43.- Testing and Calculation of Masonry: Recommendations Based on Research in Belgium

by

A. de GRAVE

Millisrere des Trm1aux Publics, Brusseh

and

H. MOTTEU

Cellfre Scielllijique et Teclmiq//e de la Constl'lIction, Brussels

A BSTRACT

Research ;nlo lhe mechallical pro­perties of masonry struclures carried OUI in Belgium sillce 1963 is SUI1I­

marized here in ( IVO synoplic tables; one giving resulls Df lhe tests on materiais, lhe olher lhe results of tesls DIl brick slrue/tlres.

011 lhe basis of this research. recommendations have been made for calculalioll alld for melhods of lesting masollry . They are principally concerned wilh lhe crushillg s lrellgth and suction rale Df lhe bricks al/d blocks; lhe clloice of morlar appro­priale lo IIIe brick or block alld lo file stresses ;11 lhe masonry; site tests; fhe determinarioll 0/ lhe ca!Culated slresses hl masonry bosed 011 lhe results Df tests 011 lI'all sectiolls and III e reduclioll faclor lo be applied 10 basic permissible stresses la lake i1110 accounllhe slelldemess of lhe masol1ry and lhe eccell lricity olloadillg.

1. INTRODUCTION

Essais et Ca/cu/ des Maçonneries: Recommandations lssues de Recher­c"es Effecluées en Belgiq/le

Les recllerches, effectuées ell Belgique depuis 1963, sur les propriétés méc­oniques des eonslruelions ell moçoll­nerie, sont résllmées ell deux tobleaux synoptiques dOl1l1al1l , respeetil'ement , les résultats des essais sur les matéri­oux ef eeux des essais Sur les COI1-

slrucliol1s en briques. Sur lo base de ces reeherehes des recommand­atiol1s ont été faUes pour le calclIl et les méthodes d'essai des maçonneries. Elles concemel1t principalemenl la résislance à I'écrasement ef le taux de sueciol1 des briques ef des blocs, le ehoix du morlier approprié à la brique ou au bloe el aux contrailltes dalls la maçollnerie, lesessais sur le chantier, la délerm;,wtiol1 des contrainles calculées dons la maçonnerie sur la base de ré­sullals d'essais effeclués sur des secl­ions de murs el le f acleur de réduclioll à appliqller aux conlraintes admiss­ibles de base pour lellir compte de I'élafl cemellt de la maçol1l1erie e f de I'excenfricifé de la charge.

Prüfen und BerecJmen von Mauenvel'k: Empfehlungen auf Grllnd von Unt .. ·­suchungen in Be/gien

Die seil 1963 in Belgien durchgeführ­len Ulltersllchungen meehanischer Eigenschaffell \'011 M auerwerks-Kon­slruk liollen sind Mer in zlI'ei übersiclrt­lichen Tabellen =lIsammengefa~t: eine mil dell Ergebnissen der Material­pri{jimg, die andere mil den Resu/lalen der Priifungen 11011 Ziegelkol1struktioll­en. Au! Grund diesel' Unlersllchungen werden Empjehlungen fü r die Bereeh­I7lmg 1'0 11 l/fld Prüfme lhoden fü r Mauen verk gegeben. Sie gehen atls VOI1 der Druckfestigkeil IIl1d dem Saugverllallell der Ziegelund B/ocke, berücksicllligell die Wah! des Morlels fi;r die Ziegel oder BlOcke ulld die im Mallerwerk zu erlt'artendell Spallllllng­ell, Situalions- Versuche. die Ermill­lung der Iheorelisehell SpanIHmgen aus Prüfergebnissen 1'0 11 Mauerwerks­abscll11il1en und dem Reduk lionsfak­lor, m;, dem die zlIlãssigen Spanlllll1gell mullipliziert werdeu miissen, lIIn die Sehlankheil des fvJauenverks wul die Exzentrizitiit der Belastung zu erfassen.

Research on the mechanical properties of masonry has been carried out jointly in Belgium since 1963 by the CSTC (Building Industry Research Centre) and the CRIC (Research Centre for the Cement Jndustry), in collaboration with the universities of Liége and Louvain, and with the help of the clay and concrete brick and block industries.

(2) To determine the effect of slenderness ratio and eccentricity.

(3) To establish recommendations for masonry cal ­culations, based on the mechanical and physical pro­perties of lhe materiaIs.

