1
Torsion Design
Slides prepared byAzhar
Lecturer of Civil Engineering
2
Torque:- (Twisting Moment)
Moment about longitudinal axis.Corresponding deformation produced is twist or torsion.
J = Polar M.O.IT = Torsional Shearing Stressr = Radial distance from center of shaft
T, Twisting Moment
T
T
T
T
JTr
=Τ
Valid only if no point on bar is stressed beyond proportional limit
3
Assumption : Plane section remains plane
JTr
=Τ
Torsional Shearing Stress Diagram
• Here the emphasis will be on design
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Reasons of Torsion:
1. Eccentric Loading
Resultant
Torsion Member
• Spandrel Beams
• Curved Stairs Slab
• Curved Beam
• Cantilever Slabs
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T
T
mt
Torsion at a cantilever slab
T
mt
Torsion at an edge beam
A B
6
Reasons (contd…)
2. Different loadings and deformations of adjacent 2-D frames ( connecting members shall be subjected to torque)
3. If members are meeting at an angle in space then part of bending moment will become torque for other member.
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Soap Film Analogy• Slope at any point is equal to shear stress at
that point• The volume between the
bubble and the original plane(by the analogy of governingdifferential equation) is proportional to the total torque resistance( applied). Steeper the slope of tangent at any point greater will bethe shear stress.
• SFA is more useful for noncircular and irregular section for which formulas are not available.
AIR
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SFA For Rectangular Section
Square Cross Section
x
y
At the mid of Longer
side
Tmax
When material behaves inelastically
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Shear Flow
Shear Flow = Shear Stress x Thickness= Shear force per unit length
Shear flow is opposite to applied shear
Shear Flow ( Resultant shear at a point)
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Shear Flow (contd...)
At one section , two directional shear flow, ONE CANCELS OTHER
BOX SECTION
At one section only one directional flow. So closed sections are very efficient in resisting torque
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Formula For Maximum Torsion
yxTx
= 3αΤ max
cTx=
Valid for Rectangular Section only (PCC)
x ( smaller side)
y
C, Torsion constant = α x3yα depends on x / y ratio.
1/3.290.267.246.219.208
∞5.03.02.01.51.0Y/x
α
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C For I-Sectionx
x
x
y
y
yC = ∑ α x3yPlastic Torsion• Whole the section will yield in torsion, T = Ty
•Plastic analysis assumes uniform shear intensity all around the surface and all around the cross section.
Ty
Plastic analysis can be envisioned in terms of SAND HEAP ANALOGY
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Sand Heap Analogy
Put sand on a plate having a shape same as that of cross section ( Circular, Rectangular, Irregular) Slope of sand
heap is constant everywhere as T=Ty throughout
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Sand Heap Analogy (contd…)
Volume under the sand heap is proportional to the torque.
yxTx
= 3p
maxα
T
αp = 0.33 for y/x = 1.0= 0.5 for y/x = ∞
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General Cracking Pattern
τ
τ
T
Less inclined Almost Vertical
45o
Inclined cracks due to torsion which spiral around the member
Cracks due to Vu, Mu, and Tu
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General Cracking Pattern…
On one web face the torque shear adds ( & on the other face subtracts) relative to the vertical shear.
Section subjected to Shear
Section subjected to Torsion
• Cracks due to shear are parallel on both sides of the member butt due to torsion cracks are just like spiral around the member.
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Types of Torsion
There are two types of torsion depending upon its sources.
1- Equilibrium Torsion2- Compatibility Torsion
Equilibrium Torsion:Required for equilibrium & can’t be redistributed.
Pa
P x a, Can’t be redistributed,Must be here to maintain equilibrium
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P
Torque at B= P x L, it can’t be redistributedIf AC is not designed for torsion it will fail.
BC
LA
D
Compatibility Torsion
Compatibility means, compatibility of deformation b/w different members meeting together.
Redistribution/Adjustment is possible.
