I have seen many entry level civil engineers desperately trying to master a few software and then demanding expertise and expecting a consultant to directly place them. This rarely works unless the consultant is desperately looking for someone to slog and finish off some modelling works. Most of the times, a seasoned structural consultant will be looking at an overall skillset and an engineer who troubles him less for decisions and day to day calculations.
Today am discussing a topic that should be of interest to every entry level civil engineer who is aspiring to be a structural engineer. This is something you should be practicing from the beginning of your career. The Load calculations that you need to be precise. This has much higher significance than you think. I will break this down to simple steps for your understanding.
Every client or an Architect looks at a civil & structural engineers’ skill with a few yard sticks.
The three important skills that general people appreciate of a structural engineer are
1. Economy of the structural design
2. The Structural engineer’s communication – The drawing
3. The structural engineer’s ability to handle revisions and site mistakes and issues.
I will be focusing on the economy of the design in this structural blog. As I already mentioned, the accuracy of the load calculation has an important role in deciding the economy.
Dead Loads
The dead load in a building structure consists of self-weight of members, self-weight finishes like screed, tiles, weight of walls and so on. Let me discuss, how we generally calculate these loads and how a wrong consideration can compound the quantity and thus affect the economy of design.
Self-weight of the members
Generally, slabs contribute most to the dead weight of structural members. Consider that during the preliminary sizing we got a thickness requirement of 125mm and we rounded off to 150 mm and decided to go ahead with that thickness in the final design. Many times, young engineers think that more thickness is safer. This is not true all the times. I will be explaining this in the next sections any ways. So, this 25mm thickness difference contribute to an increase in DL
Finishes, Screed, Tiles and false ceilings
Many times, during the entire structural design cycle, the client and the architect of the project might not have frozen the kind of tile, screed and even false ceiling requirement. Structural engineer can make his judgement based on the projects and situations can judge some of it. However, there can be drastic differences at times and result in over design. Of course, many times there can be under design too and hence it is important to freeze these requirements before structural design is frozen and constructed. Since this blog is on un economy, let us focus on extra load consideration. Let us consider that the screed + tile considered was 100mm and finally what was build was only 50mm. Here there is a difference of 50mm of thickness. This is not so small as you think.
Many times, wall thickness is also changed or conservatively considered.100mm walls are considered 200mm thinking it may be changed later by the client and the beam need to take it with that revision. But it is compounding a problem to many folds. You are considering 200mm walls on many beams when the probability of that change is only on one beam or a few beams.
Also note that, you can even factor in the large openings in walls to reduce the load.
Another aspect is about the sunken requirement. Many times, Architects would not have decided if a toilet need to be sunken or not. Also, things like a landscaped area. If these things like the kind of trees planted etc. are frozen earlier, the design can be more economic.
Not just the Dead load increase and the corresponding size and steel increase, it also increases the seismic demand of the building. I will be explaining this earth quake demand in the next section. If you expect the wall thickness to change, the right thing to do is to design the beams and the structure for gravity loads, considering 200mm thickness wall. When you design for seismic, considering larger wall thickness than currently needed is extremely unrealistic and compounding. See the video for better understanding.
Live Load
The live load considerations in IS 875 Part 2 is conservative for a global analysis and hence IS 1893 allows you to reduce Live load to realistic value. See my blog on this here. The code also allows Live load reduction in vertical members due to this conservative approach on the local member load considerations. In addition to this, at the start of the load calculations, it is important to identify the functional use of the building correctly. The client may or may not be sensitive about the structural implication of the change in usage. Hence it becomes the responsibility of the structural engineer to identify this by asking the correct questions. Architect also might not appreciate the structural implication as much as a civil engineer does. The client might tell you that a building is a library. However, it might be just a reading room. So, you need to get this right before just deciding the load from the Live load code IS 875 Part 2.
Anyone who has confusion in these aspects can join my groups here and you can ask your doubts directly and get structural clarifications. I have very engaging civil engineering forums where civil engineering professionals discusses various topics on civil engineering jobs, civil engineering internships and structural challenges.
The compounding effects of wrong loads
As all of you know, additional Dead load or Live load is going to increase the sizes of footings, columns, beams and even slabs. Of course, it depends on how much the load increase is. However, there is going to be an increase. And even the steel will increase. I will explain the compounding effect with an example. Say you took the slab thickness from 120 to 150 and assumed the screed to be 100 when it needed to be 50. This by itself is around 1.75kN/m2. Assume that in a beam, the real moment should have been 50kN-m and now it is 57kN-m. Now assume that many similar beams have a moment of 62kN-m. You are likely to group it and then design for 65kN-m. See how easily 50 became 62 and then 65. In a real project situation, you wont even realize this effect as you don’t have the value 50 with you since you considered more in step 1 itself. Assume that for this 65kN-m moment and what ever depth, you need reinforcement of say 270mm2 in a 200mm wide beam. You might be giving 2No T16 mostly as 2- T12 will be only 226mm2. However, a real load consideration would have given you a moment of 50kN-m and probably this would need only 2 number of 12 diameter rebars. This is what makes your structure uneconomic. There are round offs that we adopt at each stage from load calculation, analysis,analysis result interretation,structural design, structuraldetailing and probably even during construction. Therefore, right load calculation is very important. Learn to be precise early in your structural design career. Same way all these play a big role in structural analysis optimisation and structural design optimisation. Even the structural detailing optimisation will be more effective when the load calculations are more accurate.
Exponential Seismic forces and seismic demand
What is earth quake to a structural engineer? Ground movement is earth quake.
What is earth quake force? It is inertia. The Dead and Live load is being attracted to the center of the earth and when there is a lateral movement, the building tends to remain in place and that is inertia. And this inertia is proportional to the weight. In analysis what we consider as loads decides this inertia in calculations and correspondingly the seismic forces and the design steel and sizes depends on the gravity load, we consider. Not only steel and sizes of members, the entire scheme might be affected if the gravity load considered is not correct. You may end up providing more shear walls than needed too if the load is too conservative. In a soft ware like ETABS, it is defined as a mass source and this source of your mass is nothing but the gravity load that you have defined. Please see the structural video where I have explained this effect.
Summary
Load take down in a building project is very important. Its not just right to be always conservative and assume things. There is a huge difference in assumption and consideration. Consideration has a logic behind it and assumption is wild and can have compounding effects. Its important to engage with all stake holders of the project like an architect and the client and be realistic on the load calculation.
While coordinating the Architects, you can even consider making layouts that shows the extent of walls and the amount of fill and screed you consider. Architect may not have a feel of the load when you state it in kN/m2. It may be a good idea to communicate this as a drawing and then co-ordinate and refine the load as much as possible to make a building economic. If you adopt all the fine tuning in load calculations, many times we can adhere to all structural code provisions with out compromising. Let us make building design industry more correct, productive and economical.