The mechanical and physical properties of the whole range of c1ay and concrete bricks and blocks covered by Belgian Standards NBN 11 8, 476 and 538, have been determined (compressive strength by various methods, tensile strength , water absorption, suction rate, bulk density, shrinkage), and related to the measured per­formance of wallettes and walls.

The results of the first 2 years of research have been published e1sewhere-' The results of the second part of the research and the lessons and practical conc1usions drawn from it are described below.

2. PROGRAMME AND RESULTS OF THE TEST

The objectives of the research were :

(I) To define a metbod of laboratory tests on walls, allowing the basic strength of the masonry to be deter­mined.

266

The morta r composition was adapted to the quality of the material on the basis of the conclusions of the first 2 years of the resea rch ; its compressive strength measured on 4 x 4 x 16-cm (1·6 x 1·6 x 6·3-in .) specimens at 28 days, was taken to be about equal to half that of the brick ar block (mean of ten tests) whose test faces

A. de Grave and H. Motteu 267 were dressed with standard mortar (2 parts works sand , I part rapid hardening Portland eement, 16-20 % water) . lt is shown in Seetion 3. 1 that this eondition leads to the ehoiee of a mortar whose st renglh is equal to the eor­reeted eharaeleristie strength of the material. Table I

li sts the properties of the materiais used. Generally eaeh property was determined 011 l en specimens.

Dt'Signal ioll S/te S)'/1Ibol· 01 material L x f x h

(cm)

Solid high. strength

A. bricks 19 x 9 x 65

-Ab Perforated 29 x l4 x 9

bricks, type 1·4

Ao Hollow Ac 19 bricks, t tu'ee shapes 29 x 19 x 19 type 0·8

Ac 14

29 x l4 x l9

Ao'

29 x 9 x 19

D, Solid blocks of cellular concrele, 39 x I9 x 2'S ultra Iight-weight

Hollow Bb 19 concrete

"b blocks 39 x l9 x l9 based on expanded Db 14 clay, light type

39 x l4 x l9 I

Solid concrele 39x 19x 19

"' blocks based on expanded day, seml-heavy

Table 2 summarizes the characteristics of the construc­tion, bond , dimensions a nd slenderness ratio of the wallettes and walls, and the resuIts of eompression tests

TABLE l - PROPERTIES OF THE M ATER IALS

Diagram of/he horizontal (H) or lransverse ( T ) CompTt:s. Tel/si/e D",;,y[ I w"'''1 C'pUI.

A-Jortor (comp-.W!CtiOIl o/ tIre matcrial

sl~e strenglht ------ (Ibsorp- absorp. OSiliofl per !fIJ s/rel/g/h on True Dlllk /;011 tioll§ of llry $01/(1 U) gross areo (s /em J) (g/eml) (lI't%)(gldm2fmill)

(kgf,clII~) 8inder Watcr

\. ft 11 7S6 26 1·87 1-87 "5 16-0 C 500 19·5

CJ • ( H) (15 %) (45 %) (2-4 %) ( 10%) (, _ 4 '8)

29 56. 8·5 1·9 1 1·38 ' ·0 6·0 CSOO 19 ·5

(H) .. CIlO· · · · D·IPO .. °aOQ!l °lluO OOOD 14 (6'5%) (41 %) (0-8 %) (11 ~.) (r=2-I) O ODU0c::J.tr DOa 0 OOO"O~ :: :O"O"O

(28 % voids) \. Ac /9

(T) 67 - 1-85 0 '555 "7 .. \ {C200

22 ·2

IO~Ol ( 14 %) (1 '6 %) (12 %) (r = 4'S) 0100

°0° \. O O

Ac 19 (70 %) voids) Ac /4

47 - ' -88 0 ·658 l i " 14 ·9

[I I (14%) (1 ,6%) l7 %} (, = 4'5)

000 00 O O 00 Ad

85 - 1·87 0·785 10·5 17·8 Ac 14 (65% voids) Ac 9 (58 % voids)

(24 %) (1 -6%) ('%) (r=7'1) ---

(H)

CJ 55 (3·5 %) 2·2 0 ·683 0 ·683 67·7 29 ·2 {C ISO 22·4 t9

&65(7 %) (35 %) (0'7 %) (3 rol (, = 27) 0150

3.

whole for cuUing (H) Bb 19-,)

122 11·9 1·37 0·97 11 ·1 "3 {C250 22·3

)I:JD( f~ \000\ (22 %) (30%) (4'4 %) ('%) (r _ 17) 050

Bb 14 : 20% voids Bb 14~

Bb 19 : 29 % .. lJ2 13·6 1·40 1·15 13·3 6·2

Bb 19 : 22 -6 % " (20 %) (I3·SY.J (4-4 %) (13 %) (,=6'4)

39 ( H) 233 (14 %) 9 ·3 1·43 1-43 10 ·83 41·0 C 300 2 1'2

CJ 260(12 %) (1 5%) (3 '2 %) (22 Y.J (r=34) \.

