Download - Design of One-Way Slab
DESIGN OF REINFORCED
CONCRETE STRUCTURE
ONE WAY SLAB
Presented by -
MD. Mohotasimur Rahman (Anik)
24th batch, AUST
INTROCDUCTION
Slab in which the deflected surface is predominantly
cylindrical termed as one-way slabs spanning in the
direction of curvature. Curvatures, and consequently
bending moments, of this slab shall be assumed same for all
strips spanning in the shorted direction or in the direction of
predominant curvature, the slab being designed to resist
flexural stress in that direction only.1
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If a slab is supported on two opposite sides only, it will bent or deflect in a direction perpendicular to the
supported edge. The structure is one way, and the loads are carried by the slab in the deflected short
direction (fig-2a).2 If the slab is supported on four sides and the ration of the long side to the short side is
equal to or greater than 2, most of the load (about 95% or more) is carried in the short direction, and one-
way action is considered for all practical purposes (fig-2b).2
Figure 1. Deflection of One-Way Slab
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Figure 2. One-way slab
DESIGN OF ONE-WAY SOLID SLABS
One-way slab may be treated as a beam. A unit strip of slab, usually 1 ft. (or 1 m) at right angles to the
supporting girders, is considered a rectangular beam.3
One-way slabs shall be designed to have adequate stiffness to limit deflections or any deformations that
affect strength or serviceability of a structure adversely.4
Minimum thickness stipulated in the table 1, shall apply for one-way slabs not supporting or attached to
partitions or other construction likely to be damaged by large deflection, unless computation of deflection
indicates that a lesser thickness can be used without adverse effects.4
The total slab thickness (π) is usually rounded to the next higher 1 4 inch (5mm). For slab up to thickness
6 inch. Thickness and next higher 0.5 inch. (or 10 mm) for thicker slabs.5
Concrete cover in slabs shall not be less than 3 4 inch.6 (20 mm) at surface surfaces not exposed to weather
or ground. In this case, π = π β 34 β (ππππ πππ ππππππ‘ππ).Where, π is defined as distance from
extreme compression fiber to centriod of tension reinforcement.7
Factored moment and shears in one way slab can be found either by elastic analysis or through the use of
the same coefficients stated ACI 8.3.3. printed in table 2.8
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Table 1: Minimum Thickness of Non-Prestressed One-Way Slab
(Normal weight concrete and Grade 60 (Grade 420) Reinforcement)π
Member Minimum thickness, h
Simply supported One end continuous Both end continuous cantilever
Solid one-way slab π
20
π
24
π
28
π
10
Ribbed one-way slabs π
16
π
18.5
π
21
π
8
No
te:
(1) For ππ¦ other than 60,000 ππ π () multiplied tabulated value by 0.4 + (ππ¦ 100,000) [for ππ¦ other than
420 π ππ2 multiplied tabulated value by 0.4 + (ππ¦ 700) ]
(2) For structural light weight concrete, multiply tabulated values by (1.65 β 0.005π€π) but not less than
1.09. where π€π is range in 90 π‘π 115 ππ ππ‘3 .[For structural light weight concrete, multiply
tabulated values by ( 1.65 β 0.003π€π ) but not less than 1.09 . where π€π is range in
1440 π‘π 1840 ππ π3 .]
DESIGN OF ONE-WAY SOLID SLABS
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DESIGN OF ONE-WAY SOLID SLABS
Table:2 Approximate Moments and Shears in Continuous Beams.ππ
Positive moment
End span
Discontinuous end unrestrained π€π’ππ2 11
Discontinuous end integral with support π€π’ππ2 14
Interior span π€π’ππ2 16
Negative moments at exterior face of first interior support
Two spans π€π’ππ2 9
More than two spans π€π’ππ2 10
Negative moment at other faces of interior supports π€π’ππ2 11
Negative moment at face of all supports for (1) Slabs with spans not exceeding 10 ft. and
(2) beams where ratio of sum of column stiffness to beam stiffness exceeds eight at each
end of the span.
π€π’ππ2 12
Continued
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DESIGN OF ONE-WAY SOLID SLABS
Negative moment at interior face of exterior support for members built integrally with supports
Where support is spandrel beam π€π’ππ2 24
Where support is a column π€π’ππ2 16
Shear in end members at face of first interior support 1.15 π€π’ππ 2
Shear at face of all other supports π€π’ππ 2
Where , π€π’ = π’ππππππππ¦ πππ π‘ππππ’π‘ππ ππππ ; ππ = π πππ πππππ‘π
β’ If the slab rests freely on its supports the span length may be taken equal to the clear span plus depth of the
slab but did not exceed the distance between centers of supports.11
β’ In analysis of frames or continuous construction for determination of moments, span length shall be taken
as the distance center-to-center of supports.11
β’ It shall be permitted to analyze solid or ribbed slabs built integrally with supports, with clear spans not
more than 10ft, as continuous slabs on knife edge supports with spans equal to the clear spans of the slab
and width of beams otherwise neglected.11
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DESIGN OF ONE-WAY SOLID SLABS
Figure 4: Summary of ACI Moment Coefficient: (a) beams with more than two span (b) beams with
two spans only (c) slabs with spans not exceeding 10 ft. (d) beams in which the sum of column stiffness
exceeds 8 times the sum of beam stiffness at each end of span.ππ
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DESIGN OF ONE-WAY SOLID SLABS
The conditions under which the moment coefficients for continuous beam and slabs given in table 2 should
be used can be summarized as follows:10
β’ Spans are approximately equal, with the larger of two adjacent spans not greater than the shorter by
more than 20 percent.
