PROCEEDINGS PAPER

Urban Renewal andPrestressed Concrete

by Sepp Firnkas*

SYNOPSIS

A major aspect of urban renewal is slum clearance and rehabilitation oflow and medium income families. To meet the demanding requirementsof the Boston Building Code, the Boston Redevelopment Authority, and theFederal Housing Authorities in terms of city planning, lowest rents, per-manence, minimum maintenance, flexibility, and fire safety, a new con-struction system was developed, using, exclusively, prestressed extruded floorslabs or posttensioned floor units and precast bearing walls connected byposttensioning.

The system is competitive with conventional low budget constructionmethods for 1 and 2 story buildings and is unsurpassed for buildings ofover 3 stories high. Only a small increase in cost allows high rise buildingsup to 2B stories, using the same system and basic components.

The system, basic components, and connection details are explained; fab-rication and erection photographs are shown of 3 completed projects; photo-graphs of models of S new projects in planning and construction stage areshown; cost figures and comparison with conventional construction methodsare given.

STATEMENT OF PROBLEM

A key element in any urban re-newal project is slum clearance andrehabilitation of residential neigh-borhoods. To achieve this goal theBoston Redevelopment Authorityconsidered the development ofsound housing at low and moderatecost the keystone to the success of avast program leading to the revitali-zation of the city.

To achieve this goal without dan-ger of rebuilding a new slum the

°.sepp firnkiis engineeringBoston, Massachusetts

60

following ground rules had to beobserved;

1. Provide for a rich living envi-ronment, urban in characterand purpose.

2. Provide for good housing, at amonthly cost range low enoughto meet the requirements of therelocated and/or poorly housedfamilies.

3. Solve the above problems andpropose a program for whichconstruction could start imme-diately (Figs. 1 and 2) .

GENERAL APPROACH

The foregoing goals were studied

PCI Journal

Fig. 1-202 Units—Boston, Massachusetts. First low budget housing development. Completelyprecast—bearing walls, extruded hollow core slabs.

February 1966 61

L^^ [317 rn] -' rnJ \____

4

Ir I t n<< i G C 1 E P A{ N t w t t t i t t . L a r y

Fig. 2-120 Units, New London, Connecticut, Low income housing development. Precast concretebearing walls, post tensioned floor and roof slabs.

62 PCl Journal

Icy Architect Carl Koch & Associatesand the author as Structural Engi-neer, and were met in a buildingprogram which is based on a systemof standard, prefabricated parts.Such a system implies a repetitionof components, the ideal being aminimum number of identical ele-ments repeatable in a maximumnumber of variations. In short:standardization. In this case I willdwell only on standardization inconnection with structural elements.Standardization will first, reduceconstruction costs by taking advan-tage of volume production of stand-ard components; second, reduceconstruction time and, therefore, alsocost; third, allow maximum qualitycontrol; but fourth, sha]l not limitthe individual expression of archi-tects, owners, and tenants.

DESIGN

Standardization is frequently mis-interpreted and misunderstood, andif not followed, the results becomeevident in excessive constructioncosts. We all realize that we couldnot own a car, dishwasher, or anyother number of indispensable itemsif the basic components of thesethings were not standardized. A cus-tom made car—even if the motorcomes off the assembly line—costsabout 5 to 10 times as much as astandard product from Detroit.Keeping this in mind we approachedthe design criteria in view of thestructural system to be adopted.Three ultimate criteria served asguides:

Economy

Sizes of structural components andspans in view of optimum efficiencyof materials and erection procedurehad to be determined and the limita-tions of each to he clearly known.

Effects of various possible soil con-ditions and sites were evaluated,materials of construction weighednot only on the basis of initial cost,but with consideration of long rangeup-keep, including fire insurance,maintenance, repair, and safety forthe tenant. Also considered was thechoice of materials in view of laborpractices, time schedules, and shopfabrication versus site work. Specialemphasis has been placed on mate-rials and methods which will providemultiple use, such as structure andfinish ( Fig. 3) .