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Increased +veBending Moment
Decreased _ve B.M
Redistribution is allowed by the code
- +
PB
C• If AC is not designed for torque redistribution can be there, initially there will be some torque and if no resistance developed it will be redistributed and +ve BM in span BD will increase
A
D
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DesignACI CODE
Before 1995 1995 & Onwards
Design of shear and torsion was combined
Shear & Torsion design Separate. Reinforcement is combined
in the end.
• For shear design shear strength of concrete is considered.
• For torsion design shear strength of concrete is considered zero. Compressive strength is considered to an extent.
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Reinforcements
Two types of reinforcements are required to resist torque.i) Transverse reinforcement in the form of stirrups
( closed loop)ii) Extra reinforcement in longitudinal directions
specially in corners and around perimeter.
Open stirrups are for Shear not for Torsion
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Design (contd…)
If section is solid consider it hollow. Consider some outer surface to resist torque.
Hollow
t
Space Truss
Analogy
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Space Truss AnalogyShear stresses are considered constant over a finite thickness t around the periphery of member, allowing the beam to be represented by an equivalent tube.Within the walls of the tube torque is resisted by the shear flow q. in the analogy q is treated as constant around the perimeter.
Not in center but close to edge
t
Tt = thickness of assumed / actual hollow section yo
xo
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Space Truss Analogy (contd…)
xo & yo are measured from the center of wall.we are neglecting the internal area and using shear flow calculated from average shear stress, instead of maximum, so these two things are balancing each other. MAKING THE PROBLEM SIMPLER.
T
Aoxo
Ao = Area enclosed by the shear flow path,
Ao = xoyo
t
yo
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Space Truss Analogy (contd…)
T = Average shear stress in the wall thicknessq = shear flow = T x t
Moment about center due to shear flow must be equal to applied torque T, so
o
o
oo
oooo
oo
oo
A2T
=q
qA2=Tyqx2=T
yqx+yqx=T
2×]2y
×x×q+2x
×y×q[=TAs t×=q T
tq
=T
tA2T
=o
T
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ACI 11.6.1 CommentaryPrior to cracking due to torsion the thickness we
consider is
cp
cp
PA
75.0=t
Area enclosed by the wall center line, Ao
≈
Tensile strength of concrete is considered =( which is generally )
oA cpA32
'fc31
'fc21
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Cracking Torque
Torque at which cracking starts.For pure shear case principal tensile stress is equal to Torsional shear stress.
T
T
T
T f1 = T
cp
2cp
cp
cpcp
o
PA
'fc31
=Tcr
PA
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×A32
×'fc32
=Tcr
t×A×'fc32
=Tcr
'fc31
=Aot2T
=T
Tcr = Torque at which cracking starts
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ACI 11.6.1Neglect torsion effect if
ACI 11.6.2.2Compatibility torque is allowed to be reduced to
cp
2cp
PA
'fc12Φ
<=Tu
75.0=Φ4TcrΦ
<=Tu
cp
2cp
PA
×'fc3Φ Value of moment and shear
in adjoining member must be consider
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ACI 11.6.2.3Unless determined by more exact analysis it shall be permitted to take the Torsional loading from slab as uniformly distributed along the member (beam).
w Tu
LL
WL/2
dDesign Torque
TuL/2
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ACI 11.6.2.4
Face of the support can be considered as critical section if there is some point momentwithin d distance from face of support.
