COMPRESSION MEMBERS

3. 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

4. STEEL STRUCTURE

5. 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 trusses
3. Stability is an important consideration in design and behavior of
4.compression members
5. Area is generally spread out to maximize Radius of Gyration

6. 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.

7. 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

8. 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.

12. 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, and
I = moment of inertia of the column section.
The column will become unserviceable if the loads are larger than Pcr . In the
Euler equation, it is assumed that stress is proportional to strain, therefore,

13. Critical Stress =
Where, A= area of cross-section, and
r = radius of gyration about the bending axis
= slenderness ratio

14. Columns with length L and effective length (L) are shown in figure below:
Strengthof an Axially LoadedCompression Members
Maximum 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