47.- Design and Construction of a Prestressed 8rickwork Water Tank

ABSTRACT A 1 a brick factory which is usingbllfane gas for firing ils prodl/cls, lhe need for cooling the gas slorage lowers in case of fire il1lplies a demand for waler for in excess of lhal possible Iram lhe mains. T/lUS, a waler slorage lank of 120000-gal capacily IVas provided.

The lank is cirel/lar, 40-11 dia. in­lernally alld 16-fl deep. II is con­structed o[ 9-;11. brickwork 011 a Ilormally reinforced-concrete slob 46-119-il1. dia. The 9-ill. brickuwk is prestressed l'erUcally OIU/ circumferen­lially alld buill on a slidillg joim base. An externaI decorafive skin of 4/-il1. brickwork provided externaI S/llll ­

tering for a grouted cavifY, making an ol'eroll IVall Ihickness of 15f ill.

Vertical and circumferential sfres­sing s/eel COl1siSIS o[ 0-276-;n. wires in grouted duels l'ertical/y mui in ,Ile grouled CGVily circumferential/y.

By D. FOSTER Structural C/ay Producls Lld., Poflers Bar, Hens.

Etude el Constl'uction d 'un Réservoir d'Eau en Maçonnerie en Briques Pré­contrainte

Dans une usine de briques qui utitise te gaz bll1alle pour cuire ses Jabri­catiOIlS, la quanlilé d'eall nécessaire au refroidissemelll des lours de slock­age du gaz en cas d' illcelldie excede de beaucollp celle que peuvell( fournir les canalisations. Ainsi Ul1 résenoir d'eau d'ulle capacilé de 545 1/13 a élé COII­struit . Ce réservoir esf cil'culaire; SOIl diamerre inte1'lle es[ de J 2 m el sa profondel/r de 4·8/1/. 1/ eSI COlIslruit eu maçollllerie en briques de 22·5 cm sur une dalle de bétoll IJormalemeJ1l armée de 14·25 m de diamelre. La maçol1l1erie en briques de 22·5 cm est précol1trainle verticalemel1l el circu­lairemenl el cOflslruite sur un joinl glissam. Une enveloppe décorative extérieure en maç01111erie ell briques de J }'5 cm fou1'Ilissait U11 coJJrage exle1'lle pour une cavilé I'emplie de coulis, faisant ainsi Ull mur d 'une épaisseur totale de 39·5 (.'111. L'arm­alllre verticale el circulaire consiste ell fils de fer de 7111m de diamime insérés verlicalemelll dans Jes cOl1duilS de cOl/lis el circulairemel1f dans la cavilé à coulis.

Entwurfund Konstruktion eines Wass­e,'belriilters aus vorgespanntem Ziegel­maue,'werk

In einem Ziegelmauenrerk, das seine O/ell mil Buran-Gas beheizl, lV;re' für del1 Fali eines Schadenfeuers zwecks Kiihlullg der Gasvorralsbehãlrer eine Wassermel1ge benotigl, die lI'eil grosser ais die VOIl der Hauplll tasserleilllllg beigebrachte ist. Darum lVurde eill Wassel'vorl'atslallk mil ruml 545 m3

[nhall erforderlich. Der kl'eisformige Tank hal einell inlleren Durehmesser VOll flllld J 2111 und isl ca. 4,8 m fie! Er beslehl aus einer 22,5 cm dicken Ziegelmauer auf einer Plalle aus be~ welrrtem Be/oll mil gUI /4 m Durch­messer. Das 22,5 cmdickeZiegelmauer­lVerk iSI vertikal sOlVie im Umfang vorgespallnt ul/d au! Basis einer elast­ischen Fuge gebaut. Eine dekoratve­A ussenhaul aus J 1,5 em dickem Ziieg elmauerwerk slell! die ãussere Ver­schalung eilles ausgegossenen Hoh/­raumes dar, so dass die Gesamldicke der Walld 39,5 cm betriigl. Del' senkl'echl und peripher lVirkende Spannstahl besteht aus Drãhlell mil I'und 7 mm Durchmessel', er bejindel sich in den vergossellen senkrecltlell KalliileJl bzlV. ill dem vergossenen wnlaufenden Hohlraum.

