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Structural Engineering. Sergio F. Breña STEM Education Institute Saturday Workshop September 30, 2006. Outline. Introduction to Structural Engineering Forces in Structures Structural Systems Civil Engineering Materials Some Definitions of Important Structural Properties.

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structural engineering

Structural Engineering

Sergio F. Breña

STEM Education Institute

Saturday Workshop

September 30, 2006

University of Massachusetts Amherst

outline
Outline
  • Introduction to Structural Engineering
  • Forces in Structures
  • Structural Systems
  • Civil Engineering Materials
  • Some Definitions of Important Structural Properties

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structural engineering3
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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engineering design process
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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design process in structural engineering
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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examples of typical structures
Examples of Typical Structures

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forces in structures

Forces in Structures

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forces acting in structures
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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forces acting in structures9
Forces Acting in Structures

Vertical: Gravity

Lateral: Wind, Earthquake

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global stability
Global Stability

Sliding

Overturning

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forces in structural elements

100

lb

Compression

Forces in Structural Elements

100

lb

Tension

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forces in structural elements cont

100

lb

Bending

Forces in Structural Elements (cont.)

Torsion

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typical structural systems 1

Arch

Typical Structural Systems (1)

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typical structural systems 2

C

T

C

C

T

Forces in Truss Members

Typical Structural Systems (2)

Truss

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typical structural systems 3

Frame

Typical Structural Systems (3)

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typical structural systems 4
Typical Structural Systems (4)

Flat Plate

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typical structural systems 5
Typical Structural Systems (5)

Folded Plate

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typical structural systems 6
Typical Structural Systems (6)

Shells

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properties of civil engineering materials

Properties of Civil Engineering Materials

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definition of stress

T

Example (English Units):

T = 1,000 lb (1 kip)

A = 10 in2.

Stress = 1,000/10 = 100 lb/in2

Stress = Force/Area

Section X

Example (SI Units):

1 lb = 4.448 N (Newton)

1 in = 25.4 mm

T = 1,000 lb x 4.448 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

T

T

Definition of Stress

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definition of strain

T

DL

Lo

T

Definition of Strain

Strain = DL / Lo

Example:

Lo = 10 in.

DL = 0.12 in.

Strain = 0.12 / 10 = 0.012 in./in.

Strain is dimensionless!!

(same in English or SI units)

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stress strain behavior of elastic mats
Stress – Strain Behavior of Elastic Mats.

Stress

E

E = Modulus of Elasticity = Stress / Strain

Strain

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types of stress strain behavior

Stress

Stress

E

Strain

Strain

Stress

(a) Linear Elastic

Stress

(b) Non-linear Elastic

Strain

Strain

Plastic strain

Plastic strain

(c) Elastic-plastic

(d) Non-linear Plastic

Types of Stress-Strain Behavior

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materials used in civil engineering
Materials Used in Civil Engineering
  • Stone and Masonry
  • Metals
    • Cast Iron
    • Steel
    • Aluminum
  • Concrete
  • Wood
  • Fiber-Reinforced Plastics

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engineering properties of materials
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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concrete components
Concrete Components
  • Sand (Fine Aggregate)
  • Gravel (Coarse Aggregate)
  • Cement (Binder)
  • Water
  • Air

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fiber reinforced composites
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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important structural properties

Important Structural Properties

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engineering properties of structural elements

Compressive Failure

Tensile Failure

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)

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engineering properties of structural elements30
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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axial stiffness

T

DL

Lo

T

Axial Stiffness

Stiffness = T / DL

Example:

T = 100 lb

DL = 0.12 in.

Stiffness = 100 lb / 0.12 in. = 833 lb/in.

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bending stiffness
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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stiffness of different structural shapes

Stiffest

Stiffness of Different Structural Shapes

Stiff

Stiffer

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types of structural elements bars and cables
Types of Structural Elements – Bars and Cables

Bars can carry either tension

or compression

Cables can only carry tension

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types of structural elements beams

Loads

Compression

Tension

Types of Structural Elements – Beams

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providing stability for lateral loads

Racking Failure of Pinned Frame

Infilled Frame

Rigid Joints

Braced Frame

Providing Stability for Lateral Loads

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concepts in equilibrium

Concepts in Equilibrium

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equilibrium of forces statics
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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moment rotational equilibrium

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

A

3 ft

6 ft

Moment (Rotational) Equilibrium

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force calculation in simple structure

8 ft

10 ft

Side AC

Side BC

=

=

=

=

1.667

1.333

100

lb

6 ft

6 ft

Side AB

Side AB

A

10 ft

6 ft

Force  BC

Force  AC

1.333

1.667

=

=

Force  AB

Force  AB

C

B

Force  AC = 1.667 x 100 lb = 166.7 lb

Force  BC = 1.333 x 100 lb = 133.3 lb

8 ft

Force Calculation in Simple Structure

36.9

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graphic statics

166.7 lb

100 lb

36.9

133.3 lb

1 Square = 10 lb

Graphic Statics

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force transfer from beams to supports
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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force transfer example bridge
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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