shear stresses on beam (mechanics of solids)
TRANSCRIPT
SHREE SA’D VIDYA MANDAL INSTITUTE OF TECHNOLOGY
DEPARTMENT OF CIVIL ENGINEERING
Subject:-Mechanics Of Solids
Topic:-Shear stresses on beam
Presented by:-Name Arvindsai
Dhaval Chavda
Fahim Patel
Navazhushen Patel
Enrollment no.130454106002
130454106001
140453106005
140453106008
Topics To Be Covered1. Shear Force2. Shear Stresses In Beams3. Horizontal Shear Stress4. Derivation Of Formula 5. Shear Stress Distribution Diagram6. Numericals
Shear forceAny force which tries to shear-off the
member, is termed as shear force.
Shear force is an unbalanced force, parallel to the cross-section, mostly vertical, but not always, either the right or left of the section.
Shear StressesTo resist the shear force, the element
will develop the resisting stresses, Which is known as Shear Stresses().
= = Shear force
Cross sectional area
SA
Example:-For the given figure if we want to
calculate the ..Then it will be Let shear force be S
=S/(bxd)d
b
S
Shear Stresses In Beams Shear stresses are usually maximum
at the neutral axis of a beam (always if the thickness is constant or if thickness at neutral axis is minimum for the cross section, such as for I-beam or T-beam ), but zero at the top and bottom of the cross section as normal stresses are max/min.
NANA
NA
When a beam is subjected to a loading, both bending moments, M, and shear forces, V, act on the cross section. Let us consider a beam of rectangular cross section. We can reasonably assume that the shear stresses τ act parallel to the shear force V.
v
n
V
z
m
O
b
h
Shear stresses on one side of an element are accompanied by shear stresses of equal magnitude acting on perpendicular faces of an element. Thus, there will be horizontal shear stresses between horizontal layers of the beam, as well as, Vertical shear stresses on the vertical cross section.
m
n
Horizontal Shear StressHorizontal shear stress occurs due
to the variation in bending moment along the length of beam.
Let us assume two sections PP' and QQ', which are 'dx' distance apart, carrying bending moment and shear forces 'M and S' and 'M+ ∆M and S+ ∆S‘ respectively as shown in Fig.
Let us consider an elemental cylinder P"Q" of area 'dA' between section PP' and QQ' . This cylinder is at distance 'y' from neutral axis.
F)(FQ'Fbe, shallcylinder in force
horizontal unbalancedHence,
dA δσσQ'F
dAx σp'F be, shallQ' and'P'at
stresses these to due forces The
xyIdMMδσσ
be, will'Q'at stress bendingSimilarly,
xyI
Mσ
be will P"at stress bending Hence,
This unbalanced horizontal force is resisted by the cylinder along its length in form of shear force. This shear force which acts along the surface of cylinder, parallel to the main axis of beam induces horizontal shear stress in beam.
Aax yI
dM
dA)(σd(σpFQFHF
DERIVATION OF FORMULA: SHEAR STRESS DISTRIBUTION ACROSS BEAM
SECTION Let us consider section PP' and QQ' as
previous. Let us determine magnitude of horizontal
shear stress at level 'AB' which is at distance YI form neutral axis.
The section above AA' can be assumed to be made up of numbers of elemental cylinder of area 'dA'. Then total unbalance horizontal force at level of' AS' shall be the summation of unbalanced horizontal forces of each cylinder.
a_yx
I
dMFM
dA.y1yy
1yy.X
I
dMdA.xy
1yy
1yy I
dMHF
Here, y = distance of centroid of area above AB from neutral axis, And a= area of section above AB.
This horizontal shear shall be resisted by shear area ABA'B‘ parallel to the Neutral plane. The horizontal resisting area here distance of centroid of area above AB from neutral axis and a=area of section above AB.
Ah = AB x AA’=b x dx where ‘b’is width of section at
AB.
