Repair And Retrofit Using External Post-tensioning 255i2e

Repair and retrofit using external post-tensioning BY KAREN J. BARCHAS

agging floor slabs, cracked beams, spalled concrete. All are symptoms of a sick building that signal the need for major repairs. No construction material or type of building is immune; such stru c t u ra l problems occur in steel, reinforced concrete, prestressed concrete, and wood office buildings, condominiums, hotels, and parking garages. The causes can be natural—such as earthquakes, or man-made— such as design errors, construction flaws, or adjacent construction activity. A healthy structure also may require significant modification— for example, to increase live-load capacity if the owner wishes to accommodate a change in occupancy. Whatever the cause and whether the building was sick or healthy, until recently, the owner’s repair or retrofit options were limited—and costly.

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able to building occupants and owners. Rather than face these obstacles, some owners opt to tear down and start from scratch.

The external post-tensioning alternative There is an alternative to the steel-beam, concrete-topping, and

The traditional approaches Where access and layout permit, the usual fix is to add steel beams underneath sagging floors and a concrete topping. The beams may require steel columns, either at the building’s periphery or through its public areas. And steel beams generally must be designed for full dead and live loads, not just the strength deficiency, thus escalating material quantities and costs. Although this method may achieve its objective, the cost, disruption to utility services, and unsightly steel work may be unaccept-

Figure 1. Partial floor plan: circles indicate point of application of short saddles between existing concrete columns. Triangles show tube columns, all located approximately in the center of the 28-foot-square bays.

that requires strengthening—typically a floor slab—and steel saddles are placed between the tendon and floor at precise locations. The tendons are then tensioned, and they apply a vertical, upward force to the bottom of the floor.

The advantages of EPT

Figure 2. Base plate at top of typical tube column. tear-down-and-start-over methods of building repair and retrofit. Pioneered by Seneca Construction Systems Inc., a Canoga Park, California, engineering-construction firm, the external post-tensioning (EPT) method has been successfully applied to repair or retrofit some 30 concrete and steel structures in the United States. Ty p i c a l l y, post-tensioning is applied internally to re i n f o rc e c o n c rete slabs or beams. Pl a s t i c s l e e ved, 1⁄2- i n c h - d i a m e t e r, highs t rength cables, or tendons, are completely enclosed in the conc rete member when it is first p o u red. After the concrete cure s, high tension is applied to the cab l e. Be a ring plates transfer the tendon tensile force to the conc re t e. The compressed concre t e can then withstand design loads without exceeding allowable tensile stresses. With EPT, tendons are used on the outside of a steel, concrete, or wood structure to bolster its strength. Although the method uses the same type of tendons as internal post-tensioning, it requires coring through concrete walls and columns—avoiding damage to rebar—to attach them. The tendons are run underneath the member

Although the EPT process is t ri c k y, it allows the application of large upward forces at practically any location in a sagging floor. Its p ri m a ry purpose is to increase bending strength, but its secondary benefits include increased shear strength and elimination of further deflection. When Seneca’s founders Ken Bondy and Chris Deetz were asked to bid on what became their first EPT project in 1977, neither had ever done retrofits or repaired sick

buildings. Both, howe ve r, were structural engineers with extensive post-tensioning experience. The first EPT opportunity came in the form of an apartment building with a severely cracked and sagging, reinforced concrete slab ing three floors of wood framing. Bondy and Deetz proposed to run posttensioning cables under the ground-floor slab, anchor them at the perimeter walls, and use short steel saddles between the bottom of the slab and the cables to generate upward force. Since successfully completing this first EPT job, Seneca has been engineer-constructor on many apartment, office, condominium, hotel, and parking structure repairs and re t ro f i t s. Many of these structures had been red tagged and were candidates for demolition.

Figure 3. Section 1 of Figure 1, showing typical east/west truss. The entire posttensioning system is placed between the second-floor slab and the first-floor false ceiling.

ods,” he says. Since quality control is so critical to the success of EPT, Seneca usually provides both engineering and construction s e rv i c e s. Even when construction is performed by others, as was the case with a recently completed Figure 4. Tube column saddle after tendons were strung. condominium project, either Bondy or Deetz is always onsite. “There are many steps that must be completed perfectly, like drilling through columns, attaching the tendons, and cable tensioning,” says Bondy. “To make sure one of us is able to personally supervise each of Figure 5. Short saddle after tendons were strung. these steps, we’ve limited our EPT Bondy attributes Seneca’s success projects to two or three a year.” with EPT to the technique’s minimal disruption to building function and Luxury condominium retrofit utilities, lower cost compared with Construction of a luxury, 15-story other methods, and faster completion condominium complex in the Los times. “EPT offers a flexibility that’s Angeles area began in the early simply not possible with other meth-

Figure 6. Section 2 of Figure 1, showing typical north/south truss. Downward forces on short steel saddles are taken up by post-tensioning cables ing through existing concrete columns.

