Wisconsin Pt Workshop Notes

8/13/2019 Wisconsin Pt Workshop Notes

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Universit of Wisconsin


WELCOME

Gary Whited

Construction and Materials Support Center, U.W.

Scot Becker

Chief Development Engineer, WisDOT


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What is P/T?

Method of applying a large compression force

to a structure (concrete) before loading occurs.

Applying that force AFTER the concrete has

already hardened.

May also apply bending forces - to try to

counteract bending created when load isapplied.


Basics

of

Prestressing


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Basics of Prestressing

2 methods:

pre-tensioning

post-tensioning

2 types:

bonded

unbonded

1


Pre-tensioned:

Tendons are stretched andanchored to strong bulkheads,

then concrete is placed aroundthe stretched tendon.

tendon = a group of tension

strands or bars used

for prestressing;


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Tendons are stretched andanchored to strong bulkheads,

Anchoring

bulkhead

strands

run

between

the platesand

anchored


Anchor ing bulkhead

with strands placed

strand

chuck


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Long lineprecasting

bed;

Multiple

girders cast at

once and

separated by

bulkheads;

strands

spacing

bulkhead

stirrups for

embedment

in slab,

composite

action

then concrete is placed aroundthe stretched tendon.


Post-tensioned:

The concrete is first placedaround embedded tubes or ducts

after the concrete is hardenedtendons are placed in the ducts,stretched to desired tension,and then anchored against theconcrete at the ends.


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The concrete is first placedaround embedded tubes or ducts

End anchorages for tendons


Draped P/T ducts


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after the concrete is hardened tendons are placed in the ducts,stretched, and then anchored against the concrete at the ends.



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P/T strands anchored at end


Bonded:

The tendon is bonded to the concrete

pre-tensioned: concrete is placed directlyaround the bare exposed steel;

post-tensioned: grout is pumped into theduct after the strand is stretched.


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pre-tensioned: concrete is placed directlyaround the bare exposed steel;

P/T

DUCTS

BARE

STR

ANDS


Bare

pre-tens

strand Post-ten

ducts


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post-tensioned: grout is pumped into theduct after the tendon is stretched.

GROUT

MIXER/PUMP

GROUT FLOWS

OUT OPPOSITE

END

PUMP END OF

DUCT


Unbonded:

The tendon is coated to preventbonding to the concrete -

pre-tensioned: usually strand is coveredwith a plastic sheath;

post-tensioned: the duct is NOT groutedbut the steel is protected by pumping

grease or another coating into the duct.


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pre-tensioned: usually strand is coveredwith a plastic sheath;


Plastic sheathed/greased strand


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BRIDGES

Pretensioning and post-tensioning both used;

Pretensioning: almost always bonded, unlessspecial conditions require lessprestress strands partiallyunbonded;

Post-tensioning: should be bonded or very

specials measures taken toprevent tendon corrosion


Calculating

Effects

of

Prestress

on a

Concrete Member


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Internal forces in prestressed beam -

no external loading:No external load applied to beam: there cannot be any internal moment.

M=0

Since there is no in ternal

moment from loads -

the resultant C in concrete must

act on the same line of action

as the T in the strand.

3

strand

We separate the C and T:

C is the compression created in the CONCRETET is the tension applied to the TENDON


Internal forces in prestressed beam - no external loading:

Axial load eccentricity in concrete

Considering only forces in the concrete:

Resulting eccentric compression causes an axial force and a moment: M = Ce

No axial load,so C = T


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internal concrete stresses

When the strand is eccentric,below the cgc,

the moment is opposite that shown

= a negative moment.

the stress in concrete at any

height y above the cgc:

(compression is +)

Stress due prestressing alone

When loads are applied, the

additional moments M create

added stress,

final stress at y :

Total stress at any fiber y

e Tf y +=

My

e Tf y ++=



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Load Balancing:

Draping strand:Total stress at any fiber y

If the moment due prestress, (Te), is opposite in

sign to the moment created by the loading (M) -

the bending st resses cancel out and pure axial

compression is left.