β’ There are two or more spans.
β’ Loads are uniformly distributed.
β’ Unit live load does not exceed three times unit dead load.
β’ Members are prismatic.
The maximum reinforcement ratio, ππππ₯ = 0.85π½1πβ²πππ¦
ππ’
ππ’:ππ‘; where, βπ‘= 0.004 a minimum set tensile
strain at the nominal member subjected to axial loads less than 0.10πβ²ππ΄π where π΄π is the gross area of the
cross section, provides the maximum reinforcement ratio.13 and ππ’ = Maximum usable strain at extreme
concrete compression fiber shall be assumed equal to 0.003.14
The minimum required effective depth ππππ =ππ’
β ππππ₯ππ¦π(1;0.59ππππ₯ππ¦
πβ²π);15
Check π > ππππ; (okay) .15
Figure 5. distance from
extreme compression fiber to
centriod of tension reinforcement
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DESIGN OF ONE-WAY SOLID SLABS
Reinforcement, π΄π =ππ’
β ππ¦ π;π 2 ; where, π =
π΄π ππ¦
0.85πβ²π π; at first assumed, π = 1 to calculate π΄π ; that value
can be substituted in equation of π to get a better estimate of π and hence a new π β π 2 can be
determined.16
For structural slabs of uniform thickness the minimum area of tensile reinforcement, π΄π πππ in the direction
of the span shall be the same as temperature and shrinkage reinforcement area.17 In no case is the
reinforcement ration to be less than 0.0014 .18
Table 3: Minimum Ratios of Temperature and Shrinkage Reinforcement in Slab based on Gross Area.ππ
Slabs where Grade 40 (275) or 50 (350) deformed bars are used 0.0020
Slabs where Grade 60 (420) deformed bars or welded wire fabric (plain or deformed) are used 0.0018
Slabs where reinforcement with yield stress exceeding 60,000 psi (420 MPa) measured at a
yield strain of 0.35 percent is used
0.0018 Γ 60,000
ππ¦
In slabs, primary flexural reinforcement shall be spaced not farther apart than three times slab thickness,
nor 18 inch (450 mm).19 and Shrinkage and temperature reinforcement shall be spaced not farther apart
than five times the slab thickness, nor 18 in.18
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Check shear requirements. Determine ππ’ at the distance from π and calculate β ππ = 2β πβ²πππ. If
1
2β ππ > ππ’ the shear is adequate.20 Note that the provision of minimum area of shear reinforcement where
ππ’ exceeds 1
2β ππ does not apply to slabs .21 If ππ’ >
1
2β ππ , it is a common practice to increase the depth of
slab .22 so π can be determine, assuming ππ’ = β ππ = 2β πβ²πππ.23
Straight-bar systems may be used in both tops and bottom of continuous slabs. An alternative bar system of
straight and bent (trussed) bars placed alternately may also be used.7
The choice of bar diameter and detailing depends mainly on the steel areas, spacing requirements, and
development length.24
DESIGN OF ONE-WAY SOLID SLABS
Figure 6. Reinforcement Details in Continuous One-Way Slab: (A) Straight Bars And (B) Bent Bars.ππ
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QUESTION:
The cross-section of a continuous one-way slab in a building is shown in figure 7. The slab are supported by
beams that span 12 ft. between simple supports. The dead load on the slab due to self-weight plus 77psf; the
live load psf. Design the continuous slab and draw a detailed section. Given πβ²π= 3 ππ π and ππ¦ = 40 ππ π
Figure 7. Continuous One-Way Slab
SOLUTION:
Minimum depth,
ππππ = maxπΏ
30=
15Γ12
30= 6" ,
πΏ
10=
5Γ12
10= 6" = 6"
Dead load =6
12Γ 0.15 + 0.077 ππ π = 0.152 ππ π
Live load = 0.13 ππ π
Load, π€ = 1.4 π·πΏ + 1.7 πΏπΏ = 0.434 ππ π
Moment βππ’= max βπ€π2
12, β
π€π2
12, (β
π€π2
2) = max β8.13 , β8.13, β5.42 = β8.13 πβ²/ππ‘
Moment +ππ’= max +π€π2
14= max +6.975 = +6.975 πβ²/ππ‘
EXAMPLE OF ONE-WAY SOLID SLABS
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EXAMPLE OF ONE-WAY SOLID SLABS
SFD
BMD
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EXAMPLE OF ONE-WAY SOLID SLABS
To maintain β = 0.9, reinforcement ratio corresponding to ππ‘ = 0.004 will be selected.
