Lyra

Fig. 6. View of plaza and gardens.

Part 2

Structural Design andConstruction

Alfred A. YeeAlfred A. Yee Division,

Leo A. DalyHonolulu, Hawaii

Chang Nai KimAlfred A. Yee Division,

Leo A. DalyHonolulu, Hawaii

The challenge faced by our firm, asstructural engineers for One Hun-

dred Washington Square, was clear: towork out the logistics of balancing theoffice tower on its pedestal.

Our solution to this assignment isbased on an exterior wall system actingas an understated, 19-story precast con-crete Vierendeel wall truss, spanningfrom corner column to corner column,which provides an economical but visu-ally interesting means of lifting the bulkof the building 40 ft (12.2 m) above itsplaza, The truss opens the ground levelfor pedestrian use and provides unim-peded views of both the main head-quarters building and the gardensaround the new building (Fig. 6).

During the conceptual planning of thebuilding system, various structuralschemes were considered in developingthe Vierendeel wall truss. These in-cluded structural steel, cast-in-place

24

ml ^ 4

iR

0m

- a a

E• •r e

. lg

th

H ! II?trar^S l parking

11 ! 1. i. ^__ l 1 J11.111

7 #I entrarrce7

,^r- aSrtr..

° y

' y ti::..c4UF rr..v^JiLv.

-

cqr'

—. truck ramp do

i ll■I^lIE U^^!^^^^

^C - .. =5 ems r '# }' a; .;i p,

IiC

wasrrton ave„^aPLAZA FLOOR FLAN • 0 S S 2

Fig. 7. Plaza floor plans of office tower with shaded vertical columns and center core wallelements.

reinforced concrete, and precast con-crete sandwich panels combined witht)ost-tensioned concrete girders. Thefinal solution evolved from a careful re-view of the total design impact uponmechanical, electrical, architectural,structural and construction systems.Considering all the design and eco-nomic factors, the structural system andconstruction methodology included thefollowing major elements:

• Precast prestressed double teefloor joists and inverted tee floorbeams.

• Precast concrete sandwich exteriorwall panels,

• Slip-formed in situ concrete centercore.

• Four in situ double corner col-umns.

• Cast-in-place bottom chord perim-eter post-tensioned girders.

• Cast-in-place transverse post-ten-sioned girders through slip-formedcenter core walls.

• Construction sequences of post-tensioning girders at differentstages of building erection.

PC! JOURNALlJanuary-February 1984 25

mil

• TOT. Of 1 RGRE9 aCOLUJ. NTEI+IORS Ctla nOORS (ZSAOfJ EG FT_PER FLOp11 OFFER SMIWAPLM" 'A FLeXS&lTv

FOI11 POBI-IEMTU *O PERKDETER 0051555 (OESNMEU,-v tot SOTTO T 01010 CFTHE 0105EI51EEL 19U55 FE-TERIOR W q U •O' SOOYEcc•]• LEVEL. •GCEMl LOAD051 sROr PREC•SI ROORFR/^~ IMI 5O51A SO 050851 WALL E55118 To PE -ATE 45 EST%EIELT STROyO

Fig. 8. Diagrammatic perspective of tower structure showing major elements of thestructural system.

Fig. 7 shows the street level plan ofthe central core and double corner col-umns. A diagrammatic perspective ofthe building system is illustrated in Fig.8. A transverse building section, in-cluding dimensions of floor and towerheights, is shown in Fig. 9.

A retrospective review of the com-pleted design effort confirms that theexecution ofthe construction documentsrequired close teamwork among theconsultants electrical, mechanical andstructural engineers. Complete and fre-quent coordination of all the disciplinesby the architect was of primary impor-

tance. Any changes which occurredduring the design development re-quired a full understanding of the impli-cation upon the total building system.Some design items which affected theexterior wall system were: (1) Withinthe Vierendeel wall truss there weremany openings for the mechanical sys-tem; (2) The exterior finish of the Vie-rendeel truss was the finish of thebuilding; and (3) Within the exteriorfinish of the Vierendeel wall panel wererailing elements for the window wash-ing equipment.

