1. Structural Engineering
Sergio F. Breña
STEM Education Institute
Saturday Workshop
September 30, 2006
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2. University of Massachusetts Amherst
Outline
Introduction to Structural Engineering
Forces in Structures
Structural Systems
Civil Engineering Materials
Some Definitions of Important Structural Properties
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3. Structural Engineering
What does a Structural Engineer do?
A Structural Engineer designs the structural systems and structural elements in buildings, bridges, stadiums, tunnels, and other civil engineering works (bones)
Design: process of determining location, material, and size of structural elements to resist forces acting in a structure
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4. Engineering Design Process
Identify the problem (challenge)
Explore alternative solutions
Research past experience
Brainstorm
Preliminary design of most promising solutions
Analyze and design one or more viable solutions
Testing and evaluation of solution
Experimental testing (prototype) or field tests
Peer evaluation
Build solution using available resources (materials, equipment, labor)
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5. Design Process in Structural Engineering
Select material for construction
Determine appropriate structural system for a particular case
Determine forces acting on a structure
Calculate size of members and connections to avoid failure (collapse) or excessive deformation
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6. Examples of Typical Structures
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7. University of Massachusetts Amherst
Forces in Structures
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8. Forces Acting in Structures
Forces induced by gravity
Dead Loads (permanent): self-weight of structure and attachments
Live Loads (transient): moving loads (e.g. occupants, vehicles)
Forces induced by wind
Forces induced by earthquakes
Forces induced by rain/snow
Fluid pressures
Others
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9. Forces Acting in Structures
Vertical: Gravity
Lateral: Wind, Earthquake
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10. University of Massachusetts Amherst
Global Stability
Sliding
Overturning
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11. Forces in Structural Elements
100 lb
Compression
100 lb
Tension
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12. Forces in Structural Elements (cont.)
100 lb
Bending
Torsion
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13. Typical Structural Systems (1)
Arch
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14. Typical Structural Systems (2)
Truss C T
Forces in Truss Members
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15. Typical Structural Systems (3)
Frame
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16. Typical Structural Systems (4)
Flat Plate
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17. Typical Structural Systems (5)
Folded Plate
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18. Typical Structural Systems (6)
Shells
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19. Properties of Civil Engineering Materials
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20. University of Massachusetts Amherst
Definition of Stress
Section X T
Example (English Units):
T = 1,000 lb (1 kip)
A = 10 in2.
Stress = 1,000/10 = 100 lb/in2
Example (SI Units):
1 lb = N (Newton)
1 in = 25.4 mm
T = 1,000 lb x N/lb = 4448 N
A = 10 in2 x (25.4 mm)2 = 6450 mm2
(1 in)2
Stress = 4448/6450 = 0.69 N/mm2 (MPa)
Section X
Stress = Force/Area T
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21. University of Massachusetts Amherst
Definition of Strain DL T Lo
Strain = DL / Lo
Example:
Lo = 10 in.
DL = 0.12 in.
Strain = 0.12 / 10 = in./in.
Strain is dimensionless!!
(same in English or SI units)
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22. Stress – Strain Behavior of Elastic Mats.
E = Modulus of Elasticity = Stress / Strain
Strain
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23. Types of Stress-Strain Behavior
(a) Linear Elastic
(b) Non-linear Elastic
(c) Elastic-plastic
(d) Non-linear Plastic
Plastic strain
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24. Materials Used in Civil Engineering
Stone and Masonry
Metals
Cast Iron
Steel
Aluminum
Concrete
Wood
Fiber-Reinforced Plastics
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25. Engineering Properties of Materials
Steel
Maximum stress: 40,000 – 120,000 lb/in2
Maximum strain: 0.2 – 0.4
Modulus of elasticity: 29,000,000 lb/in2
Concrete
Maximum stress: 4,000 – 12,000 lb/in2
Maximum strain: 0.004
Modulus of elasticity: 3,600,000 – 6,200,000 lb/in2
Wood
Values depend on wood grade. Below are some samples
Tension stress: 1300 lb/in2
Compression stress: 1500 lb/in2
Modulus of elasticity: 1,600,000 lb/in2
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26. University of Massachusetts Amherst
Concrete Components
Sand (Fine Aggregate)
Gravel (Coarse Aggregate)
Cement (Binder)
Water
Air
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27. Fiber-Reinforced Composites
Composite Laminate
Polyester
Polymer
Matrix
Epoxy
Vinylester
Glass
Functions of matrix:
Force transfer to fibers
Compressive strength
Chemical protection
Fiber Materials
Aramid (Kevlar)
Carbon
Function of fibers:
Provide stiffness
Tensile strength
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28. Important Structural Properties
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29. Engineering Properties of Structural Elements
Strength
Ability to withstand a given stress without failure
Depends on type of material and type of force (tension or compression)
Tensile Failure
Compressive Failure
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30. Engineering Properties of Structural Elements
Stiffness (Rigidity)
Property related to deformation
Stiffer structural elements deform less under the same applied load
Stiffness depends on type of material (E), structural shape, and structural configuration
Two main types
Axial stiffness
Bending stiffness
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31. University of Massachusetts Amherst
Axial Stiffness DL T Lo
Stiffness = T / DL
Example:
T = 100 lb
DL = 0.12 in.
Stiffness = 100 lb / 0.12 in. = 833 lb/in.
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32. University of Massachusetts Amherst
Bending Stiffness
Displacement
Force
Stiffness = Force / Displacement
Example:
Force = 1,000 lb
Displacement = 0.5 in.
Stiffness = 1,000 lb / 0.5 in. = 2,000 lb/in.
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33. Stiffness of Different Structural Shapes
Stiffest
Stiffer
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34. Types of Structural Elements – Bars and Cables
Bars can carry either tension
or compression
Cables can only carry tension
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35. Types of Structural Elements – Beams
Loads
Tension
Compression
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36. Providing Stability for Lateral Loads
Racking Failure of Pinned Frame
Braced Frame
Infilled Frame
Rigid Joints
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37. Concepts in Equilibrium
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38. Equilibrium of Forces (Statics)
Forces are a type of quantity called vectors
Defined by magnitude and direction
Statement of equilibrium
Net force at a point in a structure = zero (summation of forces = zero)
Net force at a point is determined using a force polygon to account for magnitude and direction
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39. Moment (Rotational) Equilibrium
3 ft
6 ft A
Moment of Force = Force x Distance
To neutralize rotation about point A, moments from the two forces has to be equal and opposite:
100 lb x 3 ft = 50 lb x 6 ft
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40. Force Calculation in Simple Structure
Side BC
Side AB =
8 ft
6 ft
1.333
Side AC
10 ft
1.667
Force  BC
Force  AB
Force  BC = x 100 lb = lb
Force  AC
Force  AC = x 100 lb = lb
100 lb
8 ft
6 ft
10 ft A C B
36.9
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41. University of Massachusetts Amherst
Graphic Statics
1 Square = 10 lb
100 lb
133.3 lb
166.7 lb
36.9
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42. Force Transfer from Beams to Supports
Force, P
1/3 L
2/3 L
2/3 P
1/3 P
Span, L
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43. Force Transfer Example - Bridge
8,000 lb
32,000 lb
15 ft
45 ft
30 ft
30 ft
L = 60 ft
22,000 lb*
18,000 lb**
*Front axle: 8,000 lb x 45/60 = 6,000 lb
Rear axle: 32,000 lb x 30/60 = 16,000 lb
**Front axle: 8,000 lb x 15/60 = 2,000 lb
Rear axle: 32,000 lb x 30/60 = 16,000 lb
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44. University of Massachusetts Amherst
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