SEVERE CONSEQUENCE OF PROPOSAL OF POINT LOAD ON SLAB OF EXISTING BUILDING

“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 -24.3.2.1 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.

Thanks & regards!

MSA

                 

EXAMPLE ON DEFLECTION CALCULATION FOR CANTILEVER BEAM SUBJECTED TO UDL & POINT LOAD,SERIES-1

“Feel the structure” MSA

Dear friends!

       In this series-1, i have come up with very simple example with a cantilever beam with point load and distributed load and calculated maximum deflection at tip of beam with different methods. Well known method that i have used to calculate deflection  are:

                     1.   Unit load method /Virtual Method

                     2.   Moment Area Method

                     3.   Strain energy Method(Based on Castigliano’s Method)

                     4.   Conjugate beam method

         In this attached sheet, i have solved very simple two examples with above four method. Whatever method is best suited to you, you can adopt it and all of these will give same result. I have give name, Series-1 because i will upload many examples for calculation for deflection of beams  which will come in another Series like Series-2, Series-3 etc.

Download pdf from download pdf tab

DOWNLOAD PDF

          In order to create interest to readers, i have come with very simple examples. Definitely you are going to learn few new things.Please read the whole attachment.

           Structural engineers are more concerned about to check deflection for cantilever beams and lateral drifting of whole structure due to wind or earthquake. All buildings are vertical cantilevers. As a structural engineers, our main assignment is to check maximum drifting at the top floor. This calculation could be carried out with simple calculation. By taking shear walls as a single entity and attracting definite portion of earthquake load, having flexural rigidity  “EI”, and height “h” , we can calculate deflection at the top floor to check whether deflection at the top floor is under permissible limit or not.

         We will go in deep to earthquake and wind loading topic in my later blogs.

Here, please download attached examples on deflection calculation for cantilever beams with various method explained above.

DOWNLOAD PDF

Thanks & regards!

MSA