These and other similar items re-

26

TRANSVERSE SECTION • °a le

J

Fig. 9. Transverse section through building.

quired complete planning to insure ac-curate prefabrication of the wall panelsand timely construction performance onthe total building.

The Uniform Building Code wasadopted with amendments set forth bythe Minneapolis Building Code. Designloadings recommended by the Minne-apolis Building Code for gravity and lat-eral loads (such as live, snow and windloads) were included in the structural

design criteria. Additional superim-posed loadings were assigned to areaswhich were designated for mechanicalequipment, roof areas where potentialsnow and ice accumulation could occur,and for the earth-filled landscaped gar-den and plaza levels. Any exposed areawhere ponding could occur due toheavy rain or drainage system failurewere considered for design superim-posed loadings.

PCI JOURNAL/January-February 1984 27

H.V.AC. DUCTOPENING

STEEL REefFORCWG

PAST- OUTEDRECESSEDSPLICE SLEEVE

CAST-IN-PLACESPANDREL

POST-GRO4JTEDSPUCE SLEEVE

—PRECASTSTRUCTURAL

4 WALL PANEL

RIGID TFERFINSULATION

PRECASTCONCRETEE KTERIDNWALL PANEL

PRECASTDOUBLE—TEEFLOOR JOISTS

h

STEEL CABLETENDONS

• --!POST-TENSIONEDCONCRETE GIRDER

I

0C

Fig. 10. Structural section through typicaI exterior wall construction showing assembly ofelements in Vierendeel truss.

28

Structural Concept

The basic intent of the engineeringdesign was to provide an economicalstructural system which could beerected readily with construction tech-niques available in the Minneapolis re-gion and which would be sufficientlyversatile to accommodate a fairly widerange of weather conditions. A precastexterior wall system combined withprecast double tee floor framing was ex-plored and confirmed as feasible.

To further speed construction, theslip-formed center core walls were de-signed to precede the precast construc-tion. This permitted the entire erectionof the center core walls before any con-struction of the precast wall and floorsystem was started, thus concentratinglabor and equipment for the varioustasks at different time periods.

A diagrammatic view of the assemblyof the Vierendeel truss elements, in-cluding precast double tees, post-ten-sioned girders and precast sandwichwall panels, is shown in Fig. 10,

The third floor plan of Fig. 11 showsthe post-tensioned perimeter girdersand the transverse cantilever girders. Itfurther outlines the positions of the cor-ner columns and the central core, Thefourth floor plan (Fig. 12) shows theupper flanges of the post-tensioned pe-rimeter and cantilever girders. Exteriorprecast wall panels serve as elements ofthe Vierendeel truss, which begins atthe fourth floor. Elements of the typicalfloor framing system (Fig. 13) are pre-stressed double tees and prestressed in-verted tees.

Construction MethodologyThe foundations for the corner col-

umns and the center core are conven-tional spread footing pads founded onlimestone bedrock approximately 35 ft(10.7 m) below street level. The bedrockstratum was dense and substantial, ca-pable of supporting 70,000 psf (3346

kPa), After the center core walls wereconstructed by the slipforming tech-nique to their full height, the tower cor-ner columns were constructed to aheight 2 ft (610 mm) below the thirdfloor, which was 40 ft (122 m) above thestreet level. The corner columns abovethe street level were architecturallyshaped and tapered with a maximumoverall size of 4 x 11 ft (12 x 3.4 m) foreach segment of the double column.The reinforcement consisted of 58-#18Grade 60 reinforcing bars with a W14 x311 structural steel column placed ineach segment.

At an intermediate height betweenthe plaza street level and the third floor,a concrete tie was required to assist inthe comer column stability. The struc-tural steel column was designed to sup-port Vierendeel wall truss and perime-ter girders until the wall truss wasstructurally functional for six stories(third floor to ninth floor). During thisconstruction sequence, three stages ofpost-tensioning were performed on theperimeter and transverse girders,thereby subjecting the steel column tohorizontal and rotational displacementsdue to prestress axial shortening. Thecorner columns were originally de-signed to be constructed with the Vie-rendeel wall truss keeping not morethan four stories behind erection of thewall panels.

