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# Lec5

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### Lec5

1. 1. 2.001 - MECHANICS AND MATERIALS I Lecture #5 9/20/2006 Prof. Carol LivermoreTOPIC: FORCES AND MOMENTS TRANSMITTED BY SLENDER MEM-BERS?When will a structure fail?Recall: 1. What can the structural elements tolerate? 2. What forces and moments are experienced in the various parts of thestructure?EXAMPLE: AirplaneEXAMPLE: TrussSLENDER MEMBERS Long, skinny structural elements 1
2. 2. L >> b, L >> h EXAMPLE: Skis, Golf Clubs, I-BeamsLocal axes follow the beam.AXIAL FORCES:Forces that act along the axis of the slender member.FBD Fij (x) = N (x), i: face, j: direction, x: location SHEAR FORCES: Forces that act in the plane of a face.
3. 3. Summarize Internal Loads: Moments: Mxz (x2 ) > 0x: face, z: axis, x2 : location Mxz (x) = Mz (x) Bending Moment Sign ConventionSummarize sign conventions 3
4. 4. N >0N <0V >0V <0M >0M <0 4
5. 5. EXAMPLE: Calculating internal forces and momentsStep 1: Find reactions at supports. Fx = 0 R Ax = 0 Fy = 0 R Ay − P = 0 R Ay = P MA = 0 MA − P a = 0 MA = P a 5
6. 6. Step 2: If you have a multi-component structure, you will need to ﬁnd jointforces too.Step 3: Find internal forces and moments. 1. Set up local coordinates. 2. Identify interesting points. 3. Cut beam and do FBDs. 6
7. 7. For 0 < x < a Fx = 0 N =0 Fy = 0 P − Vy = 0 P = Vy MA = 0 P a + Mz − P x = 0 Mz = P x − P a = −P (a − x)OR (From the other end)FBD Fx = 0 −N = 0 N =0 Fy = 0 −P + Vy = 0 Vy = P M∗ = 0 −Mz − P (a − x) = 0 7
8. 8. Mz = −P (a − x)Note: This is the same as before.For a < x < L Fx = 0 −N = 0 N =0 Fy = 0 Vy = 0 M∗ = 0 −Mz = 0 Mz = 0Step 4: Plot result and sanity check. 8