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Example 7: nominal moment for reinforced concrete T beam (Si units)

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Determine nominal moment Mn for beam in figure 1.



Figure 1









Assuming a<80mm

a=(As*fy)/(0.85*fc'*b)
a=(6*1006*300)/(0.85*28*800)
a=95.1mm

the assumption is wrong, therefore a>80mm

0.85*fc'*a1*b1+0.85*fc'*a2*b2=As.fy
0.85*28*80*800+0.85*28*a2*350=6,036*300
a2=34.52

F1=0.85*fc'*a1*b1
F1=0.85*28*80*800
F1=1,523,200N=1,523.20Kn

F2=0.85*fc'*a2*b2
F2=0.85*28*34.52*350
F2=287,551.6N=287.55Kn

Center of compressive force, extreme compression fiber(top of the beam) is the reference axis

X=(F1*40+F2*97.26)/(F1+F2)
X=(1523.2*40+287.55*97.26)/(1523.2+287.55)
X=49mm

a moment can be calculated by multiplying a couple of force with equal magnitude and opposite direction by the vertical distance between them

Mn=As*fy*(381)
Mn=689,914,800 N.mm=689.9Kn.m









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