Load Calculation for Design of RCC Frame Buildings

Load Calculation for Design of RCC Frame Buildings

 

Loads In Buildings:

 

In Building construction during the design of RCC/Steel structures, the frame systems are designed to withstand the loads which the building will serve for its design life. In this post, I will illustrate the basics of the design loads considered while designing structures & the basic mathematics involved in their calculation.

 

Types of building loads:

In a typical residential building as discussed below the following given loads are present.

 

1.      Dead Load –

 

Under this category, all the load of the dead or non-moving/stationary/fixed elements of the building is considered such as the dead weight of walls, the weight of roof/floor finishes, self-weight of structural members such as beams, columns, Footings & RCC slabs, etc. The calculation of live loads must be done by taking the properties of the material & the dimensions of the element considered.

 

2.      Live Load –

 

In the live load category, the moving loads such as due to human use or furniture or any other mechanical equipment are considered. For all general categories live loads are defined in Indian Code IS 875 Part-3 rest if any shall be calculated as per the requirement.

 

3.      Dynamic Load –

 

Apart from dead & live loads, a building structure is also subjected to the application of dynamic load. Dynamic loads are the loads in which the value & nature of the application of force is not constant throughout & is subjected to changes as per the conditions governing the forces. Forces due to wind, earthquake, traffic, or heavy machinery (such as blowers or pumps) fall under this category.

 

Dead Loads:

 

1.              Self-weight-

 

It is the weight of the frame or the individual weight of the structural elements such as Beams, Columns & Slabs. In computer programs such as STAAD Pro inbuilt capability is there which allows the program to calculate the self-weight of the elements automatically in accordance with the shape, size & member dimensions.

 

Example:

Below given are examples of manual calculations of the self-weight of the structural elements.

a.              Beam:

 

Let it be of any shape Rectangular/square/tee/trapezoid anything for calculation of weight uses the given formula.

 

Weight of member = cross-section area of member x length of member x RCC density

 

For a 0.3 m wide 0.45 m deep beam section of 5 m clear length & material density = 25 kn/m³ (for RCC), the weight will be.

 

Weight of beam = (0.3 x 0.45) x 5 x 25 = 16.875 Kn

 

The above-calculated weight of 16.875 Kn is the total weight which can be further converted into Uniformly Distributed Load (UDL) by dividing the total weight by member length.

UDL = 16.875/5 = 3.375 Kn/m

b.              Column: 

 

Same as explained above the self-weight of a column can be calculated only if the terminology of member length will be changed to member height & the weight of the column will be calculated as point load only its conversion in UDL is not required.

 

c.               Slab:

 

RCC slabs, the weight of roof slabs are applied as uniform pressure in Kn/m². For analysis purposes, it is we consider a 1 m x 1 m square section & calculate the volume of the RCC & then the same is multiplied by the density for the derivation of pressure in kn/m².

 

Weight of slab = (1 x 1 x slab thickness) x RCC density

 The above formula (1 x 1) doesn’t affect the calculation thus it can be further simplified

 Weight of slab = slab thickness x RCC density

For a 0.15 m thick slab, the calculation will be as follows.

Weight of slab = 0.15 x 25 = 3.75 Kn/m²

 In the above calculation of RCC slab weight, further additional load due to floor finishes are to be included generally for stone/cement floorings 0.75 kn/m² to 1.5 kn/m² is considered.

 Distribution of slab load on supporting beams:

Depending upon the arrangement of beams (square or rectangular) triangular or trapezoidal shape distribution is carried out. For example (refer to fig-1 below) in the case of a rectangular slab of 6 m x 4 m the longer side beams spanning between A-B & D-C will carry a load of corresponding trapezoidal portion whereas the shorter span beams spanning between A-D & B-C will support the weight of roof slab coming from the corresponding triangular region.

 Load on 6 m span = area of trapezoid x thickness of slab x density

 Load on 6 m span = 8 x 0.15 x 25 = 30 Kn = 30 / member length = 30/6 = 5 Kn/m

 The above-calculated load of 30 Kn can be further converted in UDL of 5 kn/m by dividing it by member length as shown above.

 Load on 4 m span = area of triangle x thickness of slab x density

 Load on 4 m span = 4 x 0.15 x 25 = 15 Kn = 15 / member length = 15/4 = 3.75 Kn/m

Fig-1 Load distribution of slabs on supporting beam members/load-bearing walls

 

The above discussed was the case of rectangular slab whereas in the case of the square slab the slab plan will be divided into 4 nos. of equal triangles & the load for the same will be transferred to the corresponding beam members.

d.              Walls: 

Generally, in building construction brick walls of single brick (0.115 m thick) & double brick (0.230 m thick) is used, and a load of brick walls is calculated as follows.

     Weight of wall (In KN/m) = Height of wall x thickness of wall x density of brick/stone masonry

    For a 0.23 m thick wall of 3.2 m clear height the load will be.

    Weight of wall (In KN/m) = 3.2 x 0.230 x 22 = 16.192 Kn/m

The above-calculated load of 16.192 Kn/m will be applicable in the form of UDL on the beam members supporting the brick wall, in the above calculation brick wall density considered is 22 Kn/m³.

Live Loads:

Live loads or imposed loads as per building type & occupancy classification shall be provided as per Indian Code IS 875 Part-2 in table-1.

Dynamic Loads:

Mainly, the RCC buildings are subjected to two dynamic forces.

1. Wind Force: To be calculated in line with the provisions of IS 875 (Part-3).
2. Seismic/Earthquake Force: To be calculated in line with the provisions of IS 1893: 2016.

methodology for calculation of wind forces & earthquake forces will be discussed in a separate post, keep following.

Hope the above-given explanation helps in understanding the basics for the determination of dead & live loads as per the Indian Code.

 

 

Er. SP. ASWINPALANIAPPAN., M.E., (Strut/.,)., (Ph.D.,)

Structural Engineer

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