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Sagging floor slabs, cra c k e db e a m s, spalled con cre t e.

All are sympto ms of a sickbuilding that signal the

need for major re p a i r s. No con-s t ruction material or type of build-ing is immune; such stru c t u ra lp roblems occur in steel, re i n f o rc e dconcrete, prestressed concrete, andwood office buildings, condomini-ums, hotels, and parking garages.

The causes can be natura l s u c has eart h q u a k e s, or man-madesuch as design erro r s, constru c t i o nf l a w s, or adjacent construction ac-t i v i t y. A healthy stru c t u re also mayre q u i re significant modificationfor example, to increase live-load ca-pacity if the owner wishes to accom-

modate a change in occupancy.W h a t e ver the cause and whether thebuilding was sick or healthy, until re-c e n t l y, the ow n e rs repair or re t ro f i toptions we re limitedand costly.

The t raditional approaches

W h e re access and layout perm i t ,the usual fix is to add steel beamsu n d e rneath sagging floors and ac o n c rete topping. The beams mayre q u i re steel support columns, ei-ther at the buildings peri p h e ry or

t h rough its public are a s. And steelbeams generally must be designedfor full dead and live loads, not justthe strength deficiency, thus esca-lating material quantities and costs.

Although this method maya c h i e ve its objective, the cost, dis-ruption to utility serv i c e s, and un-sightly steel work may be unaccept-

able to building occupants andow n e r s. Rather than face these ob-s t a c l e s, some owners opt to teard own and start from scra t c h .

The ex ternal post-t ensioningalternative

T h e re is an alt ern a t i ve to thesteel-beam, concrete-topping, and

Repair and r etr ofit usingexternal post-tensioningBY KAREN J. B ARCHAS

Figure 1. Part ial floor plan: circles indicat e point of application of short saddlesbetween exist ing concrete c olumns. Triangles show t ube columns, all locatedapproximately in t he center of the 28-foot-square bays.

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t e a r- d ow n - a n d - s t a rt - over methodsof building repair and re t rofit. Pio-n e e red by Seneca Co n s t ruction Sy s-tems In c., a Canoga Pa rk, Ca l i f o rn i a ,e n g i n e e ri n g - c o n s t ruction firm, thee x t e rnal post-tensioning (EPT )method has been successfully ap-plied to repair or re t rofit some 30c o n c rete and steel stru c t u res in theUnited St a t e s.

Ty p i c a l l y, po st- ten sio nin g isappli ed in ter nal ly to re i n f o rc ec o n c ret e slabs or be ams. Pl a s t i c -s l e e ve d, 12- i n c h - d i a m e t e r, hig h-s t rength cables, or tendons, arecompletely enclosed in the con-c rete member when it is firstp o u red. After the concre te cure s,high tension is applied to the ca-b l e. Be a rin g plat es tran sfer thetendon tensile force to the con-c re t e. The com pres sed con cre t ecan then withstand design loads

wit hout exce eding allowable ten-si le stre s s e s.

With EPT, tendons are used onthe outside of a steel, concre t e, orwood stru c t u re to bolster itss t rength. Although the method usesthe same type of tendons as intern a lpost-tensioning, it re q u i res cori n gt h rough concrete walls andc o l u m n s a voiding damage to re-barto attach them. The tendonsa re run underneath the member

that re q u i res stre n g t h e n i n g t y p i-cally a floor slaband steel saddlesa re placed between the tendon andfloor at precise locations. The ten-dons are then tensioned, and theyapply a ve rtical, upw a rd force to thebottom of the floor.

The advantages of EPT

Although the EPT process ist ri c k y, it allows the application ofl a rge upw a rd forces at pra c t i c a l l yany location in a sagging floor. It sp ri m a ry purpose is to incre a s ebending strength, but its secondarybenefits include increased shears t rength and elimination of furt h e rd e f l e c t i o n .