• Clay bricks and blocks are designaled A; concrete blocks B. t The tensile lest was carried OUI by sticking steel plates provided with an articulaled rod to lhe opposite small faces oflhe bricks or blocks. This leSI could nOI be

carried out on lhe hol1ow blocks Ac because Iheircross-se<:lion contained voids. : The t rue densi ly mm was deduced from the bulk densily ma takins account oflhe percentase of voids ~ in the material, (brick or block).

mm=.!!!!!.. \ -,

§ The suction rate was measured by Haller's melhod. This measures the weight inceease of lhe malerial (as supplied) when it is placed for 1 min in a 'lesse I in which thc water levei is maintained at 0'5 crn (0·2 in.) above the lower face. The weight increase is rclated lo lhe horizontal surface immersed (uniu: g/dm2). The horizontal seclion either includes or does nol inc1udc voids, depending on whelher, in lhe wall, lhe malerial is p[aced on a solid bed ofmortar (perforaled bricks) or nOI (hollow concrele blocks). Since lhe results are lhe mcan of only tive tests, lhe figure in brackets is the 'range'.

11 For blocks Ba and Bc, IWO compression tcsts ","cre carríed out aI an interval of 2 months 10 show lhe elfect of age. The lattee test corresponded lO lhe age of lhe blocks in lhe wall crushing lests.

'11 For blocks Bb, lhe properties were determined on whole blocks, cxcept for lhe Icnsile st renglh leslS, measured on blocks or lhe typc for cuuing which havc plane lransverse faces.

Sand: 4 ·8% < 74 ...,m; tincness modulus 1'16; sand equivalent 32%. tt Binders a re designated: C >= ccment; O - dry fat lime. Thc figures express the weighl of binder in ki/ml of dry sand . •• The quanli ly of water is expressed as a percentage ofthe weight of dry sand and binder_ The consistency of lhe mortars was left 10 lhe judgemenl or lhe bricklayer.

268 Testing and CalcuJation of Masonry: Recommendations Based on Research in Belgium TABLE 2- RESULTS OF TESTS ON WALLS ANO WALLETfES

JI.10rlar Crushing strt'ngth Bond o/ .... 01/5· Des{gna(ion of Sll'IId",-

Material sedes n/'ss Dtsignationt Compressive ralio O" nal/",", "I M,a. Range r

(Widlh: 120 em) strength "'alls (kgf/cm2) (kgflcm2) (%) (kgf{cm 2)

A. 9-cm walls, allernale joiots CSOO 268 Aa 19- IOO-FA 4 234- 252-263 150 11,6 CSOO 293 Aa 19- 192-FA ,., 119§ 219 -

~DDDDDDDDDts CSOO 277 Aa 19-2S3-AA 15-3 250--251-264 2SS S'S CSOO 277 A. 9-169- FA 22 ·4 120- 130-157 136 27·2

19 CSOo 312 Aa 19- 270- AA 16·2 61 - 87- 88- 96 83 4H : d/6

CSOO 2.9 Aa 19- 274-AA 16-4 22- 9-5 11 15-' -d/3

OODCJDCJO Di li IOCJ

I I Ab 14-cm walls, alternale joinlS c 500 291 Ab '4- IOO-FA S'S 244-256- 159 2S3 S·9

CSOO 30S Ab 14- 162- FA 9 277-278- 295 283 6·4 29 DDDDOOOO courses CSOO 274 Ab 29- 224- AA 9 218- 228-235 227 7·S

1.3.5,7 CSOO 268 Ab 14-160- AA 14·2 252- 296--306 282 19·1 C500 283 Ab t4- 221 - AA 18·5 246--270- 278 26S 12·1 C500 "S Ab 14- 293-AA 23·6 196- 200- 204 200 4·0

t:![EJElE5 COUfses 2,6,10

~DL..JO~ courses Ao 0== O -4,8,12 { e 200 } 62 Ac 19- IOO-FA • 26,7- 26·6-26,6 26·6 O·.

