Lecture 11
Transcript of Lecture 11
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Lecture 11 – Analysis Lecture 11 – Analysis and Designand DesignFebruary 7, 2003CVEN 444
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Lecture GoalsLecture Goals
One-way Slab designResistance Factors and LoadsDesign of Singly Reinforced Rectangular Beam Unknown section dimensions Known section dimensions
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Pattern LoadsPattern LoadsUsing influence lines to determine pattern loads
Largest moments in a continuous beam or frame occur when some spans are loaded and others are not.
Influence lines are used to determine which spans to load and which spans not to load.
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Pattern LoadsPattern Loads
Influence Line: graph of variation of shear, moment, or other effect at one particular point in a structure due to a unit load moving across the structure.
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Pattern Pattern LoadsLoads
Quantitative Influence Lines Ordinate
are calculated (“exact”)
MacGregor (1997)
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Pattern LoadsPattern LoadsQualitative Influence Lines Mueller-Breslau Principle
Used to provide a qualitative guide to the shape of the influence line
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Pattern LoadsPattern LoadsQualitative Influence Lines (cont.) For moments
Insert pin at location of interestTwist beam on either side of pinOther supports are unyielding, so
distorted shape may be easily drawn.
For frames, joints are assumed free to rotate, assume members are rigidly connected (angle between members does not change)
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Qualitative Influence Lines The Mueller-Breslau principle can be stated as follows:
If a function at a point on a structure, such as reaction, or shear, or moment is allowed to act without restraint, the deflected shape of the structure, to some scale, represents the influence line of the function.
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Pattern LoadsPattern LoadsQualitative Influence Lines
Fig. 10-7 (b,f) from MacGregor (1997)
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Pattern Pattern LoadsLoads
Frame Example:• Maximize +M at point B.• Draw qualitative
influence lines.
• Resulting pattern load:“checkerboard pattern”
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Pattern LoadsPattern LoadsArrangement of Live Loads – (ACI 318-02, Sec. 8.9.1) It shall be permitted to assume that:
The live load is applied only to the floor or roof under consideration, and
The far ends of columns built integrally with the structure are considered to be fixed.
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Pattern LoadsPattern LoadsArrangement of Live Loads – ACI 318-99, Sec. 8.9.2: It shall be permitted to assume that the
arrangement of live load is limited to combinations of:Factored dead load on all spans with
full factored live load on two adjacent spans.
Factored dead load on all spans with full factored live load on alternate spans.
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MomentMomentEnvelopesEnvelopes
Fig. 10-10; MacGregor (1997)
The moment envelope curve defines the extreme boundary values of bending moment along the beam due to critical placements of design live loading.
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MomentMomentEnvelopes ExampleEnvelopes ExampleGiven following beam with a dead load of 1 k/ft and live load 2 k/ft obtain the shear and bending moment envelopes
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MomentMomentEnvelopes ExampleEnvelopes ExampleUse a series of shear and bending moment diagrams
Wu = 1.2wD + 1.6wL
0
1
2
3
4
5
0 5 10 15 20 25 30 35 40
(ft)
kips
-80
-60
-40
-20
0
20
40
60
80
0 5 10 15 20 25 30 35 40
ft
kips
-250
-200
-150
-100
-50
0
50
100
150
0 5 10 15 20 25 30 35 40
ft
k-ft
Shear Diagram Moment Diagram
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MomentMomentEnvelopes ExampleEnvelopes ExampleUse a series of shear and bending moment diagrams
Wu = 1.2wD + 1.6wL
Shear Diagram Moment Diagram
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 5 10 15 20 25 30 35 40
ft
k/ft
-20
-15
-10
-5
0
5
10
15
20
0 5 10 15 20 25 30 35 40
ft
kips
-80
-60
-40
-20
0
20
40
0 5 10 15 20 25 30 35 40
ft
k-ft
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MomentMomentEnvelopes ExampleEnvelopes ExampleUse a series of shear and bending moment diagrams
Wu = 1.2wD + 1.6wL
Shear Diagram Moment Diagram
00.5
11.5
22.5
33.5
44.5
5
0 5 10 15 20 25 30 35 40
ft
k/ft
-60-50-40-30-20-10
01020304050
0 5 10 15 20 25 30 35 40
ft
kips
-200
-150
-100
-50
0
50
100
150
200
0 5 10 15 20 25 30 35 40
ft
k-ft
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MomentMomentEnvelopes ExampleEnvelopes ExampleThe shear envelope
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MomentMomentEnvelopes ExampleEnvelopes ExampleThe moment envelope
Moment Envelope
-300
-200
-100
0
100
200
0 5 10 15 20 25 30 35 40
ft
k-ft
Minimum Moment Maximum Moment
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Approximate Analysis of Continuous Approximate Analysis of Continuous Beam and One-Way Slab Beam and One-Way Slab
SystemsSystemsACI Moment and Shear Coefficients
Approximate moments and shears permitted for design of continuous beams and one-way slabsSection 8.3.3 of ACI Code
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Approximate Analysis of Approximate Analysis of Continuous Beam and One-Way Continuous Beam and One-Way Slab SystemsSlab SystemsACI Moment and Shear Coefficients -
Requirements:Two or more spansApproximately Equal Spans Larger of 2 adjacent spans not greater
than shorter by > 20%Uniform LoadsLL/DL 3 (unfactored)
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Approximate Analysis of Approximate Analysis of Continuous Beam and One-Way Continuous Beam and One-Way Slab SystemsSlab SystemsACI Moment and Shear Coefficients -
Requirements: ( cont.)Prismatic members Same A, I, E throughout member length
Beams must be in braced frame without significant moments due to lateral forces Not state in Code, but necessary for
coefficients to apply.
