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Deflection of Beams. (Credit for many illustrations is given to McGraw Hill publishers and an array of internet search results). Parallel Reading. Chapter 7 Section 7.1 Section 7.2 Section 7.3 Section 7.6 (Do Chapter 7 Reading Assignment Work). - PowerPoint PPT Presentation
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Deflection of Beams
(Credit for many illustrations is given to McGraw Hill publishers
and an array of internet search results)
Parallel Reading
Chapter 7 Section 7.1 Section 7.2 Section 7.3 Section 7.6 (Do Chapter 7 Reading Assignment Work)
Previously on Engr 350
Bending moments deflect beamsalong the arc of a circle.
The degree of curvature depends on the magnitude of the Bending Moment, Young’s Modulus, and the beams Moment of Inertia
But as the Stomach Turns We Recently Learned that Bending Moments Vary over the Length of
Beams
We learned how to do bending momentDiagrams.
But if the bending moment changesOver the length x, then the degree ofBeam curvature must also be changing!
How Can We Get the Deflection of a Beam Under These Conditions?
That curvature is thesecond derivative ofvertical displacementat a distance x downthe beam
Time for some algebraic substitution!
By algebraBy substitution
Let Me See
y
If I have this
And I want this.
Oh Yes! How about integrating twice (Oh Gosh no – not calculus please)
Ok it May Suck But Lets Get to It
Or with substitution
If we integrate once
We get the angle on the deflectioncurve.
If We Integrate Twice
We can get the displacement of the beam
Dare We Try This?This is a cantilever beam witha simple end load.
Value of x
Lx
M
L
Our Moment Diagram
P
M=-P*x
Lets Integrate Once
= -P*x
cxPdx
dyEI
1
2
2
1
Houston – We Have a Problem
cxPdx
dyEI
1
2
2
1
I don’t see what c1 is
The first integration gives us the bend angle
Off the Wall Boundary Conditions
cLP 1
2
2
10
cxPdx
dyEI
1
2
2
1
The angle of bend at thewall is 0.
X L
So when x = L Lc P2
1 2
1
Anyone going to giveme genius credit for this?
Integrate Twice
Lx PPdx
dyEI
22
2
1
2
1
cLx xPPEIy2
23
2
1
6
1
SlopeAngle
Integrate
I get the equation for the elastic curve
We have another unknown c(I feel a boundary condition coming on).
Checking Out the Wall
cLx xPPEIy2
23
2
1
6
1
cLL PP2
33
2
1
6
10
What is the displacement at the Wall
We better hope y=0 at x=L
xL
Lc P3
2 3
1
Finishing Up Our Answer
LLx PXPPEIy323
3
1
2
1
6
1
)23(6
323
LLx xEI
Py
EI
PLy6
3
0
Get y by itself
And when x = 0
y0
Assignment 23
Do Problem 7.3.5Do Problem 7.3.16
Remember – Show and explain your work step by step. Scribbles with a circledAnswer at the end – even if the answer is right, will be marked wrong.
We Can Well Imagine Some of the Loadings and Bending Moments We Might Face Could be
Unpleasant
Are We Always Going to FaceDouble Integrations with BoundaryConditions?
Not Necessarily
As with mostcommon integralsthere is alwayssomeone makinga standard table.
Lets Try Doing One With a Table
Once upon a time there was aUniformly loaded Cantilever Beam
12 ft
100 lbs/ft
5.5 in
5.5 in
(6X6 woodBeam)
δ
We want the deflection distance δ
We Look Up the Solution On a Table
The displacement at the endOf the beam.
Plug In TimeLoad = 100 lbs/ft = 8.33 lbs/inch
Length 12 ft = 144 inches
Young’s Modulus forWood 1,750,000 psi(From Table F.2 in yourBook)
12
4
bI
76.26
3.35 in =
But Lets Face ItYou Can’t Make a Table of Everything
There are ways of extending basic tablesto complex situations using the methodof superposition. (Linear functions can beadded one atop another to make somepretty complex stuff.
How wouldyou find theanswer if youcould?
The Need for Method of Superposition
What are the chances I will findThis situation in a table?
I’ve got a bad feeling about that.
I Did See Some Stuff in the Tables that Relates
Here is a set for a uniformly loaded beam
Here is a point loadedbeam
The Method of Superposition
Lets Try a Cantilever Beam With A Uniform Load Only Over the Outer Half
Tables Give Me a UniformlyLoaded Cantilever
So How Do I Get a Half Loaded Cantilever?
We apply superposition
Of Course There Is a Problem
If the tables don’t havehalf loaded cantilevers, howdo I get a half loaded cantilever?
The Bending Moment Diagram Gives Us a Clue
6 ft
12 ft
M
How much of a bending momentdid the maid in the parlor put onthe outer end of that beam?
You say None!
So that outer part of the beam isnot bent at all and only the innernalf has anything to do with theproblem.
Therefore We Can Start to Get One of Our Superposition Components from a half
length cantilever.
Displacement halfway out to the beam end is
EI
w
EI
Lw L1288
2(4
4
)
Now for the Rest of the Story
Because there is no bending theremaining half of the beam justkeeps going straight up.
But remember – we got the slopeAngle from our first integration soThat is bound to be in the table.
Going up for a distanceOf L/2
We Are Now Able to Give the Displacement for the cantilever with loading on the inner
half
Now Back to Applying Superposition
How Might the FE Test Our Mastery of Beam Deflections?
What is going to happen with this load?
A- That eccentric 50N load is going to twistthe main rod
B- The shear diagram will show that the 50Nload will transfer and become a 50 N pointload on the main rod which will act asa cantilever.
We need to recognize that withSuperposition we can solve eachproblem separately withoutinterference from the other.
This is Just a Cantilever with a Point Load at the End
50 N
2 meters
From a deflection table(or double integration like we did in class)
Going for a Quick Plug
50 N2M
Get Young’s ModulusFrom the problem
3 cm
We just need I for aCircular rod
Moment of Inertia for a Circular Rod
From tables such as those at the back of yourBook.
4
4
rI
=3.976X10-8
And Finishing
And Pick D!
Assignment 24
Do problems 7.3-18 7.3-21
Remember to explain and show step by step how you are solving theseProblems. Putting down and answer with some scribbled steps does notCut it and will get you marked wrong even if you chosen number orExpression is correct.