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ARRANGEMENT OF REINFORCEMENT IN TYPICAL CONCRETE MEMBERS
Simple Beams and Slabs
Simply supported slab:
Curtailment of tension steel in simply supported slab construction.
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DETAILING OF BEAMS
Beam carries transverse external loads that cause bending moment, shear forces and in
some cases torsion
Concrete is strong in compression and very weak in tension.
Steelreinforcement is used to take up tensile stresses in reinforced concrete beams.
Mild steel bars or Deformed or High yield strength deformed bars (HYSD) are used.
HYSD bars have ribs on the surface and this increases the bond strength at least by 40%
Bar Bending Schedule
Drawings generally include a bar bending schedule
The bar bending schedule describes the length and number, position and the shape of the bar
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Anchorage in steel bars is normally provided in the form of bends and hooks
The anchorage value of bend of bar is taken as 4 times the diameter of bar for every 45 0 bend
subjected to maximum of 16 times the diameter of bar.
Standard hooks
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The beams are classified as:
According to shape: Rectangular, T, L, Circular etc
According to supporting conditions: Simply supported, fixed, continuous and cantilever
beams
According to reinforcement: Singly reinforced and doubly reinforced
Cover in Beam
Minimum cover in beams must be 25 mm or shall not be less than the larger diameter of bar
for all steel reinforcement including links.
Nominal cover specified in Table 16 and 16A of IS456-2000 should be used to satisfy
the durabilitycriteria.
Types of Reinforcement in Beams
Generally a beam consists of following steel reinforcements:
Longitudinal reinforcement at tension and compression face.
Shear reinforcements in the form of vertical stirrups and or bent up longitudinal bars are
provided.
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Side face reinforcement in the web of the beam is provided when the depth of the
web in a beam exceeds 750 mm. (0.1% of the web area and shall be distributed
equally on two faces at a spacing not exceeding 300 mm or web thickness
whichever is less)Specification for the reinforcement in beams is given in clause 8.1 to 8.6 of SP34
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While drawing the details of a beam following convention representation of bars are used
Mild steel bars : f; HYSD bars: # or
Main bars are shown by thick single line.
Hanger bars are shown by medium thick lines.
Different forms of stirrups used in beams
Typical drawing of a simply supported beam
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REINFORCED CONCRETE DESIGN
Assumptions
Concrete has no tensile resistance
The reinforcing resists all the tensile forces.
Maximum tensile stress in the steel:
The characteristic cube stress of the concrete = fcuand the compressive stress in the concrete =
0,4 fcu
The maximum neutral axis depth (dn) is limited to 0.5 d.
The internal moments are in equilibrium with the external moments.
Resistance moment (Mr) of a singly reinforced concrete beam orslab.
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A partial material factor of 1,15 is applied to the steel and 1,5 to the concrete.
The following formula may be derived:
Resistance moment of the concrete in the compression zone:
Take moments about the tension zone, i.e., centroid of the tensile steel.
Assume a practical limit for the maximum: maximum dn = 0,5 d
The resistance moment of the tensile zone:
Take moments about the centroid of the compression zone.
Neutral axis depth and lever arm with known beam section details:
Note that b, d and As are known.
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Horizontal equilibrium of forces:
Fc = Ft
The lever arm z
Determination of the lever arm z = a1xd and tensile area As for a known section and known
dimensions b and d with a moment Mu acting on the section.
Substitute n1 and z in the equation for Mu
Solve for a1:
This can be written in a tabular form:
0,15 0,13 0,10 0,07 0,04
a1 0,75 0,80 0,85 0,90 0,95
With a known value of a1 the lever arm z may be calculated. Once the lever arm is know, the
steel may be calculated.
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