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Our Agenda. Engineering Design ProcessBridge Terminology and BasicsDesign RequirementsMaterials TestingStart Building a TrussForces and EquilibriumContinue Building the TrussStructural AnalysisFinish the BridgeSelect Member SizesTest Bridge. Engineering Design Process. A systematic design process precedes construction.Engineers design; contractors build.The design process is informed by math and science.Design is iterative.Structures are designed to carry code-specified loads safe32631
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1. The Waddell A-Truss Bridge Designing and Building File-Folder Bridges as an Introduction to Civil Engineering
2. Our Agenda Engineering Design Process
Bridge Terminology and Basics
Design Requirements
Materials Testing
Start Building a Truss
Forces and Equilibrium
Continue Building the Truss
Structural Analysis
Finish the Bridge
Select Member Sizes
Test Bridge
3. Engineering Design Process A systematic design process precedes construction.
Engineers design; contractors build.
The design process is informed by math and science.
Design is iterative.
Structures are designed to carry code-specified loads safely and economically.
Designed to stand up, not to fail.
4. Engineering Design Process Design Requirements:
Length, width, loading, budget
Decide on bridge configuration.
Materials property testing
Perform a structural analysis.
Reactions
Internal member forces
Select member sizes based on required strength.
Draw plans.
Build the bridge.
Test
5. Our Agenda Engineering Design Process
Bridge Terminology and Basics
Design Requirements
Materials Testing
Start Building a Truss
Forces and Equilibrium
Continue Building the Truss
Structural Analysis
Finish the Bridge
Select Member Sizes
Test Bridge
6. Basic Bridge Types
7. What is a Truss? A structure composed of members connected together to form a rigid framework.
Usually composed of interconnected triangles.
Members carry load in tension or compression.
8. Tension and Compression
9. Component Parts
10. Standard Configurations
11. Structural Members
12. Truss Connections
13. Our Agenda Engineering Design Process
Bridge Terminology and Basics
Design Requirements
Materials Testing
Start Building a Truss
Forces and Equilibrium
Continue Building the Truss
Structural Analysis
Finish the Bridge
Select Member Sizes
Test Bridge
14. Beginning the Design Process
15. Let’s build this bridge…
16. Our A-Truss Bridge
17. Our Truss Configuration
18. Our Agenda Engineering Design Process
Bridge Terminology and Basics
Design Requirements
Materials Testing
Start Building a Truss
Forces and Equilibrium
Continue Building the Truss
Structural Analysis
Finish the Bridge
Select Member Sizes
Test Bridge
19. Materials & Equipment File folders or legal size card stock paper
Yellow carpenter’s glue
Building board (Styrofoam or cork)
Pins
Scissors
Metal ruler
Plastic wrap or wax paper
Plans and Parts Template
Download from TECT website
20. File Folders or Card Stock? Inexpensive.
Easy to cut, bend, and glue.
Surprisingly predictable structural behavior.
Can be used to build:
Tubes and bars.
Connections that are stronger than the attached structural members.
21. Test Specimens Bars (tension members)
3 – 2 mm (1/16”) x 15 cm (6”)
3 – 4 mm (5/32”) x 15 cm (6”)
3 – 6 mm (1/4”) x 15 cm (6”)
Tubes (compression members)
3 – 10 mm2 (3/8”) x 8 cm (3”)
3 – 10 mm2 (3/8”) x 12 cm (4.5”)
3 – 10 mm2 (3/8”) x 15 cm (6”)
22. A Hydraulic Testing Machine
23. Our Testing Machine
24. Testing Tensile Strength
25. Testing Tensile Strength
26. Testing Tensile Strength
27. Testing Tensile Strength
28. The Principle of the Lever
29. Graph the Results
30. Results of Tension Testing Tensile strength depends on:
Type of material
Thickness of cross-section
Width of cross-section
Tensile strength does not depends on:
Length of member
Shape of cross-section
31. Testing Compressive Strength
32. Testing Compressive Strength
33. Graph the Results
34. Results of Compression Testing Compressive strength depends on:
Type of material
Length of member
Width and thickness of cross-section
Shape of cross-section
35. Our Agenda Engineering Design Process
Bridge Terminology and Basics
Design Requirements
Materials Testing
Start Building a Truss
Forces and Equilibrium
Continue Building the Truss
Structural Analysis
Finish the Bridge
Select Member Sizes
Test Bridge
36. Prefabrication of Members Paste template to file folder or print to legal size card stock
Cut out bars
Cut out and assemble tubes
Cut out gusset plates
37. Template Parts Locations
38. Set up the Building Board
39. Set up the Building Board
40. Add Gusset Plates
41. Add Gusset Plates
42. Add Bars
43. Add Bars
44. Add Tubes
45. Add Tubes
46. Add Tubes
47. The Finished Half-Truss
48. Our Agenda Engineering Design Process
Bridge Terminology and Basics
Design Requirements
Materials Testing
Start Building a Truss
Forces and Equilibrium
Continue Building the Truss
Structural Analysis
Finish the Bridge
Select Member Sizes
Test Bridge
49. Forces, Loads, & Reactions Force – A push or pull.
Load – A force applied to a structure.
Reaction – A force developed at the support of a structure to keep that structure in equilibrium.
50. Equilibrium
51. Tension and Compression
52. Structural Analysis For a given load, find the internal forces (tension and compression) in all members.
Why?