G 100 62 Ac 14- tOO-FA S·4 34,9- 35,3- 36·4 35·5 4-2 62 Ac 9- IOO-FA 8'S 37 ·6- 45 ,5- 47·9 43 24 ·0

Wall s of9-14- or 19-cm: 6<l Ac 19-180-FA 7·3 24-26--27 25·7 11·7 alternale joinlS 61 Ac 19- 283- AA 16·9 23·6-24·1 - 28 ·1 2S 18·0

41 Ac 14 222 FA 12 28·5 30·4 31'5 30·1 10·0

8. { C ISO } 28 Ba 19- 100- FA 4 ' 1 39,5-43,5- 45 42·7 12·9 G 150 32 Ba 19-177- FA 7-2 34,5- 36·1 - 36,4 35·7 S'3

I' I 11 11 I 0' { C2S0 } 97 Bb 19- IOO-FA .·1 51,1 - 62,5- 64 59·2 21·8 Rb 19 G 50 83 Bb 14- 100- FA S'S 85'1 - 98-101 OS 16·7

103 Bb 19- 184- FA 7-6 52 '5- 52 '9- 57 54·1 8·3 109 Bb 19- 229-AA I' 52·8- 55- 61'3 56·4 15·1 102 Bb 14- 200- AA 17 75 '8-89· 3- 77,4 80·8 2·0 100 Bb 14- 260- AA 21 ·3 77-3- 83 '3-86·3 82·3 10·9

DI II 10

Ik C 300 128 Bc 19-100- FA 4· ' 160- 173-178 170 10·6 176 Bc 19- 187-FA 7·7 149- [58- 158 ISS S·8

For wallelles lhe bond is with alternaling joinlS. The mortars are designated according to their composition in binder per 013 of dr)' sand, as shown in Table I .

! dl6 and dl3 designate the eccentricity e of the application of the loads and correspond respectively 10 relative eccentricities o f m '" (6eld) .. I (edge of central core) and ofm = 2.

§ Tests on this series of walls have been stopped at 219 kgfJcm2• or 500 tons, as pivots capable of wilhstanding higher loads without deformation were not available. II The series oftests with eccentricity dl 3 \m '" 2) gave such low bre<lking strenglh s thal il was decided 10 ghe up the third wall, and 10 put il in to lhe previou~ sedes which

Ihen comprised four wall s.

on mortars, walls and wallettes. Each series consisted of three identieal test-pieees, designated by the following convention.

Example: Ab 14-162-FA is a wall eonstructed of lype­Ab perforated clay bricks, 162-cm (5'3-ft) high (brick­work proper), 14-cm (5·5-in .) thick, one end fixed (F) and lhe other pin-ended (A).

The wallettes were ali aboul 100-em (3'2-ft) high, 60-cm (2-ft) wide, and as thick as their materiais, exeept for Aa. The width of the walls was about 120 em (3'9 ft). Certain walls had two pivots (AA).

The slenderness ratio E of the walls was calculated as shown in Figure I. Figure 2 shows the arrangement of lhe base of a wallette and Figure 3 lhe detail of the ends of lhe wall showing lhe dividing beam. These bases are symmetrical at lhe top and bottom of the tesl wall.

3. DlSCUSSION OF THE RESUL TS ANO CONCLUSIONS

As far as the materiais, bricks or blocks, are concerned, lhe tesls most c10sely related to the mechanieal behaviour of the masonry, Were those for compressive strength,

water absorption by capillarity and by immersion, and lhe bulk density.

hf l' H ~ pivots • A4J h

fl'07[hm' )<~JE 1ix.zd II!nd ~ 1 pivot • F~

"f E • .!>,!t-

-' W r t lQCm

80" A

5C1porotlng Beom

Prl!ss P IO!\>

FIGURE l- Wal1 tesl apparatus.

A. de Grave and H. Motteu 269

.. . ..... ; .•.. '.:.; .. ,"

.,' · . Conc r llt~ ':.', . .

. . ' l'.,.~. " . ','

Mode l Wo ll

FIGURE 2- Diagram of base of wallette.

