A 3m span rectangular cantilever beam of cross section 400mm

A 3m span rectangular cantilever beam of cross section 400mm overall depth x 250mm width is made using 25MPa concrete. It contains 4N24 bars in the top tension layer and 2N24 bars in the second top layer. It also contains 2N24 bars in the compression layer. Top and bottom clear covers are 30mm each and clear distance between bars in the horizontal and vertical directions is 30mm each. A 2-legged R10 stirrup is used as shear reinforcement at 120mm c/c. All bars are required to develop yield; what embedment length of the reinforcement will be required for this purpose? If the BM varies linearly, along the length of beam,

Solution

Development length of bar is nothing but the extra length of bar require to provide for development of stress in steel at that section.

At any section whatever stress produced is need to be develop in steel through bond between steel and concrete. So that the bar will not get pull out of concrete…as one friend give very good example of finger with grease.

Thats why, when calculating development length need to account bond stress. You can find it out by equating,

(bond stress × Develop. Length × perimeter of bar) = ( yield stress of steel i.e 0.87*fy)

And bond stress as per IS 456:2000, table given with grade of concrete.

This length again we can say that, length from critical section to tangent to hook of bar plus anchorage length of bar.

Sometime length provided more than this to assure proper anchorage.