When Se n e c as founders Ke nBondy and Chris Deetz we re askedto bid on what became their firstEPT project in 1977, neither had

e ver done re t rofits or re p a i red sick

b u i l d i n g s. Both, howe ve r, we res t ru c t u ral engineers with extensivepost-tensioning experi e n c e.

The first EPT opportunity came inthe form of an apartment buildingwith a seve rely cracked and sagging,re i n f o rced concrete slab support i n gt h ree floors of wood framing. Bo n d yand Deetz proposed to run post-

tensioning cables under theg round-floor slab, anchor them atthe perimeter walls, and use shortsteel saddles between the bottom ofthe slab and the cables to genera t eu pw a rd forc e.

Since successfully completingthis first EPT job, Seneca has beene n g i n e e r- c o n s t ructor on manya p a rtment, office, condominium,hotel, and parking stru c t u re re p a i r sand re t ro f i t s. Many of these stru c-t u res had been red tagged and we re

candidates for demolition.

Figure 2. Base plate at top of typicaltube column.

Figure 3. Sect ion 1 of Figure 1, showing typical east/ west t russ. The entire post-tensioning system i s placed betw een the second-floor slab and the first -floor falseceiling.

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Bondy attributes Se n e c as successwith EPT to the techniques minimal

d i s ruption to building function andu t i l i t i e s, lower cost compared withother methods, and faster completiont i m e s. EPT offers a flexibility thatssimply not possible with other meth-

o d s, he says.Since quality

c o n t rol is so cri t i-cal to the successof EPT, Se n e c ausually prov i d e sboth engineeri n gand constru c t i o ns e rv i c e s. Eve n

when constru c-tion is perf o rm e dby others, as wasthe case with a re-cently completedc o n d o m i n i u mp roject, eitherBondy or Deetz isalways onsite.

T h e re aremany steps thatmust be complet-ed perf e c t l y, like

d rilling thro u g hc o l u m n s, attach-ing the tendons,and cable ten-s i o n i n g , saysBo n d y. To mak es u re one of us isable to personallys u p e rvise each ofthese steps, wevelimited our EPT

p rojects to two or three a ye a r.

Luxury condominium r etrofit

Co n s t ruction of a luxury, 15-storycondominium complex in the LosAngeles area began in the early

1980s with conventional, re i n f o rc e dc o n c rete slabs and columns. Afterthe building was stru c t u rally com-p l e t e, the developer went bankru p t .

In 1987, the new owner decidedto complete the building and hire dan architect to add more space toeach units layout. The owner ap-p roved the new layoutsbut unfor-

t u n a t e l y, changes in kit chen andb a t h room locations made the exist-

The sequence of EPT con-s t ruction was as follow s :

1. Fi ve-inch-diameter holeswe re cored through each ex-isting column just below thesecond-floor slab, avo i d i n gall re b a r.

2. Holes we re cored at the ap-

p roximate centers of eachfloor bay to accommodatethe tubular columns.

3. The tubular columns we ree re c t e d .

4. The steel saddles we re bolt-ed to the bottom of the sec-ond-floor slab.

5. The small tolerance gap be-t ween the tubular columnb e a ring plates and the bot-tom of the floor slabs wasd ry-packed with nonshri n k

g ro u t .6. The tendons in all the bun-

dles we re strung, but nott i g h t e n e d .

7 . The tendons we re individu-ally tensioned to 25 kips, oneat a time in each successiveb u n d l e, first in the east/we s td i rection and then in then o rth/south direction. Thisa s s u red re l a t i vely equal ten-sion on each bundle.

8. The exposed tendon bun-

dles we re fire protected byw rapping them with metallath and hand applying ave rmiculite plaster to a cov-er of about 1 12 i n c h e s.

9. Gaps around floor holeswe re patched, equipmentwas re m oved, and the sitewas cleaned.

Figure 4. Tube column saddle after t endons were st rung.

Figure 5. Short saddle after tendons were st rung.

Figure 6. Sect ion 2 of Figure 1, showing typical nort h/ south truss. Downwardforces on short steel saddles are taken up by post-tensioning cables passingthrough existing concrete columns.

