Concrete · Beams

RC T-beam flexure (flange in compression) calculator

Rectangular stress block in a T-section: neutral axis in the flange or web, moment capacity, ductility (ku) and minimum steel.

Standard: AS 3600 Cl 8.1 (rectangular stress block)

Inputs in the worked example

The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.

Result	Value	Unit
α2 = 0.85 − 0.0015 f′c (≥ 0.67)	0.802
γ = 0.97 − 0.0025 f′c (≥ 0.67)	0.890
Tension force Ast fsy	1,550	kN
Flange capacity α2 f′c bef Ds	4,620	kN
Stress block in flange?	yes
Overhang force when block in web	0.000	kN
Stress block depth γ ku d	50.33	mm
ku = a/(γ d)	0.090
φ = 1.24 − 13ku/12 (0.65 … 0.85)	0.850
Mu	937.5	kNm
φMu	796.9	kNm
Ast,min = 0.20 (D/d)² 0.6√f′c/fsy bw d (approx. for T-beams)	369.6	mm²

Diagram for the worked example

α2 = 0.85 − 0.0015 f′c (≥ 0.67) = MAX(0.67,0.85-0.0015*32) = 0.802
γ = 0.97 − 0.0025 f′c (≥ 0.67) = MAX(0.67,0.97-0.0025*32) = 0.89
Tension force Ast fsy = 3100*500/1000 = 1550 kN
Flange capacity α2 f′c bef Ds = 0.802*32*1200*150/1000 = 4619.5 kN
Stress block in flange? = IF(1550<=4619.5,"yes","no (in web)") = yes
Overhang force when block in web = IF(1550<=4619.5,0,0.802*32*(1200-350)*150/1000) = 0 kN
Stress block depth γ ku d = IF(1550<=4619.5,1550*1000/(0.802*32*1200),(1550-0)*1000/(0.802*32*350)) = 50.33 mm
ku = a/(γ d) = 50.33/(0.89*630) = 0.089763
φ = 1.24 − 13ku/12 (0.65 … 0.85) = MIN(0.85,MAX(0.65,1.24-13*0.089763/12)) = 0.85
Mu = IF(1550<=4619.5,1550*(630-50.33/2),0*(630-150/2)+(1550-0)*(630-50.33/2))/1000 = 937.49 kNm
φMu = 0.85*937.49 = 796.87 kNm
Ast,min = 0.20 (D/d)² 0.6√f′c/fsy bw d (approx. for T-beams) = 0.2*(700/630)^2*0.6*SQRT(32)/500*350*630 = 369.58 mm²

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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.