The transverse girders cantilever 19 ft(5.8 m) from the core wing walls to pro-vide a flexible support to the north andsouth perimeter girders. Reterto Fig. 11for the plan location of the transversecantilever girders, to Fig. 7 for outline ofcore wing walls, and to Fig. 9 for theelevation view of the cantilever trans-verse girder located 40 ft (12.2 m) abovethe plaza street level. The core wingwalls, which are 2 ft thick x 20 ft wide(0.6 x 6.1 m), were cast with the slip-furmed core -walls. The maximum un-supported height of the core coringwalls is 40 ft (122 m) between the plazastreet level and the third floor. Core

PCI JOURNALJJanuary-February 1984 29

222'-I I"0

DBL-TEES

POST-TENSIONEDGIRDER- -

DOUBLEPRECASTBEAMS

NED

^1.,,,JNEDGIRDER

Fig. 11. Structural plan for third floor.

C)

0Cmz

C-w

C

T

coT

CORNER COLUMNS

I^ I^MI

li 111 Ali4u

i I DOUBLETEES

li! u!

-Tr ^ _----_—

DOUBLE i ----TEES

EXTERIOR PRECASTALL PAN ELS

n

,^ f

DOUBLE TEESII

POST-TENSIONEDGIRDER------

_J'

ii II

r,

DOUEPRE AST

j BEAMS ^l lFjl II ^I

^I II j POST-TENSIONED_±I II I I TRANSVERSE

I it GIRDERS

k ^I^E, ` II III

-----_^^_------------ _^i tL

POST-TENSIONED GIRDER

r_i rI ^

DOUBLE TEES

Fig. 12. Fourth floor plan.

WN

COLUMNS EXTERIOR PRECASTPANELS1

•I

.1

i

r,

i DOUBLE-PRECASTBEAMS

r ^.I- -----z

d q^d^ a

Ili

1; h

j^ it 1

i4

Fig. 13. Typical floor plan, 7th through 12th floors.

t OF TRANSVERSE ? POUR GIRDERSGIRDERS a -^ —3rda41hFLRS 4th F10OR

3rd FLOOR _ -

BASEMENT .-SUB-BASEME'

-W k4 Cot_.

Fig. 14, Construction of north and south girders, Sequence 1.

CONCRETE STRENGTHAT TIME OF STRESSING SHALLBE 6000 PSI OR GREATER

PERIMETER GIRDER_ _ __ POST TENSIONING

-P ES RE S TRANSVERSE STAGEI

GIRDERS STAGEfII I ADJUST SHORES AFTER

r SHORING { ^J I POST TENSIDNING FORIs t FLOOR SNUG FIT

BASEMENT__SUB =BASEMENT .

Fig. 15. Construction of north and south girders, Sequence 2.

POUR 5th, 6th &7 th FLOORS C OF TRANSVERSE

7^h FLOORGIRDERS

—__ _6th FLOOR J Z— - -- - —^

— —`i^h. FL048 -o o aa-— __

4tti FLOOR 0 I- z d.3 rd FLOOR__ .__

o^w^aa^vr^^

_t —_.1^ _ _ST i GE TES R SS RA SUERS£ ADJUST SHORES T

GIRDERS STAIGE 2 BE LOOSE FIT SHORESi

I I ^

ISHORING I MAY BE REMOVED

AFTER 7 th FLR. CONC.Iat.FLOOR — STRUCTURE HAS CUREDBASEMENT TO THE REQUIRED 28

— DAY STRENGTH.SUB-BASEMENT

of north and south girders, Sequence 3.Fig. 16. Construction

L OF TRANSVERSEGIRDERS - - _

4th FLOOR'^'^_ _ELQQI3_ ^;^ _-^=

W !4 COL.--

wing wall vertical reinforcement con-sisted of #18 reinforcing bars at 6-in.(152.4 mm) spacing on each face.

In addition, two W14 x311 structuralsteel columns were added on the eastwing walls and two W14 x370 structuralsteel columns on the west walls. These

structural steel members provided im-proved stiffness at the free ends of thewing wall to cope with the large reac-tions generated by the cantileveredtransverse girders.

