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TJIE DALUATIOB OI' COPIBIJ:SSIVE S'lBEHG'.rH OP BlUC% JIASODY IB snrtr

Assoeiate Professor WANG QINGLIN Leeturer WANG XIUTI Department of Civil Engineering

Xian Institute of P1etallurgy and Construetion Engineering, Xian, China

In thiB paper uaing a apeeial flat jaek a technique for direet eTalDa­tion of strength of maaonry valIa in an existing building ia introdueed. From the teste eompleted by above .entioned teehnique the eompreeeive strength of aasonry valIa vith reatr8Ínt in tvo aides ean be obtained and transfonaed into standard eompressive strength of briek masonry. This _thoel of evaluation of eOIlp,re_ive strength is eonvenient for use, it gives accurate resul.ts and mini.mm damage of valls.

DlTROEC'l'IOB

In reeonstruetion and aeeident treatment of existing building, the aetual eOllpressive strength anel deformation beharlours need to be determined. For briek mas.onry atruetures the usual _thod is to eut out several standard apeeimens froa struetural member and to test them in laboratory. It is COBtly and difllcult. Ploreover the disturbance and damaQes to brick mas'onry in the proces8 of cut and tranaport vill reaul t in significant test error, reducing the reliability of test data. The buildin~ itself is also subjeeted to a eertain damage. Recentl,., for determining the strength grades of briek and lIortar the eo_only used methods for nondestructive testing, aueh as rebound testing, supersonie sounding give an exeessive scattering of test data. Impaet lIIethod is based on the p,rinciple of energy transformation and lia,. be uaed oruy for testing motar of high-strength grades. In addition this method requires speeial devices in laboratory. In Italy Rosmn used a thin pressure eell, isserted into bed joints to determine direetly the vorking stress of brick sasonry in situo A series of similar researeh vork have been carried out at Hunan University in China. The main disadvantage of this type of thin pressure eell ia the limitation of possible deformation, therefore, it ean not be used for briek masonry whieh hae large ultimate co~ssive deformation.

'Ilith the purpose of reliable ~ simple meaaurement of eOllpressive strength of briek masonry in situ, a apeeial flat jack vith a stroke of

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25mm is designed.The length and thickness of jack is 240x68 Mm, just the same as, a standard brick. For meaauring the compressive strength of brick masonry firstly from the valI cut two horizontal slots vith a size equal to the size of jack. The vertical distance betveen these tvo slots ia ranged from 420 mm to 480 DlII. Tvo flat jacks are p,laced in these slota. Then these jacks are provided with a synchronistic pressure till the brick maaonry between jacks fails. In this way the actual restrained comporeaaive strength of Masonry is abtained.

It must be pointed out that the load, applied by the flat jack is of local character, and the brick masonry between jacks is confined 01'1 two sides. In this case the measured compressive strength of brick masonry is different from that of standard specimen. A transformation has to be made.

Testa ahow that the method proposed in this paper is simp'le.lt gives reliable measured data. and a minimUlll repairable damage to the valI.

In figure 1 the flat jacks under vorking state is illustrated.

E'igure 1. The flat jacks under working state

TEST RESULTS

The purpose of tests is to find out the influence of restraining condition on compressive strength of masonry and to establish a relation between measured restrained compremsive strength of masonry and the 'prismatic compressive strength specified by Chinese current design Code.

Tests have been carried out both in laboratory and 01'1 site. For testing in laboratory the same batch of cement, sand and brick is used to lay standard prismatic specimens of size 720x 370x240 mm and wall specimens of size 1250x1250x240 Mm. After 28 days curing the compressive strengths of standard specimens are measured. The restrained compressive strengths of valI specimens are measured vi th flat jacka. In order to simulate the compression stress 60 caused by upper loading in valls of building, 01'1 the ends of the top surface of valI tvo jacks are provided for synchronic pressure (figure 2).

ror testing in situ the flat jacks are used for wall specimens, and for comparison the standard specimens are cut from the adjacent wall parts.

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.L

Figure 2. The experimental apparatus.

when loading the ""alI by flat jacks. the load transfers in the form of shear stress to adjacent ""alI parts. this leads to a reduction of compression stress of masonry directly under the jack and to a nonuniform distribution of stresses along the horizontal cross section. Figure 3 sho""s the distribuf.ions of measured strains Ey and stresses ~y' obtaimed by finite element analysis along seetion I-I \figure 2).