1. INTRODUCTION

In 1967, G. H. Downing & Co. Ltd* opened a new factory aI Chesterton, Staffs., firing high-quality facing and engineering bricks by butane gas. This required the installation of lhree 60-ft high by 8-fl dia. pressure storage vessels (Figure J) and suhsequently cooling water for these in lhe event of fire . .Because mains supplies were inadequate a slatic water tank of 120000-gal oapacity had to be erected. Dr J. M. Plowman, who had previously experimented with the method, suggested that the walls be buill in preslressed rather than reinforced brickwork. The many examples of thin-walled prestressed concrete tanks indicated that this would probab1y be as economical- and certainly far more interesting­than lhe aiternative, already considered, of using a prefabricated steel section tank. (Cerlainly this would have been simpler for the company and it is therefore to their credil that they encouraged the prestressed

brickwork pro posai, especially when il was realized lhat it was a relatively untried medium.)

• A member firm of Structural ClayProducts Ltd. FIGURE l- View of partly finisbed tank and butane towers,

287

288 Design and Constructlon of a Prestressed Brickwork Water Tank 2. GENERAL DESIGN CONSIDERA TIONS

2.1 Sito

Thc location of the struclUre is shown in Figure 2. It had to be as near as possible to the gas storage tanks to keep pipe work to a minimum but it was nOI allowed to be built on the concrete apron used for stacking the product. The soil is made-up ground sloping rapidly away into the old c1ay pit and consisting, in the main, of reject bricks from the old factory which the modern plaot replaced. Tbe allowable load on the soil is about 0·5 to 0·75 tonf/rt2.

slopoc- <I ... "

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2,2 Dimensions of Tank

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By plotting height against dia meter and by estimating costs ofthe reinforced-concrete base and the brickwork it was determined that with an internai diameter of 40 ft and a height to water levei of 15·25 ft , the required capacily would be achieved as economically as possible. Allowing for wall tbickness and projection the overall diameter of the base is 46 ft , and lhe resulting area implies a unil load on the soil within that allowed. The general arrangement adopted is shown in Figure 3.

2.3 Choiee of Wan Tbickness

The requirements governing the ehoiee of wall thickness were:

(a) The strength of the brickwork, both vertically and circumferentially,

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(b) The ease of incorporating the necessary vertical reinforcement.

(c) The permeability of the brickwork .

(d) The possibility of eorrosion of the tendons.

These requirements are clearly interrelated. For maximum economy a single 4t-in. nominal-4k-in. actual- wythe would have been ideal. However, its disadvantages are only too e1ear:

(a) Perforated bricks would be necessary so that vertical steel could be incorporated in the wall thickness. This implied a pattern of perforation such that the super­imposition af successive courses would form ducts at intervals for the vertical tendons, and hence a degree of accuracy on the site which might prove difficult to attain.

(b) Bricks would have to be threaded individually over tendons- clearly a clumsy arrangemem.

(c) If, to avoid (b), lhe bricks were made with slots instead of holes, then the circumferential strength would be seriously weakened. One type of perforated brick (Figure 4) gave a mean strength of 16901bf/in2 and a range of 1110 lo 23401bf/in2 when tested on end, compared with a mean of around 8000 Ibf/in' when tested on bed.

(d) The protective cover to the vertical tendons would be relatively small. 11 was known that failures of hoop tendons where protected by sprayed morta r coatings had occurred on prestressed concrete tanks ; due, probably, to

D. Foster 289

/

't:J ~ ~-,

" O~'~ ,,,....t "oba" ", ... 1 to ..... 1(& th,,~ hndi.t .. , tI. . Iet •.

FIGURE 4- Brick tcstcd 00 end.

a combination af penetration cf rain and ao aggressive atmosphere. This was disregarded becallse it had been agreed to prateet lhe structural brickwork with a decora­tive externai skin and a grouted cavity, but corrosion due to penetration af water from lhe tank was a real danger. The brick itself might well be practically imper­vious but the mortar joints and lhe interface would nol be, it was lhought, over such a short distance.

It was recognized that this difficlIlty could be overcome by a waterproof lining such as site-applied hot-asphalt tanking and that, given time, a lype of slotted brick could be devised to meet the circumferential slrenglh needs- a suggestion is shown in Figure 5. However, time

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",o,h, .. 1 1-"'''' ''' '''''1 G<1bk, '" .", .. ~"' .

FIGU RE 5-Suggesled slo tted brick .

was lacking and it was requested lhat existing stocks be lIsed .

A 9-in. actual bonded thickncss was adopted (Figure 6). In this arrangemenl solid bricks in Flem ish bond give the passibility af vertical spaces every 6i in. By increa5ing the thickness of lhe collar joinl (the cenlre vertical joinl paraI/e! with lhe wall face) i -in.-dia. sheaths could readily be incarporated.