We know that shear force is defined as S=dM/dx
Therefore, horizontal shear stress acting at any level across the cross sections.TH= Say / Ib
Ibyax
dxdM
dx.b
ayI
dM
xAF
forceshear horizontal resistingshear stressshear Horizontal
_
H
_
H
MH
SHEAR STRESS DISTRIBUTION DIAGRAM
1.Rectangular section
2.Circular section
maxNA
maxNA
3.Triangular section
4.Hollow circular section
h/2 max
avg
NA
maxNA
5.Hollow Rectangular section
6. “I” section
maxNA
maxNA
7. “C” section
8. “+” section
maxNA
maxNA
9. “H” section
10. “T” section
maxNA
maxNA
Numericals
Rectangular section sum Example-1: Two wooden pieces of a
section 100mm X100mm glued to gather to for m a beam cross section 100mm wide and 200mm deep. If the allowable shear stress at glued joint is 0.3 N/mm2 what is the shear force the section can carry ?
KN4FeShear forcKN4N4000F
10061067.6650810000F3.0
I.byFAτ
Νοω.
mm502
100Υ
2mm000,10100100A
joint. glued above beam ofpart consider
2N/mm3.0τ
4mm61067.66123200100I
:Solution
100mm
100mm
100mm
wooden piece
Circular section sumExample-2: A circular a beam of 100mm Diameter is subjected to a Shear force of 12kN, calculate The value of maximum shear Stress and draw the variation of shear stress along the Depth of the beam.
22.03N/mm
1.531.33
1.33Now.
21.53N/mm 7853.98
31012AF
27853.98mm21004πA
N3101212kNF
τmax
τavgτmax
τavg
:Solution
22.03N/mmτmax D
=100mmNA
I section sumExample-3: A rolled steel joist of I section overall 300 mm deep X 100mm wide has flange and web of 10 mm thickness. If permissible shear stress is limited to 100N/mm2, find the value of uniformly distributed load the section can carry over a simply supported span of 6m.
Sketch the shear stress distribution across the section giving value at the point of maximum shear force.
100mm
300mm
10mm
150mm
__
Y
10NA
25.63N/mminm 261.63N/mmavg
2100N/mmmax
mm10 b(givan)3N/mm100 τ
3mm257500
)7010140()14510110(yAN.A.at maximum be will stressshear
4mm61055.64Ixx
I 3xxI 2xxI 1xxIxx
4mm61029.180280012
328010I 2xx
4mm61013.232145110012
310110
2ahIgI 1xx
Solution:
kN/m56.83w
kN/mm64.836
36.501l
Wwu.d.l.
)load totalkN(36.501W2
W68.250
2WNow, F
kN68.250FN6.250679F
1061055.66257500F100
I.byFaτ
Triangular section sum Example-4: A beam of triangular section
having base width 150mm and height 200mm is subjected to a shear force of 20kN the value of maximum shear stress and draw shear stress distribution diagram.
2N/mm2
33.15.1 τave5.1τmax
2N/mm33.115000
31020AF
τave
2 mm1500020015021
bh21A
20kNF forceshear
Solution:
max=2N/mm2
avg=1.33N/mm2
200mm
150mm
h/22/3.hNA
Cross section sumExample-5: FIG Shows a beam cross section subjected to shearing force of 200kN. Determine the shearing stress at neutral axis and at a-a level. Sketch the shear stress distribution across the section.
50mm
100mm100mm 100mm
100mm
100mm
50mm50mm X
215.48N/mm 50610129.161005000310200
I.byFAτ
50mmb200kNF
100mm5050y
25000mm10050A
:line b_bat stressshear
4mm610129.6
]12
3100100[2]12
330050[xxI
:Solution
2N/mm03.5 250610129.16
812500310200
I.byFA τ
3mm812500
25)50(250100)100(50y A:axisx_x at stressShear
2mm/N09.32505015.48X
5.03N/mm2
15.48N/mm2
3.09N/mm2
100mm100mm 100mm
100mm
100mm
50mm50mm X
50mm
NA
Inverted T section sumExample-6: Shows the cross section of a beam which is subjected to a vertical shearing force of 12kN.find the ratio the maximum shear stress to the mean shear stress.