It was realized that methods ofconstruction may provide the princi-pal key to economy. Since the con-

Fig. 3—End bearing wall with exposed aggre.gate finish

February 1966 63

Fig. 4—Typical New England Setting

struction industry hardly utilizesmass production techniques, it be-came obvious that the economicalsuccess of our design was depend-ent on the development of a systemin which each part could be massproduced.

Livability and Aesthetics

These latter criteria were of great-er concern to the architects andplanners than to the engineer, butoverlapped frequently, especially interms of privacy (sound transmis-sion) and safety (fire ratings), etc.Aesthetics and diversity became im-portant considerations in connectionwith a system based on mass pro-duction. The system must allow theindividual architect the liberty to ex-press himself and incorporate thebuilding into the environment. Itmust be possible to retain local tra-dition and flavor or even to reinforceit, if the system is valid. The systemmust be able to represent the back-bone of the whole development(Figs. 4 and 5) .

BASIC STRUCTURAL SYSTEM

Based on the above outlines, ex-tensive research, soul searching, and

64

plain hard design work was done.Steel and wood framing, concreteblock masonry, precast concrete andany possible combinations thereofwere considered, and typical designdata and estimates established.Keeping the minimum room size re-quirements in mind as set forth bythe Federal Housing Agency, a spanmodule of 32 ft. was developed asthe most practical and allowing themaximum number of variations with-in a typical unit. Concrete as con-struction material emerged as themost promising and adaptable, and

Fig. 5—Courtyard—Neighborhood Center

PCI Journal

if used as a precast system offeredeconomical and aesthetic advan-tages. In addition, future applica-tions and developments could easilybe accommodated within the system.Furthermore, it was felt there waslittle room for significant advance incost reduction techniques in wood,steel, or masonry structures, but thepotential for precast concrete hasonly begun to emerge. Coincidingwith the architectural optimum wasa maximum structural efficiency andeconomy to be obtained with the useof a pretensioned, extruded, hollow-core slab of 8 in. thickness.

The party and separation walls be-tween apartments and stairtowerswere developed as bearing membersof 8 in. and 6 in. thickness and offloor height - Acting as deep girdersthese bearing walls allow the adap-tion of the most economical founda-tions designed for any possible soilcondition (Fig. 6). If continuousfootings are possible, the wall formsgrade beams at the same time andrests on a small continuous pad,transmitting loads uniformly on thesoil. If spread footings, caissons, orpiles are required, one block can reston four points of support withoutrequiring a change in size or detailsof the bearing wall itself. On slopingsites a typical bearing wall can beused as a retaining and basementwall (Fig. 7) .

SCIE ME^ of'q T- 4 5*la^^iTY D"^^

S1V WM

5^'I

nR $aA- SyvOY-eFri^^I

'r2a'ic9I - sway fc.wwurr. s ^ fowl o^^rs

Fig. 6—Basic structural components and foun.dations

Lateral stability is provided byprestressed shear walls of 4 in. thick-ness extending over full height of thebuilding up to six floors or by a lay-out in which the bearing walls alsobecome shear walls.

CONSTRUCTION DETAILS

The basic structural system is pic-tured in Figs. 8, 9 and 10. Founda-tions and grade beams are cast inplace, as well as the slab on grade,if soil conditions permit it. The struc-ture is then assembled from 8 in.thick wall panels extending the full

Fig. 7—Bearing walls at sloping riles become also retaining walls

February 1966 65

Fig. 8 '--8asic structural system

Fig. 9—Corner units

depth of the building, and the heightof one floor; a prestressed shear wall,4 in. thick, placed at right angle tothe walls, and prestressed; and 8 in.deep floor and roof planks spanning32 ft. from wall to wall,

The building as shown of 32 ft.length and 32 ft. depth would con-tain Four three-bedroom flats or twofour-bedroom duplexes or combina-tions thereof. Basic changes in floorplan for two or one bedroom unitsare achieved by reducing the depthof the building from 32 ft. to 28 ft.

and 20 ft., respectively, and reduc-ing the hearing wall length to 32 ft.and 30 ft. The distance betweenhearing walls and also the floorspans remains the same to ensurethe most efficient use of wall andfloor units. Public access and doubleegress to and from each apartmentis through the stair tower. Privatestairs within duplexes and utilitiespass through slots between the floorpanels.