In this 9-in. wylhe lhe cables are alleasl 4 in. from lhe water. lt was tho ughl lhal the hydraulic head was too great to leave even this thickness unprotected, 50 it was de­cided to render the tank internallywilh one oflhe cement­styrene- bUlaueine compounds* now available, which

(a)

(b)

Tl IO ICOL ,,"co

, NYbe.OCOt:?'

C.OR.I( i'I LL ~!I.

FIGURE 6- (a) Parl plan ; (b) Sl iding joint.

are claimed to have high adhesive and waterproofing properties. This arrangement, together with lhe grouting of the cables in lheir shealhs, was deemed to be a reasonable safeguard . Many reinforced-brickwo rk water­reta ining structures have been built in the USA without , apparently, any reported failures a nd lhe writer has examined a number of these without noticing signs of deterioralion even where lhe brickwork is unprotected .

2.4 Junction of Walls and Base

1t was decided at the beginning to build the walls on a sliding joint rather than to fix them to lhe reinforced­concrete base, primarily on lhe grounds of simplicity af

*Revinex 29Y40. by Reverlex Lld.

290 Design and Construction of a Prestressed Brickwork Water Tank design and construclion (Figures 6 and 7). By this melhod fixing moments were avoided which otherwise would have been difficult lo develop due to lhe small lever arm possible wilh steel in the cenlre. Even with a sliding joinl, CP 2007 slill requires' lhat a moment in the wall 'be assumed on lhe basis of a reslrainl equal to one-half af lhat provided by a pinned foo!', presumably because

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(b) FIGURE 7- Part wall section; (a) bottom; (b) topo

frictional restraint is unavoidable.

3. PROPERTIES OF MATERlALS

3.1 Bricks

The bricks used were 'Slaffordshire Blues' a Class-A engineering brick to BS 3921. 2 The mean strength of a sample crushed on end was 11 990 Ibf/in2 with a range of 8600 to 15420 Ibf/in2. The strenglh 00 bed is similar. The initial rale of absorption (suclion rate) was nol measured but is certainly very low 00 the basis af previous determinalions with a brick of lhis kind.

3.2 Morlar

The mortar used for the stressed waJl was I :t:3 ordinary grade Portland cement :hydrated lime :sand , lhe sand complying with BS 1200. '

3.3 Brickwork

Three laboratory 9-in. brickwork cubes were made from the bricks and morlar to be used. Their crushing strengths at 28 days were 4900, 5780, and 6190Ibf/ in2• Three mortar cubes had crushing strengths at 28 days of 2700, 2180 and 1950Ibf/ in2. Sile brickwork cubes were not made. While lhe strength of the brickwork itself was nol measured directly, extensive tests on walls and brick­work cubes' have indicaled thal for this mean slrenglh of cubes, a waJl strenglh under axial loading of belween aboul one-half to two-thirds of the cube slrenglh may be expecled.

Allhough no experimenlal delermination of lhe E value was made, brickwork of lhis strenglh and quaJily has been taken as 3·0 x 10'lbf/ in2 • LENczNER' has shown lhat with similar strength brick and morlar il is 3·5 x lO' Ibf/ in2 but the maximum aJlowed by CP 111' is 2·33 x lO' Ibf/ in2. However, lhis was deemed lo be too conservative. The SCPI code' aJlows a maximum of 3·0 x 10'lbf/ in 2•

3.4 Concrete

The concrete for lhe base was I :2:4 nominal wilh an assumed 28-day works cube slrength of 3000Ibf/in2 •

That for the ring beam was I :1·6:3·2 nominal with a corresponding preliminary cube slrength of 5400 Ibf/ in2

and a works strenglh of 3600 Ibf/ in2. The average strenglh of two cubes laken from the si te while casting the top beam was 4420 Ibf/ in2 at 28 days .

4. CALCULA TIONS'

4.1 Base Slab

CaJculalions for the base slab foJlow normal procedure lIsing ordinary reinforced concrete in accordancc with the relevant British Standard Codes of Practice. They would be out of place in lhe presenl texl but at least lhe foJlowing point is worth noting.

CP 2007' requires lhal any reinforced-concrele seclion in a liquid-retaining structure should be 50 proportioned thal in any face in contact wilh lhe liquid the tensile stress in lhe COllcrele (designed as though the steel were nol lhere) does nol exceed a permissible figure. For I :2: 4 nominal concrele mix lhis is 1751bf/ in2 for direcl lension and 245 Ibf/in2 for lension due to bending. The purpose af such restr iction is, af course, to provide resistance to cracking.

