Updated: Feb 25, 2021
The Live load in a structure is assessed based on IS-875 Part 2
The code in Table 1 specifies the occupancy classification and the corresponding udl and concentrated loads that need to be considered.
There are various sub clauses,notes and even discretion that an engineer need to take based on the building occupancy and the way of usage.
However generally Table 1 can be taken as a guideline for normal buildings.
Clause 3.2 talks about the Live load reduction applicable to column, foundation and all vertical load carrying members.
It is very much important to read this table correctly. This means that if the column under consideration has only 1 slab above then no live load reduction is applicable.
If there are 2 floors/slabs above then 10% LL can be reduced in the column in the lower level.
If there are more than 10 floors above then 50% of Live load can be reduced.
This aspect can be more clearly understood if you look at Last page (FIG 1) of the same code.
Why does the code allow this Live Load reduction?
Many of us during our studies do not think about the reasons.We are more interested to solve the question not the logic!
Let us take a practical example and look at the logic.
Consider a residential building common area room of size say 3mx3m =9m2. The code stipulates a load of 2kN/m2. Practically thinking,in a space of 3mx3m, even if you consider 10 people of 75kg weight, total load is around 750kg (7.5kN)
7.5 divided by 9m2(area of room) = 0.84kN/m2 Even after considering some furniture and some more people, 2kN/m2 looks a higher load.
However code is being conservative. The building may have a life span of 50 years and any one day the room can be full of people shoulder to shoulder.This plus some impact created by dancing or jumping can result in 2kN/m2.
However the same kind of load acting on all floors of the building and in all rooms of the building is exceptionally rare. So more the number of floors a column is carrying, less the probability that it is fully loaded. The probability of a 1 floor building is fully loaded is very high and hence no reduction for roof level slab supporting column or for 1 floor building.
Please note that same logic is not applicable to beam and slab since these are local members carrying 1 level load and probability of full load coming on it is high.
There is another kind of Live load reduction mentioned in IS-1893-2016 in the mass source. Both reduction shall NOT be applied simultaneously.
Even when you analyse and design a building using etabs or staad pro, one need to rightly do these reductions. Therefore it is important for you to decide where you do the etabs training.
Will discuss this topic in another blog on another day.