Design of compression members in steel structures - civil Engineering

DESIGN OF COMPRESSION
MEMBERS IN STEEL
STRUCTURES

CONTENTS
• Abstract
• Introduction
• Columns & beams
• Compression members in trusses
• Different types of cross section shapes
• Design Methods
• Design Procedure by example problem
• Conclusion

ABSTRACT
• The design methodologies for the steel structures
namely, working stress design method and limit
state design methods are briefly explained. The
importance of limit state design method is
highlighted.
• This focuses entirely to the procedure involved in
design of compression members. A problem have
been worked out.

INTRODUCTION
• The strength of steel compression members is usually limited by their
tendency to buckle.
• The load at which a compression member becomes unstable is the
buckling load.
• The buckling load depends on the length, cross-section, and end
conditions of the column and the stiffness of the material
• Steel Compression members are
a) Building columns & Beams
b) Frame Bracing
c) Truss members
• Useful in pure compression as well as in beam columns
• Design Clauses from IS codes

COLUMNS & BEAMS
• Columns form the main component of a structure
which serves the basic purpose of supporting and
transmitting the entire loads both vertical and horizontal
for which the overall structure is intended to the
foundation system.
• Beams are generally subjected only to flexure about the
horizontal axis whereas columns are subjected to axial
load along with bending moment about the major axis.
• The minor axis moment in columns are generally nil or
very nominal since in standard structural system, the
columns are so oriented that the frames along the major
axis of the columns are moment resistant frames, and
column bracings are provided in the frames along the
other perpendicular direction.

Different types of Cross sections

Compression members in trusses

DESIGN METHODS
ALLOWABLE STRESS DESIGN
• With the development of linear elastic theories, the stress-strain behavior of new
materials like wrought iron & mild steel could be accurately represented.
• The allowable stress is defined in terms of a “factor of safety” which represented a
margin for overload and other unknown factors which could be tolerated by the
structure.
Allowable stress =
LIMIT STATE DESIGN
• An improved design philosophy to make allowances for the shortcomings in the
“allowable stress design” was developed in formulating design standards and codes.
Although there are many variations the basic concept is broadly similar.
• The probability of operating conditions not reaching failure conditions forms the
basis of “Limit States Design” adopted.
• In order to reduce the probability of its occurrence to a very low level.”
Serviceability limit state” refers to the limits on acceptable performance of the
structure.
Yield Stress
Factor of Safety

DESIGN PROCEDURE
 DESIGN A MEMBER SUBJECTED HAVING A SPAN OF 3M
WHICH IS FIXED @ BOTH ENDS LSM (As per IS: 800-2007)
Solution:
Let us take ISMB 200 @ 254 N/m
Area = 3233mm2
Depth (d) = 200mm
Width of flange (b) = 100mm
Thickness of the flange (tf) = 10.8mm,
Thickness of the web (tw) = 5.7mm

Buckling Load
• Pcr is the load at which the compression member
becomes unstable
• E is modulus of elasticity of steel
• I is moment of inertia of the cross section
• L is the length of the compression member
• K is the effective length factor
2
2
)(KL
EI
Pcr



Step 1: Type of the Section
The section is Compact
Step 2: Determination of Effective
Length
Leff = 0.65 x 3000 = 1950mm
Step 3: Calculate the Slenderness
Ratio
Step 4: Determination of Non
Dimensional
Step 5: Calculation of ø
428.82d cdP Axf kN 
178
31.22
5.7w
d
t
 
1950
23.43
83.2
x
x x
lKL
r r
 
1950
90.69
21.5
y
y y
lKL
r r
 
2
2
2 2 5
250 23.43
0.2638
2 10
y
y
x
cc
KL
f
f xr
f E x x

 
 
 
    
2
2
2 2 5
250 90.69
1.020
2 10
y
y
y
cc
KL
f
f xr
f E x x

 
 
 
    
  2
0.5 1 0.2        
  2
0.5 1 .34 1.02 0.2 1.02 1.16       

Step 6: Calculation of Stress
Reduction Factor
Step 7: Determination of design
Compressive Stress fcd
Step 8: Determination of
Compressive stress Pd
 
2 20.52 2
1 1
0.58
[1.16 (1.16 1.02 )]

  
  
     
0.52 2
/y mo
cd
f
f

  

   
132.64y y
mo mo
f f
 
 
428.82d cdP A f kN  

CONCLUSION
• The load carrying capacity of the compression
members as per IS 800-2007 is controlled by
‘stress reduction factor, inclination of tension
field stress in web and effective slenderness ratio.
• The slenderness ratio is inversely proportional to
the stress reduction factor. The design
compressive stress is directly proportional to
‘stress reduction factor’.

Design of compression members in steel structures - civil Engineering

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