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Module 4D8Prestressed Concrete

Lent Term 2010Dr C.J.Burgoyne

Syllabus1. History2. Current examples3. Elastic Section

Design4. - continued –5. Ultimate Strength

Analysis6. - continued –7. Composite

Construction

8. Losses9. Creep10. - continued –11. Continuous

beams12. - continued –13. New Materials14. - continued –

Housekeeping• 14 Lectures• 2 Examples Classes• 1 Laboratory session (2 hours)• 75% marks by examination• 25% marks by coursework

• 1 design exercise• 1 laboratory write-up

Laboratory ClassesDone in groups of 13 (maximum)

Laboratory should take 2.5 hours (maximum)Stress one beam – test 2 beamsWrite up results within two weeks

Web pagehttp://www-civ.eng.cam.ac.uk/cjb/4D8

will (or does) contain:-• Copies of lecture notes• Useful computer programs• Examples papers and cribs• Exam papers and cribs• Interesting web pages

Winterton HouseTorridge Bridge VR modelHistory of PSC in the UK

Reinforced ConcreteInvented by

Hennebique in 1892

Steel bars carry tensile stresses

Concrete carries compression

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Reinforced Concrete Beam

Concrete cracks on tension face• Unsightly• Durability worries

Pre-stressed Concrete

Pre-compression of concrete eliminates tension

+ =

Stresses due to load + prestress = total

P.S.C. is fundamentally different from R.C.

• Stress in prestressing steel does not vary much at working load

• Prestress puts the concrete into a state where the concrete can carry the tension

Prestressing?

The application of a force to the structure, other than the applied load, which assists the performance of the structure.

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Athenian Trireme• Jason’s ship Argo (1500 BC) and galleys of

the Athenian wars (500 BC)• These vessels were long and thin• Ships flex as waves pass along them• Trees are short – joints open and close• Prestressed by a hypozoma – “undergirdle”• The rope passed from bow to stern and was

twisted to tension it• Put the hull into compression keeps joints

closedHypozoma here

Modern reconstruction Wooden Barrel

Bicycle Wheel Bicycle wheels- rely on three different types of prestress

1. Tensioned spokes to carry compressive load

2. Pneumatic tyres to keep side walls in tension

3. Tyre cord to retain tyre on rim

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London Eye

Hack-saw

blade

Deployable StructuresOften use prestress to stiffen the deployed structure

Foyle Bridge

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Requirements• All these systems have some

property that is deficient, and some property that they have in abundance

• Make use of one to overcome the other

• Concrete is weak in tension but very strong in compression

Prestressed Steel?• Foyle Bridge is a rare example of a

prestressed steel structure• Steel is weak in compression due to

buckling• Prestress would have to be in

tension• Reaction system would thus be in

compression – difficult in practice

Early attempts at prestressing concrete

Engineers quickly realised that if concrete could be compressed by an external load, cracking could be reduced or eliminated

Jackson’s patent 1896A self-stressed tied arch

Steiner (1908) proposed curved tendons

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Lund (1911) Early attempts failed

• Cracks would be prevented at first but would then open up

• Behaviour of concrete not properly understood

Breakthrough by Freyssinet (1927)

Eugene Freyssinet

• In the uniform (!) of the EcoleNational de Pontset Chausees, Paris

Floating formwork Plougastel, near Brest

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Boutiron Bridge, Vichy• One of 3 similar bridges over the R.

Allier but the only one that survives• River floods in spring and danger of

washing away formwork• Freyssinet decided to build stronger

falsework and to jack the bridge away rather than lowering falsework

• Installed jacking pockets at mid-span

Problem• A year after construction Freyssinet

was cycling across the bridge to work when he realised that the parapet was no longer straight

• He concluded that the arch had got shorter, so he reinstalled the jacks and lifted the bridge again

The inspiration!• Conventional wisdom said that concrete

was an elastic solid with constant E• Freyssinet realised that the concrete

must have been creeping over a period of a few months

• Did tests to confirm his suspicionsIn England, Glanville was doing tests at

BRE and coming to the same conclusion

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Prestress requirementsFreyssinet realised that:-• Creep would relieve prestress unless:-• He used a very high strength tendon

• High strain in tendon, so effect of creep is reduced

• He used high quality concrete• To reduce amount of creep in the first place

• This is why early attempts had failed

Prestressed telegraph poles

Prestressed poles• High strength mortar – wall thicknesses ~

10mm• Very high strength steel (piano wire)

~3000 MPa• Highly industrialised – central former was

a collapsible mandrel• Financial disaster• Most electricity and telephone poles in

France are today prestressed concrete

Today

Tomorrow with CFRP

• Then developed system with pairs of wires secured with deformed head or wedges.

• Fixed over brackets on moveable soffit shutter Even Stirrups were prestressed

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Licensed in Germany1938 - Bridge at Oelde, (E34, nr Dortmund)

Elsewhere in Germany

• Dischinger tried to use ordinary steel to prestress concrete

• Needed restressing due to creep • Also included untensioned steel• Forerunner of partially prestressed

concrete

Finsterwalder’s scheme

Freyssinet’s new systemOld system very complex – could only be used for straight tendons

Freyssinetdeveloped new anchorage system

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U-boat factory in Germany. Bow string girders prestressed with old system. Placed side by side and infilled.

4.5 m thick roof. RAF bombs could penetrate so thickened to 7.5 m. Extra prestress added with Freyssinet new system.

Karl Mautner

Came to UK in 1938/39Conducted first beamtest in UK at Southall

Monkton Farleigh Mine

Overburden

Prestressedbeams

Hangers

Only part completed since pillars removed too quickly

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Monkton Farleigh Beams

300

500

20 hard steel wiresin two layers

Beams 5.1 m longdesigned to support falling

rocks, not overburden

Webs tapered longitudinally

Shear links individually spaced

Sinderby BridgeNew bridge added 1943

Soffit corrosion on one beam

Today, railway closedand void filled

Only edge beam visible

Luzancy Bridge

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Freyssinet Hinge

FerrybridgeFerrybridge, W , W YorksYorks

Patents

• Freyssinet’s ownership of good patents led to the development of many different systems

• The good ones have survived

Magnel system

First continuous bridge (Magnel)Modern prestressed concrete

• Uses very high strength steel • (~ 2000 N/mm2)

• Stressed to very high permanent stress • (~ 70% ultimate)

• High strength concrete ( ~ 60 N/mm2)• cf Reinforced Concrete ( ~ 30 N/mm2)

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BBRV system BBRV coupler

7-wire strand12 or 15 mm

Dy-form strand

19-wire strand Banned!Banned!

7-wire strand Single strand stressing

Typical multi-strand tendon Multi-strand jack

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Dywidag Bar Macalloy bar