flexure strength of rc beams
TRANSCRIPT
![Page 1: Flexure Strength of RC Beams](https://reader031.vdocuments.site/reader031/viewer/2022021922/577ccde61a28ab9e788cdb3b/html5/thumbnails/1.jpg)
CE 331, Spring 2011 Flexure Strength of Reinforced Concrete Beams 1 / 5
A typical reinforced concrete floor system is shown in the sketches below. The floor slab is supported by the beams, which in turn are supported by the columns. If the width of a concrete slab (= column spacing below) is greater than twice its span (= beam spacing below), then the slab is said to be a “one‐way slab” (bends in principally in one direction only). The beams run left to right in the Plan View below. Section A‐A below indicates that the beams are T‐shaped: the beam web extends down below the soffit (underside) of the slab.
Plan View of Floor System
Elevation View
Figure 1. Plan View, Elevation View and Cross‐section of typical cast‐in‐place RC floor system.
Isometric views of the reinforced concrete floor system are shown on the following page.
Beam Span
Column Spacing
Bea
m
Spa
cing
B
eam
S
paci
ng
Bea
m
Spa
cing
slab
sp
an
A
A Bea
m
Spa
cing
Exterior Span Interior Span Exterior Span
ColumnSpacing
ColumnSpacing
Ln (clear span) slab
Ln
Column Spacing
Column Spacing
ColumnSpacing
beam web
colu
mn
Ln
Section A‐A: Slab
& Beam Elevation
Bea
m
Spac
ing
Bea
m
Spac
ing
t slab
h
b w
slab
be
am w
eb
![Page 2: Flexure Strength of RC Beams](https://reader031.vdocuments.site/reader031/viewer/2022021922/577ccde61a28ab9e788cdb3b/html5/thumbnails/2.jpg)
CE 331, Spring 2011 Flexure Strength of Reinforced Concrete Beams 2 / 5
Figure 2a. Isometric View of Reinforced Concrete Floor System
Figure 2b. Cut‐away showing beam dimensions
![Page 3: Flexure Strength of RC Beams](https://reader031.vdocuments.site/reader031/viewer/2022021922/577ccde61a28ab9e788cdb3b/html5/thumbnails/3.jpg)
CE 331, Spring 2011 Flexure Strength of Reinforced Concrete Beams 3 / 5
Placement of Reinforcement Concrete is strong in compression but weak in tension and cracks under relatively small tensile stresses. The crack patterns in a three‐span continuous beam are shown below. Steel reinforcement is placed in the tension zones of reinforced concrete beams, as indicated in the next figure.
Elevation View
The steel reinforcement is covered with a minimum thickness of concrete to protect it from moisture which can lead to corrosion. Reinforced concrete design is governed by the American Concrete Institute (ACI). ACI clear cover requirements for cast‐in‐place concrete are shown below.
Beam Cross‐Section
ACI Clear Cover Requirements:
Clear Cover, in
Concrete cast against and permanently exposed to earth
3
Concrete exposed to earth or weather
• #6 bar and larger • #5 bar and smaller
2 1.5
Concrete not exposed to earth or weather
• Slabs, walls, joists • Beams and columns
0.75 1.5
φbar / 2
φstirrup
clear cover
stirrup
+'ve M steel -'ve M steel
![Page 4: Flexure Strength of RC Beams](https://reader031.vdocuments.site/reader031/viewer/2022021922/577ccde61a28ab9e788cdb3b/html5/thumbnails/4.jpg)
CE 331, Spring 2011 Flexure Strength of Reinforced Concrete Beams 4 / 5
Factored Moments due to Dead + Live Loads (Mu)
Because concrete frames are highly indeterminate, the moments due to factored dead and live loads are typically calculated with the aid of a computer program. Alternatively, designers often use the American Concrete Institute (ACI) moment coefficients (shown below) which represent the envelope of moments due to dead load plus various live load span load patterns. (See Pg. 144 of the FE reference for the moment coefficients.)
ACI Moment Coefficients
Stress and Strain in a Reinforced Concrete Beam Beam flexure strength is usually calculated considering the stress and strain distributions across the section. The stress‐strain relations for steel and concrete are shown below.
A reinforced concrete beam must be analyzed differently than a wood or steel beam. Differences include the presence of two different materials, non‐linear stress‐strain behavior, and tensile cracking of concrete. Simplifying assumptions exist for the analysis of a reinforced concrete beam an imminent flexure failure, as shown in the table below. .
Mu
10
2nu Lw
14
2nu Lw
16
2nu Lw
11
2nu Lw
16
2nu Lw
Ln (clear slab
Ln
beam web co
lum
n
Ln
![Page 5: Flexure Strength of RC Beams](https://reader031.vdocuments.site/reader031/viewer/2022021922/577ccde61a28ab9e788cdb3b/html5/thumbnails/5.jpg)
CE 331, Spring 2011 Flexure Strength of Reinforced Concrete Beams 5 / 5
Complication Simplifying Assumption
Concrete ruptures under relatively small tensile stresses.
The strength of the concrete in tension is neglected. Steel reinforcement carries all of the tensile force.
The steel stress‐strain curve is bi‐linear The steel has yielded at failure
The concrete stress‐strain curve is non‐linear
An equivalent rectangular stress distribution (stress block) is used to approximate the curvilinear stress distribution at failure.
0.002 0.005
0.65
0.90
εs
φ
0.48 + 83 εs
min for beams = 0.004
f'c, psi β1 <= 4,000 0.85
5,000 0.80 6,000 0.75 7,000 0.70
>=8,000 0.65
Beam Cross-Section
Neutral Axis
Strain Distribution
Actual Stress Distribution
Stress Resultants
.003
c
εs
f'c
fs
Cc
Ts
a/2
d
Equivalent (Whitney Stress
Block) Stress Distribution
0.85f'c
fs
a=β1c
Cc Ts