Design principles of Reinforced concrete | Civil engineering

Design Principles of reinforced concrete.

Introduction :

Any member should design the following two requirements.

• Strength ( Maximum load carrying capacity )
• Serviceability should not undergone excessive deflections.

Strength (safety) :

Note: Collapse may lead to various reasons such as exceeding the load carrying capacity, overturning etc.

Serviceability :

Serviceability satisfactory performance of structure under service load, without discomfort o user due to excessive deflections, cracking, vibrations.

Analysis :

Analysis: Calculation of Axial forces, shear force, Bending moment and torsion.

Design :

The main aim of Design is to decide the size of members ( Beam, column, slab) and amount of reinforcement is required so that the structure will perform during its life period with minimum cost.

Requirement of structure :

• Sustain all types of loads.
• Sustain deformation during & after construction.
• Should have adequate durability.
• Should resist secondary stresses ( Temp, creep, etc).

Basic methods of Design :

• Working stress method (IS 456 : 1964).
• Ultimate load method.
• Limit state method (IS 456 : 2000).

Working stress method :

• First method accepted by all national codes.
• Based on elastic theory, assumes that the structural material behaves in a linear elastic manner (Stress is linearly proportional strain).
• Strength criteria material only considered. .
• Factor of safety taken accounts for any uncertainties involved in estimation loads and material.

\[{F_{os}} = \frac{{{M_{strength}}}}{{{\sigma _{allowable}}}}\]

Material	Factor of safety
Concrete	3
Steel	1.5

Ultimate load method :

• Improvement over the WSM.
• Ultimate load method.
• Loads are designed for ultimate loads only.
• Ultimate or Design loads =loads factor x Service loads.
• Safety in the design of structure taken care by load factor. Non-linear stress-strain Curve accounted for stress induced in structure at the verge of failure.
• It gives more economical sections, i.e. slender sections of beams & columns compared to WSM.
• Slender sections sometimes result in excessive deflection of crack width under service loads.

Limit state method :

It is the state of about to collapse, beyond which the structure is of no practical use, i.e either the structure collapses / becomes unserviceable.

Limit state of collapse :

It is state shall be able to withstand design loads against collapse collapse may arise from i) Rupture of one/more critical sections, ii) loss of static equilibrium due to overturning/ sliding. It deals with

• Compression.
• Flexure.
• Torsion.
• Shear.

Limit state of serviceability :

It is the state shall be able to perform under service load without discomfort to the occupants.

• Deflection.
• Cracking.
• Vibrations.

Loads considered in design of structure :

For the design of building following types of loads are considered as shown below.

• Dead load.
• Live load.
• Wind load.
• Snow load.
• Earthquake load

How to calculate dead loads for Design :

To calculate dead load we need to consider ( IS 875(Part 1):1987 ) code .

• For beams dead loads are calculated as : B x D x 25.
• For slabs dead loads are calculated as : D x 25.

• Where,B = Breadth of beam in mm.
• D = Depth of beam in mm.
• 25 = Density of concrete kN/m².

How to calculate live loads for Design :

To calculate live load we need to consider ( IS 875(Part 2):1987 ) code. The examples are shown below

• Residential buildings: 2kN/m².
• Hospital & hostels: 3kN/m² .
• Class rooms, Restaurants: 4kN/m² .

How to calculate wind loads for Design :

To calculate wind load we need to consider ( IS 875(Part 3):1987 ) code. The calculations are shown below

• Horizontal loads caused by wind. Design of wind velocity } V_Z = k₁ . k₂ . k₃ . V_y.
• Where, k₁ = Risk co-efficient.
• k₂= Based on terrain, height.
• k₃ =Torography factor.
• Design wind force, P_Z = 0.6 V_Z

How to calculate snow loads on roof :

To calculate snow load we need to consider ( IS 875(Part 4):1987 ) code. The minimum design snow load on a roof area or any other area above ground which is subjected to snow accumulation is obtained by multiplying the snow load on ground, ssub>0, by the shape coefficient μ, as applicable to the particular roof area considered.

• S=μ x S₀.
• where, s = Design snow load in Pa on plan areaof roof.
• p = shape coefficient.
• so = ground snow load in Pa.
• ( 1 Pa = lN/m². )

Special loads and combination :

To consider special loads and combination we need to consider ( IS 875(Part 5):1987 ) code. A load combination results when more than one load type acts on the structure.

Building codes usually specify a variety of load combinations together with load factors (weightings) for each load type in order to ensure the safety of the structure under different maximum expected loading scenarios.

How to calculate earthquake load for design :

To design earthquake resistant building we need to consider ( IS 1893 :2016 ) code. Ground vibrations during earthquakes cause forces and deformations in structures. Structures need to be designed to withstand such forces and deformations.

Seismic codes help to improve the behaviour of structures so that they may withstand the earthquake effects without significant loss of life and property.