The design capacity of the section against compressive force, the guiding phenomenon is the flexural buckling.
Limit State Method
The buckling strength of the member is affected by residual stress, initial bow and accidental eccentricities of load.
To account for all these factors, the strength of the members subjected to axial compression is defined by buckling class a, b, c or d as per clause 188.8.131.52 and Table 7 of IS 800:2007.
Imperfection factor, obtained from buckling class, and Euler’s Buckling Stress ultimately govern compressive force capacity of the section as per clause 7.1.2 of IS 800:2007.
The buckling class of a solid rod section is determined per Table 10 of the specification.
Working Stress Method
The actual compressive stress is given by:
fc = FX/Ae
- Ae = The effective section area as per Clause 7.3.2 of the code. This is equal to the gross cross sectional area, AX, for any non-slender (plastic, compact, or semi-compact) section class. In the case of slender sections, this is limited to value of Ae as described below.
The permissive compressive stress is calculated by first determining the Buckling Class of the section per Table 10 of the code and αYY & αZZ based on Table 7.
Fac = 0.6·Fcd
- Fcd = the minimum of the values of Fcd calculated for the local Y and Z axis.
Fcd = (FYLD/γmo)/ [φ + (φ2 + λ2]
- λ = the non-dimensional slenderness factor is evaluated for each local Y and Z axis.
λ = (FYLD/Fcc)1/2
φ = 0.5[ 1 + a(λ - 0.2) + λ2]
- Fcc = the Euler Buckling Stress.
Fcc = π2·E/(Kl/r)2
- K = the effective length factor for bending about either the local Y or Z axis, as provided in the KY and KZ parameters, respectively.
- r = radius of gyration about the local Y or Z axis for the section.
- FYLD = The yield strength of steel specified in the FYLD parameter.
For member with slender section under axial compression, design compressive strength should be calculated on area ignoring depth thickness ratio of web in excess of the class 3 (semi-compact) limit.
Refer to clause 7.3.2 and Table 2 of IS 800:2007, (corresponding to Internal Element of Compression Flange)
Ae= Ag - (d/tw - 42ε) · tw 2
- Ae = Effective area of section.
- Ag = Gross area of section.
- d = Depth of web.
- tw = thickness of web.
Flexural-Torsional Buckling for Single Angles
In the case of single angles (ST and RA), a axial compression load does not pass through the member centroid. In practical applications, the load must pass through one of the legs. The deviation between the load and the centroid can be considerable, and thus the effect of flexural-torsional buckling must be considered.
The parameters of ANG, FXTY, and NBL are used to control these calculations.
The calculations for the flexural-torsional buckling strength of a single angle member in compression are performed per section 184.108.40.206 of the code (either limit state method or working stress method are applicable).