Sir I have a doubt about multi storey building. When they cast one slab after another , and load of upper slab is supported on the slab below it. So I am asking that , is there any provision for lower slab to carry upper slab with load of shuttering & formwork. Means how they cast upper slabs supported upon lower slab , even though lower slab is yet to achieve it's full strength.
Please explain.
Also please explain ' Soft Storey ' .
Thanking you..
To be completely correct,you will have to calculate the load from the formwork coming on to the lower slab.Please consider the points below.
1) In some cases,site engineers keep the lower level slab supported by props when the upper level slab is concreted.This will ensure that there are no much loads in the slab when upper level is being constructed.
2) In more important buildings and taller buildings,even construction stage loading needs to be computed and analysed for that. This is slightly different and more significant than the exact point you are asking. It involves not just the gravity analysis but also the sismic behavior and wind loads when buildings are errected in stages.
3) In smaller and normal buildings,generally only the gravity load considerations are applied. In such cases, the loads are point loads from the props.However there will be large number of props and hence this will be not of a huge magnitude as it is more of uniformly distributed because of the number of props.
You need to remember that,during construction, the finishes on slab is not yet applied in the lower level. Screed,tiles,slab soffit plastering etc is not yet applied. Live load is not fully acting. In fact, when you design a slab,the Live load considered is huge in magnitude if you follow IS 875 Part 2. You can read about the margin in LL in one of my previous blog here. Though the blog is talking about the Live load reduction, under the title ''Why does the code allow this Live Load reduction'', you can read the point am talking about.
Generally because of this reason, loads from the upper level may not become critical.
However as I said,if you want to be completely correct, you can decide the formwork scheme and then do a quick analysis and arrive at the point loads.Rarely a scope for consultants in India.
Your question on soft storey is answered below.
Its a situation when a buildings lower level is less stiff (weak) and one or more upper level is very stiff. An example is a building with stilt or a building using mivan technology in construction with a stilt floor. The lower level you just have columns and upper levels all have stiff walls.
Think what happens if humans had very weak legs and a very strong upper body. You will not be able to walk or you will fall down at the slightest of lateral push! This is the effect on buildings.
You can refer IS 1893 : 2016 which has provisions for its analysis and design considerations.