Pre Stressed Concrete Tcm45-343849

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The novelty of p re s t re s s e d

c o n c rete ha s largely bee n

disp elled by this time. Mere

e m p l oym ent of th e pro ces s

is no longer n ews. Progre ssive con-

t ra c t o r s, engine ers an d arc h i t e c t s

have blazed the trail and their more

conservative colleagues have fallen

in line.

When confronted with a new

building technique, the accepted

p ro c e d u re seem s to be: (1) test its

pra cticality; and (2) per fect an d ex-

tend th e technique if it turns out to

be sound enough to stand up u nder

the rigors of field service.

Like airplanes, steam h eating an d

sweate r girls, which we like to th ink

of a s strictly prod u cts of Ame ri c a n

k n ow- h ow, pre s t ressed concret e is

m o re u niversal in origin an d earlyd e velopm ent than is sometimes

conced ed. A Ge rm an , Doehri n g ,

conceived an d paten ted the idea in

1888; a Frenc hm an , Freyssin et, was

the first to ma ke the idea pract ica l;

and today builders all over the

world are whole-hea rtedly seeking

ways to imp rove an d widen its ap-

plication .

A TWO-WAY STRETCH

But just what is the idea? Brief ly, it

is a met hod by which m ore effective

use is made of the con crete in rein -

f o rced con cre t e. Let’s look at th re e

hypothetical beam s to un derstand

som e of the gen eral principles that

govern the a ction of concrete me m-bers un der loads.

Unreinf orced. In th e accompan y-

ing drawing, Beam 1 re p resents a

c o n c rete girder withou t re i n f o rc e-

ment. If we assume an imposed

load, the b eam will tend to deflect

d ow n w a rd. The interior forces set

up by this action a re of two nat ures:

c o m p re s s ive an d ten sile. Co n c re t e

along the top of the beam is

“s q u e eze d” by com p re s s i ve forc e,

c o n c rete along the bottom is

“stre tched” by a force ref erred to astensile.

It mu st be kept in m ind t hat while

the com pre s s i ve strength of con -

c rete is qu ite high, u p t o a m axi-

mum of 18,000 psi, its tensile

s tren gth is low, usu ally som ewhere

in th e neighbor hood of 550 psi. In

the unrein f orced beam, the bottom

edge would crack soon a fter an y ap -

preciable load had b een ap plied; it

would be literally torn ap art.

Re i n f o rc e d . Re i n f o rcem en t is

placed in concrete to absorb these

ten sile forces. Becau se th ese force s

are concentrated in the bottom por-

tion of mem bers, the steel is placed

t h e re (see Beam 3). Un f o rt u n a t e l y,

considerable stress is placed o n th e

c o n c rete before the tensile forc e s

a re tra n s f e r red to th e steel. This

causes the concrete to crack under

loading even th ough it is reinf orced.

As a m atter o f fact, on ly on e-third

the con crete in a rectan gular girder

with con ventional re i n f o rc e m e n t

(the top part that is in com pression)

is assu m ed to carry st re s s. It is as-

sumed that the bottom two-third s

will crack un der load, an d th eref ore

will no t con tribute to th e bea m’sstru ctural strength.

No appreciable gain in tensile

f o rce resistan ce is ach ieved with

c o n c retes which have ve ry high

c o m p re s s i ve stren gths ( say, ove r

8,000 psi ) . Use of high- stre n g t h

steel, rather th an m ild steel, would

only p ostp one the tra n s f e rence of

tensile forces from the concrete to

Let’s ta ke a look a t. . .P r e s tr e s se d co n cr e t e

• what it is

• how it works• some problem s

• the outlook

This is concrete? A secretary at the Skokie

Laboratories of t he Port land Cement Associat iondemonstrates the remarkable strength and resilienceof a prestressed concrete plank.



the reinf orcing bars. If an ything, this

would increase the tend ency of the

concrete to crack under load.

These facts, coupled with the

s t ringen t building codes pre va l e n t

in the Un ited Stat es, usu ally rende r

o rd i n a ry re i n f o rced con crete im-

p ractical1 for fram es of b u ildin gs

that are over 25 to 30 stories, or that

call for ve ry wide ba ys. Beam s forsuch stru c t u res would have ve ry

l a rge cross sections an d would be

extrem ely heavy. These limiting fac-

tors affect the use of concrete in

building fram es, bridge girders, roof

an d floor pan els, and almost eve ry

load-bearing application .