Integral with the construction of theperimeter post-tensioned girders was

PC! JOURNAUJanuary- February 1984 33

R

_

=MENT

--PRESTR $5 RA SVERSEGIRDERS STAGE3

SHORINGI 51 FLOQR

BASEMENSUB -BASI

7th FLOU6th FLOOR5th FLOOF4tfi F^003rd FLOOF

-SHORES MAY BE REMOVEDAFTER 7 t h FLR. CONCRETESTRUCTURE HAS CURED TOTHE REQUIRED 28 DAYSTRENGTH

{F --t OF TRANSVERSE91h FLOOR I GIRDERStth FLOOR ----

Fig. 17. Construction of north and south girders, Sequence 4.

POUR GIRDERS

41h ELOOR3rd 8, 4th FLRS.

I--

rd FLOOR ____W 14 COL.

^ 1 I I I^--SHORING iTh

LS1 FLOOR

7E)BASEMENTSUB-BASEMENT EEitEIEt;bFig. 18. Construction of east and west girders, Sequence 1.

the requirement for shoring the post-tensioned girders, precast wall panelsand floor system, This shoring was re-quired from the third floor to the seventhfloor with superimposed loading ac-counting for construction to the ninthfloor, until the Vierendeel wall was con-structed and structurally effective. Atthis point in the construction sequence,three post-tensioning stages were to becompleted and shoring removal wouldtherefore be permitted.

The construction sequences for thenorth and south post-tensioned perim-eter girders are shown in Figs. 14 through17. East and west post-tensioned perim-eter girders are shown in Figs. 18through 20. East and west post-ten-sioned cantilever girders are shown inFigs. 21 through 24.

After completing the third stagepost-tensioning of the east and westcantilever girders (with six stories ofVierendeel exterior precast wall assem-bled), it was planned to place the in situcomer columns from the third floor tothe sixth floor so that the corner columnconstruction would be progressivelycompleted within three floors of theon-going installation of precast wallpanels and precast floor framing.

At this stage, the contractor would alsobe permitted to remove the perimetershoring, thereby activating the partiallyconstructed Vierendeel truss to supportthe external loads. However, during theactual construction the contractor re-quested that this restraint he modified topermit continued construction of theprecast wall panels and floor framing

34

PERIMETER GIRDERPOST TENSIONINGSTAGE I

4th FLOOR3 rd FLOOR

W14 COL:

CONCRETE STRENGTH ATTIME OF STRESSING SHALLBE 6000 PSI OR GREATER

ADJUST SHORES AFTERPOST TENSIONING FORSNUG FIT

I St FLOOR

BASEMENT _SUB-BASEMENT

Fig. 19. Construction of east and west girders, Sequence 2.

POUR 5th, 6 th B,7 th FLOORS ti

7 th FLOOR6th FLOOR IIi5^h _FL_OOR _4th FLOOR3rd FLOOR_

Ist FLOOR __

BASEMENT SUB -BASEMENT

—PERIMETER GIRDER• POST TENSIONING

STAGE 2

TL uzrTi Wu)(

ADJUST SHORES TOBE LOOSE FIT SHORESMAY BE REMOVEDAFTER 7 th FLR CONC.STRUCTURE HAS CUREDTO THE REQUIRED 28DAY STRENGTH.

Fig. 20. Construction of east and west girders, Sequence 3.

before corner columns were constructedto their intended schedule.

To accommodate this request, theVierendeel wall system was reviewedand the contractor was permitted to pro-ceed with a new criterion. This revisedconstruction sequence allowed the con-tractor to proceed with the Vierendeelwall tntss to the 13th floor but necessi-

SEGIRDERORING IstELQc

111 I SUB-BASEMENT

Fig. 21. Construction of east and westtransverse girders. Sequence 1.

4th FLOOR _ ^r

T 11 I RESTRESS TRANSVERSE

SHORING ^1 GIRDER STAGE I

Ls1 FLOOR

EASEMENTSUB-BASEMENT

Fig. 22. Construction of east and west transverse girders, Sequence 2.