ERECTION

First, a wall panel, then a shear

66 PCT ,Journal

Fig. il--Erection nucleus

^^ f

.. ate-

Fig. 12—Typical wall panels

Fig. lo—Bearing wall and floor plank details

wall and theadjacent wall panel iserected, forming a rigid, stable nu-dens from which the further erec-tion continues without requiring fur-ther bracing of individual members,except the party walls (Fig_ 11).

The basic wall panel is 36 ft. longand weighs between twelve andfourteen tons. The crane used is ofsuch size that panels can be placed70 ft. from the center of the cab. Atoffsets of the buildings, larger panelsare used with a maximum length ofup to 48 ft. (Fig. 12).

The casting of the panels is donein the conventional flat position witha tilt-up form for stripping. Fromthis point on, storage, handling, andshipping are always done in an up-right position.

The floor slabs comprise one fullunit and a half unit cut to length.Only two different lengths are re-quired: 32'-6" (32 ft. clear span and3 in. support on each side) and $'-0" (for stair landings). The half unitis used to work out the differentfloor and roof plans.

The shear walls are 6 ft. wide andall are uniform, except for length.Up to 6 floors in length for low rise,

February 1966 67

they are replaced for high rise builci-ings by the precast elevator towerunits. Shear wall connections arewithin the floor thickness and atfloor levels only (Fig. 13).

Fig. 13—Shear wall connection detail

CONNECTIONS

Two-inch diameter conduitsplaced in the wall panels accommo-date Stressteel posttensioning rodsfor the connection of wall panels tofloor units. Length of the rods isequal to the floor height. The rodsare connected at each floor level bycouplers and serve as rough align-ment and as placement guide for thewall panels (Fig. 14), AFter plumb-ing and final positioning with theaid of the shear wall or braces, therods are posttensioned and thus sta-bilize each panel and allow the floorplanks to be set on Teflon bearingpads (Fig. 15). Then the couplerfor the posttensioning rod and thenext rod of floor height is installedfor the placement of the next panel.Floor to wall connection is throughprestressing forces plus additionalcontinuous or hooked reinforcingrods placed in the shear keys of thefloor planks and into the grout spacebetween the wall panels. Erectionthus becomes independent of anygrouting and provides a stable struc-

tore. Final connection and protec-tion of posttensioning and reinforc-ing steel, as transverse connection offloor panels through shear keys isdone by pumping expansive groutinto all openings. The grout mixturefor this purpose was specially devel-oped and tested and guarantees noshrinkage and a minimum strengthof 30(0 psi. Strength that can betransmitted through a shear key toan adjacent panel is great enoughto permit the omission of headersat planks that have to be cut forfloor openings.

While originally a cast-in-placetopping was provided to take up dif-ferent camber and unequal thicknessof floor slabs, we were able to omitthe topping in the Iast project, dueto improved production quality con-trol of the planks, plus the reliable

IIWfr. M1G

I.: . 1 W LL

Fig, 14—Wal I•floor connection detail

`vR4 JT^ f"

t

Fig. 15--Connection details

68 PCI Journal

strength of the grout in the shearkey, permitting an adjustment of thedifferential camber.

TIME-METHOD STUDY

Beginning with the first project, atime-method study as a joint ventureof architect, engineer, and producerwas started, having as final goal topinpoint basic errors in design con-cept, fabrication methods, and erec-tion procedures. One timekeeperand one photographer equippedwith a time-lapse movie camerawere continuously present at the joband recorded, down to the second,each movement made by each mem-ber of the crew, plus crane andtrucks delivering the units. The re-sults were truly amazing and hardto believe.

For example, 20% of total timewas used up for wasted operationssuch as unnecessary switching ofslings, unloading of slabs on theground at the wrong place, huntingfor the right tools, just plain stand-ing around, and two percent of totaltime was consumed by coffee breaks.Seventeen percent of total time wentinto crane moving, moving of equip-ment, required changing of equip-ment, and tools.