Re s e a rch devoted to these pro b-

lems ha s resulted in the evolvem ent

of design and construction tech-

niqu es that rend er ord i n a ry re i n-

f orced concrete practical and com -

p e t i t i ve for a wide ran ge of a p p l i c a t i o n s. Gi rde r sh ape s h ave

been developed to make better use

of the m aterials employed, and con-

siderable work has been done o n the

q u a l ity, p lacem ent an d size of re in-

f orcemen t. In so m e cases, extra large

re b a r s, over 2 inches in diam eter,

have pro du ced excellent results, es-

pe cially in bridges. Recently, howev-

e r, the Bu reau of St a n d a rds an -

nounced that tests they have

conducted indicate that in most

c o n s t ruct ion, use of a num ber of small rebars results in less crack ing

than fewer bars of greater diam eter.2

Pre s t re s s e d . These improve-

m ents, despite their importance an d

va l u e, m inimize rath er than elim i-

na te th e sho rt c o m i n g s. An en tire l y

f resh app roach to the p roblem was

ne eded to take a real stride forward .

Pre s t ressed con crete has prov i d e d

tha t fresh app roach. Ind eed, in sev-

eral ways it has already proved to b e

superior to ord inary rein f orced con-

crete in m any bu ilding fields.

To un derstand the technique we

can start by breaking down th e word

1Ultimate load design has changed thispicture somewhat. For example, a 40-storyapartment building under construction inChicago will have a reinforced concreteframe.

2See CONCRETE CONSTRUCTION,October 1956, page 10.

1. Simple concrete beam, underenough load, will bend andcrack.

2. Concrete under load actslike wood. Top iscompressed, bottom cracks.

3. Steel rod can take tension inbottom of concrete beam.Top takes compression. Forsmarter design. . .

4. Squeeze can be used as inlifting books. Applied toconcrete, that idea is. . .

5. Prestressing. High-strengthcable, under high tension,strengthens beam, preventscracking.



Extensive use of prestressed memberswas made in the Pryor (Oklahoma) HighSchool gymnasium. Four prestressed 100-foot girders support the roof channels andeight 35-foot prestressed beams supportthe bleacher seats.

Fifty beams were required for theEndicott Street Bridge, Danvers,Massachusetts, the first prestressedbridge in New England. In thebackground, the studs of the twolower strands have been pulled andthe nuts run up. The jack is attachedto the draped strand. Note that the

top of the end plate is bent slightlyinward; this brings the bearing face ofthe stud exactly perpendicular to theaxis of the draped strand.



itself: p re - s t ressed . Red uced to its

simp lest ter m s, that is exactly the

reasoning behind the new pro c e-

d u re; stress is app lied to the con-

crete before load is ap plied.

This is accom plish ed by stre t c h-

ing the re i n f o rceme nt, usually by

hydrau lic jacks. As a result, it ceases

to act as re i n f o rceme nt at n orm a l

loads and becom es merely an agen tto app ly a comp re s s ive force a t the

ends of the mem ber. The same re-

sult could be achieved by a giant

clam p.

Und er overloads seve re en ough

to stretch th e cables, som e of the

characteristic properties of ordinary

re i n f o rcemen t are assum ed. In or-

der to ta ke fullest advantage of the

cables if overloading shou ld occu r, it

is desirable to h ave them con cen-

trated near the bottom of the girder,

becau se this is where the gre a t e s ttensile stresses build up.

To oversim plif y, we m ight say that

the squeezing effect achieved by

p re s t ressin g a beam causes the

center to bend upw a rd—just the

opposite reaction from loading.

Then, when a load is app lied, as in

Beam 5, the beam re t u rns to a

n o rm al straight line. In this way,

p re stressin g anticipates the deflec-

tion of a concrete mem ber tha t will

result from loading and compe n-

sates for it by setting up forces with-in the m ember that are directly op-

p o s i t e. On e force thus cancels out

the oth er.

ADVANTAGES

Pre s t ressed conc rete is tra c k l e s s

Even u nd er ap preciable ove rlo ads,

the concrete in a prestressed mem -

ber is held u nder com pression by

the sq ueezing effect of the stretched

cables. Even when cracks do a pp ear,

after a load several times the d esign

load has been app lied? they will dis-

appear when the overload is re-

m oved, unless the cables have been

s t retched beyond the poin t fro m

which they can sn ap b ack to their

f orm er length. Tha t critical stress is

called th e cab le’s yield p oint .

