maximum bending moment in an RC beam?

Question

how do you work out the max. bending moment in an RC beam, and is it the same as the resistant of bending moment? any help would be welcome.

Answer 1

What means RC rectangular??

The maximun bending moment is affect with a parameter, so is smaller than the resistant bending moment.

Moment max=0.90 Moment resist

Answer 2

The maximum bending moment on a beam is the same no matter what the beam materials are. It depends only on the external loads and the manner of support for the beam

For a steel reinforced concrete beam, the resistance of bending moment is different than in a beam consisting of only one material.

Pure Flexure Design Example:
Determine the bending moment resistance of a rectangular beam with tension reinforcement and no axial loads.



Depth to reinforcing = d = 21 inches
Beam width = b = 12 inches
Reinforcing steel area = As = 3.0 in2
Concrete compressive strength = f'c = 4000 psi
Reinforcing yield strength = fy = 60000 psi

Determine moment capacity per ACI 318
Maximum concrete strain = e = 0.003
Reduction factor for flexure = f = 0.90
Ratio of average concrete stress = a1 = 0.85
Ratio of stress block depth = b1= 0.85

Solve for a using the SF = 0.

As fy = a b b1 f'c
3.0(60000) = a (12)(0.85)(4000)
a = 4.41 inches

Determine moment from force couple

f Mn = f Asfy (d - a / 2)
f Mn = 0.9(3.0)(60)(21 - 4.41 / 2) / 12
f Mn = 254 k-ft

Determine moment capacity per CSA A23.3
Maximum concrete strain = e = 0.0035
Concrete resistance factor = fc = 0.60
Reinforcement resistance factor = fr = 0.85
Ratio of average concrete stress = a1 = 0.81
Ratio of stress block depth = b1= 0.9

Solve for "a" using the SF = 0.

fr As fy = a b b1 fc f'c
0.85(3.0)(60000) = a (12)(0.81)(0.6)(4000)
a = 6.56 inches

Determine moment from force couple

Mr = fr Asfy (d - a / 2)
Mr = 0.85(3.0)(60)(21 - 6.56 / 2) / 12
Mr = 226 k-ft