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My understanding and explanation is as follows:
Mass of the specified live load:
In IS: 875, the live load is specified as uniformly distributed load over the slab area. The real live load is not uniformly distributed but small loads are distributed as concentrated loads on finite area. They are converted into uniform loads by considering EUDL on a simply supported span. Thus total actual load is less.
As an example, if I consider 10 kN (mass = 10*1000/9.81=1019 kg) central point load has a maximum moment of (WL/4) =10*4/4= 10 kNm. Equivalent UDL for moment = wL2/8=10, i.e., w = 5 kN/m, i.e. W = 5*4=20 kN. For moment calculation, I consider 20 kN distributed load having its mass = 1019 kg.
This is an example, how we consider UDL on floors. Nevertheless, the specified loads are not double, but 25-40 percent more, since loads are not at center.
𝑊 Whatever the UDL is specified by the code, its mass is 30-40 percent less than w/g .
The old or present code do not say to reduce the live load by 75% or 50% as the case may be. It says “use appropriate live loads “.
Thus while calculating the seismic mass, we are not reducing live loads, but by using the appropriate factor, to calculate the real mass of the live load acting, since earthquake knows mass and not the equivalent UDL.
Reduction of live load in load combinations:
In old code, they considered to take appropriate live loads for combo also. This is now withdrawn from code.
I was not comfortable with this. Our arguments were:
EQ does not give any sign. When it comes, all live load is there. How can we reduce?
The eminent EQ engineers explained like this:
When EQ is from left to right, the free live loads like furniture, sleeping coat etc. will move in opposite direction resulting in reduction in EQ effects. Therefore, you may reduce the LL in combo also.
Since written in code- we had no other alternative.
Now the revised code has removed that note and we have to consider full live load in combo
One Reply to “Appropriate Live Load”
Thanks for your blog, nice to read. Do not stop.