** All these requirements must be met to use the coefficients!**
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Approximate Analysis of Approximate Analysis of Continuous Beam and One-Way Continuous Beam and One-Way Slab SystemsSlab Systems
2
)( 2
nuvu
numu
lwCV
lwCMwu = Total factored dead and live
load per unit lengthCm = Moment coefficientCv = Shear coefficientln = Clear span length for span in
question for –Mu at interior face of exterior support, +Mu
and Vu
ln = Average of clear span length for adjacent spans for –Mu at interior supports
ACI Moment and Shear Coefficients – Methodology:
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Approximate Analysis of Continuous Approximate Analysis of Continuous Beam and One-Way Slab SystemsBeam and One-Way Slab Systems
ACI Moment and Shear Coefficients
See Section 8.3.3 of ACI Code
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ExampleExampleDesign the eight-span east west in figure. A typical 1-ft wide design strip is shaded. A partial section through this strip is shown. The beams are assumed to be 14 in. wide. The concrete strength is 3750 psi and the reinforcement strength is 60 ksi. The live load is 100 psf and dead load of 50 psf.
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Example – One-way Example – One-way SlabSlab
Use table 9.5(a) to determine the minimum thickness of the slab. 12 in15 ft 180 in
ftl
180 in.min. h = 7.5 in.
24 24l
End bay:
180 in.min h = 6.43 in.28 28l
Interior bays:
Use 7.5 in.
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Example – One-way Example – One-way SlabSlab
Compute the trial factored loads based on thickness.
D 3 2
1 ft lb lb7.5 in 150 93.75 12 in ft ft
w
u D L1.2 1.6 1.2 50 psf + 93.75 psf 1.6 100 psf332.5 psf
w w w
Factored load
L D3w wCheck ratio for 8.3.3
OK!
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Example – One-way Example – One-way SlabSlab
Compute factored external moment.
22U
U
332.5 psf 15 ft6801. lb-ft/ft
C 11 81.61 k-in/ft
w LM
UN
81.61 k-in/ft 90.68k-in/ft0.9
MM
Nominal moment
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Example – One-way Example – One-way SlabSlab
The thickness is 7.5 in. so we will assume that the bar is located d = 7.5in – 1.0 in. = 6.5 in. (From 3.3.2 ACI 318 0.75 in + ~0.25 in( 0.5*diameter of bar) = 1.0 in
N s y
2Ns
y
0.92
90.68 k-in/ft 0.258 in /ft0.9 60 ksi 0.9 6.5 in
aM T d A f d
MAf d
Assume that the moment arm is 0.9d
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Example – One-way Example – One-way SlabSlab
Recalculate using As = 0.2 in2
s y
c
NN s y s
y
s
2
0.258 in. 60 ksi0.405 in.
0.85 0.85 3.75 ksi 12 in
22
90.68 k-in/ft0.405 in.60 ksi 6.5 in.
20.240 in /ft
A fa
f b
MaM A f d Aaf d
A
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Example – One-way Example – One-way SlabSlab
Check the yield of the steel
1
t cu
0.405 in. 0.476 in.0.85
6.5 in. 0.476 in. 0.0030.476 in.
0.038 0.005
ac
d cc
Steel has yielded so we can use = 0.9
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Example – One-way Example – One-way SlabSlab
Check to minimum requirement for every foot
s
y
min minc
y
0.24 in. 0.0030112 in. 6.5 in.
200 200 0.0033360000
0.003333 3 3750 0.0031
60000
Abd
f
ff
Problem!
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Example – One-way Example – One-way SlabSlab
What we can do is rework the spacing between the bars by change b Use a #4 bar As = 0.2 in2
2
s s 0.2 in 9.23 in.0.00333 6.5 in.
Use b = 9 in.
A Abbd d
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Example – One-way Example – One-way SlabSlab
Check for shrinkage and temperature reinforcement for min = 0.0018 As = minbh from 7.12.2.1 ACI
2s min
2
2
0.0018 12 in. 7.5 in. 0.162 in /ft
0.2 inspacing = 12 in. =14.8 in.0.162 in
A bd
Use 1 # 4 bar every 9 in.
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HomeworkHomework
Homework problems on the web