60mm
20mm
60mm
20mm
46
23
23
xx2xx1
21
2211
22
2
12
1
mean
max
3
mm101.36
30)1200(501212
602010)(30120012
20 60
II I inertia ofMoment
30mm 12001200
50 1200 10 1200AA
yAyA y
50mm202
60y 1200mm6020 A
10mmy 1200mm2060 A AxisNeutral of Position
ττ:find To
N101212kNs :Given:Solution
2.21ττ
511.029
ττ
stressshear mean to stressshear maximum of Ratio
5MPa2400
1012area c/s
forceShear τ
stressshear Mean
11.029MPa101.3620
2510001050 τ
axis neutral above areaConsider bI
SAy τ
axis, neutralat produced stressshear Maximum
mean
max
mean
max
3
mean
6
3
max
max=11.29MPA
avg=5 MPAmin =2.21 MPA
20mm
60mm
20mm
NA
L section sum Example-7: An L section 10mm X 2mm show in the fig.
is subjected to a shear force F. Find the value Of shear force F if max. shear stress developed is 5N/mm2.
4208.09mm
24)(6.771612
382
2ahgIxx2I
4106.12mm26.77)(92012
3210
2ahgIxxI
6.77mm1620
416920
2a1a2y2a1y1a
y
4mm2y
216mm282a
9mm1y
220mm2 101a
:Solution
2mm
2mm
10mm
10mm
6.77mm __Y
3.23mm
68.14NF 2314.21
46.11F5
I.byFA τ
46.11mm 2
1.232)(1.232.232)(10yA
N.A.at occur will stressshear Maximum
314.21mm 208.09106.12
III
3
4
xx2xx1xx
Tee section sum Example-8: A beam is having and subjected to
load as shown in fig. Draw shear stress distribution diagram across the section at point of maximum shear force, indication value at all important points.
100kN
A B
3m 3m
100200 200
300
100
25
275mm __Y
K.N 502100F
4mm 3750000275)-(350(500x100)xy A
: web & flange of junction theat stressShear
4mm 91.02x10
6693.57x106322.91x10 12
2150)-100)x(275x (500 3100x(300)
12 275)-(350x (500x100) 3500x(100)I
mm 275 80000
612x10
100)x (300100)x (500 100)x15x (300100)x350x (500Y
XX
2
29max
3
2avg
29min
N/mm85.1 500x10x02.1
3781250x3)10x(50 I.b
yFA τ
mm3781250 )5.12x25x100()75x100x500 (yA
axis, neutralat stressShear
mm N/84.1 100
500x367.0 τ
to, increase suddenly will stressShear
mmN/367.0 500x10x02.1
3750000x3)10x(50 I.b
yFA τ
mm500b
max=1.85N/mm2
avg=1.84N/mm2
min=0.367N/mm2100200 200
300
100
25
275mm __Y
NA
I section sum Example-9: Find the shear stress at the
junction of the flange and web of an I section shown in fig. If it is subjected to a shear force of 20 kN.
100mm
200mm
20mm
100mm
20
2
26
3
3_
463
xx
mm/N57.4915.0x2
100 =
to, increased suddenly will stressShear
mm915.0100x10x36.39
180000x10x20b.I
_yAF
mm18000090x)20x100(yA
:web and flang of junetionat streesShear
mm10x36.3912
200x100I
N.k20F
:Solution
100mm
200mm
20mm
100mm
20NA
2N/mm 0.915τmin
2N/mm915.0avg
2N/mm75.4max
Rectangular section sum Example-10: A 50mm x l00mm in depth
rectangular section of a beam is s/s at the ends with 2m span the beam is loaded with 20 kN point load at o.5m from R.H.S. Calculate the maximum shearing stress in the beam.
20kN
RB
B
0.5m
2.0mR
A
A
2N/mm5.4
00.35.1 τave5.1τmax
2N/mm00.3100x5031015
AF
τave
kN51520B
B
B
RkN15R
5.1x202xRat Amoment Taking
Solution:
max=4.5N/mm2
50mm
100mm NA