Updated: Feb 25, 2021
The name live load itself gives an indication that the load is not same all the time.It do vary from time to time.These loads vary from occupancy.The load in a house and that in a ware house will be different. Most knows this and in India , these loads are available in IS-875 Part 2.
Most considers IS-875 Part 2 as a reference for the live load values and do not read through the code.However there are more important clauses in IS-875 Part 2 that needs attention. Some of the most important clauses pertaining to Live load and live load reduction is discussed in this blog.
1. The live load indicated needs validation and is discretionary
The client may say that a building is a shop and eventually may rent it out to a storeage. One need to engage well with the client and communicate these load considerations and then decide the loading. Reading rooms can become full fledged libraries, storeage can become a ware house and so on.Again the code might give a particular load value for a warehouse and if that warehouse has a machinery with point legs and hence point loads, this loading is special and you will have to estimate this. All loads mentioned in the code cannot be blindly taken and used.Some times you can take a lesser load than written in code if you are sure that the actual functional useage is different.However this needs careful validation and justification
2. Disproportionate collapse control
Disproportionate collapse control is an important risk mitigation principle and is a must now. There are different possibilities of accidents and it is important to ensure that a smaller accident do not create a larger failure. Can this collapse control be attended in structural design? Yes is the answer. It is all about bridging and finding an alternative load path.
If any one is interested to know more about disproportionate collapse control, please read this blog till the end and then go to the links that I have provided there.
The point is, many feel that disproportionate collapse control is not needed in Indian projects as it is not mentioned in any code.However there is a mention about this in IS 875 Part 2
The clause 0.3.3 says that ''The buildings and structural systems shall provide such structural integrity that the hazards associated with progressive collapse such as that due to local failure caused by severe overloads or abnormal loads not specifically covered therein are reduced to a level consistant with good engineering practice''
Simplifying this means that the engineer should ensure that the building do not experience disproportionate collapse. It says the collapse chances are reduced to a very low level that a normal good engineering practice should take care of this.This further means that the scheme and structural system of the building should be good enough to ensure disproportionate collapse do not occur. So emphasis is on the scheme. Contact us at firstname.lastname@example.org if you want an ebook on disproportionate collapse control.
3. Live Load reduction & reduction of LL in mass source for seismic forces.
There are 2 kind of Live load reductions.One that is mentioned in IS-875-Part 2 and another as per seismic code 1893. After reading this blog,the entire logic of Live load reduction as per IS-875-Part 2 you can read in another blog by clicking here.
The code mandates that the Live load reduction as per IS-875 Part 2 and as per 1893 shall not be adopted at the same time.Any one can only taken at a time. It is suggested to take the mass source reduction in to considerations as that will be making the structure more economic.
One more point to note is that the 1893 -2002 allowed to reduce Live load in combinations also
ie, in 1.2DL+1.2LL+1.2Eq , the live load reduction was allowed to be applied in LL part aswell as in the Eq part. However the latest seismic code,1893-2016 has discarded this and allows live load reduction only in the seismic weight calculations.
4. Live Load Reduction if the Live load is more than 5kN/m2
Some engineers are confused if live load reduction can be taken for a building if the load is more than 5kN/m2. This is due to clause 3.2.1 saying that ''Except as provided for in 220.127.116.11, live load reduction can be applied.''
IS 875-Part 2 clause 18.104.22.168 further says, ''No reduction shall be made for any plant or machinery which is specifically allowed or in buildings for storage purposes, warehouses and garages.However, for other buildings where the floor is designed for an imposed floor load of 5 kN/m2 or more, the reductions shown in 3.2.1 may be taken, provided that the loading assumed is not less than it would have been if all the floors had been designed for 5.0 kN/m2 with no reductions.''
The sentences are complicated. However it can be summarised this in 2 points
a) What ever be the load less or more than 5kN/m2, if the functional useage is a plant or machinery then there shall be no Live load reduction as per clause 3.2.1
b) If the live load is more than 5kN/m2 and if the useage is any thing than plant and machinery, live load reduction can be taken. However the total vertical load considered on columns or footings shall not be less than that you get by considering 5kN/m2 in all floors.This means a minimum of 5kN/m2 should be considered in such cases.
Though the code IS-875 Part 2 is often refered only for load values,there are far more important technical points discussed in this code.Same is the case for any code and all clauses and even notes under clauses should be read with care and required attention.
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