department of civil, architectural and environmental

Drexel UniversityDepartment of Civil, Architectural and

Environmental Engineering 21 February 2006

Introduction to Civil/Structural Introduction to Civil/Structural EngineeringEngineering

Franklin Moon, Ph.D.Franklin Moon, Ph.D.

Assistant ProfessorAssistant Professor

Department of Civil, Architectural, andDepartment of Civil, Architectural, andEnvironmental EngineeringEnvironmental Engineering

Drexel UniversityDrexel University



ObjectivesObjectives

Provide an introduction to EngineeringProvide an introduction to EngineeringIntroduce the mechanics of ‘Spanning Introduce the mechanics of ‘Spanning Across’ (the ‘science’ side of engineering)Across’ (the ‘science’ side of engineering)Introduce the concept of Introduce the concept of PrestressedPrestressedConcrete (the ‘creative’ side of Concrete (the ‘creative’ side of engineering)engineering)Provide insight into the scale (overall size, Provide insight into the scale (overall size, forces, etc.) of Structural Engineeringforces, etc.) of Structural Engineering



Engineering versus Applied ScienceEngineering versus Applied Science

What is the goal of Engineering?What is the goal of Engineering?Create structures, machines, processes, and Create structures, machines, processes, and

networks networks To make things that previously did To make things that previously did not existnot exist

What is the goal of Science?What is the goal of Science?Explain natural systems Explain natural systems To discover things that To discover things that

have long existedhave long existed

Billington, D.P. (1983) “The tower and the bridge,” Basic Books, Inc., Publishers, New York



History of (Civil) EngineeringHistory of (Civil) Engineering

Military Engineering(mid-1600s)

Civil Engineering

1775 1775 -- King Louis XV authorized a King Louis XV authorized a School of Bridges and HighwaysSchool of Bridges and Highways

1794 1794 -- Napoleon developed Napoleon developed EcoleEcolePolytechniquePolytechnique

1835 1835 –– First class of US civil First class of US civil engineers graduatesengineers graduates

ShelterTransportationWater

3 keys to civilization

Structural Engineering

Transportation Engineering

Environmental Engineering

Water Resource Engineering

Architectural Engineering

Geotechnical Engineering

Grayson, L.P. (1993) “The Making of an Engineer: An Illustrated History of Engineering Education in the United States and Canada”



Challenges of Structural EngineeringChallenges of Structural EngineeringBuilding UpBuilding Up Spanning AcrossSpanning Across

Taipei 101 (1667 ft)(www.ErikInfoBase.com) Akashi Kaikyo (6,527 ft)

(www.pbs.org)



Spanning AcrossSpanning Across

LR1 R2

R3

Truck weight subjects bridge to both BENDING and SHEAR



Bending (Flexural) DeformationBending (Flexural) DeformationL - ∆L

L + ∆L

Top gets shorter (squeezed in compression)

Bottom gets longer (stretched in tension)

R1 R2

R3



L - ∆L

L + ∆L

C = compressive force

T = tension force

d = distance between T and C

R1 R2

R3

Bending MechanicsBending Mechanics



C

T

d

Apply the principle of equilibrium (statics) to solve for the internal forces

ΣFx = 0 ( [+])

-C + T = 0

C = T

R1

o

ΣMo = 0 ( [+])

-Cd + M = 0

M = Cd

or

M = Td

Bending MechanicsBending Mechanics



L - ∆L

L + ∆L

Cracks perpendicular to tension

Crushes perpendicular to compression

T T

CC

Bending FailuresBending Failures



Shear (Racking) DeformationShear (Racking) Deformation

L

R1 R2

R3Diagonal ‘strut’ gets shorter (squeezed in compression)

Diagonal ‘tie’ gets longer (stretched in tension)



Shear MechanicsShear Mechanics

R1R2

R3

L

V= internal shear force




Apply the principle of equilibrium (statics) to solve for the internal forces

ΣFy = 0 ( [+])

2V - P = 0

V = 0.5P



P

Shear MechanicsShear Mechanics



Shear FailureShear Failure

L

R1 R2

R3



PrestressedPrestressed Concrete BridgesConcrete BridgesAttributes of PC Girder BridgesAttributes of PC Girder Bridges

PrefabricatedPrefabricated--Minimal erection timeMinimal erection time--Short traffic interruptionShort traffic interruption--Good quality controlGood quality control--Transportation issuesTransportation issues

Typical Span Lengths ~150 ftTypical Span Lengths ~150 ft

Typical Girder Spacing ~8 ftTypical Girder Spacing ~8 ft

Good DurabilityGood Durability



Introduction to Prestressed ConcreteIntroduction to Prestressed Concrete

MotivationMotivation -- Concrete is “strong” in Concrete is “strong” in compression and “weak” in tensioncompression and “weak” in tension

GoalGoal –– Introduce “internal” forces into the Introduce “internal” forces into the concrete to keep the concrete in concrete to keep the concrete in compression under “everyday loads”compression under “everyday loads”

--Delays crackingDelays cracking--Improved durabilityImproved durability



Step 1 – Stretch the steel tendonsLs

Ls + ∆Ls

Jacking force

Introduction to PC: Method of ConstructionIntroduction to PC: Method of Construction




Step 2 – Cast concrete around the steel tendons and wait until the concrete cures (hardens)

Ls + ∆Ls

Jacking force Lc



Step 3 – Remove the jacking force

Ls + ∆Ls - ∆Lj = Ls + ∆L’s

Lc - ∆Lj

Steel contracts by Lj Steel remains in tension

Concrete is placed into compression




Validation of ConceptValidation of Concept

Canfield, S. and L. Kahn (2005) - School of Civil and Environmental Engineering, Georgia Institute of Technology



Test SetupTest SetupTest 1

Bending Force

Shear Force




Bending BehaviorBending Behavior

Mid-span displacement

Force

T = tension force in steel

C = Compressive force in concrete

dd




Flexure and Shear CracksFlexure and Shear Cracks

Bending cracks Shear

cracks

Centerline of point load




Structural FailureStructural Failure




PostPost--Processing ResultsProcessing ResultsLoad-Deflection Plot for the Type IV Girder

0

100

200

300

400

500

600

700

0 2 4 6 8 10 12 14 16 18

String Potentiometer Deflection (in.)

Load

(kip

s)

Cracking Test Ulitimate Test

First Visible cracks at 363 kips

Ultimate load = 592 kips




Structural FailureStructural Failure




Scale of Structural EngineeringScale of Structural Engineering592,000 lb

~4.0 ft.

Weight ~ 1000lb

~2.5 ft.

Weight ~ 1000lb




Scale of Structural EngineeringScale of Structural Engineering592,000 lb

~1480 ft.

Weight ~ 592,000 lb

~1453 ft.




Scale of Structural EngineeringScale of Structural Engineering

~1480 ft.

Weight ~ 592,000 lb

~1453 ft.

Questions ???

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