Procedure:
Model the structure:
Define supports & loads.
Draw a free body diagram.
A sketch of the geometry, loads & supports
Calculate reactions.
Calculate internal forces.
53. Model the Structure
54. Draw a Free Body Diagram
55. Calculate Reactions Total downward force is 24.5 N.
Total upward force must be 24.5 N.
Loads, structure, and reactions are all symmetrical.
56. Calculate Reactions
57. Our Agenda Engineering Design Process
Bridge Terminology and Basics
Design Requirements
Materials Testing
Start Building a Truss
Forces and Equilibrium
Continue Building the Truss
Structural Analysis
Finish the Bridge
Select Member Sizes
Test Bridge
58. Assemble the Two Halves
59. Assemble the Two Halves
60. Assemble the Two Halves
61. Our Agenda Engineering Design Process
Bridge Terminology and Basics
Design Requirements
Materials Testing
Start Building a Truss
Forces and Equilibrium
Continue Building the Truss
Structural Analysis
Finish the Truss
Select Member Sizes
Test Bridge
62. Structural Analysis Methods Graphical Method
Less Mathematical (but not less exact!)
Method of Joints
Analytical (trigonometry based)
Vector Components
Linear Algebra (more complex structures)
Preferred Engineering Methodology
Computer Modeling & Simulation
63. Some Basic Vector Concepts All forces are vectors that have both a magnitude and a direction.
Arrows are used to graphically represent vectors.
The arrow indicates the direction.
In trusses, the vector is always in the direction of the truss member (i.e. they follow the truss geometry).
The arrow length represents the magnitude.
Multiple vectors can be graphically added to determine one equivalent vector.
To maintain static equilibrium, forces meeting at a common point must add to zero.
64. Graphical Method First, select a joint at one of the reactions.
This selects three vectors; AD, AB & the reaction.
65. Graphical Method Draw the known vector, the reaction, at its proper length and direction.
66. Graphical Method Draw lines representing the direction of the remaining vectors.
Vectors must be connected “tail to head”.
The magnitudes are unknown, so the length doesn’t matter at this point.
Since Joint A is a common point, the three vectors must add to zero.
This means that they must form a closed loop.
67. Graphical Method Measure the length of the unknown vectors.
Length = Magnitude
68. Graphical Method Move vectors to match actual joint geometry.
69. Graphical Method...Again Select another Joint.
70. Graphical Method…Again Draw the two known vectors, the load and AB, at their proper lengths.
71. Graphical Method…Again Draw the two unknown vectors, BD and BC.
Only possibility is a rectangle as shown.
Measure the lengths & move vectors to match geometry.
72. Graphical Method…Again Continue same process for all other joints.
Caution: Select joints with no more than 2 unknowns.
Final results as shown.
73. Computer Modeling & Simulation Use computer modeling to support math and science concepts.
Many programs available; all require check on mathematical determinacy.
Truss Bars = 2(Truss Joints) – 3 [ok]
Truss Bars > 2(Truss Joints) – 3 [indeterminate]
Requires more advance analysis techniques
Truss Bars < 2(Truss Joints) – 3 [unstable]
Truss will not support itself
Truss Analysis Wizard
http://mathonweb.com/
Freeware & easy to use
Provides numerical results (most don’t)
Let’s try it & check our previous answers......
74. Method of Joints Isolate a Joint.
75. Method of Joints
76. Equations of Equilibrium The sum of all forces acting in the x-direction must equal zero.
The sum of all forces acting in the y-direction must equal zero.
For forces that act in a diagonal direction, we must consider both the x-component and the y-component of the force.
77. Components of Force If magnitude of FAD is represented as the hypotenuse of a right triangle...
Then the magnitudes of (FAD)x and (FAD)y are represented by the lengths of the sides.
80. Equations of Equilibrium
81. Method of Joints...Again Isolate another Joint.
82. Equations of Equilibrium
83. Results of Structural Analysis
84. Our Agenda Engineering Design Process
Bridge Terminology and Basics
Design Requirements
Materials Testing
Start Building a Truss
Forces and Equilibrium
Continue Building the Truss
Structural Analysis
Finish the Bridge
Select Member Sizes
Test Bridge
85. Assembly the Bridge
86. Our Agenda Engineering Design Process
Bridge Terminology and Basics
Design Requirements
Materials Testing
Start Building a Truss
Forces and Equilibrium
Continue Building the Truss
Structural Analysis
Finish the Bridge
Select Member Sizes
Test Bridge
87. Tensile Strength of AB & BD
88. Strength of Member AD
89. Critiquing Our Design
90. Factor of Safety Ratio of Strength vs. Load.
Member with smallest ratio will likely fail first.
Our factors of safety:
AB = 2.11
AD = 4.62
BC = 2.11
BD = 2.12
CD = 4.62
Ratio should not be less than 1.5.
Members AD & CD likely oversized.
Modify design and repeat analysis
91. Our Agenda Engineering Design Process
Bridge Terminology and Basics
Design Requirements
Materials Testing
Start Building a Truss
Forces and Equilibrium
Continue Building the Truss
Structural Analysis
Finish the Bridge
Select Member Sizes
Test Bridge
92. Place the Structure into Service
93. Acknowledgements PowerPoint and Activity Adapted from Work Created by:
COL Stephen Ressler, P.E., Ph.D.
Department of Civil & Mechanical Engineering
U.S. Military Academy, West Point