3.1 Compressive Slrenglh of Bricks or Blocks

3.1.1 Test Methods

We propose the adoption of the compressive test 00

whole blocks, wilh the test faces dressed with cement mortar (I parI P500, 2 parts sand, 18 % waler). We designate the mean strength obtained a ' bm '

3.1.2 Corrected Strength

The compressive strength results are inftuenced by the shape and size of lhe malerials. They undergo a re­straining effect from the pia tens, lhe flatter the material the more marked il is. To take accounl of Ihis efrecl and lo put ali materiais on the same footing it is proposed:

(a) To change the lest values by a dividing faclorf, varying with dimensions af the specimens .

(b) For very flal malerials (h < 5 em) to make lhe lesl on half unils only, with lhe faces dressed wilh slandard mortar.

The 'corrected strength', o ' be> af a material will be defined by the expression a' b,! = (f' hmlf.

The Belgian formula for.r (due to DUTRoN) was adopled:

where

Rx 0·70 f= R=0'65+ ( d) (h)"25

c:!o 1+--20 d

Rx = the compressive strength value obtained using a specimen of heighl h:

R ('20= the compressive strength measured on a cube specimen (x = 20 em);

d = the anthmetic mean of the dimensions of the rectangular section of the specimen. (For a cylindrical specimen il is lhe dia­meler of the circular section).

Some calculaled values of.r are given in Table 3.

3.1.3 Characlerislic Slrenglh

To take accounl of the dispersion of lhe results, the characteristic strength of a material is defined on the basis of a sufficient number af experimental results assumed to follow the normal Gaussian distribution. It depends on the mean value (7 ' bm of the measurements, on the coeffi.cient of variation (ar dispersion à) and on lhe a priori probabilily (generally laken as 95 %) of having lesl results grealer Ihan this slrenglh.

Table 4 shows the corrected characlerislic slrenglhs of the materiais taking into account their geometry and the dispersion of lhe results. 11 should be nOled Ihat Ihese strengths are Df the same order Df magnitude as the basic slrengths oflhe corresponding masonry, and this provides

Divl d ing Beom

Pivot ----,al==Io=''

FIGUR E 3- Detail orthe ends orthc wall.

very practical and valid indications af strength . lt should also be nOled Ihal the compressive slrenglhs

Df lhe mortars chosen (and whose composition is con­sidered lhe optimum) are of the same order of magnitude as lhe characteristic strength of lhe materiais.

3.2 Waler Absorption

3.2.1 By lmmersiofl

The properly of waler absorplion by immersion of lhe material is important in determining the ability of the masonry to withstand natural wetting and drying cycles and allowing lhe passage of waler vapour from the inside lo lhe outside of lhe houses.

3.2.2 By Capillarity

The capillary absorption capacily (Haller's melhod) ­suction rale - of lhe material afrecIs lhe strenglh of lhe masonry. If Ihis absorption is high lhe material lakes parI of the waler from the fresh joinling morlar and Ihis can break lhe bond in lhe joinls, parlicularly in lhe parI near the face. This can be overcome by moderate pre­wetting of lhe faces lo be laid (e.g. for materiais wilh a Haller number grealer Ihan 20 gfdm2) . Because wetting concrele blocks leads lo subsequenl drying shrinkage.

TABU 3- CALCULATED VALUES OF f

Material shape L x I x " (em)

19 x 9 x 6-5

19 x 9 x 5-7

29 x l4 x 9

29 x l9 x l9

39 x !9 x 19

39x14 x 19

39 x 9 x 19

f

1·72

1·91

1·65

1·08

I J

·11

1·08

270 Testing and Calculation of Masonry: Recommendations Based on Research in BeIgium TABLE 4-CORRECTED CHARACTERISTIC STRENGTHS OF THE MATERIALS

Material (nominal MeulI Correcred Disper- Correcled Basic slrellgrh of COfllpressive siu in em) Compressive Dividing slreng'" !iioll clwracteristic lhe masonry srrellgth of

slreng/h factor (I'bIl/1(1 S strellgt"· measl/red on ,lte /1/or/ar (10 ICSIS) f U'bc=-- q 'bk of lhe

a ' b/1/10 f Wafl of walleltes (kgf/cm' ) Walletfe low s/el/d- (kgflcm')

(kgf/cm') erlless ratio (kgflcm' )