Tests also sho"" tbat theJre are significant lateral tellllSicl!Il strail!lls in the Drlddle part of sectiOl!ll ll-J[ (figure 2). Si.ilar to local c08pressiol!ll, the lateral c08pJressive stresses oe.cur near the locations of flat jacklJ and lateral tention stressear in the middle part (figure 3).

Flcure 3. The distributions of strains and stresses.

In the presence of vertical compression 60 the restraining effect on the specimen in ""alI is increased. This leads to a reduction of lateral tension stresses of specimen in ""alI.

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It .:is obsel!'ved in teata that the dürtance between two flat jacks (specimen height) hae signi.ficant e1'1'ect on the co.preeive strength 01' lIlasonry. W i th the increaae 01' thie dietance. more load wil 1 trane-fer to adjacent wa11 parta and cQmpreseive strength approaches to etrength under local compree-s'.Íon. Very slllal! height 01' epecimen leadllt to more uniform dist,ribution 01' masonry stresses, but the restraining ef1'ect caUl'led by friction between jack and epecimen is etill presente

Let f' be the restrained compreesive strength of maeonry and f be the compresslve strength of standard prismatic specimen. In figure 4 tbe relatiouship between the ratio f'/f and specimen he1ghte is given. It Ls seeJll froa figure 4 that the moet euitable height is aOOut twice the width 01' specimen. At thie epecimen height both the influence cf friction and ratio f'/r are smal1. AJlId the inappropriate failure 01' adjacent paris 01' wall can be avoid fn thie cae-e.

f'/f

2

~ 1

I ! ! ! • • ! ! ! .. Itpecimen O 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 . 8 bei&ht (m)

Figure 4. The relati.onshi p between the ratio 1"/1' a nd epecimen he1ghte.

Ae mentioned above. the l oad irom the upper part of building will increaee the vertical co.:preeeion st:rese and reduce the lateral teneion st:reee in epecim.eJll. 'Nhen 6'0 fe relativaly smalI, tbe iucraa_ 01' lateral restraint with 6 iA aore impo,rtant than the additiona1 cOiBpression atrese. In ~igure 5 thetest reeuH.s with dif'feren.t 6'0 are shown.

In add.ition. similar to local compreal!lion the widthe of' wall parte in. both sidee of' epeciBen alao ha,ve certain effect on the cOlllpreesive strength of maeonr,y. The wider the adjacent wall parte, the more the tranaferred 10ad to them, and the higber the cOlapreasive et:re~h of aasonry. But the test reeulte and finite eleaent analydl!!' show that when the widtb of' adjacent valI partir exce~de a certain value, th1e effect beco_a 1.'e15. brportant. Generally, when the width 01' adjacent wall part in each !!lide ie gre.ater than the height of epeciaen or twice the wiCllth or apeciaen, the effect of width (}f adjacent wall on etrength g,f lllalronry oan be' neglected .

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2.0

1.5L----+---;~-

1.0 formula (1)

0.5

O 0.1 0.2 0.4

Figure 5. The relationship bet1o'een the coefficient K ane: 60 •

T1i.AS:i'ORtlATIO» COEFFICIE)I'l' FOR STREliGTH OI' MASOJIRT

Define tne ratio of restrained eompressive strength of 1o'all epeciment r' to standard compressive strength r as the transroraation coefficient for strength of Basonry, i.e. K = f'/f. ThUB", the corresp'onding prislllatic compresaiv .. strength can be calculated by dividing co,efficient K into tilte restrained cOlllpre841;ive I!Itrength, obtained by tel!l;ting 1o'ith flat jade in aitll.

The magnitude of coefi"icient K dependa mainly on the reatraining condition of masonry bet1o'eelll t1o'o flat j 'acka. Beeides that a slze effect is. als:o reflectecl! iIlI cerlain degree. The svrf"ace area of nat jackl!r is 240X240 mm 1o'hich ia smaller tban area of standard specimen 240x 370 mm.

