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2.001 - MECHANICS AND MATERIALS I
                           Lecture #9 10/11/2006
                            Prof. Carol Livermore

Review of Uniaxial Loading:




¦Ä = Change in length (Positive for extension; also called tension)

Stress (¦Ò)

                                    ¦Ò = P/A0
Strain ( ) at a point


                                    L ? L0   ¦Ä
                                =          =
                                      L0     L0
Stress-Strain Relationship ? Material Behavior




                                     ¦Ò=E



                                       1
Force-Displacement Relationship




                                  P = k¦Ä
For a uniaxial force in a bar:


                                       EA0
                                  k=
                                        L0
Deformation and Displacement
   Recall from Lab:




   The springs deform.
   The bar is displaced.

EXAMPLE:




                                       2
du(x)
                               (x) =
                                        dx

                               (x)dx =     du

Sign Convention




Trusses that deform
   Bars pinned at the joints
   How do bars deform?
   How do joints displace?

EXAMPLE:




Q: Forces in bars?    How much does each bar deform?   How much does
point B displace?
Unconstrained degrees of freedom
   1. uB
       x
   2. uB
       y




                                   3
Unknowns
  1. FAB
  2. FBC

This is statically determinate (Forces can be found using equilibrium)
FBD:




                                 Fx = 0 at Pin B
                            ?FAB ? FBC sin ¦È = 0

                                        Fy = 0
                                                           ?P
                     ?P ? FBC cos ¦È = 0 ? FBC =
                                                          cos ¦È
                                         P
                           ?FAB +             sin ¦È = 0
                                        cos ¦È
So:


                                 FAB = P tan ¦È
Force-Deformation Relationship


                                   P = k¦Ä
                                      EA
                                   k=
                                       L
                                          FAB
                                  ¦ÄAB =
                                          kAB
                                          FBC
                                  ¦ÄBC   =
                                          kBC

                                         4
AE
                  kAB =
                           L sin ¦È
                            AE
                   kBC    =
                             L
So:


                           P tan ¦È
                 ¦ÄAB =
                                AE
                               L sin ¦È

                          ?P cos ¦È
                 ¦ÄBC =
                                AE
                                 L

                        P L sin ¦È tan ¦È
                ¦ÄAB =
                             AE
                            ?P L
                 ¦ÄBC    =
                          AE cos ¦È
Check:




Compatibility




                           5
Algorithm to ?nd B
    1. If we only have uB , what ¦Ä AB and ¦Ä BC would result?
                        x
    2. If we only have uB , what ¦Ä AB and ¦Ä BC would result?
                        y
    3. What is the total ¦Ä AB and ¦Ä BC is I have both uB and uy ?
                                                       x
                                                              B
                                                                    4. Solve
for uB and uB from known ¦Ä AB and ¦Ä BC .
     x        y
Step 1




                               ¦Ä BC = uB sin ¦È
                                       x



                 LN EW     =             BC
                                   (L + ¦Ä1 )2 + ¦¤2
                                             BC      2
                                           2¦Ä1   ¦Ä BC   ¦¤2
                           =   L (1 +           + 1 2 )+ 2
                                             L     L    L
                               ¡Ì         x
                                   1+x¡Ö1+
                                         L
                                  B      BC 2
                                 ¦Ä1 C   ¦Ä1      ¦¤2
                   LN EW   =L 1+      +       +
                                  L      2L2    2L2
                                       2
                                     BC
                                    ¦Ä1
                                         ¡ú0
                                     2L2
                                     ¦¤2
                                         ¡ú0
                                     2L2
For ¦Ä << L:


                                    ¦Ä BC ¡Ö D
So:


                                           BC
                               Lnew = L + ¦Ä1


                                       6

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Lec9 MECH ENG STRucture

  • 1. 2.001 - MECHANICS AND MATERIALS I Lecture #9 10/11/2006 Prof. Carol Livermore Review of Uniaxial Loading: ¦Ä = Change in length (Positive for extension; also called tension) Stress (¦Ò) ¦Ò = P/A0 Strain ( ) at a point L ? L0 ¦Ä = = L0 L0 Stress-Strain Relationship ? Material Behavior ¦Ò=E 1
  • 2. Force-Displacement Relationship P = k¦Ä For a uniaxial force in a bar: EA0 k= L0 Deformation and Displacement Recall from Lab: The springs deform. The bar is displaced. EXAMPLE: 2
  • 3. du(x) (x) = dx (x)dx = du Sign Convention Trusses that deform Bars pinned at the joints How do bars deform? How do joints displace? EXAMPLE: Q: Forces in bars? How much does each bar deform? How much does point B displace? Unconstrained degrees of freedom 1. uB x 2. uB y 3
  • 4. Unknowns 1. FAB 2. FBC This is statically determinate (Forces can be found using equilibrium) FBD: Fx = 0 at Pin B ?FAB ? FBC sin ¦È = 0 Fy = 0 ?P ?P ? FBC cos ¦È = 0 ? FBC = cos ¦È P ?FAB + sin ¦È = 0 cos ¦È So: FAB = P tan ¦È Force-Deformation Relationship P = k¦Ä EA k= L FAB ¦ÄAB = kAB FBC ¦ÄBC = kBC 4
  • 5. AE kAB = L sin ¦È AE kBC = L So: P tan ¦È ¦ÄAB = AE L sin ¦È ?P cos ¦È ¦ÄBC = AE L P L sin ¦È tan ¦È ¦ÄAB = AE ?P L ¦ÄBC = AE cos ¦È Check: Compatibility 5
  • 6. Algorithm to ?nd B 1. If we only have uB , what ¦Ä AB and ¦Ä BC would result? x 2. If we only have uB , what ¦Ä AB and ¦Ä BC would result? y 3. What is the total ¦Ä AB and ¦Ä BC is I have both uB and uy ? x B 4. Solve for uB and uB from known ¦Ä AB and ¦Ä BC . x y Step 1 ¦Ä BC = uB sin ¦È x LN EW = BC (L + ¦Ä1 )2 + ¦¤2 BC 2 2¦Ä1 ¦Ä BC ¦¤2 = L (1 + + 1 2 )+ 2 L L L ¡Ì x 1+x¡Ö1+ L B BC 2 ¦Ä1 C ¦Ä1 ¦¤2 LN EW =L 1+ + + L 2L2 2L2 2 BC ¦Ä1 ¡ú0 2L2 ¦¤2 ¡ú0 2L2 For ¦Ä << L: ¦Ä BC ¡Ö D So: BC Lnew = L + ¦Ä1 6