Aa (l9 x9x6'5) 756 1·72 440 0·15 30lt 250 219 268 Ab (29 x 14 x9) 568 1·65 344 0 ·065 300 253 283 291 Ac 19 (29x 19 x 19) 67 1·08 62 0 ·14 43 26·6 25·7 62 14 (29 x 14 x 19) 47 1·04 45 0·14 33 35'5 62 9 (29 x 9 x 19) 85 1·01 84 0·24 44 43 62 ................... - .. .. . . .. . ........ ... .... .. . . ..... .. . . . . . . . . . . . . Ba (39 x I9 x24) 55 1·14 48 0 ·035 44 42·7 35·7 28

65 1·14 57 0 ·07 48 Bb (39xI4 x I9) 132 1·11 119 0·20 70 95 83

(39x 19 x 19) 122 1·14 107 0·22 59 59·2 54·1 97 Bc (39 x 19 x 19) 260 1·14 144 0 ·14 144

260 1·14 228 0·12 170 170 155 128 I

• o'bk canno! be determincd on lhe basis of ten tests. We have applied lhe method of Section 4.1 which gives an acceptable estimale of q'bk. From the point of vicw ofstatistical theory lhe reasoning is nol rigorous. t Example of the calculation:

(]'blllIo = 756 kgfjcm2

sI0= Ô.(]'bmlo= O·15 x 756= 113 kgfjcm2

(]'bm= Q'bIllIO- l·S3 (s I0/3)= 687 kgfjcm2

.'bk ~ (.'bm/f) (1- 1·64 8) ~ (687f1· 72) x 0·754~ 301 kgf/cm2

the manufacturers of lightweight concrete have intro­duced an adhesive-type mortar allowing cellular blocks to be laid dry.

3.3 Bulk Density

Tbe resuhs of measurements of density on clay bricks and aerated concrete blocks show very little variation (0,7 to 2-4 % maximum). For concrete blocks other than aerated, the bulk density shows more variation (3 to 4 %), which can be explained by the usual heterogeneity of concrete and differences in compaction duriog manu­facture. These dilferences in bulk density are easily showo up by weighing lhe blocks, this being a simple method of distinguishing blocks varying widely from ihe mean weight. In this connection a variation in bulk density involves a variation in slrength , which reduces the characteristic streogth a ' bk of lhe material , and there­fore the value used to calculate the masonry strength.

lt is important to adapt the strength of the mortars to the strengths of the bricks ar blocks. The compressive strength of the mortar, measured after 28 days on speci­mens 4 x 4 x 16 cm (1'6 x 1·6 x 6·3 in.) should be equal to approximateJy half lhat of the material (brick or block), a 'bm measured 00 a whole unit with lhe faces dressed with standard mortar and referred to as the net sectioo of the material.

3.4 WalJettes and T esl Walls

3.4.1 Comparisol1 01 Slrellglh

The ratio of strength values obtained 00 wallettes and 00 walls with low slenderness ratios is about 1 (from 0·90 to 1'20), although the walleltes are usually stronger. Tests on wallettes, which are rapid, cheap and simple to carry out, can therefore replace lests on high walls with low slenderness ralios for the determination of the basic strength of the masonry. This consideration is

useful in so far as one can base building sa fety on the preservation and testing of these wallettes.

The walls shou Id consist of at least four courses of bricks ar blocks, with a minimum height of 1 m (3·3 ft), and their width should be at least 60 cm (2 ft), and at least two bricks (ar blocks) long.

3.4.2 The EJ!ect oi the Geometric Slelldemess Ralio oi lhe Wa/l

This elfect is clear for high slenderness ratios (about ar greater lhan 20), but is less marked for values of E < 15. The range over which the ratio (R' wall : R' base) retains lhe value unity is continued up to E~ 15, then decreases noticeably.

For high slenderness ratios the effect of eccentric loading on the reduction in the streogth of the wall is very great, and this is probably why the Brilish aod Swiss regulations which define reduction coefficients for slenderness are much more strict than those resulting from our tests. .

3.4.3 The EffecI 01 Eccel1lricily 01 Loadil/g

The series Aa lO-270- AA (d j6) and Aa 19- 274-AA (d j3) show that eccentric loading has a marked elfect 00 the ratio (R' base: R' wall). The value of /1/= I (dj6 = limit of central core) reduces this ralio to one-third, while for 111 ~ 2 the strength of the wall is reduced almosl to zero for a geometric slenderness ratio of 16.

These reductions in strength agree with those proposed by Swiss regalations. In ali cases it appears seosible that with masonry the tensile streogth of which is low, eccentricity should be allowed only inside the central core (111~ I) except for low slenderness ratios (E < 10) where it cao be tolerated ap to 111 = 2.