CONSTRUCTIONSEQUENCE

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ing plumbing chases unusable.To correct the problem, the con-

s t ructor would have to core aboutl,000 holes, up to 6 inches in diame-t e r, through each floor. Since it

would be pro h i b i t i vely expensive tolocate the buried re b a r, the engineerre q u i red a conserva t i ve assumptionthat 2,000 bars (both the top andbottom layers) would be seve red oneach floor. The engineer also deter-mined that, to re s t o re the ori g i n a lfloor strength, 10 kips (one kip

equals 1,000 pounds) of upw a rdf o rce would need to be concentra t-ed on the center of each of the 270floor bays, which measured about28 feet square. It was determ i n e dthat the traditional steel-beam solu-tion was impractical because itwould seve rely interf e re withplanned utilities and result in exc e s-s i ve cost.

Unusual EPTapplication

Seneca wasb rought in at thispoint. After exam-ining the plansand consulting

with the engineer,Bondy and De e t z

decided that EPTcould be used tos t rengthen thebuilding per thee n g i n e e rs specifi-c a t i o n s. Howe ve r,minimal head-room betwe e nthe bottom ofeach floor slaband the top of thesuspended ceil-ing was a major

o b s t a c l e. Theresimply was noway to place ten-dons under eachfloor to genera t e10 kips of upw a rdf o rce on each baywithout lowe ri n gthe suspendedceiling an unac-ceptable amount.

Yet the firstfloor was 18 feet

high, and the ceiling space underthe second-floor slab was about 8feetample space for an EPT sys-tem. Se n e c as solution was to run 18tube columns from the second floorto the roof near the center of eachbay (Fi g u re 1), weld bearing platesto the tube columns undern e a t heach floor (Fi g u re 2), and grout thespace between the plates and floor.Since the tube columns we re locat-ed in partition walls as close as pos-sible to the center of each bay, nomodifications to the arc h i t e c tsplans we re re q u i re d .

Vert ical load,no physical support s

Only one challenge remained: toapply a 150-kip ve rtical load (10 kipsper floor per bay) to the bottom ofeach tube column without placing

physical supports below the falseceiling between the first and secondf l o o r s. This was acco mplished byplacing the entire post-tensioningsystem between the second-floorslab and the first-floor false ceiling,as shown in Fi g u re 3, an east/we s tsection. The tube columns are sup-p o rted at their lower ends by steel

s a d d l e s with bundles of eight ten-dons running under we l d e d - o npipe sections (Fi g u re 4). Be t we e neach set of tube columns under-neath the second floor are shortsteel saddles, each with tendonsa b ove and below it (Fi g u re 5).

Fi g u re 6 (a north/south cross sec-tion at the second floor) shows howthe dow n w a rd forces on the shortsteel saddles are taken up. Thebuilding is three bays wide, so therea re three short saddles lined up with

the existing re i n f o rc e d - c o n c re t ec o l u m n s. Bundles of eight tendonsa re attached to the two outsidecolumns via bearing plates (Fi g u re7). These tendons run under eachsaddle and through holes cored atthe top of each column under thesecond floor.

The tensioning of each bundle to200 kips (25 kips per cable) results ina small, residual 3-kip upw a rd forc eat each of the short saddles and a150-kip upw a rd force at the bottom

of each tube column. Fully ten-sioned, the EPT system provides atotal upw a rd force of about 2.7 mil-lion pounds to the building floors l a b. At the same time, it adds virt u-ally no weight to the building.

K a ren J . Barchas is a fre e l a n c ewriter in San Rafael, CA.

Ac k n owledgementThis re p rinted article ori g i n a l l y

a p p e a red in the Fe b ru a ry 1991 issueof Ab e rd e e ns Co n c rete Repair Di-g e s t .

Figure 7. Tendon anchor plate bearing on column.

Figure 8. Tendons and saddles in place just beforefireproofing

Publication # C910536Co py right 1991, The Ab e rd e e n

Gro u p. All rights re s e rve d