12745475 the flexural and shear design of deep beam
TRANSCRIPT
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10 - 1
10.1 INTRODUCTION Deep beam is a beam having large depth/thickness ratio and shear span depth ratio less than 2.5
for concentrated load and less than 5.0 for distributed load. Because the geometry of deep beams, they
behavior is different with slender beam or intermediate beam.
The structural element that might be classified as deep beam are :
Transfer Girder, is a girder that carry all the vertical load without any vertical element below the
girder.
Pile Cap, is a structural element that connect the vertical element with the deep foundation such
as bored pile.
Vertical Wall, wall slab under vertical load can be designed as deep beam.
10.2 BEHAVIOR OF DEEP BEAM The followings are the major different of deep beam element compared wth ordinary beam based on
the design assumption, as follows :
Two-Dimensional Action, because of the dimension of deep beam they behave as two-
dimensional action rather than one-dimensional action.
Plane Section Do Not Remain Plane, the assumption of plane section remain plane cannot be
used in the deep beam design. The strain distribution is not longer linear.
Shear Deformation, the shear deformation cannot be neglected as in the ordinary beam. The
stress distribution is not linear even in the elastic stage. At the ultimate limit state the shape of
concrete compressive stress block is not parabolic shape again.
The followings are the major behavior of deep beam element, as follows :
Cracking of deep beam will occur at c31 'f or c2
1 'f
The distribution of tensile stress at bottom fiber is constant over the span. In other word the
value of tensile stress at bottom fiber at support and at mid span is only little different, for
this reason in deep beam the tension reinforcement must be extend to the end of support
although that region is small bending moment region (in ordinary beam we can cut off the tension
reinforcement and not all of the tension reinforcement in mid span is extended to the end of
support, practically only two for anchor the stirrups.
The maximum tensile stress at the bottom fiber is far exceed the magnitude of compressive
stress.
CHAPTER
10 THE FLEXURAL AND SHEAR DESIGN OF DEEP BEAM
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10 - 2
SUPPORT STRESSMID SPAN SUPPORT
FIGURE 10.1 STRESS DISTRIBUTION OF DEEP BEAM
The cracks is vertical follows the direction of compression trajectory, in deep beam we must
provide both vertical stirrups and horizontal stirrups.
COMPRESSIVE ARC ACTION
FIGURE 10.2 CRACKS OF DEEP BEAM
10.3 FLEXURAL DESIGN OF DEEP BEAM 10.3.1 GENERAL
The flexural design for deep beam is not described in the ACI code, the method explained in this
section is from Euro – International Concrete Committee (CEB).
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10 - 3
10.3.2 CEB DESIGN OF DEEP BEAM
The flexural design procedure is for simply supported beams and for continuous beams.
TABLE 10.1 FLEXURAL DESIGN OF DEEP BEAM
TYPE SIMPLY SUPPORTED BEAMS CONTINUOUS BEAMS
Moment
Strength ( )jdfAM ysn = ( )jdfAM ysn =
Positive
Reinforcement ( )jdfMAy
us φ
=+
+ ( )jdfMAy
us φ
=+
+
Negative
Reinforcement – ( )jdf
MAy
us φ
=−
−
As
Minimum db
f4.1db
f4'f
A wy
wy
cmins ≥= db
f4'f
dbf4.1A w
y
cw
ymins ≥=
Lever
Arm
( ) 2hL1h2L2.0jd <≤⇒+=
1hLL6.0jd <⇒=
( ) 5.2hL1h5.1L2.0jd ≤≤⇒+=
1hLL5.0jd <⇒=
Positive
Reinforcement
Distribution
h20.0L05.0h25.0y <−= h20.0L05.0h25.0y <−=
Negative
Reinforcement
Distribution
– s1s A1
hL5.0A ⎟
⎠
⎞⎜⎝
⎛−=−
1ss2s AAA −=−
where :
jd = lever arm
Mu+ = positive ultimate flexure moment
Mu- = negative ultimate flexure moment
As+ = positive reinforcements area
As- = negative reinforcements area
h = beam depth
f’c = concrete cylinder strength (MPa)
fy = yield strength of reinforcements (MPa)
Where L is taken the minimum of effective span measured center to center of supports or 1.15 Ln.