This brings us to anoth er advan -

tage of prestressed u nits: their abili-

ty t o re c over a fter seve re overlo a d-

ing. Under equal loads, a pre-stressed bea m will deflect no m ore (

usua lly som ewhat less) tha n an or -

d i n a ry re i n f o rced bea m designed

for the sam e du ty. After the load is

re m oved, the pre s t ressed un it re-

turns m uch m ore nearly to its origi-

n al sha pe. Since pres t ressed m em -

bers can stand greater ove r l o a d s

than ord i n a ry re i n f o rced beam s,

they develop m uch more severe de-

flection s before failure takes place.

BEFORE OR AFTER?Two constru ction pro c e d u re s

with reg ard to t im e of stressing are

possible in pre s t ressed work: pre-

tensioning and post-tensioning. In

p retensioned work, the cab les are

stretched b efore the con crete is cast;

in post-ten sioned un its, cables are

s t retched after the concrete has

hard ened.

The ten sile force of th e cab les istransf erred to the concrete by bond

in pretensioned conc rete. When ca-

bles are stretched, their diame ter is

d e c reased in direct pro p o rtion to

the amount they are lengthened.

Cables tend to snap back to their

f orm er length wh en released, but if

encased in hardened concrete they

are rigid ly re s trained. This strug g le

be t ween th e cab les, which attem pt

to return to their original shape a nd

the concre t e , which holds them at

their extended length and re d u ced

cross-section, sets up th e comp res-

sive force.

In post-ten sioned wo rk, force is

t ra n s f e r red to the concrete by at-

taching plates to the ends of the ca-

bles. This sets up a squ eezing action

similar to that wh ich occu rs when

you h old a nu m ber of books, with-

out a bottom support , by simply

Beverly Road Bridge, near Elgin, Illinois, was built as a test. As a result of thefindings, 225 of the 285 bridges on the Northern Illinois Toll Route are to be ofprestressed concrete.



pressing with your palm s on th e two

end volum es.

Co m p a ring the two appro a c h e s,

it can be seen tha t post-tensioning

w o rk re q u i res end anch orages to

m aintain cable tension. Co n s i d e r-

able work has b een don e in d eve l-

oping simp lified an d low-cost de-

vices, but th ese extra accessories do

increase expenses in post-tension edwork .

Often, howe ve r, whe n ve ry larg e

girders are to be job-cast because of

transportation difficulties, post-ten-

sion ing is the only solution . Also, in

po st-tensioning, there is an adva n-

tage in the ease with which cables

may be draped.

Both the com pre s s i ve an d the

tensile forces in a be am supp orted

at each end vary considerably alon g

its length. They are least inte nse a t

the end s and increase to a maxi-mu m at the center of the beam . Due

to this characteristic, most efficien t

use is m ade of the pre s t ressing ca-

bles if they are draped in a catena ry

cu rve rath er than m erely stre t c h ed

stra ight.

As can b e readily ima gine d, com -

plications are in store for an yo n e

plann ing to deflect cables an d also

tension them before the concre t e

has been cast. Som e m ethods have

been evo l ved, thou gh, th at perm i t

one to ha ve some cake and eat it ,too.

One met hod em ploys invert ed-U

positioners of va rying height tha t

ho ld the cab les to a curve. Position -

ers are bolted to the base of the cast-

ing bed un til after the co ncrete has

h a rdened. Another me thod uses a

yoke-like de vice from which b ars

extend dow n w a rd into the form to

hold the pre s t ressing cables in an

a p p roximat e arc. Howe ve r, b oth of

these m ethods retain on e of the dis-

advantages of post-ten sioned work :

they re q u ire that high-strength ac-

c e s s o ries be incorporate d in each

un it fabricated.

GETTING INTO FOCUS

Se ve ral facts are eviden t fro m

even such a cur sory look at the p re-

s t ressing concep t and its app lica-

tion as th is. For on e th ing, it is n o

WHAT’S THE SCORE ON PRESTRESSING?

THE ADVANTAGES THE DISADVANTAGES

1. Elimination of cracking. 1. Highest quality materials required.

2. Reduced cross-section of2. Closer control needed.

members.

