Why Dead Load Calculations has to be precise?

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