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

Direct and Indirect Ranging

Measurement of distances with Chain and Tape on ground with Direct and Indirect Ranging Principle of Chain Surveying: The principle of chain surveying is derived from principle of triangulation. The area to be surveyed is divided into a number of triangles and the sides of the triangle are measured in the field. To get good results in plotting, the framework should consist of triangles which are as nearly equilateral as possible. Preferably all the sides of a triangle should be nearly equal having each angle nearly 60 o to ensure minimum distortion due to errors in measurements of sides and plotting. Theory: The process of establishing intermediate point on a straight line between two end points is known as ranging . Ranging must be done before a survey line is chained. It may be necessary to establish a number of intermediate points prior to chaining when chain line is much longer. Ranging may be done by direct observation by the naked eye or by lin

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 the angle between a pair of plane mirrors. Construction:

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 Channel Sections 3. Rolled Steel An

Basics In Civil Engineering

Search This Blog