Face of support d
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Space Truss Analogy
N
NN
N
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Aoh = Area enclosed by the centerline of the outermost closed transverse reinforcement
N = Tensile force in longitudinal bars
Aoh = Shaded area
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fcd = Stress acting over compression diagonalN2 = longitudinal force required for equilibrium
at face 2
Face 2
θ
fcd Ni/2 = N2/2
V2yo
Total Area
Ni/2 = N2/2
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o2
2
A2Ty
=V
y×q=VFor Face - 2
Vi is resolved in Di & Ni
Di
θ
θCosθtSinA2T
=f
θCosθtSinyy×A2
T=f
t×θCosy1
×θSin
V=f
AreaDi
=f
ocd
o
oocd
o
icd
cdVi
θSinVi
=Di
θSin=DiVi
Ni
θθ
y
y cos θArea of this face is (y Cos θ) t
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Face - 2At = Area of one leg of
closed stirrup.n = number of stirrups
intercepted by Torsional crack
θ
Atfyv
Atfyv
V2 yo
sθCoty
=n o2
sFor vertical equilibrium yo Cot θ Tn = Nominal Torsional
moment strength
Ao ≈ 0.85Aoh, or get by exact analysis
θ = 30o to 60o
= 45o better for non-prestressed members
θCot×s
fAA2=Tn
A2yT
=fA×s
θCotyV=fA×n
yvto
o
onyvt
o
2yvt2
ACI 11.6.3.6
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Face – 2 (contd…)
Di
θ
Vi
Ni
ACI 11.6.3.7Al = Total area of longitudinal steel to resist torsion
1
( )
θ=
×θ
=×
+θ
=
×θ=
θθ=
θ=
θ=
2
y
yvh
t
h
2yvt
y
oo
2yvt
i
oyvt
2
o
oyvto2
oo
2
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Cotff
Ps
AA
PsCotfA
fA
y2x2sCotfA
N
yCotsfA
N
A2Coty
]s
CotfAA2[N
CotyA2Tn
N
CotVN
2
ll
ll
∑
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θ=
CotfA2T
sA
yvo
ntGet and put in Eq-1
• Smaller θ value we use, lesser shall be the stirrups but more longitudinal reinforcement & vice versa. θ Cot θ
Total Reinforcement (Shear + Torsion)
sA2
sA
sA tvtv +=+
For closed loop stirrup
2 is not here, because we use twice the area of shear reinforcement
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ACI 11.6.5.3Min longitudinal steel
]ff
[Ps
A_
fA
12'fc5
Ay
yvh
t
y
cpmin
lll ×=
y
w
fb
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sAt
>=
ACI 11.6.5.2
y
w
y
w
tv
fb
31
)ii
fb
'fc483
)i
)s
A( min
+is larger of
Where
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ACI 11.6.6 (Spacing requirement)
Transverse stirrups spacing should not be more thani) Ph / 8ii) 300 mm
Longitudinal bar spacing should not be more than 300 mm.
Dia of longitudinal bar should not be less thani) s / 24ii) 10 mm
•Distributed around the perimeter of closed stirrup.•At least one in each corner.
s = spacing of shear reinforcement
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ACI 11.6.6.3Torsional reinforcement shall be provided for a distance of at least ( bt + d) beyond the point theoretically required
bt = width of torsion section
Check for the x-sectional dimensions for combined shear and torsion
ACI 11.6.3.1
)'fc32
dbV
()A7.1pT
()db
V(
w
c22
oh
hu2
w
u +Φ<=+
If this equations does not satisfy increase x-sectional dimensions
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Design Procedure1. Plot SF & BM diagram, design for flexure, do partial detailing.2. Draw factored torque diagram, get torque at different sections ad
also get critical torque.3. Neglect torsion if
4. If compatibility torsion it can be reduced to
5. Check the x-sectional dimensions for combined actions of shear and torque, if not ok increase dimensions.
6. Design for shear and calculate Av/s.7. Compute the torsion transverse reinforcement At/s.8. Calculate total transverse reinforcement
4T
TucrΦ
<=
cp
2cp
PA
×'fc3Φ
sA2
sA
sA tvtv +=+
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Design Procedure (contd…)
9. Calculate extra longitudinal reinforcement
10. Combine with flexural reinforcement and decide placing according to the code requirement.
θ= 2
y
yvh
t Cotff
Ps
AA
ll
]ff
[Ps
A_
fA
12'fc5
Ay
yvh
t
y
cp
minll
l ×=&
If depth of beam is >0.9m provide skin reinforcement.
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ExamplePanel C.L
450
525175
133
d
100.3
65
25
fc’ = 25Mpa
fy = 420Mpa
d = 450mm
Cover up to center of stirrup =45 mm
Cover up to center of stirrup for flange (slab part) = 25mm
150 3000
118 113.6
150
SFD (kN)
Torque D (kN-m) 18
110 Moment Envelop(kN-m)
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Statement : Design part of beam closer to junction and mid span section for combined action of shear, bending and torque.