In simply supported beams, the positive tension reinforcement is distributed in the lower of beam
section along the distance :
h20.0L05.0h25.0y <−= [10.1]
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10 - 4
POSITIVE REINFORCEMENTS
yh
LLn
FIGURE 9.9 DISTRIBUTION OF POSITIVE REINFORCEMENTS IN SIMPLY SUPPORTED BEAMS
In continuous beams the distribution of positive reinforcements is similar as in the simply supported
beam, the difference is the distribution of negative reinforcements.
hh3
0.2h
0.6h
As1As2
FIGURE 9.10 DISTRIBUTION OF NEGATIVE REINFORCEMENTS IN CONTINUOUS BEAMS
As1 is distributed along height h1=0.2h and As2 is distributed along h2=0.6h.
Reinforcements in zone h3 are come from the tension reinforcements that continued from the mid
span to the support section.
10.3.3 STEP – BY – STEP PROCEDURE The followings are the step – by – step procedure used in the flexural design for deep beam, as follows
:
Classified the structure as simply supported beam or continuous beam.
Calculate the approximate lever arm jd.
TYPE SIMPLY SUPPORTED BEAMS CONTINUOUS BEAMS
Lever
Arm
( ) 2hL1h2L2.0jd <≤⇒+=
1hLL6.0jd <⇒=
( ) 5.2hL1h5.1L2.0jd ≤≤⇒+=
1hLL5.0jd <⇒=
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10 - 5
Calculate the required positive or negative reinforcement As+, As
-.
TYPE SIMPLY SUPPORTED BEAMS CONTINUOUS BEAMS
Positive
Reinforcement ( )jdfMAy
us φ
=+
+ ( )jdfMAy
us φ
=+
+
Negative
Reinforcement – ( )jdf
MAy
us φ
=−
−
Check the required steel bars area with minimum steel bars area Asmin.
TYPE SIMPLY SUPPORTED BEAMS CONTINUOUS BEAMS
As
Minimum db
f4.1db
f4'f
A wy
wy
cmins ≥= db
f4'f
dbf4.1A w
y
cw
ymins ≥=
Choose the number of bars and the reinforcement is distributed as follows :
TYPE SIMPLY SUPPORTED BEAMS CONTINUOUS BEAMS
Positive
Reinforcement h20.0L05.0h25.0y <−= h20.0L05.0h25.0y <−=
Negative
Reinforcement –
s1s A1hL5.0A ⎟
⎠
⎞⎜⎝
⎛−=−
1ss2s AAA −=−
10.4 SHEAR DESIGN OF DEEP BEAM
10.4.1 GENERAL
The shear design of deep beam is similar as shear design of ordinary beam, the difference is only at
the concrete shear strength, limitation of ultimate shear force and horizontal and vertical
stirrups distribution.
10.4.2 BASIC DESIGN EQUATION
According to ACI code the design of deep beam due to shear force must follows the following condition
:
un VV ≥φ [10.2]
where :
Vn = nominal shear strength
nVφ = design shear strength
φ = strength reduction factor (0.85)
Vu = ultimate shear force, factored shear force
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10 - 6
As shear design of ordinary beam, the shear force is resisted by the concrete component and by the
shear reinforcement component, as follows :
scn VVV += [10.3]
where :
Vn = nominal shear strength
Vc = concrete shear strength without shear reinforcement
Vs = shear reinforcement (stirrup) shear strength
10.4.3 CONCRETE SHEAR STRENGTH
The concrete shear strength of deep beam is taken as :
dbM
dV120'f71
dVM5.25.3V w
u
uwc
u
uc ⎟⎟
⎠
⎞⎜⎜⎝
⎛ρ+⎟⎟
⎠
⎞⎜⎜⎝
⎛−=
50.2dV
M5.25.30.1u
u ≤⎟⎟⎠
⎞⎜⎜⎝
⎛−≤
[10.4]
where :
Vc = concrete shear strength (N)
Mu = ultimate flexure moment (Nmm)
Vu = ultimate shear force (N)
f’c = concrete cylinder strength (MPa)
d = effective depth
bw = width of beam web
ρw = longitudinal reinforcement ratio
Or the concrete shear strength can be determined as :
db'f61V wcc =
[10.5]
The maximum limit of concrete shear strength is :
db'f21V wcmaxc =−
[10.6]
