members are subjected with load after few days or month of construction . But suddenly when building is subjected to huge load, it undergoes huge deformation exceeding limit limiting values of deformation.
2. CHANGE IN BEHAVIOR/FAILURE MECHANISM DURING EARTHQUAKE:
As per clause 7.3.1 of Indian standard IS:1893-2016(Part-1 for Criteria for earthquake resistant design of structure) , lump mass at floor level shall be dead load + some percentage of live load. Here in this case proposal of heavy point load at any floor will increase dead load. Consequently, lump mass will increase significantly. This will result in attracting more earthquake forces as earthquake forces are inertial forces. Also, greater super dead load at floor result in deviation in center of mass (C.M) & center of rigidity(C.R) .This will create torsion in building during earthquake. As it is well- known fact, column are very weak in torsion/twisting. Hence, columns are most likely to fail during earthquake as provision of torsion/twisting resistance system may have not considered in design.
Also, as per clause 7.1, table-6 of Indian standard IS:1893-2016(Part-1) , putting heavier load to upper floor will create soft storey at lower and upper floor and will distort failure mechanism in building. This kind of soft storey will change the mode shapes of vibration of building resulting in failure of column at soft storey level.
Mohammad Sohel Akhtar(MSA)