Methods of Structural Design

Methods of Structural Design

There are many methods of design out of them following two are more popular:

1. Working Stress Method.

2. Limit State Method.

1. Working Stress Method:

In this method, uncertainties such as correct estimation of load, possibility of occurrence of higher design load, material imperfection and geometrical imperfections in the structural members were taken into account in one single factor known as factor of safety.
A particular section just reaching to yield stress at a maximum stressed fiber is considered to be a benchmark for estimating the capacity of the steel.
Structural members are not allowed to stress more than allowable stress to ensure perfect elasticity.
Different factors of safety were used to arrive allowable stresses for different types of stresses in structural members such as beam, column, tension member, base plate and fastners.
Using estimated working/ service loads and their logical combinations the sections are designed using allowable stresses.

2. Limit State Method:

In the limit state design method, the structure shall be designed to withstand safely all loads likely to act on it throughout its life.
The objective of design is to achieve a structure that will remain fit for use during its life with acceptable target reliability.
Limit state design method is technologically sound method which results in significant economy in design of structures.
The design of a structure to satisfy all appropriate requirements derived form probability considerations is referred as Limit State Design.

Definition of Limit State:

The acceptable limit for the safety and serviceability requirements before failure occurs is called limit state.

Types of Limit State:

The limit states are classified as:

1. Limit State of Strength

2. Limit State of Serviceability

1. Limit State of Strength:

Limit state of strength using appropriate factor of safety are those connected with failures under the action of probable and most unfavorable combinations of load on structure which may endanger the safety of life and property.

It includes:

Plastic Collapse
Stability against sway, overturning and sliding.
Fatigue

2. Limit State of Serviceability:

Limit state of serviceability is related to satisfactory performance of structure at working load.

Types of serviceability limit states applicable to steel structures:

Deflection
Durability
Vibration
Fire resistance

Comments

Rolled Steel Sections

Rolled Steel Sections Structural steel sections are hot rolled into different shapes and sizes to form the different sections. Cross section and size are governed by functional requirements, dimensional and weight capacity of rolling mills, arrangement of material for optimum structural efficiency. In a rolling mill molten steel from furnace is poured into continuous casting system where steel solidifies but it is not allowed to cool completely. Then hot steel passes through series of rollers. Such series of rollers squeezes steel into desired cross sectional shape. If the load to be resisted is small single rolled steel sections are used. If the single section is insufficient, single sections are combined or built up with angles/ channels/ I sections with or without cover plates and /or laced, providing sufficient resistance to heavy loads. Most commonly used hot rolled steel shapes are as follows: 1. Rolled Steel I -Sections 2. Rolled Steel C...

How to Use Optical Square and Open Cross Staff

Offsets can be accurately set up, particularly long offsets, by use of optical instruments like cross staff or optical square etc. a) Cross Staff: Two pairs of vertical slits, mounted orthogonally on a pole staff constitutes a simple instrument for setting out normal offsets as depicted in Figure. Each pair of slits can form a line of sight. It is fixed on a stand. Two persons are required to operate cross staff. Use of cross staff: 1. To find a foot of perpendicular on chain line 2. To set out the perpendicular from a given point. b) Optical Square: It may be noted that at least two surveyor’s are required to set normal offsets while using cross staff. This disadvantage can be overcome if optical square is used for this purpose as only single surveyor can perform this task. Principle of optical square: The angle between the first incident ray and the final reflected ray is twice that of t...

Difference Between Flexible Pavement and Rigid Pavement

Difference Between Flexible Pavement and Rigid Pavement The term pavement means the surfacing layer only. It is defined as a structure having several layers bound together and placed at the top of the soil subgrade so that it provides a smooth surface for the vehicles. So the main objective behind construction of pavement is to provide a smooth surface for the vehicles. Pavements are divided into two types with respect to their structural behavior: 1. Flexible Pavements 2. Rigid Pavements Sr. No. Particular Flexible Pavement Rigid Pavement 01 Flexural strength Flexible pavements are those which on the whole have low or negligible flexural strength and are rather flexible in their structural action under the loads Rigid pavements are those which posses flexural strength or flexural rigidity. 02 Damage It has self healing properties as it deforms elastically ...

Basics In Civil Engineering

Search This Blog