The section must be enlarged if the ultimate shear force is not follows the condition below :
⎟⎠
⎞⎜⎝
⎛φ≤ db'f32V wcu
for 0.2d
Ln <
[10.7]
Or
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10 - 7
⎭⎬⎫
⎩⎨⎧
⎟⎠
⎞⎜⎝
⎛ +φ≤ db'fd
L10181V wc
nu
for 0.5d
L0.2 n ≤≤
[10.8]
10.4.4 STIRRUP SHEAR STRENGTH
The shear reinforcements must be provided in the deep beams follows the condition below :
cu VV φ≤ [10.9]
The strength of horizontal and vertical shear reinforcements is :
df12
dL11
sA
12d
L1
sAV y
n
h
vh
n
v
vs
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛⎟⎠
⎞⎜⎝
⎛−+
⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛⎟⎠
⎞⎜⎝
⎛+=
[10.10]
where :
Vs = horizontal and vertical stirrups shear strength (N)
Av = area of vertical stirrups
sv = spacing of vertical stirrups
Ln = clear distance of beam
d = effective depth
Avh = area of horizontal stirrups
sv = spacing of horizontal stirrups
fy = yield strength of stirrups
10.4.5 LIMITS OF SHEAR REINFORCEMENT
The minimum shear reinforcement area is :
( )vminv bs0015.0A =−
( )hminvh bs0025.0A =−
[10.11]
where :
Av-min = minimum vertical stirrups
Avh-min = minimum horizontal stirrups
b = width of beam
sv = spacing of vertical stirrups
sh = spacing of horizontal stirrups
The maximum spacing of shear reinforcement is :
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TABLE 10.2 MAXIMUM SPACING OF SHEAR REINFORCEMENT
VERTICAL STIRRUPS HORIZONTAL STIRRUPS
5dsv ≤
mm500sv ≤
3dsh ≤
mm500sh ≤
10.4.6 CRITICAL SECTION IN DEEP BEAM
The critical section to determines the ultimate shear force in the deep beam is :
TABLE 10.3 CRITICAL SECTION OF DEEP BEAM DUE TO SHEAR
UNIFORM LOAD CONCENTRATED LOAD
( )nL15.0x = ( )a50.0x =
10.4.7 STEP – BY – STEP PROCEDURE
The followings are the step – by – step procedure used in the shear design for deep beam, as follows :
Determine the critical section to calculate the ultimate shear force Vu.
UNIFORM LOAD CONCENTRATED LOAD
( )nL15.0x = ( )a50.0x =
Check the ultimate shear force, enlarge the section if the condition is not achieved.
⎟⎠
⎞⎜⎝
⎛φ≤ db'f32V wcu for 0.2
dLn <
⎭⎬⎫
⎩⎨⎧
⎟⎠
⎞⎜⎝
⎛ +φ≤ db'fd
L10181V wc
nu for 0.5
dL0.2 n ≤≤
Calculate the concrete shear strength Vc
dbM
dV120'f71
dVM5.25.3V w
u
uwc
u
uc ⎟⎟
⎠
⎞⎜⎜⎝
⎛ρ+⎟⎟
⎠
⎞⎜⎜⎝
⎛−=
50.2dV
M5.25.30.1u
u ≤⎟⎟⎠
⎞⎜⎜⎝
⎛−≤
If cu V5.0V φ< then no shear reinforcements needed, but for practical reason provide minimum
shear reinforcement.
( )hminvh bs0025.0A =−
( )vminv bs0015.0A =−
If cu VV φ> then provide the shear reinforcements.
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10 - 9
Calculate the ultimate shear force carried by the stirrups Vs.
cu
s VVV −φ
=
Choose the vertical and horizontal stirrups until the condition achieved.
df12
dL11
sA
12d
L1
sAV y
n
h
vh
n
v
vs
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛⎟⎠
⎞⎜⎝
⎛−+
⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛⎟⎠
⎞⎜⎝
⎛+=
Check the spacing of shear reinforcement sv and sh.
VERTICAL STIRRUPS HORIZONTAL STIRRUPS
5dsv ≤
mm500sv ≤
3dsh ≤
mm500sh ≤
If necessary check the chosen shear reinforcements for the basic design equation for shear
design.
scn VVV +=
df12
dL11
sA
12d
L1
sAV y
n
h
vh
n
v
vs
⎪⎪⎭
⎪⎪⎬
⎫
⎪⎪⎩
⎪⎪⎨
⎧
⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛⎟⎠
⎞⎜⎝
⎛−+
⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛⎟⎠
⎞⎜⎝
⎛+=
The design procedure above is repeats until the basic design equation for shear design is
achieved.
