Transcript Lean Construction - Swedish College Of Engineering

Reinforced Concrete Design-6
Shear Design of Beams
By
Dr. Attaullah Shah
Swedish College of Engineering and Technology
Wah Cantt.
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
νcr =Vcr/bd= 1.9√f’c + 2500 ρ Vd/M<3.5 √f’c
Alternatively νcr = 2√f’c
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
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.
Vs = Vc + Vw = (1.9√f’c + 2500 ρ Vd/M<3.5 √fc’)bd +Avfyd/s
− 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.
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 ϕVc/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