7.3 Relations between Distributed Load, Shear and Moment 7.3 Relations between Distributed Load, Shear and Moment Distributed Load Consider beam AD subjected to an arbitrary load w = w(x) and a series of concentrated forces and moments forces and moments Distributed load assumed positive when loading acts downwards
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7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Distributed Load� Consider beam AD subjected to an arbitrary
load w = w(x) and a series of concentrated forces and momentsforces and moments
� Distributed load assumed positive when loading acts downwards
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Distributed Load
� A FBD diagram for a small segment of the beam having a length ∆x is chosen at point x length ∆x is chosen at point x along the beam which is not subjected to a concentrated force or couple moment
� Any results obtained will not apply at points of concentrated loadings
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Distributed Load
� The internal shear force and bending moments shown on the FBD are assumed to act in the positive senseassumed to act in the positive sense
� Both the shear force and moment acting on the right-hand face must be increased by a small, finite amount in order to keep the segment in equilibrium
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Distributed Load
� The distributed loading has been replaced by a resultant force ∆F = w(x) ∆x that acts at a fractional distance k (∆x) from the right end, fractional distance k (∆x) from the right end, where 0 < k <1
( )[ ]2)()(
0)()(;0
)(
0)()(;0
xkxwxVM
MMxkxxwMxVM
xxwV
VVxxwVFy
∆−∆=∆
=∆++∆∆+−∆−=∑
∆−=∆
=∆+−∆−=∑↑+
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Distributed Load
Slope of the = Negative of
xwdx
dV−= )(
Slope of the = Negative of
shear diagram distributed load intensity
Slope of = Shear moment diagram
Vdx
dM=
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Distributed Load
� At a specified point in a beam, the slope of the shear diagram is equal to the intensity of the distributed loaddistributed load
� Slope of the moment diagram = shear
� If the shear is equal to zero, dM/dx = 0, a point of zero shear corresponds to a point of maximum (or possibly minimum) moment
� w (x) dx and V dx represent differential area under the distributed loading and shear diagrams
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Distributed Load
Change in = Area under
∫−=∆ dxxwVBC )(
Change in = Area under
shear shear diagram
Change in = Area under
moment shear diagram
∫=∆ VdxM BC
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Distributed Load
� Change in shear between points B and C is equal to the negative of the area under the distributed-loading curve between these distributed-loading curve between these points
� Change in moment between B and C is equal to the area under the shear diagram within region BC
� The equations so not apply at points where concentrated force or couple moment acts
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Force
� FBD of a small segment of the beam
FVF −=∆=∑↑+ ;0
� Change in shear is negative thus the shear will jump downwards when F acts downwards on the beam
FVFy −=∆=∑↑+ ;0
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Force
� FBD of a small segment of the beam located at the couple momentmoment
� Change in moment is positive or the moment diagram will jump upwards MO is clockwise
OMMM =∆=∑ ;0
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Example 7.9
Draw the shear and moment diagrams for the
beam.beam.
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
Support Reactions
� FBD of the beam� FBD of the beam
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
SolutionShear DiagramV = +1000 at x = 0V = 0 at x = 2Since dV/dx = -w = -500, a straight negative sloping Since dV/dx = -w = -500, a straight negative sloping line connects the end points
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
Moment Diagram
M = -1000 at x = 0
M = 0 at x = 2M = 0 at x = 2
dM/dx = V, positive yet linearly decreasing from
dM/dx = 1000 at x = 0 to dM/dx = 0 at x = 2
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Example 7.10
Draw the shear and moment diagrams for the
cantilevered beam.cantilevered beam.
