What is Working Stress Method?

 WSM (Working Stress Method ) is a traditional method. WSM was introduced in 1900's. This is based on linear elastic theory.This method was not only for structural steel but also for reinforced concrete and timber design.

Assumption:-

a) Structural member behave in linear elastic manner.

b) Permissible (Allowable) stress are kept well below the material strength.

c) In this design method, members are designed to never beyond their elastic range.

d) The allowable stress is determined by suitably restricting the materials yield or ultimate strength.                                

Allowable stress=	yield stress or ultimate stress
	factor of safety

Yield Stress - Ductile materials (i.e, Steel)

Ultimate stress - Brittle materials (i.e, Concrete)

Factor of safety (FOS):- As per IS (456-2000)

Concrete = 3

Steel = 1.78

** When we apply loads, stress induced in materials and we can calculate them using the principles of strength of materials (SOM).

**Reinforced cement concrete is a composite material.It has two different material concrete and steel.

Draw back of WSM :-

a) The actual stress distribution of concrete is not described. It is assume to be elastic but in reality, concrete is not elastic.

b) Same factor of safety is used for different type of loads.

c) The failure mode cannot be observed.

d) The warning before failure cannot be studied precisely.

e) We cannot account shrinkage and creep effects by using Working Stress Method.

Note:- As per IS-456 clause 18.22 , working stress method may be used only if it is not possible to adopt the limit state design.

 

What is design philosophy?

                                                                 Philosophy

                                                   a) Few fundamental assumption

                                                  b) Procedure (Reflect the way of thinking)

 The design philosophy of a structure may be regarded as the process of selecting proper materials, proportioned element of the structure according to the art, engineering science & technology.

A design philosophy is a set of assumptions and procedures which are used to meet the conditions of serviceability, safety and economy of the structure. Several design philosophies have been introduced from different parts of the world. Some of the design philosophies that has been used by designers are

a) Working Stress Method (WSM)

b) Ultimate Load Method (ULM)

c) Limit State Method (LSM)

                                                             

 

Loads

 The forces that act on a structure are called loads.

For the safe design of structure, it is essential to have a knowledge of various types of loads and their worst combinations to which it may be subjected during its life span.

Types of loads

1. Dead load - Loads of constant magnitude that remains in one position.

Includes the weight of the structure as well as other parts that are permanently attached to structure.

eg; Beam & column , Wall , Slab , Stairways etc.

2. Live  or Imposed load - Loads that can change in magnitude and position.

eg; a)  Loads of people, furniture & storage itemes.

b) Loads of traffic on bridge.

c) Impact load due to vibration of moving load.

d) Longitudinal loads due to stopping train on bridge.

e) Soil pressure on wall and hydrostatic pressure on dams.

3. Environmental load - Loads caused by the environment in which structure is located.

All environmental loads are not caused by gravity.

a) Snow & Ice

b) Rain

c) Wind

d) Seismic or Earthquake loads

Stress - Strain Curve For Mild Steel

 Curve OABCDEF in figure represents the stress-strain curve when a mild steel specimen is subjected to a gradually increasing tensile load.

OAB - represent a straight line curve (stress is proportional to the strain, i.e., it obeys Hooke's law)                       

A - proportional limit

B - represents the elastic limit

C - upper yield point (In many of the structural steel hot rolled sections, the upper yield  point is not obtained due to residual stresses from the hot rolling  process. Hence, it has no practical significance)

D - lower yield point (The stress corresponding to the lower yield point 'D' is the yield stress with a typical magnitude of 250 N/mm²  for mild steel.)

DE - represents plastic yielding (It is the strain which occurs after the yield point, with no increase in stress)                                                        

EF - represents strain hardening region

F - represents the ultimate stress (After reaching the ultimate tensile stress, a localized  reduction of cross section of the specimen take place.)

FG - represents necking region (It is also knows as minimum ultimate tensile strength. Stress falls with rapid increase in strain till the specimen breaks)

G - represents breaking point

Mechanical properties of structural steel

1. Modulus of elasticity (E) = 2 x 10⁵   N/mm²

2. Shear modulus (G) = 0.769 x 10⁵  N/mm²

3. Poisson's ratio (µ) 

   a. Elastic range = 0.3

   b. Plastic range = 0.5

4. Coefficient of thermal expansion (α) = 12 x 10^-6 / ֯C

5. Density of steel = 7850 Kg/m³

About Steel

 Alpha-ferrite is a term in material science use for pure form of iron. When we adding  2 percent of carbon content to pure form of iron. Then it becomes steel.

Steel is an alloy of iron and carbon. Carbon content of steel is 2%.  Steel is the most important engineering and construction material.

Steel is used in buildings, tools, ships, trains, machines and weapons. 

Advantage of steel structure

* High strength

*High elasticity

*Light in weight

*Easy fabrication

 *Prefabrication

*Faster erection

*Low maintenance

*Ease of transportation