So, want : (Te) + M = 0

or, strand eccentricity e: e = - M / T

the strand location e should vary opposite to the

moment diagram due to applied loads.


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For a typical moment diagram on a simply supported beam (above)the strand should be draped as below:side view ofbeam and strand

Precasters, however, cannot obtain a curved drape in pre-tensioned beams.The strand pattern in a precast beam is likely to look like:

Plotting the resulting moment diagrams:

There are critical locations where the moments do not cancel

and bending stresses must be controlled!

2


sing

legird

er

second girder

some strands draped

others

straight


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For a typical moment diagram on a simply supported beam (above)the strand should be draped as below:side view ofbeam and strand

With post-tensioning:It is easy to get the tendon in exactly the shape desiredto cancel out the bendingA hollow duct is placed in the beam in the desired shape.Since the tendon is not tightened yet, it doesnt try togo into a straight line shape.



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Prestressing:

From the calculations -

we can see that the actualstresses in a member can be

CONTROLLED

by adjusting the tendon locatione and the applied prestress

force T.


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Aim of Prestressing

control amount of tension in concrete

limit concrete tension to a level below cracking

keep uncracked cross sectionmoment of inertia stays = Igross

Igross Igross

cracked


Why use post tensioning?

durability / crackfree

stiffness / crackfree

allows longer spans

can possibly reduce substructures/piers

lower maintenance

elimination of joints


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summary

pretension & post-tension . both create prestress,-- pretension in precasting plant,-- post-tension at job site,

post-tensioning . can exactly cancel out the bending effectfrom loads,

aim to prevent cracking: increased I allows

spanning longer distances,better durability;


Purpose of Workshop

what is post-tensioning?

PROGRAM: - best situations for using P/T

DESIGN: - special designs steps

CONSTRUCTION: - components and use


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How is P/T Applied?


eed strands through duct,place anchor head and wedges

anchor casting

embedded in

concrete

figures from VSL Inc.


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jack & anchor head


anchor head(bears on anchorcasting)

wedgesseated inconical holes


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place jack feed strands thruover anchor head and wedges



jack pulls set of strands

second head and wedges

grip strands here



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jack pulls set of strands



extended strands are locked in position

after strands are pulled

(to desired force/elongation)

2ndjack pushes against wedges

anchoring strands against head

---- then main jack is released

mix grout and pump groutinto duct to protect strand



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alternate to strand

pull bar with jack,

tighten nut in place

bearing plate applies

compression to concrete

figures from DSI


when to use

CRACKING -- possible situation where

cracking might develop and

affect:

durability, or

performance


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ideal application of post-tensioning with bars

plan from WisDOT


target piers

plan from WisDOT


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section BB

plan from WisDOT


36 - #11epoxycoatedrebar

Section

BB

plan from WisDOT


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girder girder

GIRDER 4


54


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Girder 4

Girder 3


543


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Between 2 & 3


354

2

Cracking:

due to weight of girders alone(deck not formed or poured)

pierdeflection


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6-8 above footing


10-11 above footing


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pier cracking additional softening and deflection

cracking =reduced moment of inertia &

increased deflections


cracking =reduced moment of inertia &

increased deflections

pier 2 deflections

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0 100 200 300 400

distance from end of cap (inch)

deflection(

inch)

measured

cracked sub

1.2 end deflection


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post-tension

to prevent

cracking

dead end

embedded

anchor

dead endembedded

anchor

thread bars

live end

jack ing

Replace 36 - 11 barswith

20 1.25 threadbar

36 - #11epoxycoatedrebar

no cracking

much more efficient


Other Prime ApplicationsBrady Street Bridge: pedestrian pathway on Milw lakeshore