-To slide-rule accuracy.

D. Foster 291 4.2 Walls

4.2.1 Stresses in Walls: Circumjerential

Weight of water=62'5Ib/ft l

Read of water, say, 15·75 ft InternaI diameter of tanks, 40·0 ft Roop tension at base ofwall = (15'75 x 40·0 x

62'5)/2= 19700 Ibf

".Tensile stress per foot of height at the base in the 9-in.­thick (nominal) wall= 19 700/(12 x 8,625)= 191 Ibf/in2

Roop steel must be provided to resist this. AIso, with the tank full. CP 2007 requires (Clause 318 h iii)' a residual compression at ali points of at least 100 Ibf/in'. ... Total hoop compression required at base = 29 I Ibf/in2.

4.2.2 Stresses in Walls: Vertical (CP 2007, Clause 318)'

Allowance for bending induced by differential hoop stressing, l.e. partially wound =0 ·3 x ring compresslOn stress

=0 '3 x 291 =87'3 Ibf/ in2

Allowance for bendiog due to frictional restraint: with a sliding joint this is assumed to be equal to half that if the foot were pinned.

WL M(max)= -

7-8

M = WL = 62·5 x (16'75)' x 12 2 15·6 15'6 x2

= 67441b. in.

bd2 (8·625)2 Section modulus of wall = 6" = 12 x 6

= 149 in l

.·.Stress due to bending ± ~:~4 = ± 45·3 Ibf/in2

Compression due to self weight of wall =

0·58 x 16·75 (height to top of coping) x 100 12 x8'625

=9'35Ibf/in2

.'. Vertical stress required = 87'3+ 45 '3 - 9·35 = 123'3Ibf/ in2

4.2.3 Losses in Steel Prestress

Before determining the layout of the steel it is necessary to ascertain losses. In doing this certain assumptions have had to be made about some brickwork characteristics for which there is very little experimental evidence. A ttention is drawn to these assumptions as they oecur.

4.2.3 .1 Vertical sleellosses

(i) Relaxation of steel itself = 15 0001bf/in2

r (ii) Elastic contraction at transfer:

Modular mtio x Stress in brickwork 2

9·33 x 123·3 2

5751bf/in2

Note: The elastic contraction is half where post tensioning is used and/or where wires ar bars are not ali stressed simultaneously.

(iii) Creep of brickwork: The creep of brickwork under such low stresses will be almost negligible. For post-tensioned concrete the loss per unit léngth

0·25x 10- 6x 6000 Min. cone. cube strength at transfer

with a minimum of 0·25 x 10-' per Ibf/in2• LENczNER' gives a figure for similar strength br ickwork of 17 x 10- ' at a stress leveI of 247 Ibf/in' measured over 80 days. Although this is thought to be higher than the present scheme warrants it will be used.

17xI23·3x28·0xlO' .' .Loss due to creep 106 x 247

=238Ibf/in2

(iv) Shrinkage of brickwork: Clay hricks do not shrink, but joints do. Vertically, the proportion of height represented by the joiots is one-seventh (i in./ 2i in.). If the joint is considered to have the same shrink­age as concrete then the loss will be given by:

Coeff. x E of steel 7

CP 1158 gives, for concrete which is post tensioned, a coefficient of 200 x 10-6

L 200 X 10- 6 x 28·0 X 106 :. 085 7 8001bf/in2

(v) Losses due 10 slipping while aochoring: Say 6 % of final applied load

=0·06 x 0·65 x 100 x 2240 87001bf/in2

Note: In prestressed cylindrical liquid-retaining struc­tures the maximum allowable stress in tendons is 65 % of ultimate.' The percentage of the applied load is determined from the manufacturer's recommendations for the type of anchorage. The range for the equipment used here is not linear but varies from 10% at very low forces to 5 % at normal working loads. At 6 % it is a surprisingly high figure compared with the other losses. .'.Total relaxatiol1 loss per in' = 253131bf/in2

.'.Loss per wire (0·276-il1. dia.) =0·0598 x 25313= 1520 Ibf.

This loss per wire can be added to the prescribed working maximum per wire 50 that the residual force is equal to this maximum. From the residual must be deducted the frictional loss to obtain the design stress. Max. =0,65 x 13 400=8700 Ibf per wire (13 400 Ibf is the ultimate for one wire of 100 tonf/in2 steel). 8700+ 1520= 10220 Ibf = force aI jack.

