Question

A reinforced concrete beam is subjected to the following bending moments:

Dead load - 20 kN-m
Live load - 30 kN-m
Seismic load - 10 kN-m

The design bending moment for limit state of collapse is

• A. 60kN-m
• B. 75 kN-m
• C. 72 kN-m
• D. 80 kN-m

Answer 1

The correct option is B 75 kN-m

The various load combinations are as follows:

(i) For Dead Load and Live Load the ultimate bending moment is given by,
$M_u=1.5(DL+LL)$
$=1.5\times (20+30)=75kN-m$

(ii} For Dead Load and Earthquake (Seismic) Load the ultimate tending moment is given by,
$M_u=1.5(DL+EL)$
$=1.5\times (20+10)=45kN-m$

(iii) For Dead Load, Live Load and Earthquake (Seismic) Load the ultimate bending moment is given by,
$M_u=1.2(DL+LL+EL)$
$=1.2\times (20+30+10)=72kN-m$

So, design BM is maximum of all three
combinations i.e. 75 kN-m.