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
Support Reactions
� FBD of the beam� FBD of the beam
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� At the ends of the beams,
when x = 0, V = +1080
when x = 2, V = +600when x = 2, V = +600
� Uniform load is downwards and slope of the shear diagram is constant
dV/dx = -w = - 400 for 0 ≤ x ≤ 1.2
� The above represents a change in shear
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
6004801080)480(
480)2.1(400)(
02.1=−=−+=
−=−=−=∆
==
∫VV
dxxwV
xx
� Also, by Method of Sections, for equilibrium,
� Change in shear = area under the load diagram at x = 1.2, V = +600
600
6004801080)480(02.1
+=
=−=−+===
V
VVxx
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� Since the load between 1.2 ≤ x ≤ 2, w = 0, slope dV/dx = 0, at x = 2, V = +600
Shear Diagram
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� At the ends of the beams,
when x = 0, M = -1588
when x = 2, M = -100when x = 2, M = -100
� Each value of shear gives the slope of the moment diagram since dM/dx = V
at x = 0, dM/dx = +1080
at x = 1.2, dM/dx = +600
� For 0 ≤ x ≤ 1.2, values of the shear diagram are positive but linearly increasing
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� Moment diagram is parabolic with a linearly decreasing positive slope
Moment Diagram� Moment Diagram
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� Magnitude of moment at x = 1.2 = -580
� Trapezoidal area under the shear diagram = change in momentchange in moment
58010081588
1008
1008)2.1)(6001080(2
1)2.1(600
02.1
−=+−=
+=
+=−+=
=∆
==
∫
xxMM
VdxM
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� By Method of Sections,
at x = 1.2, M = -580
� Moment diagram has a constant slope for 1.2 ≤ x ≤ 2 since dM/dx = V = +600
� Hence, at x = 2, M = -100
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Example 7.11
Draw the shear and moment diagrams for the
shaft. The support at A is a thrust bearing
and the support at B is a journal bearing.and the support at B is a journal bearing.
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
Support Reactions
� FBD of the supports� FBD of the supports
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� At the ends of the beams,
when x = 0, V = +3.5
when x = 8, V = -3.5when x = 8, V = -3.5
Shear Diagram
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution� No distributed load on the shaft, slope dV/dx = -
w = 0� Discontinuity or “jump” of the shear diagram at
each concentrated forceeach concentrated force� Change in shear negative when the force acts
downwards and positive when the force acts upwards
� 2 kN force at x = 2m changes the shear from 3.5kN to 1.5kN
� 3 kN force at x = 4m changes the shear from 1.5kN to -1.5kN
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� By Method of Sections, x = 2m and V = 1.5kN
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� At the ends of the beams,
when x = 0, M = 0
when x = 8, M = 0when x = 8, M = 0
Moment Diagram
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� Area under the shear diagram = change in moment
===∆ ∫
� Also, by Method of Sections,
mkNMmx
MM
VdxM
xx
.7,2
7707
7)2(5.3
02
==
=+=+=
===∆
==
∫
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Example 7.12
Draw the shear and moment diagrams for the
beam.beam.
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
Support Reactions
� FBD of the beam� FBD of the beam
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� At A, reaction is up,
vA = +100kN
� No load acts between A and C so shear remains � No load acts between A and C so shear remains constant, dV/dx = -w(x) = 0
� 600kN force acts downwards, so the shear jumps down 600kN from 100kN to -500kN at point B
� No jump occur at point D where the 4000kN.m coupe moment is applied since ∆V = 0
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
Shear Diagram
� Slope of moment from A to C is constant since dM/dx = V = +100
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
Moment Diagram
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� Determine moment at C by Method of Sections where MC = +1000kN or by computing area under the moment
∆MAC = (100kN)(10m) = 1000kN
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� Since MA = 0, MC = 0 + 1000kN.m = 1000kN.m
� From C to D, slope, dM/dx = V = -500
For area under the shear diagram between C and � For area under the shear diagram between C and D, ∆MCD = (-500kN)(5m) = -2500kN, so that MD
= MC + ∆MCD = 1000 – 2500 = -1500kN.m
� Jump at point D caused by concentrated couple moment of 4000kN.m
� Positive jump for clockwise couple moment
7.3 Relations between Distributed Load, Shear and Moment
7.3 Relations between Distributed Load, Shear and Moment
Solution
� At x = 15m, MD = - 1500 + 4000 = 2500kN.m
� Also, by Method of Sections, from point D, slope dM/dx = -500 is maintained until the slope dM/dx = -500 is maintained until the diagram closes to zero at B