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1/4STRUCTURE magazine

PRODUCTWATCHupdates on emerging technologies, products and services

January 200848

Expanding the Use ofPost-Tensioning in BuildingsBy John Crigler, P.E.

Part 1 of this series of articles on Post-Tensioning ran in theJuly 2007 issue of STRUCTUREmagazine.

Visitwww.STRUCTUREmag.orgfor archived articles.

Transfer Plates

The suspended floors in multistorybuildings are generally supported bycolumns and load bearing walls. Thelayout and size of these elements helpdefine the space available within thebuilding. Floor framing systems areselected with consideration for bothspace needs and economics. In multi-story buildings, the requirements maychange from floor to floor. Therefore,the selection of a single framing system

for an entire structure is not alwaysideal. As the population grows, the costand availability of building sites willbecome more challenging and the needfor multifunction multistory buildings

will increase. This trend is seen in manyareas of the U.S., where multifamilyresidential buildings are built withunderground parking. These structuresare built with hybrid framing systemsconsisting of post-tensioned slabs for

the underground parking area, with a loabearing wood frame for the living area aboveIn Hong Kong, where the populatio

density is high and good building sites arscarce, multifunction high rise buildingare more common. For these buildingspost-tensioned transfer plates have beeextensively used to adjust the floor framinsystem as the requirements change withinthe height of the building.An early example of a large transfer plate i

the Pacific Place building in Hong Kong. Th222-meter (728-foot) tall building consists oa large retail/commercial/parking area thaextends up to the 57-meter (187-foot) leve

with residential and hotel space above. Thupper part of the building utilizes closelspaced supports which were suitable for thuse, whereas the base of the building requirelong spans. A 4.5-meter (14.7-foot) thicpost-tensioned transfer plate is used at the 57meter (187-foot) level to transfer the vertica

Buildings are created to achieve

well defined goals. While there aremany different systems used torealize these goals, the structural

system plays a key role. For more than 40years, post-tensioned concrete has been usedin buildings. It is widely accepted by structur-al engineers for offices, parking garages, con-dominiums and apartments. Post-tensioningis particularly well suited to reinforce slabs.When compared to conventionally reinforced

slabs of equal strength, post-tensioned slabscrack at a higher demand level which reducesdeflection under service loads, permittingthinner slabs and/or longer spans. The resultis a reduction in floor system weight which inturn reduces demand on the other structuralmembers (columns and shear walls) and thefoundations, while longer spans enhance thevalue of the building. For these reasons, theuse of post-tensioning for building slabs willcertainly continue to be the dominant appli-cation for post-tensioning in buildings. A rendering of the Pacific Place Building showing the transfer plate in relation to the

various building sections.

The Pacific Place Building in Hong Kong.


2/4STRUCTURE magazine January 200849

loads between the building zones. While this isan early example, there have been more than80 post-tensioned transfer plates constructedin Hong Kong over the last 20 years.While the residential transfer plates thatare more common here in the U.S. and thestructures built in Hong Kong represent therange of possibilities, they both demonstratethe advantages:

Accommodate irregular loading/support

paths, which can be challenging withbeam/girder framing systems Significant reduction in reinforcement Reduced construction time In many cases, reduction of depthwhich reduces the buildingheight and weight

Greater stiffness, which results in bettercracking and deflection control.

Foundations

The use of post-tensioning in single family

residential slabs on ground is currently thelargest market segment for the industry. Forthis application, the post-tensioning tendonsare usually installed concentrically in the slab

without the benefit of the deviation forceswhich result from draped tendons that arenormally used in elevated structures. Whilethis approach is adequate for lighter loads,a different approach is needed for moreheavily loaded foundations.Two-way mat foundations are used to

support multistory buildings. The prin-ciple of mat foundations is very similar to

that of a floor slab turned upside-down.The distributed soil pressure applies upliftpressure to the bottom surface, which isheld in equilibrium by the vertical con-centrated loads from columns and walls.Similarly, strip foundations behave likeupside-down beams.Post-tensioning of mat and beam foun-

dations has similar advantages to those infloors and beams: More uniform soil pressure distribution Increased shear resistance Reduction in thickness and

reinforcement quantity Compressed construction schedule Improved cracking and

deflection behavior Reduced excavation quantity and depth Improved water tightnessBasically, the designer has the choice be-

tween unbonded monostrand and bondedmultistrand tendons. For thin slabs, includ-ing residential slab on ground, industrialfloor slabs and low rise buildings, unbondedmonostrands are a logical choice. Bonded

multistrand tendons are best for larger foun-dations with heavier loads. They have highercapacity and are bonded, which improvescrack control and local flexural behavior.