Efficiency was thus reduced to61%. After the first preliminarystudy, we were able to increase effi-ciency to 80%, even without any ba-sic changes in the whole system.

The time study plus the time lapsemovie—one picture every three sec-onds—allowed us, then, to approachthe third project, a high rise apart-ment house, with the experiencegained.

We were able to simplify the de-sign and increase the efficiency ofthe system to about 90%G. (Coffeebreaks and some human idiosyncra-sies will probably always remain.

In addition, the overall time require-ments for erection were reduced toone-third. Where it previously took51 minutes 30 seconds average timefor a typical wall panel to be placedand connected in Project No. 1, ittook only 16 minutes average timefor the latter project for the sameoperation. An average of 4000 sq.ft. of building—bearing walls andfloor panels—were set per workingday.

DEVELOPMENT OF SYSTEMTHROUGH THREE PROJECTS

Project No. 1 in Boston, Massa-chusetts (Figs. 1, 4 and 5), com-prised 202 units in the form of du-plexes and row houses on a steeplysloping site. Site precasting versusplant fabrication was tested andwith it four different types of hollowcore slabs and a solid, posttcnsioned,room-size slab. In this case, a diffi-Cult site and the nearness of a first-rate precasting plant made plantfabrication the most economical andqualitatively the most reliable sourceof supply. In addition, the plant in-stalled an extrusion machine for floorplanks and offered the necessaryspeed for production. Connections offloor to wall panels were by dowelsand in very early stages by weldplates. These weld connections weremost unsatisfactory from the pointof view of appearance and did notproduce the necessary elasticity fortemperature and shrinkage move-ments. Cracks resulted at an earlystage. The finally adopted dowelconnection proved structurally ade-(plate, but had disadvantages inerection; i.e., the structure requiredbracing until all grouting was com-pleted and set; tolerances, even ifamply given, still had to be main-tained, and a certain amount of fieldwork and plant work had to he co-ordinated. To straighten out dowels

February 1966 69

Fig. 16—Site precast walls and shear walls

F',

_ !r „

"

rim `^^, -^ 4 ..,

; 4

-rte

Fig. 17—Posttensioned Floor slabs

bent in transport and handling wasone of the most time-consumingparts in the erection of the panels asthe time-method study showed.

The floor panels, even if machineproduced, had camber differencesup to 314 in, and thickness variationsup to 'kz in. Plenty of patching andtopping were required.

Project No. ? in New London,Connecticut, under construction atthe same time as Project No. 1, com-prised 120 units, all flats in this case.Wall panels and shear walls weresite precast, while the floors werecast-in-place, posttensioned slabs

(Figs. 16 and 17). Thickness of wailsand floors as all other dimensionswere exactly retained. The great dis-tance of precasting plants, lower la-bor rates, and a vast stock of floorforms that the general contractorhad left over from a previous jobmade site precasting of wall panelsand posttensioned, cast-in-place floorslabs competitive with plant fabrica-tion. Erection was very simple, andwe had no problem with displaceddowels. The posttensioned floor slabproved very satisfactory and againwas nnore economical than any otherwood or steel floor system.

70 PCI Journal

Project No. 3 (Fig. 18 ), a highriseapartment house with 380 units, fol-lowed immediately after the comple-tion of the other projects. All previ-ously gained experiences were takeninto account from the planning stageon. To reduce required tolerancesto a minimum, the dowel connectionwas completely abandoned in favorof posttensioned connections. Practi-cally no bracing is required, improv-ing quality to such an extent thateven though 1 in. topping was pro-vided for, the Owner omitted it afterthe floor contractor assured him thathe would not require topping forthe installation of tile floors. Erectiontime was cut by a factor of three,and efficiency increased to 90% aspreviously pointed out (Figs. 19, 20and 21).

ECONOMIC CONSIDERATIONS

The ultiniatc• measure of Successof this prestressed—precast buildingsystem—which we will call "Tech

-Crete" system—in connection withthe low budget requirements of ur-ban renewal projects—was, natural-

ly, the overall economy. Before theAuthority was convinced, we had tosupply substantiating data based onactual designs for comparable sys-tems. Alternate proposed systemswere: wood or steel frame withstressed-skin plywood panels forfloor system, and concrete block withbar joists, and concrete slab, andvariations thereof.

