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Column design as Per BS 8110-1:1997
SHORT – both lex/h and ley/b < 15 for braced columns
< 10 for unbraced columns
BRACED - If lateral stability to structure as a whole is provided by walls or bracing designed to resist all lateral forces in that plane.
else – SLENDER
else – UNBRACED
Effective length le = ßlo ß – depends on end condition at top and bottom of column.
emin = 0.05 x dimension of column in the plane of bending ≤ 20 mm
Deflection induced moments in Slender columns
Madd = N au whereau = ßaKh
ßa = (1/2000)(le/b’)2
K = (Nuz – N)/(Nuz – Nbal) ≤ 1
Nuz = 0.45fcuAc+0.95fyAsc
Nbal = 0.25fcubd
Value of K found iteratively
Design Moments in Braced columns :-
b) Mi+Madd Mi = 0.4M1+0.6M2
Columns where le/h exceeds 20 and only Uniaxially bent Shall be designed as biaxially bent with zero initial moment along other axis.
Braced and unbraced columns
Design Moments in UnBraced columns :-
The additional Moment may be assumed to occur at whichever
end of column has stiffer joint. This stiffer joint may be the
critical section for that column.
Deflection of all UnBraced columns in a storey
auav for all stories = Σ au/n
Axial Strength of column N = 0.4fcuAc + 0.8 Ascfy
Biaxial Bending Increased uniaxial moment about one axis
Mx/h’≥ My/b’ Mx’ = Mx + ß1 h’/b’My
Mx/h’≤ My/b’ My’ = My + ß1 b’/h’Mx
Section Stress Strain
N = 0.402fcubx + f1A1 +f2A2
f1 and f2 in terms of E and f1 = 700(x-d+h)/x
f2 = 700(x-d)/x
The solution of above equation requires trial and error method