Aceordilllg to the statistical analyl!til!t on the 24 cOlllparative test data, coerfícient K caJIi be formlllated as rouove (figure 5, formula (1):

Ae mentiollJed above. ou the' (fIM! bancl, the s 'trel!ll!!' tramlfer reduces the coapreesion strees of specimen in 1o'all, this, resultl!l in an enhancement of' c()ç.resaive strength aDd eLacIr rel!liatance. On. the other band. thia part of maaonry is under a biaxial combined stress state 1o'ith a compression in one direction anti a tension in the other. redueing strength of masonry.

It i5 obvious that at the cross section I-I the streases 6 and 6 are 01' the same magnitude and direction of the principal atress~1!I; 6

1 Jd

6~ respectively. It has been indicated in referenee (2) that in certain ránge the strength of lIasonry 1n principal stress rtta7 be deterained as for brittle material 1o'ithout consideration of anisotropy. Under combined suess state brick maaonry stnl have plastic failure behaviour. For tbe biaxial stress probl_ 1o'itb a cOll!pre&Bion in one direction and a tension in tbe other, in reference (2) the following failure equation ls given:

lfuere

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j6~ + 6 ~ - 6163 = a + b·,ue+ C'Ó16/(<51 - ( 3)2(1 - Me ) (2)

!-l e = Lode parameter; fle= -2 o: /(61 - 6 ) - 1; for 61> 6 , a, b and c are constants de t .etmined ac60rding to the un!axial strength in compression, tension and shear respectively.

Tests have show!!: that before cracking the behaviour of Ilasonry wall appears to be of elastic charaeter. Elastie finite element analysis shows tbat lhe central point of wall apeei_n il!! the point of maxi_ principal teD!!l'ion stress. With the ratio of principal stresl!res at this point known, tbe cracking principal stresses for biaxial eotllp·resl!rion and tension stress state can be found for corresponding craeking load.

A comparison of failure elW'elope of brick 118.sonry for biaxial cOllpression and telll8ion stresB state, calculated fro. fo·rmula (2) showa a good açeell!ent witb the test resulta of craeking load, as indicated in fig1ll!'e 6.

o;/f

• e~ rp ~. i. 1°·1 ~/f 1.0 0.8 0.6 0.4 0.2 o

Figure 6. Camparilsolm 01 failure envelope calc1!l.lated by formula (2) with test results.

It ia ass1!lllred for calclIlation that a failure at point A, locatiou of which is shown in figure. 2, represents the failure o·f wbole walI speei_n. According to the principal stresses at point A failure 10ad can be determi ned using a failure enYelope for biaxial compression and teDSion. stress state, obtained by form1l1a (2). There i5 a general siaílarity of these ana]ytical faiIare loads to the test resulta.

Tbe following tranaf'onaatlon coef'ficienta are obtained related to additional atreasea 60 :

60 (MP a) K

O 1.44 2 1.51

3.5 1.55

COllCLUS IOX

1. Dete.rmi.Dation of coapre.ssive strengtb of briek DaEOnry in I!ritu by lISing flat jaclca is a reliable and simple approach.

2. Size of flat. jaclc , widths of adjaeent wa11. parta anel the distanee between jacks are the l18.in influenee !aeto·rs on aasonry I!rtren~h. It is preferable to take the dis1anee between jaekl!r twice the width o f

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jack, amd the width of adjacent waII parts no Iess than the di~tance betweeJ]) jacks.

3. Tbe restrained cocpressive stren'gth of brick masonry f·. determined by flat jaclits in sitll is greater than the cOlIPresaiye strength of standard speclmen f. A transformation coefficient K = ft If should be introduced. Coefficient K can be determined by formula (1).

1. Rossi, P.P., AmialJ'sis of Mechanical Characteristic of Brick Masonry Tested by Means of Non-Destructiye in Situ Test, The Proceeding!! of 6th IBMAC, Rome, 1982.

2. Wang Qinglin, Yi Wenzong and Cherr Huiyi, The Investigation of Shear Strength in COlllbined stresses, ~ Masonn Structure, 1986.1.

3. WaJ])g Jichuan, Measure.ent and Analysis of Mechanical Characteristic5 of Brick MaSODr7 bJ' Means of Non- Destructive in-situ Test, J. of Hunan Uniyersitl, Aug. 1986. -- --