University of Massachusetts Amherst Structural Engineering Sergio F. Breña STEM Education Institute Saturday Workshop September 30, 2006.

University of Massachusetts AmherstUniversity of Massachusetts Amherst

Structural EngineeringStructural Engineering

Sergio F. BreñaSergio F. Breña

STEM Education InstituteSTEM Education InstituteSaturday WorkshopSaturday WorkshopSeptember 30, 2006September 30, 2006


OutlineOutline

• Introduction to Structural EngineeringIntroduction to Structural Engineering

• Forces in StructuresForces in Structures

• Structural SystemsStructural Systems

• Civil Engineering MaterialsCivil Engineering Materials

• Some Definitions of Important Structural Some Definitions of Important Structural PropertiesProperties


Structural EngineeringStructural Engineering

• What does a Structural Engineer do?What does a Structural Engineer do?

– A Structural Engineer designs the structural A Structural Engineer designs the structural systems and structural elements in buildings, systems and structural elements in buildings, bridges, stadiums, tunnels, and other civil bridges, stadiums, tunnels, and other civil engineering works (bones)engineering works (bones)

– Design: process of determining location, material, Design: process of determining location, material, and size of structural elements to resist forces and size of structural elements to resist forces acting in a structureacting in a structure


Engineering Design ProcessEngineering Design Process

• Identify the problem (challenge)Identify the problem (challenge)• Explore alternative solutionsExplore alternative solutions

– Research past experienceResearch past experience– BrainstormBrainstorm– Preliminary design of most promising solutionsPreliminary design of most promising solutions

• Analyze and design one or more viable solutionsAnalyze and design one or more viable solutions• Testing and evaluation of solutionTesting and evaluation of solution

– Experimental testing (prototype) or field testsExperimental testing (prototype) or field tests– Peer evaluationPeer evaluation

• Build solution using available resources (materials, Build solution using available resources (materials, equipment, labor)equipment, labor)


Design Process in Structural EngineeringDesign Process in Structural Engineering

• Select material for constructionSelect material for construction

• Determine appropriate structural system for a Determine appropriate structural system for a particular caseparticular case

• Determine forces acting on a structureDetermine forces acting on a structure

• Calculate size of members and connections Calculate size of members and connections to avoid failure (collapse) or excessive to avoid failure (collapse) or excessive deformationdeformation


Examples of Typical StructuresExamples of Typical Structures


Forces in StructuresForces in Structures


Forces Acting in StructuresForces Acting in Structures

• Forces induced by gravityForces induced by gravity– Dead Loads (permanent): self-weight of structure Dead Loads (permanent): self-weight of structure

and attachmentsand attachments– Live Loads (transient): moving loads (e.g. Live Loads (transient): moving loads (e.g.

occupants, vehicles)occupants, vehicles)

• Forces induced by windForces induced by wind• Forces induced by earthquakesForces induced by earthquakes• Forces induced by rain/snowForces induced by rain/snow• Fluid pressuresFluid pressures• OthersOthers


Forces Acting in StructuresForces Acting in Structures

Vertical: Gravity Lateral: Wind, Earthquake


Global StabilityGlobal Stability

Sliding Overturning


Forces in Structural ElementsForces in Structural Elements

100 lb

Compression

100 lb

Tension


Forces in Structural Elements (cont.)Forces in Structural Elements (cont.)

100 lb

Bending

Torsion


Typical Structural Systems (1)Typical Structural Systems (1)

Arch



TrussC

T

CCT

Forces in Truss Members



Frame



Flat Plate



Folded Plate



Shells


Properties of Civil Engineering MaterialsProperties of Civil Engineering Materials


Definition of StressDefinition of Stress

Section X

T

T

Section X

Stress = Force/Area

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 = 4.448 N (Newton)1 in = 25.4 mm

T = 1,000 lb x 4.448 N/lb = 4448 NA = 10 in2 x (25.4 mm)2 = 6450 mm2

(1 in)2

Stress = 4448/6450 = 0.69 N/mm2

(MPa)


Definition of StrainDefinition of Strain

L

T

T

Lo

Strain = L / Lo

Example:

Lo = 10 in.L = 0.12 in.

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

Strain is dimensionless!!(same in English or SI units)


Stress – Strain Behavior of Elastic Mats.Stress – Strain Behavior of Elastic Mats.

Stress

Strain

E

E = Modulus of Elasticity = Stress / Strain


Types of Stress-Strain BehaviorTypes of Stress-Strain BehaviorStress

Strain

E

(a) Linear Elastic

Stress

Strain(b) Non-linear Elastic

Stress

Strain(c) Elastic-plastic

Stress

Strain(d) Non-linear Plastic

Plastic strain Plastic strain


Materials Used in Civil EngineeringMaterials Used in Civil Engineering

• Stone and MasonryStone and Masonry

• MetalsMetals– Cast IronCast Iron– SteelSteel– AluminumAluminum

• ConcreteConcrete

• WoodWood

• Fiber-Reinforced PlasticsFiber-Reinforced Plastics


Engineering Properties of MaterialsEngineering Properties of Materials

• SteelSteel– Maximum stress: 40,000 – 120,000 lb/inMaximum stress: 40,000 – 120,000 lb/in22