10.5 APPLICATIONS 10.5.1 APPLICATION 01 – FLEXURAL DESIGN OF SIMPLY SUPPORTED DEEP BEAM
500
275
50470
PROBLEM
Design the flexural reinforcement of simply supported deep beam above.
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10 - 10
MATERIAL Concrete strength = K – 300
Steel grade = Grade 400
Concrete cylinder strength = 9.243083.0'f c =×= MPa
85.01 =β
DIMENSION
b = 500 mm
h = 2750 mm
Concrete cover = 50 mm
d = 2700 mm
DESIGN FORCE
2625056000814.1qL
814.1Mu 22 =⎟
⎠
⎞⎜⎝
⎛ ××=⎟⎠
⎞⎜⎝
⎛= kgm
DEEP BEAM CHECKING
74.127004700
dLn == 0.574.10.1 ≤≤ Deep Beam Action
LEVER ARM
( ) ( )( ) 21002750250002.0h2L2.0jd =+=+= mm
POSITIVE REINFORCEMENT
262500000Mu = Nmm
( ) ( ) 34821004009.0
262500000jdf
MAy
us =
×=
φ=
++ mm2
421127005004004
9.24dbf4'f
A wy
cmins =×
×== mm2
47252700500400
4.1dbf4.1A wy
mins =×== mm2
4725As = mm2
Use 10D25, 4906254110D
4110A 22
s =⎟⎠
⎞⎜⎝
⎛ ×π=⎟⎠
⎞⎜⎝
⎛ π= mm2
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10 - 11
DISTRIBUTION OF POSITIVE REINFORCEMENT
275
50
43.75
5D25
( ) ( ) ( ) 550275020.0h20.05.437500005.0275025.0L05.0h25.0y ==<=−=−=
5.437y = mm
The longitudinal positive reinforcement must be distributed at the lower base of the beam with a
distance 437.5 mm from the bottom fiber.
We place 5D25 at each face of the section.
10.5.2 APPLICATION 02 – SHEAR DESIGN OF SIMPLY SUPPORTED DEEP BEAM
500
1077
015
000
d=70.5
1077
0 DESIGN SHEAR FORCEDIAGRAM
DIAGRAMSHEAR FORCE
1500
0
275
50470
PROBLEM
Design the web reinforcement of simply supported deep beam above.
MATERIAL Concrete strength = K – 300
Steel grade = Grade 240
Concrete cylinder strength = 9.243083.0'f c =×= MPa
85.01 =β
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10 - 12
DIMENSION
b = 500 mm
h = 2750 mm
Concrete cover = 50 mm
d = 2700 mm
DESIGN FORCE
705470015.0L15.0x n =×== mm
( ) 15078107704.1Vu == kg
150780Vu = N
LIMITATION CHECKING
381734227005009.243285.0db'f
32150780V wcu =⎟
⎠
⎞⎜⎝
⎛ ××=⎟⎠
⎞⎜⎝
⎛φ≤= N
The section is not enlarged.
CONCRETE SHEAR STRENGTH
88.12700150780
2625000005.25.3dV
M5.25.3u
u =⎟⎠
⎞⎜⎝
⎛×
−=⎟⎟⎠
⎞⎜⎜⎝
⎛−
0036.02700500
4906db
A
w
sw =
×==ρ
( ) N20521432700500262500000
27001507800036.01209.247188.1V
dbM
dV120'f71
dVM5.25.3V
c
wu
uwc
u
uc
=⎭⎬⎫
⎩⎨⎧
×⎟⎠
⎞⎜⎝
⎛ ××+=
⎟⎟⎠
⎞⎜⎜⎝
⎛ρ+⎟⎟
⎠
⎞⎜⎜⎝
⎛−=
1744322205214385.0Vc =×=φ N
87216117443225.0V5.0 c =×=φ N
DESIGN OF STIRRUPS
872161V5.0150780V cu =φ<= Provide minimum web reinforcement
For horizontal and vertical stirrups we choose 2 legs φ10.
15710412
412A 22
v =⎟⎠
⎞⎜⎝
⎛ ×π=⎟⎠
⎞⎜⎝
⎛ πφ= mm2
HORIZONTAL STIRRUPS VERTICAL STIRRUPS
sh Avh-min sv Av-min
125 ( )hvh bs0025.0A =
( )1255000025.0Avh ×=
156Avh =
200 ( )vv bs0015.0A =
( )2005000015.0Av ×=
150Av =