1980s with conventional, reinforced concrete slabs and columns. After the building was structurally complete, the developer went bankrupt. In 1987, the new owner decided to complete the building and hired an architect to add more space to each unit’s layout. The owner approved the new layouts—but unfortunately, changes in kitchen and bathroom locations made the exist-

CONSTRUCTION SEQUENCE The sequence of EPT construction was as follows: 1. Five-inch-diameter holes were cored through each existing column just below the second-floor slab, avoiding all rebar. 2. Holes were cored at the approximate centers of each floor bay to accommodate the tubular columns. 3. The tubular columns were erected. 4. The steel saddles were bolted to the bottom of the second-floor slab. 5. The small tolerance gap between the tubular column bearing plates and the bottom of the floor slabs was dry-packed with nonshrink grout. 6. The tendons in all the bundles were strung, but not tightened. 7. The tendons were individually tensioned to 25 kips, one at a time in each successive bundle, first in the east/west direction and then in the north/south direction. This assured relatively equal tension on each bundle. 8. The exposed tendon bundles were fire protected by wrapping them with metal lath and hand applying a vermiculite plaster to a cover of about 1 1⁄2 inches. 9. Gaps around floor holes were patched, equipment was removed, and the site was cleaned.

Unusual EPT application Seneca was brought in at this point. After examining the plans and consulting with the engineer, Bondy and Deetz decided that EPT could be used to strengthen the building per the engineer’s specifiFigure 7. Tendon anchor plate bearing on column. cations. Howe ve r, minimal headroom between the bottom of each floor slab and the top of the suspended ceiling was a major obstacle. There simply was no way to place tendons under each floor to generate 10 kips of upward force on each bay without lowe ri n g the suspended Figure 8. Tendons and saddles in place just before ceiling an unacfireproofing ceptable amount. Yet the first floor was 18 feet ing plumbing chases unusable. high, and the ceiling space under To correct the problem, the conthe second-floor slab was about 8 structor would have to core about feet—ample space for an EPT sysl,000 holes, up to 6 inches in diametem. Seneca’s solution was to run 18 ter, through each floor. Since it tube columns from the second floor would be prohibitively expensive to to the roof near the center of each locate the buried rebar, the engineer bay (Figure 1), weld bearing plates required a conservative assumption to the tube columns underneath that 2,000 bars (both the top and each floor (Figure 2), and grout the bottom layers) would be severed on space between the plates and floor. each floor. The engineer also deterSince the tube columns were locatmined that, to restore the original ed in partition walls as close as posfloor strength, 10 kips (one kip sible to the center of each bay, no equals 1,000 pounds) of upward modifications to the architect’s force would need to be concentratplans were required. ed on the center of each of the 270 floor bays, which measured about Vertical load, 28 feet square. It was determined no physical s that the traditional steel-beam soluOnly one challenge remained: to tion was impractical because it apply a 150-kip vertical load (10 kips would severely interfere with per floor per bay) to the bottom of planned utilities and result in exceseach tube column without placing sive cost.

physical s below the false ceiling between the first and second floors. This was accomplished by placing the entire post-tensioning system between the second-floor slab and the first-floor false ceiling, as shown in Figure 3, an east/west section. The tube columns are ed at their lower ends by steel “saddles” with bundles of eight tendons running under welded-on pipe sections (Figure 4). Be t we e n each set of tube columns underneath the second floor are short steel saddles, each with tendons above and below it (Figure 5). Figure 6 (a north/south cross section at the second floor) shows how the downward forces on the short steel saddles are taken up. The building is three bays wide, so there are three short saddles lined up with the existing re i n f o rc e d - c o n c re t e columns. Bundles of eight tendons are attached to the two outside columns via bearing plates (Figure 7). These tendons run under each saddle and through holes cored at the top of each column under the second floor. The tensioning of each bundle to 200 kips (25 kips per cable) results in a small, residual 3-kip upward force at each of the short saddles and a 150-kip upward force at the bottom of each tube column. Fully tensioned, the EPT system provides a total upward force of about 2.7 million pounds to the building floor slab. At the same time, it adds virtually no weight to the building. K a ren J. Barchas is a fre e l a n c e writer in San Rafael, CA. Acknowledgement This reprinted article originally appeared in the February 1991 issue of Ab e rd e e n’s Concrete Repair Digest.

Publication # C910536 Copyright © 1991, The Aberdeen Group. All rights reserved