1. CE8501 Design Of Reinforced Cement Concrete Elements
Unit 1-Introduction
Design of Doubly reinforced beam
[As per IS456:2000] & SP -16
Presentation by,
P.Selvakumar.,B.E.,M.E.
Assistant Professor,
Department Of Civil Engineering,
Knowledge Institute Of Technology, Salem.
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2. Problem#09
β’ Design the doubly reinforced beam with b= 300mm, D= 600mm.
Factored moment = 320 kN.m. Assume fck= 20 N/mm2, fy= 415 N/mm2.
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b= 350mm
D = 600 mm
Ast
Given :
b = 300mm
D = 600mm
fck = 20N/mm2
fy = 415N/mm2
M = 320 kNm
To find
Ast = ?
Asc = ?
dβ = ?
Asc
3. Step 1 : Determination of required dimensions of beam
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Assume effective cover of 40mm, Hence overall depth D = 600mm
d = 600- 40
d = 560 mm
4. Step 2: Limiting moment of resistance (Mu,lim)
Mu,lim = 0.36
ππ,πππ
π
[1 β 0.42
ππ,πππ
π
] b d2 fck
Mu,lim = 0.36 * 0.48 [1 β 0.42 β 0.48 ] * 300 * 5602 * 20
= 259.59 x 106 N.mm
Mu,lim = 259.6 kN.m [Mu>Mu,lim Over reinforced section]
[Consider as Mu1]
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5. Step 3 : Area of steel at tension zone (Ast1)
For under reinforced section, Ast1 derived from Mu
Mu1 = 0.87 fy Ast1 d [1 -
π¨ πππ
ππ
π π πππ
]
259.6 * 106 = 0.87 * 415 * Ast1 * 560 [1-
Ast1
β
415
300β560β20
]
= (- 25 Ast1
2 )+ (202.19 * 103 Ast1 ) β (259.6 * 106)
Ast1 = 1600 mm2
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Ast1
6. Step 3 : Area of steel at tension zone (Ast2)
Mut= Mu1 + Mu2
Mu2 = Mut - Mu1
= 320 - 259.6
Mu2 = 60.4 kN.m
60.4 x 106 = Ast2 (0.87 * 415) (560-56)
Ast2 = 331.9 β 332 mm2
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Mu2 derived from
β’ Mu2 = Ast2 (0.87 fy) (d-dβ)
Ast2
πβ²
π
= 0.10
πβ²
= 0.10 * 560
= 56 mm
7. Step 3 : Area of steel at tension zone (Ast2)
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Ast2
Ast = 1600 + 332
Ast = 1932 mm2
Ast1Ast
8. Step 4 : Selecting Rebar size (Tension Zone)
Area for tension zone
Ast = 1932 mm2
Assume 25mm dia bar,
No.of bars =
1932
490.9
= 3.93 β 4
Hence use 4 numbers of 25mm dia bar
Area provided,
Ast = 1963.6 mm2 > 1932 mm2
Hence ok
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Area of rebar
Area =
π
4
(π2)
= 50.3 mm2 (8mm Ο)
= 78.5 mm2 (10mm Ο)
= 113.1 mm2 (12mm Ο)
= 201.1 mm2 (16mm Ο)
= 314.2 mm2 (20mm Ο)
= 490.9 mm2 (25mm Ο)
= 804.2 mm2 (32mm Ο)
10. Step 6 : Selecting Rebar size (Compression Zone)
Area for compression zone
Asc = 350 mm2
Assume 12mm dia bar,
No.of bars =
350
113.1
= 3.1 β 4
Hence use 4 numbers of 12mm dia bar
Area provided,
Asc = 452.4 mm2 > 350 mm2
Hence ok
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Area of rebar
Area =
π
4
(π2)
= 50.3 mm2 (8mm Ο)
= 78.5 mm2 (10mm Ο)
= 113.1 mm2 (12mm Ο)
= 201.1 mm2 (16mm Ο)
= 314.2 mm2 (20mm Ο)
= 490.9 mm2 (25mm Ο)
= 804.2 mm2 (32mm Ο)
11. Area of Steel
At tension zone,
Ast = 2049 mm2
At compression zone,
Asc = 452 mm2
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Ast
AscAst
Asc
12. Step 7 : Check for depth of neutral axis
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xu =
0.87 β415 β1963 β 353β13.38 β452
0.36 β20 β300
xu = 257.05 mm
xu =
0.87 ππ¦ π΄π π‘ β π π π
βπππ π΄ π π
0.36 πππ π
Xu,max = 0.48 * d
Xu,max = 0.48 * 560
Xu,max = 268.8 mm
[xu<xu,max Hence it is under reinforced section]
13. Step 6 : Reinforcement details of Singly
reinforced beam
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N A
Compression Zone
Tension Zone
d= 560mm
Effective cover = 40mm
b= 300mm
25mm dia main bar
12mm dia main bar
Cross section
xu= 257 mm
D= 600mm
14. Assignment#07
β’ Design a rectangular beam as doubly reinforced section subjected to a
moment of 200kN.m. Consider concrete of grade M20 and steel of grade
Fe415. Assume b = 250mm.
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