(vi) Loss due to friction in the duct: This is given in CP 115' (and CP 2007') by the formula

Px=Poe- Kx

where Px=prestressing force at x ft from the jack Po = prestressing force at the jack

e=2'718 K =conslant depending on type of duct (5 x 10- 4

here).

For this case it is sufficiently accurate to assume that

292 Design and Construction of a Prestressed 8rickwork Water Tank

e- Kx = 1 - Kx and since K is given, then

P, = Fo[l -(5 x 16·75 x 10- 4)]

= Fo (I - 0·008) 1055 = 0·008 Fo

Now P is the force required at the jack. which is 10270 Ibf. c.Friction loss = 0 ·008 x 10 220 =821bf ".Residual force per \V ire for purposes of determining the number ofwires = 8700 - 82

= 8618 1bf

4.2.3.2 Hoop sreellosses

(i) Relaxation = 15 000 Ibf/ in '

(i i) Elast ic contraction

Modular rat io x Stress in brickwork

9·33x291 2

L

I 350lbf/in'

(iii) Loss due to creep

17 x 291 x 28 x lO. 562 lbf/in'

10· x 247

(iv) Shrinkage.

(v)

Circllmferentially, the joints form H 8t of the work. Thu s the coef-ficient must be l1lultiplied by (3/69)

Loss = 3 x 200 x 10- 6 x 28 x 106

244lbf/ in ' 69

Loss dlle to slippage while anchor-ing- as before 8 700lbf/ in '

258561bf/ in '

Total relaxation losses per wire. i.e. before friction

= 0·0598 x 25 856 = 1542 Ibf

Required force per wire at jack

= 8700 (max. working allowed)+ 1542 = 10 2421bf.

From this mllst be deducted the loss due to friction. (vi) Loss due to friclion due to circular tendons- per

tendon. From CP 115.8 fr ictiol1 loss is given by the formula

Px= Poe- II.l:IR

where Px- the resultan1 stressing force ai point x Po= prestressing force at the langent point near

the jacking end e=2·718

R = radius of curvature = 20 ft For steel riding 0 11 steel as in this job fi- is given as 0·25 Distance x=! circumference =;r D/4

I'X = 0·25 x " x 40 = 0.39 1 R 20 x 4

Px= Pox 2.71 8- 0'3 91

~ = ~_= 10 242 = 6990 Ibl" 2·718°·391 1-476 1-476

c. Friction loss per lVire = 10 242 - 6990= 3252 Ibf . ... Residual wo rking force = 8700 - 3252 = 5448 Ibf.

4.2.4 Layou/ of Srressillg Sreel 4.2.4.1 Verrical sreel layour

The possi ble spacing of the vertical steel is governed uy the bonding arrangement (Figu re 6). From this it wi ll be seen lhat wires in pairs could be spaced aI a minimuITI of 6i-ill. celltres-giving the maximum number of wires. Try 13~-in. centres.

Force required = 13 ·5x8·625x 123 ·65 1bf

= 14400 Ibf

Force supplied =2 (wires) x 86 18 Ibl"

= 17 2361bf

which is clearly adequate (Figu res 6 and 7).

4.2.4.2 Hoop s/eellayou/

The hoop steel required can best be determined by tabulation (Table I).

TABLE 1- DETERM IN AT10N OF Hoop STEEL REQOlREO - ANO I NSTALLEO

I I Deplh D Hoop temile TOlal force· No.of No . of of U'''w force (/bf) ~ I req/lired i/lc//ldillg 0·276-il/. wires

(fI) D x 40 x 62·5 reJidual of 11 ires per fI iUSlalle(j 2 10350 (Ibl) (5448/blp"

! wire)

t5·75 19700 i 30050 5·6 8 t4·75 18500 28850 5·25 8 13·75 17200 27550 5·1 7 12 ·75 16000 26350 4·82 7 I t ·75 14700 25050 5·35 7 10·75 13500 23850 5·05 6 9·75 12200 22550 4·8 6 8·75 10950 21 300 4·55 6 7·75 9700 20050 4·28 5 6·75 8460 18800 4·0 5 5·75 7200 17550 3·72 4 4·75 5950 16300 3·47 4 3·75 4700 15050 3·20 4

· The resid ual hoop slress required is 100 Ibf/ in2 when lhe lank is fu i\.

