Estimating Beam Depth for Initial trial

Deflection criteria: Use simplified form of IS456,2000 cl23.2.1


Span/deff= BV*mft*mfc*mff

For Both End Simply supported , Basic Value(BV) =26

mft= 0.8 for Assumed 2.5% tension steel

Assume; mfc = 1.25 for 1% Compression steel

Assume ;mff= 0.8 for web width/ flange width<0.3

Span/depth = 20*0.8*1.25*0.8=16 (approx), for simply supported

Span/depth = 26*0.8*1.25*0.8=20( approx), for both end continuous

Span/depth= 23*0.8*1.25*0.8=18( approx), for one end simply supported and one end continuous

Sample Calculation;
Beam span=4200mm
Boundary Condition; one end simply supported other end continuous
deff= span/18= 4200/18= 233.33 mm


Overall depth = deff+(assumed bar dia)/2+Clear cover = 233.33+25/2+25= 270.83 Assume 300mm ( 12”)

Ductile Detailing criteria( IS13920;1993)
 
The spacing of hoops over a length of 2d at either end of a beam


S<[ deff/4, 8*φsmall longitudinal bar, 24*φhoop, 300 ]min

Sample Calculation

Assume, s =75 mm, φsmall longitudinal bar =12mm, 24*φhoop =8mm


Effective depth of the beam/4 =75 mm


Effective depth of the beam =300mm


Overall depth of the beam = 300+25/2+25=337.5 =350 mm (approx round off value)


Check for S(75mm)<[ (deff/4=75), (8*φsmall longitudinal bar =96), (24*φhoop =192), 300 ]min=75mm; Satisfied

Conclusion: Thumb Rule
Span/depth = 16 (approx), for simply supported

Span/depth = 20( approx), for both end continuous
Span/depth= 18( approx), for one end simply supported and one end continuous