ππππ₯ = 0.85π½1πβ²
π
ππ¦
ππ’ππ’ + ππ‘
= 0.85 Γ 0.85 Γ3
60Γ
0.003
0.003 + 0.004= 0.0155
ππππ =ππ’
β ππππ₯ππ¦π(1;0.59ππππ₯ππ¦
πβ²π)=
8.13 Γ12
0.9Γ0.0155Γ60Γ12(1;0.59Γ0.0155Γ60
3) = 3.45" < πππππ£ππππ = 6 β 1 " = 5" (okay)
βπ΄π =ππ’
β ππ¦ π;π 2 =
8.13 Γ12
0.9Γ60Γ 5;0.75 2 = 0.39 ππ2 ππ‘ (ππ π π’ππππ π = 1) ; π =
π΄π ππ¦
0.85πβ²π π=
0.39Γ60
0.85Γ3Γ12= 0. 76ππ
+π΄π =ππ’
β ππ¦ π;π 2 =
6.975 Γ12
0.9Γ60Γ 5;0.64 2 = 0.33 ππ2 ππ‘ ; π =
π΄π ππ¦
0.85πβ²π π=
0.38Γ60
0.85Γ3Γ12= 0.65 ππ
Minimum Reinforcement π΄π πππ= 0.0018 Γ 12 Γ 5 = 0.11 ππ2 ππ‘ ; π΄π =
β0.39 ππ2 ππ‘ (controls)
+0.33 ππ2 ππ‘ (controls))
Check π =π΄π
ππ=
0.39
12Γ5= 0.0065 > ππππ₯; so β = 0.9 (okay);
At π distance ππ’ =π€π
2βπ€π =
0.434Γ15
2β 0.434 Γ 5 = 1.085 πππ
1
2β ππ =
1
2Γ 2β πβ²
πππ = 0.85 Γ 3000 Γ 12 Γ
5
1000= 2.79 πππ > ππ’ ( so the provided depth is okay).
Use β 12 ππ @ 5"π/π at two supports, β 12 ππ @ 6"π/π at mid span and β 10 ππ @ 12"π/π for temperature
and shrinkage reinforcement.
1. BNBC-1993; pg. 6-146.
2. STRUCTURAL CONCRETE: THEORY & DESIGN 5th ed.; M. NADIM HASSOUN; pg. 300
3. STRUCTURAL CONCRETE: THEORY & DESIGN 5th ed.; M. NADIM HASSOUN; pg. 302
4. BNBC-1993; pg. 6-155.
5. DESIGN OF CONCRETE STRENGTHS 13TH EDITION; ARTHUR H. NILSON; pg. 416
6. ACI 318-95; SECTION 7.7.1;pg. 318\R318-68
7. STRUCTURAL CONCRETE: THEORY & DESIGN 5th ed.; M. NADIM HASSOUN; pg. 304
8. DESIGN OF CONCRETE STRENGTHS 13TH EDITION; ARTHUR H. NILSON; pg. 395
9. NOTE ON ACI 318-08 BUILDING CODE REQUIREMENTS FOR STRUCTURAL CONCRETE 10th ed.; pg. 10-2
10. BNBC 1993; pg. 6-147
11. ACI 318-95; SECTION 8.7.4; pg.318/318R-82~83
12. DESIGN OF CONCRETE STRENGTHS 13TH EDITION; ARTHUR H. NILSON; pg. 396
13. DESIGN OF CONCRETE STRENGTHS 13TH EDITION; ARTHUR H. NILSON; pg. 81
14. ACI 318-11; SECTION 10.3.5.; pg. 138
15. DESIGN OF CONCRETE STRENGTHS 13TH EDITION; ARTHUR H. NILSON; pg. 418
16. BNBC-1993; pg. 6-148.
17. BNBC-2011; pg. 6-26.
18. DESIGN OF CONCRETE STRENGTHS 13TH EDITION; ARTHUR H. NILSON; pg. 417
19. DESIGN OF CONCRETE STRENGTHS 13TH EDITION; ARTHUR H. NILSON; pg. 416
20. STRUCTURAL CONCRETE: THEORY & DESIGN 5th ed.; M. NADIM HASSOUN; pg. 308
21. STRUCTURAL CONCRETE: THEORY & DESIGN 5th ed.; M. NADIM HASSOUN; pg. 310
22. STRUCTURAL CONCRETE: THEORY & DESIGN 5th ed.; M. NADIM HASSOUN; pg. 262
23. STRUCTURAL CONCRETE: THEORY & DESIGN 5th ed.; M. NADIM HASSOUN; pg. 424
24. STRUCTURAL CONCRETE: THEORY & DESIGN 5th ed.; M. NADIM HASSOUN; pg. 305
REFERENCE
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