.values for the reduction factor k as a function of the geometric slenderness ratio of the wall and the relative eccentricity, are given in Figure 4.

A. de Grave and H. Motteu 271 TABLE 5- PRELlMINARY (P) AN O 5ITE CONSTRucnON (C) TESTS

Obligatory tests Ofl masQnry as a funCliorr ofã'm

No.of

lã'm> 20kgflcm 2

Bricks ar Blocks Compression on material with tesl faces

dressed with standard mortar Capillary absorption (Haller) Absorption by immersion

Mortar Compression and bending 00 half·specimens

in groups of three specimens 4 x 4 x 16 em Consistence

Model walls Compression on wallettes (for tesl method sce

Sect. 2.3 , pp. 11 - 12 of C.R.Rech. No. 4 CSTC-CR1C 19663)

Walls Compression of slorey-height waUs built

between knife-edges. The properties af lhe walls (materiaIs, bond thickness, mcthod af laying) should be lhe same as for lhe in-tended construction. Concentric loading.

k

tA m~Otol

08 m ·1 0 .

O

0"r-__ ~m~. 2~ ____ ~,

I 0·5 ----- - --- - - I-- - - ---

025

O

I I I I

--- ______ 1-

10 20

_ __ :0.50

I I I 0-35

I 025 I I I

24E'h /d

FIGURE 4 - Load factor k. Linear interpola/ion of k as a function ofm = 6e/d.

4. CALCULATION OF MASONRY AND THE QUALITY CONTROL OF MATERIALS

4.1 The Characteristic Compressive Strength u' bk of Bricks and Blocks

The introduction of the idea of characteristic strength supposes that the manufacturers of the material subject it to regular control tests at the factory ; acceptance of the material on site can lhen be reduced to simple testing for conformity of the delivery.

Notation

a' m = basic strength of the masonry (compressive breaking) determined on a wallette

u'M = crushing strenglh of a wall

rests

10 10 10

1 1

3

3

ã'm ~ 12kgf/cm2 12 kgflcm2 < ã'm :s;. 20kgf/cm 2

p P&C P&C - P P P P P

- P P&C - P P&C

- P P&C

- - P

ã' m = basic working stresses of the masonry (no account taken of the slenderness ratio of the wall and of eccentricily of loading)

ã' = calculaled stress allowed for the masonry (account taken of the slenderness ratio of lhe wall and of eccenlricity of loading)

a ' bma' bca' bk = compressive strengths of the materiais (bricks or blocks) mean value (subscript m), corrected value (subscript c) or charac­teristic value (subscript k).

If the materiais are not subject to supervision in the factory, a'bm, D and a 'bk can be determined on the basis of thirty tests on material sampled from the site stock, by applying the relationship

u 'bk= uf (1 - 1·64 3)

For small sites one may rely on carrying out ten com­pressive tests from which one can determine the mean strength, the standard deviation S10 and the coefficient of variation (S10Ia' bml0) = D10 One then takes

and takes for the value af a 'bk the expression a'bm/

(1 - 1.64310)

4.2 Preliminary and Site Tests

Table 5 shows tests to be carried out aimed at charac­terizing the masonry to be erected from the mechanical point afview.

* The value of 1·83 is taken from Student·Fisher tablcs and co rresponds to a probability of 90 % that the mean of ten tests O'bmlO should ditfer from thegeneral mean value O'hm by less than 1·83 (SiO/-vfõ - l) in absolute value.

272 Testing and Calculation of Masonry: Recommendations Based on Research in Belgium 4.3 Permissible Slress in lhe Masonry ã'

The following types of stress were defined.

(I) The basic working stress ã'm ofthe masonry (taking no aceount of the slendemess ratio of the wall nor of eecentrie loading) was determined by tests on wallettes of low slenderness ratio where it could be determined by applying Table 6 for values of ã' '" not exeeeding 12 kgf/cm2 (170 Ibf/ in2).

(2) The permissible stress ã' for the masonry (taking aeeount of the slendemess ratio of the walls and of the eeeentrieity of loading) was determined, either by tests on walls (see below). or from the basie working stress ij'm by applying the reduction ã'=kã'm where the load facto r k takes into account the slendemess ratio of the wall and the eeeentrieity of loading (Figure 4).

It is proposed that the British method deseribed in CP 1I1 :19642 be used to caleulate lhe slenderness ratio of lhe wall.