Precast

Today many bridges take advantage ofthe combined benefits of precast and post-tensioning, but the use of post-tensioning inprecast buildings has been largely unexploited.

While the benefits of precast elements arewell known, the advantages of combiningprecast with post-tensioning are somewhat

unrealized. Post-tensioning can be usedsplice precast members together to bustructures with fewer joints and bearin

which reduces maintenance and provides structural benefits of continuous memband frames.Construction of fully hybrid precast/c

in-place buildings is perhaps the best wayoptimize both. An example of this approis the new Venetian Macao Resort. For

structure, continuous main beams with meter (66-foot) and 30-meter (98-foot) spwere constructed using 4 inverted pre

continued on page

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The precast secondary beams are installed on top of the corbels of precast main beams afterstressing the continuity tendons. The beams are T- or I-shaped to minimize the weight to 28 tonnes(30 tons) for erection. Half joints at either end of the beams are detailed by using strut-and-tiemodels to ensure the load transfer at the critical zones. In between the typically 4m (13ft) spacedsecondary beams, 75mm (3 in) thick precast planks with cast in lattice girders are installed. Topslab reinforcement is fixed directly on top of the lattice girder to receive the in-situ cast topping.

Level 3 and Level 6 have typical bay layouts of 20m x 20m (66ft x 66ft) and 20m x 30m (66ft x98ft). The structure consists of continuous post-tensioned framing beams, made up of semi-precast

pre-tensioned beams supported on column heads. Simply supported precast and pre-tensioned Tor I shaped secondary beams are supported on the continuous corbels on either side of the mainbeams. Precast planks are placed in between the secondary beams to receive a cast-in-situ topping.The precast beams and planks are installed by custom made high capacity traveling cranes, whichallow placing 25 tonne (27.5 ton) beams at a 50m (164ft) radius.

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Figure 1: 4-in-1 semi-precast main beams in the sequence of works.

The details of the 4-in-1 semi-precast main beams play a vital role to the success ofVSLs alternative design. The sequence of work is as follows:

Installation of 4 nos. of semi-precast main beams by heavy duty tower craneon column heads.

Stitching of flanges of precast beams to ensure the structural integrity. Fixing of steel reinforcement cages and P-T ducts in between precast beams.

In-situ concrete in-fill of main beam. Span by span stressing of continuity tendons across columns. Installation of precast secondary beams supported by corbels on main beams. Erection of precast planking supported by secondary beams. Fix top reinforcement and cast topping.

Typical weights of precast beams are 20 to 28 tonnes (22 to 28 tons).

An aerial view of the transfer plate under construction.

beams (see Figure 1 below) combined withpost-tensioning and cast-in-place concrete.These beams support the highly loadedfloors, and serve as elements in the seismic/

wind resisting frames. For this project, thesystem was selected to improve constructionspeed and reduce the impact of formworkand shoring on other trades.

Conclusion

The use of post-tensioning in buildings willcontinue to expand, with traditional applica-tions dominating. Post-tensioned structuresinherently use less material when compared

with conventionally reinforced concrete. As wegain a better understanding of the impact ofcement production on the environment, thisfactor will become increasingly important.The next article in this series will address

what to consider when incorporating post-tensioning into a structures design. Theseinclude reviewing the applicable Post-

Tensioning Institute (PTI) and AmericanSociety for Testing and Materials (ASTM)standards and their adequacy for theintended application, and selecting theappropriate materials.

John Crigler, P.E., is Senior Vice President andTechnical Manager of VStructural LLC (VSL).Crigler serves as the Secretary/Treasurer of the

American Segmental Bridge Institute (ASBI).He also serves on the Post-Tensioning Institute(PTI) Technical Activities Board (TAB). Inaddition, he is a member of the AmericanConcrete Institute and the American Society ofCivil Engineers. John can be reached via [email protected].

STAGE 7:

ERECT PLANKING

STAGE 8: CAST

INSITU TOPPING COUPLING BARS AT CORBEL

BENT OUT BARS FOR

SHEAR CONNECTION

CONCRETE PLINTH

WITH RUBBER STRIP

HALF

JOINT

HALF

JOINT

CORBEL CORBEL

500

75

100

5004700 1200

200

1225

STAGE 6: ERECTPRECASTSECONDARYI / T - BEAMS

STAGE 2:STITCHINGBETWEENSEMI-PRECASTMAIN BEAMS

STAGE 4:IN-FILLCONCRETE

STAGE 1:ERECTION OFSEMI-PRECASTMAIN BEAMS

STAGE 3:FIX IN-SITUSTEEL CAGE

STAGE 5:MULTISTRAND TENDON(6 - 19s / 6-22s)

PRETENSIONINGSTRANDS(0.6)

vsl product watch crigler jan 08.532

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