– Maximum strain: 0.2 – 0.4Maximum strain: 0.2 – 0.4– Modulus of elasticity: 29,000,000 lb/inModulus of elasticity: 29,000,000 lb/in22

• ConcreteConcrete– Maximum stress: 4,000 – 12,000 lb/inMaximum stress: 4,000 – 12,000 lb/in22

– Maximum strain: 0.004Maximum strain: 0.004– Modulus of elasticity: 3,600,000 – 6,200,000 lb/inModulus of elasticity: 3,600,000 – 6,200,000 lb/in22

• WoodWoodValues depend on wood grade. Below are some samplesValues depend on wood grade. Below are some samples– Tension stress: 1300 lb/inTension stress: 1300 lb/in22

– Compression stress: 1500 lb/inCompression stress: 1500 lb/in22

– Modulus of elasticity: 1,600,000 lb/inModulus of elasticity: 1,600,000 lb/in22


Concrete ComponentsConcrete Components

• Sand (Fine Aggregate)Sand (Fine Aggregate)• Gravel (Coarse Aggregate)Gravel (Coarse Aggregate)• Cement (Binder)Cement (Binder)• WaterWater• Air Air


Fiber-Reinforced CompositesFiber-Reinforced Composites

PolymerMatrix

Polyester

Epoxy

Vinylester

Fiber Materials

Glass

Aramid (Kevlar)

CarbonFunction of fibers:

•Provide stiffness•Tensile strength

Functions of matrix:

•Force transfer to fibers•Compressive strength•Chemical protection

Composite

Laminate


Important Structural PropertiesImportant Structural Properties


Engineering Properties of Structural ElementsEngineering Properties of Structural Elements

• StrengthStrength– Ability to withstand a given stress without failureAbility to withstand a given stress without failure

• Depends on type of material and type of force (tension or Depends on type of material and type of force (tension or compression)compression)

Tensile Failure Compressive Failure


Engineering Properties of Structural ElementsEngineering Properties of Structural Elements

• Stiffness (Rigidity)Stiffness (Rigidity)

– Property related to deformationProperty related to deformation

– Stiffer structural elements deform less under the same Stiffer structural elements deform less under the same applied loadapplied load

– Stiffness depends on type of material (E), structural shape, Stiffness depends on type of material (E), structural shape, and structural configurationand structural configuration

– Two main typesTwo main types• Axial stiffnessAxial stiffness

• Bending stiffnessBending stiffness


Axial StiffnessAxial Stiffness

L

T

T

Lo

Stiffness = T / L

Example:

T = 100 lbL = 0.12 in.

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


Bending StiffnessBending Stiffness

Stiffness = Force / Displacement

Example:

Force = 1,000 lbDisplacement = 0.5 in.

Stiffness = 1,000 lb / 0.5 in. = 2,000 lb/in.

Displacement

Force


Stiffness of Different Structural ShapesStiffness of Different Structural Shapes

Stiffest

StifferStiff


Types of Structural Elements – Bars and Types of Structural Elements – Bars and CablesCables

Bars can carry either tensionor compression Cables can only carry tension


Types of Structural Elements – BeamsTypes of Structural Elements – Beams

Tension

Compression

Loads


Providing Stability for Lateral LoadsProviding Stability for Lateral Loads

Racking Failure of Pinned Frame

Braced Frame Infilled Frame Rigid Joints


Concepts in EquilibriumConcepts in Equilibrium


Equilibrium of Forces (Statics)Equilibrium of Forces (Statics)

• Forces are a type of quantity called vectorsForces are a type of quantity called vectors– Defined by magnitude and directionDefined by magnitude and direction

• Statement of equilibriumStatement of equilibrium– Net force at a point in a structure = zero Net force at a point in a structure = zero

(summation of forces = zero)(summation of forces = zero)

• Net force at a point is determined using a Net force at a point is determined using a force polygon to account for magnitude and force polygon to account for magnitude and directiondirection


Moment (Rotational) EquilibriumMoment (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


Force Calculation in Simple StructureForce Calculation in Simple Structure

100 lb

8 ft

6 ft

10 ft

A

CB

36.9

Side BC

Side AB=

8 ft

6 ft=1.333

Side AC

Side AB=

10 ft

6 ft=1.667

Force BC =1.333Force AB

Force BC = 1.333 x 100 lb = 133.3 lb

Force AC =1.667Force AB

Force AC = 1.667 x 100 lb = 166.7 lb


Graphic StaticsGraphic Statics

1 Square = 10 lb

100 lb

133.3 lb

166.7 lb

36.9


Force Transfer from Beams to SupportsForce Transfer from Beams to Supports

Force, P

Span, L

1/3 L 2/3 L

2/3 P 1/3 P


Force Transfer Example - BridgeForce Transfer Example - Bridge

8,000 lb 32,000 lb

22,000 lb* 18,000 lb**

L = 60 ft

30 ft 30 ft

15 ft 45 ft

*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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University of Massachusetts Amherst Structural Engineering Sergio F. Breña STEM Education Institute Saturday Workshop September 30, 2006.

Documents

stress strain slide

structural engineering

structure slide

arch slide

frame slide

labor slide

earthquake slide

tension slide