By Dr. Attaullah Shah Swedish College of Engineering and Technology Wah Cantt. Reinforced Concrete Design-6 Shear Design of Beams

Flexure Shear Cracking When both the shear force and bending moments are large, flexural tension cracks form first. When the load is increased further and the tension flexural force at theses cracks increases than the tensile strength of concrete, shear flexural cracks. of RC beams It has been shown by research that flexure shear cracking develops at The shear strength of reinforced concrete Beams is developed by the following: 1.Concrete contribution 2.Aggregates interlocking 3.Dowel action due to longitudinal steel ν cr =V cr /bd= 1.9 √f’ c ρ Vd/M<3.5 √f’ c Alternatively ν cr = 2 √f’ c

RC beams with web reinforcement −Before development of diagonal shear cracks, there is no noticeable effect of web steel. −When the diagonal cracks are intercepted by the web steel, these are restricted from further extending into compression zone. −For the cracked part the web steel restricts the cracks width and lead to interface shear. − The web reinforcement also check the cracks development along the main reinforcement, thereby providing restraint against the slitting of concrete and leads to shear strength contribution also called dowel action. − The total shear strength of web/ transverse reinforced RC beams is sum of the concrete contribution and web steel contribution. i.e. −Minimum web reinforcement given by ACI is −This provision holds unless Vc is one half or less than the design shear stress −For floor beams system, beams with total depth of 10in or less the maximum spacing may not exceed 2 ½ times the slab thickness or one half of width of the beam whichever is greater. − Vs = Vc + Vw = (1.9 √f’ c ρ Vd/M<3.5 √f c ’)bd +A v f y d/s

Types of we reinforcement

Example of Beam without web steel

Region where web reinforcement is required −When the applied shear force Vu is greater than the shear strength of the beam, web reinforcement is provided till the shear strength of ϕ Vc/2. −The minimum web reinforcement must be provided till the above cited region

Assignment −A rectangular simple beam 12in x18in having a span of 20 ft is subjected to service live load of 1.5k/ft and superimposed dead load of 1 k/ft. Design the web reinforcement of the beam if fc’=3000 psi −Draw the cross section of the beam