PCI JOURNAL/January-February 1984 35

55 MORTAR

DEFORMED BAR

DEFORMED BAR

SLEEVE

Fig. 25. NMB splice sleeve.

7 th FLOOR6 th FLOOR5th FLOOR4!! FLOO R3 rd FLOOR

SHORING

I st FLOOR

BASEMENTSUB-BASEMENT

STRESS TRANSVERSEDER STAGE 2

Fig. 23. Construction of east and west transverse girders, Sequence 3.

9th FLOOR8th FLOOR7th FLOOR6th FLOOR5th FLOOR

4AFLOOR --3rdFLOOR

SHORING

I St FLQOR _

BASEMENTSUB-_BASEMENT

TRANSVERSE^E 3

Fig. 24. Construction of east and west transverse girders, Sequence 4.

tasted the existing perimeter shoring toremain in place until the corner columnswere installed to the 10th floor. At thisstage of construction, the contractorwould be permitted to remove the pe-rimeter shoring. This revised criterionwas actually followed by the contractor.

Connection of the precast wall panelsto the post-tensioned girders at thefourth floor and to upper floor wall pan-els from the fifth floor to the roof wasachieved by using the NMB SpliceSleeve Post Grout System (see Fig. 25).This permitted installation of wall pan-els on each exterior side before groutingthe connections. Further, the capacity ofthese connections allows developmentof the ultimate strength of the reintorc-

36

1,I J-I r71I

Fig. 27. Explosion detail of precast wall and floor assembly.

ing steel, which was a necessity for theVierendeel wall truss.

The typical precast Vierendeel trusswall panel was 10 ft wide x 13 ft high (3 x4 m). Details of the wall panel areshown in Fig. 26. Jointing was located at

mid-depth of the spandrel and midspanof alternate windows in plan. The pre-cast sandwich panel consisted of 12-in.(305 mm) thick structural concrete, 2½in. (64 mm) of polystyrene insulation,and 3 in, (76 mm) of precast concrete

38

exterior facing. Details of the concretejointing may be seen in Fig. 27. Themaximum vertical reinforcing in thecolumns was 8414 bars and the maxi-mum spandrel beam reinforcing was6411 bottom bars and 2411 top bars.

During installation of the precast pan-els as many as 28 reinforcing bars re-quired proper fitting and connectionsfor one panel assembly. The NMBsplice sleeve was able to accommodatethis assembly resulting in the efficienterection of the precast panels. Refer toFigs. 28 through 32 for installation ofprecast wall panels. Actual installationof panels took place from November1979 to June 1980, during both verycold, windy, wintry conditions and mildspring weather. Grouting of the sleeveswas permitted when ambient tempera-tures were 40 F (4 C) or higher. During

certain periods of erection, artificialheating was required to keep the ambi-ent temperature at the acceptable mini-mum level.

Fig. 28. Top view of installed precast wallpanels.

Fig. 29. Precast wall panel being lowered into splice sleeves.

PCi JOURNAUJanuary-February 1984 39

Fig. 30. Precast wall panel with horizontal splice sleeves prior to setting.

I;

Fig. 31. Precast wall panel with horizontal splice sleeves in final setting.

40

Fig. 32. Erection of prestressed double tees on precast wall panels.

Other Construction Details

The arrangement of reinforcing steel,structural steel, post-tensioning tendonsand anchorages at the corner columns isillustrated in Figs. 33, 34, and 35. Be-cause of the numerous and dense steelrequirements, accurate detailing andplacement of the steel was submitted toa very complete review. Full-scaledrawings were performed to insureproper location for all the steel ele-ments. Splices for the reinforcing steeland the structural steel were carefullyexamined for their strength require-ments and their positioning to avoid in-terference. Careful detailing minimizedthe difficulties and prevented manypotential delays in the construction ofthe corner columns and perimeterpost-tensioned girders.

Further significant assistance wasprovided by the use of superplasticizers.Slump at the time of pouring was in-

Fig. 33. Location of reinforcing steel andstructural steel in corner columns for plazato third floor levels.