Proposals ha ve slill to be drafted for calculalion of the eccentricity of loading.

Depending on the value of the basic working stTess ã 'm. three cases may be distinguished:

Case I ã'm< 12 kgf/cm2 (170 Ibf/ in')

In this case only lesls on lhe materiais (brieks or blocks) are necessary.

One can allow as a maximum the basic working slresses ã'", of Table 6, lo whieh lhe load faetor k of

T ABLE 6-BASIC WORKING STRESS OF MASONRY

Characteristic Bas;c working stress of masonry (kgffcm1)

strengtlrs of bricks or b/ocks CI50 l C2oo C 250 C 300 C 400

(kglJcm2) G 150 G 100 G SO ,

2S 2' 3 ~ ~ ~

SO 3 S' 6 ~ ~

100 ~ 6 8' 10 ~

ISO ~ ~ 10 12' ~

200 ~ ~ 12

I

12 12 2S0 ~ ~ 12 12 12

·These figures correspond to the compositions of mortar re­commended for the quality of lhe materiais used.

Figure 4 should be applied lO take into account the effeet af slenderness ratio and eccentricity of loading.

The values of ã' '" are limited to 12 kgf/cm 2 (170 Ibf/ in2). If it is desired to go to higher calculated stresses, Ihis must be justified by lests on wallettes or on walls (Cases 2 and 3).

Case 2 12 kgf/cm2 (170 Ibf/ in2) < ã'm';; 20 kgf/em2 (285 Ibf/ in')

In this case lests on the materiais and on model walls are necessary.

The basic working stress ã 'm can be obtained by dividing lhe basic ~trength a'm as measured on wallettes by a safely faelor of 6.

The basic strength a ' l/I is taken to be equal to the lowest of the three following values:

(1) The median value of the breaking stress for three wallettes.

(2) The mean value of the breaking slress for three wallettes.

(3) 1·15 times the minimum value if the range belween the extreme values is greater than 0·3 times the minimum value.

The basic working stress (ã' ~ a' ",/6) should in any case be limited to 20 kgf/cm2 (285 Ibf/ in') unless tests are carried oul on full-seale walls (see Case 3).

The permissible wall stress ã' is expressed by the relalionship

ka'nr k -' -, - = a m a = 6

with an upper limit to ã' of k X 20 kgf/em2 (285 Ibf/ in2).

Case 3

ã'm > 20 kgf/cm2 (285 Ibf/in2)

In this case preliminary tests on walls are necessary, as well as contrai tests on the materiais and on wallettes before and during construetion (see Table 5).

The walls intended for the tests should be constructed by a bricklayer from the contraetor, and lhe foreman should see thal the masonry of lhe building is erected under lhe same canditians as the test , i.e. quality and choice ofbricks ar blocks; composition and mixing of mortar; thickness and bond af the walls; moisture condition of the materiais as used; accuracy in building (whether the wall is vertical and the joinls horizonlal, and the thiekness of the joints); protection for the masonry after building. So far as possible lhe heighl of the test wall should be equal to lhe height between floors in lhe building to be ereeted. If this is not possible the strenglhs of lhe tesl walls are multiplied by

k wafl 10 be buifl

ktesl wafl

The test walls should have end condilions consisling oI'

(a) two pin-ends if ", ~ ";

(b) one pin-end and one flal-end if h,~ 31z/4.

The breaking stress a' M obtained from lhe crushing of three test walls corresponds lo lhe smallesl of the three values (median, mean , ar I ·15 x minimum as indicated for Case 2 above).

The caleulated slress ã' is laken equal to (k t!k2) (a',,/6) with an upper limit lO a',,/6 of 30 kgf/cm' (427Ibf/ in') where k, and k2 respeetively are the load factors to be applied to take account of the eecentricity of loading of the wall to be constructed, and of lhe zero eccentricity of loading adopted in the te,t , for the value of slender­ness ratio used in lhe test.

REFERENCES 1. MorrEU, H., Research on Load-bearing Masonry in Belgium

Since 1963. 'Designing, Engineering and Constructing with Masonry Products. Houston, Texas, Gulf Publishing. 1969. pp. 171 - 184.

2. BRITlSH STANDARDS INSTlTUTION, Structllral Reçommendations for LOM-bearing Walls. c.P. 111 :1964.

3. eSTe- CRIe (Brussels), Etude des Maçonneries Portantes, C.R. Rech .• No. 4,1966; No. 12.1970.