compression members

DESIGN OF COMPRESSION

MEMBERS

MAHREEN ZAMIR

ABRO

14 CE-16

Steel Structure :

design of steel structures is only about finding answers to a few questions like will the structure be able to withstand.

1. Bending moments due to applied and self loads.

2. Shear force due to the same.

3. Torsion, etc

STEEL STRUCTURE

COMPRESSION MEMBERS :

Structural elements that are subjected to axial compressive forces only are called columns . Columns are subjected to axial loads thru the centroid.

Compression Members :

Primarily Occur as:1. Columns in buildings;2. Chord Members in trusses and diagonal members in end panels of trusses3. Stability is an important consideration in design and behavior of 4.compression members5. Area is generally spread out to maximize Radius of Gyration

CONT… The behavior of compression members is more complicated than tension members as they are subject to various buckling modes. In steel construction, pure compression members do exist, but they are often subject to combined compression and bending actions.

Strength Design of a Compression Member :For a member subject to a design axial compression force N*, the following limit state requirement must be satisfied.

N* ≤ φ Ns

  N* ≤ Φ Nc

Where

φ = the capacity reduction factor = 0.9 Ns = the Nominal section capacity in compression Nc = the Nominal member capacity in compression These capacities depend on the possible buckling modes that could occur in compression members

DESIGN OF STEEL COMPRESSION MEMBERS

A structural member loaded axially in compression is generally called a compression member. Vertical compression members in buildings are called columns, posts or stanchions. A compression member in roof trusses is called struts and in a crane is called a boom.Columns which are short are subjected to crushing and behave like members under pure compression. Columns which are long tend to buckle out of the plane of the load axis.

THEORY OF COLUMNS :Euler’s formula for critical load for a pin-ended column subjected to axial load is

Where, L = length of column between the hinged ends,E = modulus of elasticity, andI = moment of inertia of the column section.The column will become unserviceable if the loads are larger than Pcr  . In theEuler equation, it is assumed that stress is proportional to strain, therefore,

Critical Stress =

Where, A= area of cross-section, andr = radius of gyration about the bending axis = slenderness ratio

VARIOUS END CONDITIONSColumns with length L and effective length (L)  are shown in figure below:

Strength of an Axially Loaded Compression MembersMaximum axial compression load permitted on a compression member,

               Where, P = axial compressive load (N),       = permissible stress in axial compression (MPa)(mm2)A = effective cross-sectional area of the member 

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