PCI JOURNAL/January-February 1984 41

SPC PANELSTRESSING ANCHORPLATES FOR VSL ES-31POST- TENSIONINGTENDONS

w14, 311STL. COL.

a ♦

c. VSL TENDONS

SECOPSTAGIPOUR

Via'•:''--^:^ .- . ,- q .

FIRST STAGE POUR

,' ,^o

--FIRST STAGE POUR

OUTLINE OF 3•-O"WIDE GIRDER

C~ —T' -€ VSL TENDONS

Fig. 34. Corner column junction with post-tensioned girder at third floor level.

f ?.' `FIRST STAGE POUR

OUTLINE OF CORNERCOLUMN BELOW 3R0 FL-// /r,\/// / */ / QO

/ y

FIRST STAGE POUR

Fig. 35. Corner column junction with post-tensioned girder below start of precastVierendeel wall panels.

creased to 6 in. (152 mm) with this addi-tive. The installed concrete exhibitedhigh quality placement and compressivestrength.

The post-tensioned perimeter girderconcrete and steel arrangement areshown in Fig. 36. The post-tensionedtransverse girder cross section is shownin Fig. 37. Construction of the west andsouth perimeter girders is shown in Figs.38 and 39, respectively. Shoring of theseperimeter girders is also shown, Theseshorings are kept in place and adjustedto bear snugly to the girders after eachpost-tensioning stage. The slipformedcentral core walls are visible in thebackground of Figs. 38 and 39.

Fig. 40 shows construction of the cor-ner columns being performed after thethird and final post-tensioning of perim-eter girders and transverse girders hasbeen completed. Splicing of columnreinforcing steel was performed by me-chanical connectors. Due to the densityof the reinforcement, the use of smallerbars and lapping of column verticalreinforcing was not possible. A furtherrefinement of the corner column wasthat the finish marble chip cladding wasused as the form for the in situ column.

Placement of exterior cladding wasdone prior to the second stage of post-tensioning. However, two panels weredelayed in their placement on the north

PCI JOURNAL/January-February 4984 43

N1

C'

Fig. 36. Post-tensioned north and south girder sections.

Fig. 37. Post-tensioned transverse girdersection.

and south girders, as shown in Fig. 41.After completion of the third stagepost-tensioning of the transverse gird-ers, the two remaining cladding panelswere installed. Views of the exteriorprecast wall during the final stages ofconstruction are shown in Fig. 42.

The typical floor framing consists of18-in, deep x 7 ft 6 in. wide (457 mm x2.3 m) double tees. In situ topping was2 1/2 in. (64 mm) thick minimum. Thetypical span for this system was 41 ft(12.5 m). Interior inverted tee beamssupporting the double tees (see Fig. 13for floor framing plan) were 2 ft 3 in.(686 mm) deep. The total compositedepth at the inverted tee beams, in-cluding in situ slab thickness, was 2 ft 9in. (838 mm). The inverted tee beamswere designed to span 39 ft (11.9 m) onthe east side and 48 ft (14.6 m) on thewest side. Midspan shoring for the in-verted tee beams was required duringconstruction.

Along the south and north side of thetower building the double tee was sup-ported on the precast exterior wall panel(Vierendeel wall truss) at one end andon the prebuilt center core at the otherend. Bearing connection of these floormembers was accomplished by weldingembedded metal devices. (See Figs. 43and 44 for details of double tee support

44

Fig. 38. West perimeter girder prior to first stage of post-tensioning.

Fig. 39. South perimeter girder indicating layout of post-tensioning tendons.

PCI JOURNAL/January- February 1984 45

f A

r S _rc11!

Fig. 40. Preparation of corner columns Fig. 41. Placement of two cladding panelsafter third stage of post-tensioning of was delayed until after post-tensioning ofperimeter girders. perimeter girders.

.^i

connections.) Overall structural inte-gration of precast exterior wall panels,precast double tee joists, inverted teebeams and the interior prebuilt outercore walls was performed by reinforcedcomposite in situ concrete. Thesejoineries were located in beam sectionsbetween precast wall panels and in thetopping over precast floor members.

Computer AnalysisAn important contribution to the sue-

cessful structural design of the Vieren-deel wall was the application of com-puter analysis. A modified in-house ver-sion of the SAP program was applied inconjunction with a General AutomationSPC 16/40 minicomputer with accessorydisc drive peripheral equipment.