Solution:M-ve = 133 kN-m
Asmin =
= 675.0 mm2
As = 815 mm2
M +ve = 110 kN-mAs = Asmin = 675.0 mm2
dbf wy
4.1dbf wy
4.1l
25x450x45010x85
'fcbdMu
'fcR
0506.0fy
'fc85.0s
2
6
2==
==
45
b is lesser of :i) bw + 4hf = 450 + 700ii) bw + hw = 450 +350 = 800 mm
So b = 800 mmTu 65 kN-m
b =800 mm
525 hw = 350175
450
cp
2cp
PA
'fc12
4/TcrΦ
=
Acp = 450 x 525 + 175 x 350 = 297500 mm 2
Pcp = ( 800 + 525 ) x 2 = 2650 mm
kNm44.1010
12650
29750025
1275.0
PA
'fc12
4/Tcr 6
2
cp
2cp
=××=Φ
= < Tu
We cannot redistribute because we don't know about the other members. Their design will change if we redistribute.
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730
Aoh = 210050 mm2
Ph = 2370 mm 125525–25-45= 455Step # 6
Check for the x-sectional dimensions 330
2)A7.1pT
()db
V(LS 2
oh
hu2
w
u +=
450 – 90 = 360Mpa
xxx
xxLS 113.3)
2100507.123701065()
4504501030.100( 2
2
62
3
=+=
For right hand side we need Vc
RSLS
Mpa125.3'fc65
x75.0'fc65
xRS
]'fc32
'fc61
[RS
db'fc61
V wc
<
==Φ=
+Φ=
=
So dimensions are O.K.
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Step # 7 (independent design for shear)
(Vu)max = 100.3 kN
0s
A0 V
2
3.632
56.1262
56.12675.16875.075.168
1000/4504502561
'61
v
s
=
=
Φ<<Φ
==Φ
==Φ=
=
=
cuc
wc
VVV
kNVckNxVc
kNVc
xxVc
dbfcV
So
For the actually applied shear force. However, min shear reinforcement is required for combined shear and torsion. To be calculated latter.
All shear is taken by concrete
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Step # 8 (Transverse reinforcement required for torque)
legmmmmxxxx
xsAt
xfAxxTu
sA
AA
yvoh
t
oho
o
//2578.042021005085.0275.0
1065
85.02
85.045
6
==
Φ=
==θ
Step # 9 (Total Transverse reinforcement)
mmdmms
mms
legmmmmxsx
sA
sA
sA tvtv
2252
4502
120
123
//156.1578.020712
2
2
==<=
=
=+=
+=+
Area of #10 bar
Check is required because Vu>ΦVc/2
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Minimum shear + Torsional reinforcement
14.1375.0420450
31min)(
5.28',31)( min
<==+
<=+
stAv
Mpafcfb
sA
yv
wtv
O.K.
Transverse stirrup spacing should be less than:
1. Ph / 8 = 2370 / 8 = 296 mm
2. 300 mm O.K.
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Step # 10 (Longitudinal Reinforcement)
θ= 2
y
yvh
t Cotff
Ps
AA
ll
22 1370112370578.0 mmxxxCotff
sAA
y
yvh
t ==
= θρ
ll
][_12
'5min
lll
y
yvh
t
y
cp
ff
sA
fAfc
A ρ×=
210434204502370157.0
42029750025
125][_
12'5
min mmxxxff
PsA
fAfc
Ay
yvh
t
y
cp ==×=ll
l -
Replaced by bw/(3fy)Al = 1370 mm2
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(Longitudinal Reinforcement)
Maximum spacing = 300 mmMinimum dia = 10 mmFor three layers, Al / 3 = 1370 / 3 = 457 mm2
Top Flexural + Torsional Steel = 815 + 457 = 1272 mm2
Bottom Flexural + Torsional Steel = 1/4A+ + 457 = 626 mm2
5 # 19 for top layer
4 # 19 + 1 # 13 for bottom layer
For middle layer = 457 mm2 2 # 19
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5 # 19
2 # 19# 10 @ 120 mm c / c
4 # 19 Support Section
1 #13
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Concluded
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