3. Reduced weight of members. 3. Skilled workmen required.

4. Less concrete needed. 4. Greater capital investmentfor prestressing equipment.

5. Longer spans possible.5. High cost of end anchorage

6. Greater recovery after overloads. units (in post-tensioned work).

BED: the form and abutments that

are used to cast prestressed mem -b e r s. Fo rm s m ust be rugged to

withstan d m any re u s e s. Ab u t-

ments through which the pre-

s t ressing cab les pass an d against

which they bear un til the con crete

unit has ha rdened, have to be able

to withstand th e tremend ous pres-

s u res exerted by the pre s t re s s i n g

jack s.

CABLE: a com posite of several ex-

tra high-strength wires.

CAT E N ARY EFFECT: both tensile

and com pre s s i ve forces a re least

s e ve re at the ends of a beam and

mo st sever in th e m iddle. In view

of this, fu ll advan tage is taken of

prestressing cables only when the y

are draped in a caten ary curve (the

c u rve a flexible piece of ro p e

would assume when h eld horizon-

tally at bo th e nd s). This is ref erred

to as the cat ena ry effect.

COMPRESSIVE STRENGTH: resis-

tan ce to sque ezing forces.

C R E E P : the tendency of elastic

m ateria ls, e.g., conc rete an d ste el,

to “g i ve” o r “re l a x” slightly whe n

und er continu ed stress.

DEAD LOA D : weight that a me m-

ber mu st be expected to supp ort at

all time s, e.g., its own weight an d

that of roo f top pin g, walls, etc.

DESIGN LOAD: weight which u nit

was planned to support.

END ANCHORAGE:device at each

end of a pre s t ressed u nit to hold

cables at proper tension.

LIVE LOAD:weight of a temp orary

nature that m emb ers are expected

to su pp ort, e.g., snow, wind , peo-

ple and vehicles.

RECOVERY:ability of a m em ber to

return to its original shap e after it

has b een deflected.

S E RVICE LOA D : dead load plus

live load.

STRAND: a factor y-produced unit

of helically-soun d, sm all-diam eter

wires.

TENSILE STRENGTH: re s i s t a n c e

to stretch ing forces.

ULTIMATE LOAD: least weight un -

der wh ich a m em b er will fail.

YIELD POINT: smallest amou nt of

stretchin g from which a ca ble will

not be ab le to snap back to ap-

proximate ly its o riginal len gth.

Some Common Terms in Prestressed Concret e



longer m erely an idea or a n ove l t y;

it has becom e an imp ortant real ity.

Pre stressing has be en a nd will con-

tinue to be a subject of conjecture,

experim entation and developm ent,

bu t it is alread y a widely used con -

struction technique.

We need ed experience to bri n g

into focus the p roblems th at mu st

be solved to extend an d imp rove theapp lication of prestressed concrete.

Builders all over the world are m ak-

ing imp ortan t contri b u t i o n s, an d

the wh ole pace of the developm en t

is toda y alm ost jet prop elled. Bu t

even ap art from th e apathy and re-

sistance with which most new radi-

cal design co ncepts are greeted, pre-

stress has a few prob lems all its own.

One of the m ost difficult is the very

high capital investmen t necessary

to man ufacture pre s t ressed un its.

Creep in con crete and cables poses

some difficulties, and much re-

mains to be learned about va ri a b l e

bon d an d the effect of vo l u m e

changes in concrete.

That pre st ressing is m aking p he-

nom enal pro g ress despite theseproblems is evident even in th e dai-

ly newsp ap ers. Not lon g ago pap ers

all over the country heralded the

open ing of the longest bridge in th e

world—the Lake Po n t c h a rt ra i n

Ca u s e w a y. Pre s t ressed p iles an d

deck un its were credited with a size-

able saving in b oth time and m oney.

Of cours e, in m an y categories of

c o n s t ruct ion the re is still little u se

being m ade of pre s t ressed un its. A

notable example is multi-story

building con struction. But with the

i n c reasin g accept ance of pre-

s t ressed con crete this pro m i s i n g

and versatile material seems almost

certain to gain wide acceptance for

v irtu ally eve ry type of con structi o n

w o rk. It is m ovin g rap idly into th ef o re f ront of our m ajor m aterials of

construction.

PUBLICATION #C570404

Copyright © 1957, The Aberdeen Group

All rights reserved

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Pre Stressed Concrete Tcm45-343849