·.Force per fool of heighl = 12 x8·625 x tOO ~ 10 3501br

The layaut of these wires- and the vertical steel- is shown on Figures 6 and 7. The increased number Qver lhat calclllated is due to revisions af the latter- which reslIlted in an increased alJowable force per wire- and a lso to the l1lethod of anchorage. The anchor blocks. which are of Meehallite, accoml11odate faur wires in each direction and are spaced up the wall to give lhe nearest to the desired arrangement. The actual spacing is shown in Figure 7(a). It will be SEen (Figure 3) that two twin jacks are needed to slress any one cable and lhat stressi ng is to be alternated at 90°. Thus, four wires are tensioned and locked at positions 1 and then lhe next four a1 positions 2.

5. CONSTRUCTION

The followi ng notes record the problems encountered 10 Lhe time af writing during lhe construction of the tan k.

5.1 Sequence of Erection

The erect ion procedure proposed was as follo ws.

D. Foster 293

FIGURE 8- The base and shuttering for the bottom ring beam .

FIGURE 9- Raising the tank walls.

(i) Excavate, lay hardcore of broken brick and lay a 3-in.-thick cement: sand as a blinding.

(ii) Lay foundation steel, cast the slab and form the upstand.

(iii) Form the sliding joint at the base of the inner 9-in. wall.

(iv) Set formwork for bonom reinforced-concrete ring beam and locate its steel.

(v) Erect scaffolding and top template and then locate vertical stressing steel , anchors and spirals. Clt was intended originally to stress lhe wires from lhe top, fixing the tapered sleeves into the anchors at the bottom before doing so, but aner the erection of the walls. This

F IGURE IO- The shuttering for lhe top ring beam.

FIGURE 11 - The walls before hoop steel was fixecl.

proved impracticablc so the vertical cables were stressed and the bottom anchorages made before delivery as complete sheathed \Vires incorporating grout tubes).

(vi) Build the inner brickwork and cast the top beam.

(vi i) Carry out vertical stressing when the top bcam, and hence ali the inner wall , is 28 days old.

(viii) Locate and stress the hoop steel.

(ix) Form the watertight joiot between the bottom ring beam and the concrete llpstand from the base.

(x) Render the inside of the tank with the Revinex 29Y 40 mix in two coats to the manufactllrer's directions.

(xi) Fill tank and check for leaks.

(xii) Build externaI walls and grout the cavity.

294 Design and Construction or a Prestressed Brickwork Water Tank

11 I ='"

F IGURE 12- Inside b:!fore rendering.

FIGURE 13- Completed tank.

5.2 Comment

So far Ihis programme has proved praclicable bul care­lessness on lhe parI of lhe bricklayers and scaffolders has caused some trouble and there was one mishap which caused further delay. Figures 8-12 show the work done so far and the following notes on these are given in conclusion.

Figure 8: After casting the base slab and upstand a useful amount of rain water co llected and fortuitously served to help cure the slab and keep shrinkage to a minimum.

Figure 9: The tank walls, when about 3-fl high, were rising well out of plumb because the bricklayers were following the vertical cables which had nol been held properiy by the templates.

Figure 10: After the top ring beam was cast care was nol taken in removing the wooden plugs which formed the holes for the 10p anchors to the cables. Some of the sheaths were blocked as a resull and high-pressure air was needed to clear them.

Since lhe conference lhe lank has been completed (Figure 13).

REFERENCES J. BRIT1SH STANDARDS I NSTlTUTION, Design and Construction of

Reinforced and Prestressed Concrcte Struclures for lhe Slorage of Water and Other Aqueous Liquids. c.p, 2C07: 1960.

2. BRITISH STANDARDS I NSTITUTION, Bricks and BJocks of F ired Brickearth, Clay Of Shale. 8 .5. 3921: 1965.

3. BRITISH STANDARDS I NSTITUTION, Bui lding Sands from Natural Sources. B.S. 1198- 1200: 1955.

4. WEST, H. W. H. and others, The Performance o f Walls Built of Wirecut Bricks With and Without Perrorat ions. Proe. Brit. Ceram . Soe. (17),1,1970.

5. LENCZNER, D. This vo lume, page 44. 6. BRITiSH STANDARDS INSTlTUTION, Structu ra l Recommendations

for Load-bearing Wall s. c.P. 111: 1964. Table 6, Page 21. 7. STRUCTURAL CLAY PRODUCTS lNSTITUTE, Building Code Re­

quirements for Engineered Brick Maso nry, 1969. 8. BRITISH STANDARDS INsTITuTloN, The Structllral Use of Pre­

stressed Concrete in Building. c.p, 115 : 1959.