By Dr. Attaullah Shah Swedish College of Engineering and Technology Wah Cantt.

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By Dr. Attaullah ShahSwedish College of
Engineering and Technology Wah Cantt.

• Reinforced Concrete Design-6
• Shear Design of Beams

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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
• Concrete contribution
• Aggregates interlocking
• Dowel action due to longitudinal steel

?cr Vcr/bd 1.9vfc 2500 ? Vd/Mlt3.5
vfc Alternatively ?cr 2vfc
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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 (1.9vfc 2500 ? Vd/Mlt3.5
vfc)bd Avfyd/s
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Types of we reinforcement
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Example of Beam without web steel
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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 ?Vc/2.
• The minimum web reinforcement must be provided
  till the above cited region

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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
  fc3000 psi
• Draw the cross section of the beam

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