03-01-2013, 03:16 PM
INTRODUCTION TO STRUCTURAL STEEL DESIGN
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Introduction
A Structural component must possess both Strength and Ductility
Steel gets its strength by shearing of its layers
Unit weight of steel is 78.5 KN/m³
Steel has high strength to weight ratio
Steel has ferrite (ductility) + Pearlite (mechanical strength) micro-structure
Steel stress- strain graph has elastic region, plastic region and strain hardening region. Yield strain of steel is around 0.0012
Strain hardening is the increase of strength of hardness due to the changes in micro-structure
From Von Mises yield criterion,
In state of pure shear, √
For normal stress,
Limit State
Limit states are failure states. Two limit states; Strength and Serviceability
Strength limit states; ultimate, buckling and fatigue
Serviceability limit states; deflection, vibration and corrosion
Limit state assumes variable safety and it follows probabilistic approach
Characteristic Load – Value of load above which not more than 5% of the test results may be expected fall
Characteristic Strength – Value of resistance, below which not more than 5% of the test results may be expected fall
In limit state the material is expected to go till the ultimate strength. The load is increased and the strength reduced by multiplying characteristic load and strength by partial safety factors
Beams
In beam, load is applied perpendicular to the axis
Load is transferred by bending and shear
Every section should be able to take shear and moment
In I-section it is assumed that the moment is carried by the flanges and shear by web.
If (span/depth) , then the beam is deep beam. In deep beam, load transfers by shear. In thin beam, flexure dominates.
Modes of Failure; flexure, shear, crushing failures and lateral torsional buckling
Beam can be laterally restrained or un-restrained. Lateral torsional buckling occurs in un-restrained beam.
Lateral torsional Buckling
1. It is the buckling due to the compression flange
2. To avoid it, compression flange has to be restrained
3. Each restraint should carry at least 1% of the force in the compression flange and a total of 2.5% all restrained combined.
In a plastic section, as the moment increases, the section reaches plastic moment capacity and plasticisation starts. Till plastic hinges form, it acts as a flexure member but after hat it behaves as a truss member.
Combined Forces
Beam column – member subjected to both bending and axial forces
Axial load reduces the stiffness of the member. It reduces moment capacity.
As axial load acting increases, moment capacity decreases. The figure shows the relation between axial force and moment