For the north and south Vierendeelwall truss, the analysis consisted of3949nodal points and 1591 members. TheFig. 42. Partial view of Vierendeel truss

46

DOUBLE-TEE

Fig. 43. Precast double tee connection at Vierendeel wall truss.

4-S/8"0 L4x3x3/8

BOLTS DBL-TEE

STIFF PLL6x6xI/2xO'-7"

PL I/2 x 3 x I -O

PL I /2 x 8 x 1'- 0"10"

AT CORE WALLDOUBLE-TEE CONNECTION DETAIL

f- -- [Li_

Fig. 44. Precast double tee connection at slipformed center core wall.

east and west Vierendeel wall trussanalysis consisted of 2011 nodal pointsand 1952 members. A subprogram wasadded to SAP to permit story by storyadjustment of the Vierendeel wall trussin the same manner that the construc-tion sequence was performed. Historicaldata of internal forces were accumulatedand stored until the total structure wascompleted.

One complete cycle of each Vieren-deel wall truss analysis consisted of 22runs, which considers in sequence 19structural configuration levels and threepost-tensioning stages. Total computer

running time for each cycle of the northand south wall truss was 20 hours andfor each cycle of the east and west wallwas 12 hours. The total design effort torealize an efficient structure requirednumerous cycles of computer analysis.

The selection of construction se-quence and post-tensioning stages wasoptimized through analysis of the com-puter's calculations. Maximum deadload and total load displacements werecalculated at 1.0 and 128 in. (25 and 33mm), respectively. Actual field mea-surement of maximum deflections was1.12 in. (28 mm). Computer tabulations

PCI JOURNAL/January-February 1984 47

yielded satisfactory and realistic resultsand were invaluable to the engineeringdesign.

During installation of the Vierendeelwall truss, the construction sequencewas varied according to the availabilityofmaterial, equipment and men. Cornercolumn completion lagged behind thepanel installation. Precast wall con-struction also preceded precast floorconstruction, thereby requiring modifi-cation of the post-tensioning stages andshoring removal sequence. The com-puter data were changed for each varia-tion and the analysis was rerun on theSAP program. These changes were sim-ple to perform since the basic structurewas modeled in the design phase. Thestructure was then scrutinized to insurethat its performance was within the cri-teria of the original design. In this re-gard, the computerized engineeringplayed a significant role in determiningthe flexibility permitted in the con-struction sequence.

Concluding Remarks

Based on the early studies of the officetower and an evaluation of the differentconstruction methods and materialsavailable to implement the design, theprecast concrete system selected wasthe most feasible method. After projectcompletion and observation of the con-struction and structural performance, itmay be concluded that the precast sys-tem did indeed provide an exceptionallysound solution. The combination of pre-cast concrete with composite cast-in-

place concrete provided a rigid systemcapable of resisting substantial verticaland lateral superimposed loads. Thesystem further demonstrated the instal-lation efficiency of precast concrete andthe high quality of its finish.

The computer analysis of the Vieren-deel wall truss clearly depicted themany redundancies available in thestructure. Studies were performed toprovide hinges at various locations ofhigh bending. Results showed that loadtransfer could he directed to othermember of the truss without any drasticchange in displacements or build-up ofinternal forces on the adjusted structuralconfiguration.

There was an obvious chronologicalaccumulation of internal forces in theVierendeel wall truss lower floor mem-bers due to the story-by-story erection ofwall panels. To provide a wider dis-tribution of forces, the post-tensioningof the perimeter and transverse girderswas divided into three different con-struction stages. Further distributionwas accomplished by timing the re-moval of shoring so that the partiallyconstructed structure would accommo-date and integrate with the remainingupper portion of structure yet to bebuilt. There appeared to be a sensitivedemarcation of architectural proportionsof the wall panel sizing, window open-ing, corner column dimensions and pe-rimeter girder depth with the structuralrequirements of strength and rigidity.The final design provided a buildingsystem which satisfied both the ar-chitectural concept and requirementsfor structural integrity.

48