Live Load is one among the many types of loads that acts on a building structure. Most entry level civil and structural engineers take live load lightly as a minor load. Most refer the code IS 875 Part 2 and copy the load values given in it and adopts the live load value for applying in ETABS model or what ever structural design software they use for analysis and design of the building. Most engineers apply a uniformly distributed live load all over the floor slab in all types of buildings.
However, Live loading has a lot of additional technical points to be mindful about. (I have written about some of those points in other blogs and have summarized it towards the end of this blog. Please ensure you read those blogs too after reading this blog on pattern loading.) One among the important point is pattern live loading which only a few students and structural engineers observe and appreciates. Some of the software like ETABS can partially incorporate pattern loading in the analysis. However, we need to input this as a requirement in order ETABS does this. One need to be careful to use this in the analysis software. Unless you are sure how to use it, you should not be using it. In the video I have shown the location where pattern load is possible while using ETABS for design of frames.
Also, as you know, experience makes a structural engineer perfect and many rarely used or occurring code provisions in civil engineering are encountered only during real projects. This is the reason; it is important for all structural design engineers to have practicing experience.
In the past, I also have got questions from students asking if Load pattern in ETABS is what is meant by pattern loading. No, they are not the same. It’s completely different. While ETABS has some ability to consider pattern loading, load pattern in ETABS is some thing related to Load case. An easy way to separate different types of loads. This is only to separate load and see or visualize them separately. Pattern loading should not be misunderstood for load pattern.
IS code provision for Pattern Load
In fact, I decided to write this blog when a structural engineer asked me this question on pattern live load. You can refer to his original question here and also comment your answers or questions there.
IS 456 Clause 22.4.1 talks about pattern load.
The clause says to consider two different combination or considerations for Live load application on continuous slabs and beams.
IS 456 Clause 22.4.1 has two parts 22.4.1 a and 22.4.1 b
In this 22.4.1 a has two divisions 22.4.1 a 1 and 22.4.1 a 2
IS 456 22.4.1 a 1 says that you need to consider full dead load and full Live load only on 2 adjacent spans. Other spans need not have full live load
22.4.1 a 2 says that only alternate spans need to have full live load.
These clause provisions are provided to ensure all the worst conditions of gravity load is captured in the structural analysis and design of slabs and beams of the building.
There are many points to be noted. One is that the live load that we consider is way more than actual probable live load that may act. IS code specifying a larger value considering that there is a probability of a fully load condition in a room or some parts of a room when people are jam packed and also impart an impact load. However, this is highly conservative. Many think that more load is critical and safer. However, the variation of load in two adjacent bays will have a different effect. I have explained and demonstrated this in the video. Same is the case when only alternate bays are loaded.
When to consider Pattern Loading?
Pattern loading may be critical for buildings like that used for storage and as ware houses where dead load and live loads are extremely high. Pattern loading will be very critical in such buildings and not while calculating Live load for residential loadings. In fact, the clause 22.4.1 b asks to consider full live load if the live load is not more than 3/4th of the Dead load of the floor. This means pattern load needs to be applied only in buildings where the live load is really high as that in a warehouse. I have explained all these points too in the structural video I made specially for this topic with live load examples.
Other points on Live Load
In addition to pattern loading, some of the other important Live load related aspects are listed below.
Live Load reduction for design of global structural members like columns
It is quite important to know why the live load reduction is employed only for global members like columns and not while designing local members like beam and slab. I have mentioned this in the blog here.
Live load reduction for seismic mass
I have a separate blog for this here. Its all about ensuring that an appropriate and required live load is only considered as lateral mass. The logic of this live load reduction is also same as that of live load reduction for vertical members. The live load as specified by IS 875 Part 2 is slightly more than probable load that can occur during an earth quake. Also, the chances of people being in the terrace is less during an earth quake. For these reasons, the live load is reduced as per IS 1893 2016 clauses.
Disproportionate collapse control
This is not exactly related to live load alone. Its only that this particular clause on collapse control is mentioned in the code IS 875 Part 2 which is meant for Live load considerations. In the same above blog on live load reduction I have mentioned about this point too. Probably disproportionate collapse is some thing related to lives of occupants and hence it is covered in this particular code.
Summary
While live load calculation appears to be a simple topic, there are important points like live load reduction, seismic mass source reduction, pattern loading etc. which can change your design requirement. It is required to attend all this correctly and accurately to ensure the building is safe and stable. The strength as well as the deflection can become critical if these structural loading cases are not attended to. We have to be careful and should note that the probability of gross over load is high in storage buildings and ware houses. Missing out many smaller points like this can add up and create criticality and even failure of buildings.
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