LONG SPAN125 ft center span eliminated need for a midspan pier

5 tendons, each with ni ne 0.6 strands

from ASPIRE W2007


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Lakeshore State Park Bridge - near Milw lakeshore

from AECOM

Universit of Wisconsin from AECOM


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MUST HAVE:

vent tubesat high pointto allow airescape asgrout is

pumped in


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feed strand through ducts

1. strand spool

2. feeder3. into duct


anchor head& wedges


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grout vents

grouttubes



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Post-Tensioning Bridge Workshop

EXAMPLE

Bridge Design

&

Construction Controls



Spliced Girder Bridge

using precast girders to allow long span construction to improve long term durability to simplify transport of girders


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Florida - Choctawhatchee Bay Spliced girders

b a c k s p a n s u p p o r t

C a n t i l e v e r e d

g i r d e r s

p o s t -t e n d u c t s

PCI J V38, N4

LONG SPAN POSSIBILITIES


Florida - Choctawhatchee Bay Spliced girders

t a p e r e d c a n t i l e v e r g i rd e r

d r o p - i n s pan

g i r d e r

PCI J V38, N4


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Spliced girders

I - 15

S a l t L a k e C i t y

Ascent, Sp 99

Ascent, Sp 99

over 200 ft.

span


Spliced girders

Ascent, F 97

Rock Cut Bridge, Washington


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Rock Cut Bridge, three 63, 40 ton girders spliced for 190 span

Spliced girders

Ascent, F 97


Spliced girders

Tw i s t b r i d g e , Wa s h i n g t o n

1 7 6 s p an , 9 5 g i rd e r

eliminated acenter pier

in stream bed


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Spliced girders

Me t h o w R iv e r , WA , d o u b l e 1 8 0 s p a n s , 8 3 g i rd e r


Spliced girders

Neb r a s k a B r i d g e : 2 0 7 ft . s p a n , 7 9 I -g i r d e r s

PCI Journal Nov-Dec 06


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Spliced girders


Neb r a s k a B r id g e : 2 0 7 ft . s p a n , 7 9 I -g i r d e r s


Spliced girders


Neb r a s k a B r id g e : 2 0 7 ft . s p a n , 7 9 I -g i r d e r s


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splicing girders is particularly effective if:

1. it is possible to completely eliminatea middle pier by using long spans ($$$)

2. it makes shipping long span girders possible



design & construction problem:


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-- a single 180ft girder weighs 92 tons

-- a 180 ft. length is difficulty totransport around turns in a road


Temporary bents - support girders

joints

filledwhendecked


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I - 15

S a l t L a k e C i t y

Spliced girders

Ascent, Sp 99

3 95 p r e c a s t g i r d e r s s p l i c e d f o r a s in g l e s p a n o v e r 2 0 0

temporary const.supports atsplice points


Alternate to temporary supports:


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post-tension

decking



suspended girder

girder hangs from steelstrut beam


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good for situations whereP/T is applied before deck

is placed

(then deck has no prestress)

temp supports struts: 1. capacity to carry beam + deck?2. interfere with deck placement


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after deck and joints are placed:post-tension strands are fed through ducts


** the deck is prestressed with the girders:

we cant easily rip it off and replace it in 20 years,make provision for an overlay in design loads(similar to method used in segmental box bridges)


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FOR DESIGN: analyze this structurewith girder and deck weight

(solve for internal moments andinitial locked-in stresses in girder)

girder carries all load, deck is wet


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FOR DESIGN: all loads that are appliedafter the deck has hardenedand P/T applied -

create moments in this two spancontinuous composite beam


Construction Sequence:

the construction sequencedirectly affects the design process


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Design Process:

the design process shown

here follows

the same steps as

shown in WBM

design of W72 beam

in Chapter 19



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after deck and joints are placed:post-tension strands are fed through ducts

then tendons are pulled with the jack

- tendency is for strand to straighten

post-tensioning lifts girders off temp supports!


