A guide to structural load calculations – 1: Dead Load

How a structure is designed?

• Loads are assessed: basic loads are defined as which loads will be coming on the structure.
• Structure is analyzed upon applying these loads and combination of it.
• Structure is designed for the results of analysis. Structure is designed to withstand the stresses caused by these forces and its combinations. Structure is designed for the shear force and bending moments found through analysis done based on these load cases and combination.

In this article, let us learn the first part of structure design, how to assess the load. How to know as to which load will be coming on the structure in its life span.

Load: every object in universe have some mass, this mass when multiplied with gravity exerts some weight. This weights are creating pressure on the structure they are resting on. These are called loads.

Types of loads:

• Dead Load: Self weight of the structure. It can be find by density x volume of the member. Every country has defined densities of different materials in specific codes. In India, it is defined in IS 875 Part-1. Densities of some common material for structure design are
  • Concrete: Plain concrete with sand and gravel or crushed natural stone aggregate has density between 22 to 23.5 KN/m³. Reinforced cement concrete with 1 percent reinforcement has density between 22.75 to 24.20 KN/m³. Likewise, you can find density of different concrete type from IS 875-1.
  • Brick Masonry: Brick masonry with common burnt clay brick has density of 18.85 KN/m³, Brick masonry with engineering bricks has density of 23.55 KN/m³. As you can find in the code, partition brick masonry of 100 mm has loading of 1.91 KN/m².
  • Mortar: Cement mortar has density of 20.40 KN/m³.
  • Floor finish: Clay floor tiles have loading of 0.1 to 0.2 KN/m². Remember, floor finish load is given as KN per square meter and not as density. Moreover, this load excludes screeding(layer of binding material below tiles.) Terrezo has loading of 0.23 KN/m².
  • Roofing: Mortar Screeding of 100mm has loading of 0.21 KN/m².
• Super dead load: Loading except the self-weight of structure is said to be super dead load. Based on the density mentioned above Super dead load like, wall load, floor finish, water proofing on terrace, water tank load comes is super dead load category. Let us see how these loads are calculated.
  • Wall load: Let us calculate the loading of 230mm thick brick masonry.
    • The density of brick is 18.85 KN/m³ and the density of mortar is 20.4 kKN/m³.
    • 230mm thick masonry wall + 20 mm plaster (Total assumed plaster on wall)
    • So, loading shall be 0.23 x 18.85 + 0.02 x 20.40 = 4.75 KN/m per m height. Now when you are calculating the UDL to be applied on the beam, you have to multiply this loading with the height of the masonry wall.
    • The other way is, IS 875-1 has given loading of 100mm partition wall as 1.91 KN/m² (KN/m per m height). We add the plaster in it. So, the loading shall be 1.91 + 0.02 x 20.40 = 2.32 KN/m per m height. Now, if your wall has 230mm thickness then your load will be 2.3 x 2.32 = 5.33 KN/m per m height.
  • The conservative value to be taken for loading.
• Floor finish:
  • It is calculated based on the layers above the slab viz. Screed, tile, water proofing etc.
  • Generally, the floor finish is defined in design basis.
  • Let us assume floor finish in normal floor is 1 KN/m² and on the roof as 2.5 KN/m². This has to be applied as floor load on the floor slabs.
• Water tank load:
  • Self-weight of the tank + Water load when the tank is full.
  • Let us say 1000 ltr tank. So the load of the tank shall be 1000 kg = 10 KN + Self-weight of the tank.
  • This load has to be applied on the slab area, hence you have to distribute it on the slab panel by dividing this 10+ load to slab panel area. The will give you xx KN/m² load which you have to apply as floor load to the panel.
• Live load:
  • As per IS 875 pert 2. Live load is given for different type of occupancy.
• Seismic Load: As per IS 1893, we have to find the base shear due to structural mass and contribute the horizontal forces to the respective joints. The method of defining seismic load and finding structural mass will be covered in articles that will follow.
• Wind Load: Pressure is to be calculated as per IS 875 Part 3. The forces are to be calculated based on the area resisting that pressure. We will cover wind load in upcoming articles.
• Snow Load: It is applied where snow fall is common. Snow accumulation load is to be applied as per IS 875 part 4.
• Temperature Load: It is to be applies when temperature difference is too high. We will cover this in upcoming articles.
• Vehicular load: It comes in to picture when vehicular movement is deemed to be happen on the structure. Some of the examples are podium, culvert etc.

There are many other load comes, one has to assess all the load and make sure that the structure will be safe for all the possible loads and combinations of them.