Lecture 25 Shear Design for Oneway Slabs

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Lecture 25 - Shear Design for One-way Slabs

• March 24, 2003
• CVEN 444

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Lecture Goals

• Project
• One-way slab design

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Discontinuities at Bar Cutoff
ACI 12.10.5
Prohibits flexural bar cutoffs in zone of
flexural tension, unless 1 of the following is
satisfied.
(1) (2)
Extra stirrups are provided at the cutoff points.
(See Sec. 12.10.5.2 for details)
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Discontinuities at Bar Cutoff
Prohibits flexural bar cutoffs in zone of
flexural tension, unless 1 of the following is
satisfied.
ACI 12.10.5
For No 11 bars smaller Increase shear
strength is required when bars are cutoff in a
tension zone.
(3)
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Joist Design
Flat Slab reinforcement is calculated for bending
or minimum reinforcement for shrinkage and
temperature.
(1)
(ACI Sec 7.12.2 )
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Joist Design
Shear Design of Joist Ribs (Joist - Section 8.11)
(2)
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Joist Design
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Joist Design
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One-Way Slab Design
Design of one slabs is like design of parallel
12 beams.
Thickness of One-Way Slabs
Minimum thickness for solid one-way slabs not
supporting or attached or attached to partitions,
etc. Likely to be damaged by large deflections
ACI Table 9.5(a)
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One-Way Slab Design
Thickness of One-Way Slabs
The table calculates the minimum thickness t
( l span length in inches) (normal weight
concrete fy 60 ksi see code for
modification factors)
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One-Way Slab Design
Thickness of One-Way Slabs Table A-14 tmin,
when damage to non-structural components may occur
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One-Way Slab Design
Thickness of One-Way Slabs
Fire Rating
This is equal to the number of hours for
unexposed surface to rise a set amount usually
250 o F
3.5 in. 1 hour 5.0 in.
2 hours 6.25 in. 3 hours
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Cover for Slab Reinforcement
ACI Sec. 7.71 (min. cover for corrosion
protection)
( 1.) Concrete exposed to earth or weather.
5 and smaller 1.5 in. 6 and larger 2.0
in.
( 2.) Concrete not exposed to earth or weather.
11 and smaller 0.75 in.
Min. covers for fire ratings should also be
considered.
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One-Way Slab Design
Reinforcement
Typical Reinforcement in a one-way slab
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One-Way Slab Design
Cutoffs
If requirements for use of ACI Moment
Coefficients
Figure A-5 one-way slab
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One-Way Slab Design
Need to confirm thickness is adequate for one-way
shear. Difficult to place shear reinforcement in
a slab. Minimum area of shear reinforcement
required in slabs if
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One-Way Slab Design
Need to confirm thickness is adequate for one-way
shear. Difficult to place shear reinforcement in
a slab.
Minimum area of shear reinforcement required in
slabs if
ACI Sec. 11.5.51
Usual use
ACI Eqn. 11-3
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One-Way Slab Example
The cross section of a continuous one-way solid
slab in a building is shown. The slabs are
supported by beams that span 24 ft. Between
simple supports. The dead load on the slabs is
that due to self-weight plus 60 psf The live
load is 120 psf. Design the continuous slab and
draw a detailed section. Given fc 3 ksi, fy
40 ksi.
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One-Way Slab Example
Determine the thickness of the slab.
Minimum thickness of interior bay
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One-Way Slab Example
Use h 6 in.
Dead weight
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One-Way Slab Example
Determine the load on the floor
For slab design r is seldom gt 0.01. Use c/d 0.2
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One-Way Slab Example
Find the coefficient of the moment diagram
The maximum moment is
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One-Way Slab Example
Find d for the design
If b 12 in. then d 3.46 in., which for c/d
0.2 so d 5 in., the slab is OK.
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One-Way Slab Example
Find the coefficient of the shear
The maximum shear is
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One-Way Slab Example
Check the shear capacity of the slab
Use h 6 in., d 5 in. and wu 0.354 k/ft
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One-Way Slab Example
Minimum about of steel is (use ACI 7.12 )
Moment capacity of the beam
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One-Way Slab Example
Compute the moments
Moment capacity of the beam
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One-Way Slab Example
Compute the moments
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One-Way Slab Example
Check the assumption
Assumption will work!
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One-Way Slab Example
Use 4 bar Ab 0.2 in2 The maximum spacing ACI
7.6.4
The spacing on the first location
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One-Way Slab Example
Select 15 in. for the spacing to calculate the
provided area.
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One-Way Slab Example
Actual spacing.
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One-Way Slab Example
Summary
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Homework (Due 3/31/03)
Homework Problem 6.6