simulate the effect of temporary

support removal

by applying reverse loads


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temp support moment

-6000

-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

0 20 40 60 80 100 120 140 160 180

distance (ft)

moment(ft-kip)

a ctu al in t ac tu al ex t

moments fromremoval of

temporary supports

(reaction gone)


temp

bent

reaction

on girder

Moment from Girder and Deck

(on temporary supports)

-1500

-1000

-500

0

500

1000

1500

20002500

3000

3500

0 20 40 60 80 100 120 140 160 180 200

distance (feet)

moment(kip-ft)

temp support moment

-6000

-4000

-2000

0

2000

4000

0 50 100 150

distance (ft)

m

oment(ft-kip)

unit load actual int actual ext


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Service 1 Moment due DL on non-composite girder

-1500

-1000

-500

0

500

1000

1500

2000

2500

3000

3500

0 20 40 60 80 100 120 140 160 180

distance (feet)

moment(kip

-ft)

Service 1 DL M on composite girder

-8000

-6000

-4000

-2000

0

2000

4000

0 20 40 60 80 100 120 140 160 180

distance (feet)

moment(kip-ft)

Service 1 Total DL M

-8000

-6000

-4000

-2000

0

2000

4000

6000

8000

0 20 40 60 80 100 120 140 160 180

distance (feet)

moment(kip-ft)



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How to approach the design? where to start


pretension in girders only needs to besufficient to carry girder and deck

weight on the initial short temp spans!

controllingcse

forpretension

design


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checkthiscase

notlikelytocontro

ldesign


cracking in deck over pier (-M)

& comp in bottom flange at pier

thiswillbecontrollingca

seforPTdesign


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summary design approach:

design pre-tension to eliminatecracking of girders when simplysupported, girder + deck weight

design P/T to avoid tension in deckover top of pier



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Temporary bents - support girders and deck

Temporary bents - support girders

concrete deck is pl ced


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non-composite girders

-1000

-500

0

500

1000

1500

2000

2500

3000

3500

4000

0 20 40 60 80 100 120 140 160 180 200

distance (ft)

moment(kip-ft)

interior ex terior

girder weight plus concrete deck

before

deck is

hardened

weight is

carried by

girders

(non-composite)



-1000

-500

0

500

1000

1500

2000

2500

3000

3500

4000

0 20 40 60 80 100 120 140 160 180 200

distance (ft)

moment(kip-ft)

i nt er io r e xt er io r



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Initial Stresses due Non-Comp. DL

(without pretension)

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 20 40 60 80 100 120 140 160 180

distance (ft)

stress(ksi)

to p bo ttom ten a ll ow


high tension on bottom is unacceptable = cracking



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Note: dontuseapprox.lossequationasinWBM,

overestimateslosses 75yrs)



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1.266ks

i

needed



bottom tension is acceptable = no cracking

Temp: Girder and Deck Weight Initial Stresses

(with pretension)

-1

-0.5

0

0.5

1

1.5

2

0 20 40 60 80 100 120 140 160 180

distance (ft)

stress(

ksi)

top bottom ten al lo w

pre-tensioned


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-1000

-500

0

500

1000

1500

2000

2500

3000

3500

4000

0 20 40 60 80 100 120 140 160 180 200

distance (ft)

moment(kip-ft)

i nt er io r e xt er io r



IF - the girder is liftedat its ends:

high bending at middlewould require bottom

pre-tension

moment diagram BUT: later on withfull loading on the

structure the M overthe pier will create

high compression at bottom

- dont want comp. at bottom

from pre-tension -


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IF - the girder is liftedat its ends:

moment diagram


moment diagramequal +M an M

both smaller


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Pre-tension design

Summary:

the 2 end span girders have 12 strandsfor pre tension at the bottom;the middle span girder does notrequire any pre tension;


Post-tension Design


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Note:with W82 girderweb is 6.5 wide,3.4 duct is largestto fit!! limit ~ 12 strands



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girder end - anchorages

ducts over pier


note: the e is less than the kern,that means that the P/T will cause compressionat the bottom of the beam, the pier moments will also!