The result of a thorough investi-gation into all aspects is best sum-marized in the verdict of the jurythat judged the system: "Reinforcedconcrete is preferred because of itsfire resistance in high-density areas;because of its strength, low mainte-nance, and availability; because ofits suitability to industrial produc-tion and aesthetic potential in vari-ety of surface treatment. The longspans of the prestressed concreteminimize foundation work. Theseprestressed floor slabs rest on hear-ing walls 32 ft. on centers. Curtainwalls are non-bearing, leaving choiceof material to individual preferenceor cde requirements. Mechanicalcore and vertical circulation are

h '^ Sty^

Fig. 18— Project Number 3—High Rise Apartment House

February 1966 71

Fig. 19—Parrly comp'eted srructure

combined and centralized. Economyof space and construction ... Indus-trialized unit suggests an infinite va-riety of applications."

In quoting cost figures for thethree projects in which for the firstand second difficult foundation con-ditions (piles and caissons) wereencountered, we find the economyof the system substantiated, ForProjects 1 and 2, which had a maxi-mum of S stories height, the averagesquare foot cost based on apartmentarea was between $9.65 and $10.75.For Project No. 3, high rise andmore expensive treatment of non-bearing curtain walls, the averagecost was 512.80 per sq. ft.

Of greater significance are the fig-ures for the structural system alone.Since all architectural, plus site re-quirements, can be eliminated, theygive a true picture of the economyof the system. For low rise buildingsthe square foot cost is 92.64 to $2.76,while for high rise a slight increaseup to $3,00 can occur. These costsare based on material and labor ratesas currently in use in the metropoli-tan Boston area.

FUTURE DEVELOPMENTS

From the designer's point of view

fig. 20— Erection of floor units

72 PCI Journal

Fig. 21 —End view of completed structure

Fig. 22—High rise project in working drawing stage

the "Tech-Crete" system as present-ed here must be considered as partof a continuous process. Each jobdone adds to knowledge and know-how and must be exploited corre-spondingly for the following project.The desirable goal is a building sys-tem using industrialized standardcomponents compatible with 20th

February 1966

century production methods andtechnological advances to allow bet-ter housing at a lower cost.

At this time, having completed thestructure of Project No. 3, the sys-tem can be considered as de-hugged,allowing us to concentrate on refine-ments to further increase the effi-ciency of the total system.

73

IJ

Fig. 23—High rise waterfront project in planning stage

If the anionnt of additional workgenerated by these pilot projectsshall serve as a yardstick for the suc-cess of the "Tech-Crete" system andfor prestressed concrete in the lowbudget field of housing, we can sayit was a tremendous success. Withdifferent architects we are in theworking drawing stage of B newprojects (Fig. 22), totaling a build-ing cost of about $12 million, whileseveral others are in the planningstage (Fig. 23).

74

These new projects reconfirm thevalidity of the concept as set forthin the general approach to the sys-tem, mainly to take advantage ofstandardization to reduce costs, butnot to limit the individual's expres-sion. The extent of the systemreaches from garden-type rowhouses to urban waterfront sky-scrapers.

ACKNOWLEDGEMENTS

For the development of the Tech-

13CI Journal

Crete System the Architect, CarlKoch and his associates Fritz Day,Gardner Ertman, Leon Lipsbutz,and Margaret Ross, and job captainJohn Cummings were vitally in-

volved. Credit must also be givento the Developer, Mr. James P. Line-han, Jr., and to Project Engineer Ul-rich J. Boehlke of seep firnkas engi-neering.

Presented at the Eleventh Annual Convention of the PrestressedConcrete Institute, Miami Beach, Florida, December 1965

Discussion of this paper is invited. Please forward your discussion to PCI Headquartersbefore May 1 to permit publication in the August 1966 issue of the PCI JOURNAL.

i ebruaiy 1966 ;'