ENGI 4312 Mechanics of Solids I Prof. Steve Bruneau, EN.4013 Ph 737-2119 [email protected]T.A.s Rizk, Dawood, Guo [email protected][email protected][email protected]COURSE TEXT READING INSTRUCTOR TOPIC NOTES OK, Midterm stuff: Class average was huge! = 86% - including 26 aces (100s)! Much better results than last year Beam Diagrams were very well done. A few had a bad day – don’t worry you can make it up in the final. Now lets go over it . . .
enseña a localizar el esfuerzo cortante maximo en vigas
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The two cylindrical rods AB and BC are welded together at B and loaded as shown. Knowing that d1 = 50 mm and d2 = 30mm, find the average normal stress at the midsection of rod AB and rod BC
40 kN
30 kN
BONUS for 5 marks: What will be the total extension of the two cylinders if AB is brass with E = 101 GPa and BC is Aluminum E = 68.9 GPa?
The torques shown are exerted on pulleys A, B, and C. Knowing that both shafts are solid determine the maximum shearing stress in shaft AB and shaft BC
The torques shown are exerted on pulleys A, B, and C. Knowing that both shafts are solid and made of brass (G=39 GPa), determine the angle of twist between A and B, and, A and C.
400 Nm
1200 Nm
800 Nm(same sketch as Problem 2)
Twist (rad) = TL/(JG)
Twist(AB) = 400x1.2/(7.95x10^-8x39x10^9)
Twist(AB) = 0.155 radians or 8.88 deg - ccw
Twist(BC) = 800x1.8/(2.5x10^-7x39x10^9)
Twist(BC) = 0.147 radians - cw
Twist(AC) = 0.155-0.147 = .007 radians or 0.40 deg
If the beam in Problem 4 was an 8 inch x 5 inch aluminum beam with the properties indicated on the data sheet, determine the absolute maximum bending stress in the beam for both load cases.
If the beam were oriented on its side as shown below instead of the upright position, what would be the absolute maximum bending stress for both load cases.
Side orientation:
Correct answers 5 marks each = 20
BONUS for 5 marks: Will the beam fail in either case, why or why not?
Sigma(xx) = M(max)*Cxx / Ixx
20.6*101*10^6/28x10^6 = 73.6 MPa or 10.6 Ksi
32.4*101*10^6/28x10^6 = 115.7 MPa or 16.7 Ksi
Is Sigma greater than Sigma(ultimate) = 45 KSI = 311 MPa ?
For the aluminum beam in question 4 and 5, what would be the maximum P(ultimate) load it could support before failing if the load was applied at a point in the middle of the simply supported beam as sketched below?
HERE’S THE DEAL!!
I have purchased this aluminum beam and it is in the structures lab. We are going to break it in the lab next week!
The student that gets closest to the actual ultimate bending load in this test will be awarded a cash prize equal to the value of the scrap aluminum (probably around $50), or, will be given a bonus of 20% on their midterm mark (not to exceed 100%) – their choice.
P(ultimate)
BIG BONUS PRIZE
Answer = I don’t know. It’ll be interesting to see.
I’m going to check bending, direct shear and transverse shear
Bending Max: M=PL/4 gives us M=500P when L=2000 mm
Sigma= M y / I and Sigma ultimate is 310 MPa while Ixx= 28x10^6mm^4 and y = 101mm
Therefore: 310 = 500*P*101/28x10^6 gives us P of 172 kN for bending
tau = VQ/It = 207=V*[(96.4*127*10.4) + (45.6*91.2*6.4)]/(28*10^6*6.4) therefore, V = 241 kN and thus P = 482 kN
So it will fail in flexure and I’d say that the imperfections and local flaws will be cancelled by the conservative thicknesses from manufacturing therefore stick with the 172kN = just over twice the weight of the entire class!
In this chapter we will develop a method of finding the shear stress in a beam. Also, shear flow, will be discussed and examples will be worked.
Shear in a beam subject to bending may be longitudinal and transverse. Longitudinal can be illustrated by the bending beam below:
If the boards are bonded then shear stresses build up and the cross section warps. This condition violates our assumption of sections remaining plane when bent but warping is relatively small especially for a slender beam.
We will now use the assumptions or homogeneity and prismatic cross section to develop a shear formula similar to the flexure formula. . .
It is important to recall that shear stress is complimentary meaning transverse and longitudinal shear stresses are numerically equal.
The derivation and proof of the shear formula are detailed in Hibbeler Chapter 7 section 2. Review this in your own time because I want to get straight to the formula and how it is used. It
VQ=τ
The shear stress in the member at the point located y’ from the neutral axis. This stress is assumed to be constant and therefore averaged across the width t of the member.
The Internal resultant shear force, determined from the method of sections and the equations of equilibrium
The moment of inertia of the entire cross sectional area computed about the neutral axis.
The width of the members cross sectional area, measured at the point where is to be determined
τ
VI
t
''''
AydAyQA
== ∫τ
Where A’ is the top or bottom portion of the member’s cross sectional area, defined from the section where t is measured, and is the distance to the centroid of A’, measured from the neutral axis
It is necessary that the material behave in a linear elastic manner and have a modulus of elasticity that is the same in tension as it is in compression.
ItVQ
=τ
Shear Stresses in Beams
Applying the shear formula for common beam cross-sectional situations:
Rectangular: ( )[ ]( )[ ]bbh
byhV
ItVQ
3
22
12/1
4/21
−⎟⎠⎞
⎜⎝⎛
==τ
⎟⎟⎠
⎞⎜⎜⎝
⎛−= 2
2
3 46 yhbhVτ
AV5.1max =τ
AV
avg =τMaximum shear acts on the neutral axis (centerline here) and near the ends where V is greatest.
So that it can be well understood it can be shown that integrating the shear stress, , over the entire cross-sectional area A yields the shear force V.τ
Wide Flange Beams:
A wide flange beam consists of two flanges and a web. An analysis of the shear in a wide flange beam results in the illustration below:
Built-up members are often used in engineering applications. If loading on built-up members causes bending then fasteners are usually required to keep the pieces from sliding over each other. Nails, screws, glue, bolts, welds etc must then resist the shear at along the length of the member. This shear loading along the member is called SHEAR FLOW and is computed as a force per unit length.
q = the SHEAR FLOW (force per unit length along the beam)
V = Shear Force at that section of beam
I = moment of inertia of ENTIRE cross section
Q =
''''
AydAyQA
== ∫
Where A’ is the top or bottom portion of the member’s cross sectional area, defined from the section where t is measured, and ybar is the distance to the centroid of A’, measured from the neutral axis
This chapter serves as a review of the stress-analysis that has been developed in the previous chapters regarding axial load, torsion, bending and shear. The solution to problems where several of these loads occur simultaneously will be studied. Prior to this, the stresses in thin-walled vessels will be analyzed.