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Post-Tensioning


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Post-Tensioning


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Post-Tensioning


unit load

moment diagram

225k

A tendon that was in the exact shape ofthe moment diagram shown is concordant.

BUT - there is a big kink over the pier


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unit load

Need to eliminate the 225k reaction:

225k


unit load

upward 6.25k/ft

moments

-3000

-2000

-1000

0

1000

2000

3000

4000

0 50 100 150 200 250 300 350

distance

moment(ft-k)

resulting moment diagram


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tendon profile

-80

-60

-40

-20

0

20

40

0 50 100 150 200 250 300 350

distance (ft)

in

ches

+18.4 in.

-13.2 in.


tendon profile

-80

-60

-40

-20

0

20

40

0 50 100 150 200 250 300 350

distance (ft)

inches

+18.4 in.

-13.2 in.


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1

2

162ft

18ft



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Tendon Stress

(with friction los s)

185

190

195

200

205

210

215

0 40 80 120 160 200 240 280 320 360

distance (ft)

stress(ksi)

-20

0

20

40

60

80

100

tendon stress tendon profile

jack atthis end



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Friction loss



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Tendon Stress

(with friction loss)

185

190

195

200

205

210

215

0 40 80 120 160 200 240 280 320 360

distance (ft)

stress(ksi)

-20

0

20

40

60

80

100


---slope---


Tendon Stress(with friction l oss)

185

190

195

200

205

210

215

0 40 80 120 160 200 240 280 320 360

distance (ft)

stress(ksi)

-20

0

20

40

60

80

100


X


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Tendon Stress(with friction lo ss)

185

190

195

200

205

210

215

0 40 80 120 160 200 240 280 320 360

distance (ft)

stress(ksi)

-20

0

20

40

60

80

100


X


Tendon Stress(with friction lo ss)

185

190

195

200

205

210

215

0 40 80 120 160 200 240 280 320 360

distance (ft)

stress(ksi)

-20

0

20

40

60

80

100


X


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Tendon Stress(with friction and seating loss)

185

190

195

200

205

210

0 40 80 120 160 200 240 280 320 360

distance (ft)

stress(ksi)

-20

0

20

40

60

80

100


200 ksi allowedmax stress



185

190

195

200

205

210

0 40 80 120 160 200 240 280 320 360

distance (ft)

stress(ksi)

-20

0

20

40

60

80

100


200 ksi allowed max stress


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185

190

195

200

205

210

0 40 80 120 160 200 240 280 320 360

distance (ft)

stress(ksi)

-20

0

20

40

60

80

100


200 ksi allowed max stress

move

down

extra

releas

e


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Ad justed Tendo n Stress(with initial release, friction, seating and all tim e dependent loss)

184

186

188

190

192

194

196

198

0 40 80 120 160 200 240 280 320 360

distance (ft)

stress(ksi)

-20

0

20

40

60

80

100



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Elevation at beam end:


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733.9

328.7

590.3

442.3

Tpt0



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STRUTS:


Add ties:


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Add secondary members triangulation:


Co-ords for computer model:


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EXAMPLE

Bridge Design

&

Construction Controls



OTHER DESIGNS


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Multi Span Slab Bridges:


Multi Span Slab Bridges:

longitudinal reinforcingis replaced with post-tensioning ducts


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with P/T: length of middle span can be extended aslong as 80-90ft.

piers can be moved from in water to stream sides


Same design process is used:1. find maximum eccentricity for tendon at critical section2. solve for prestress force needed at that section3. find a concordant tendon layout using a M diagram shape


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Simpler than previous example:

no need for temporary supports; no pretensioning combined with P/T; no variation between non-composite

and composite sections;


P/T Concepts

Wisconsin Pt Workshop Notes

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