COMPRESSIVE BEHAVIOUR OF MASONRY WITH LONGITUDINAL HOLES IN BED JOINTS

Transcription

1 PUSOBENÍ ZDVA S PODÉLNYM DUTNAM V LO~NYCH PR ZATÍ~ENÍ TLAKEM SPÁRÁCH COMPRESSVE BEHAVOUR OF MASONRY WTH LONGTUDNAL HOLES N BED JONTS D. PUME CSc., Leadng Scentfc Worker Buldng Research nsttute, Techncal Unversty, prague, Czechoslovaka SOUHRN predmetem vysetrování bylo chování jednovrstvych zdenych sten, tlustych 365 az 400 rnrn, ze svsle derovanych chel / rnrn/, jak s podélnym dutnam v loznych spárách, tak s plnyrn spárarn, pr zatízení tlakem. Bylo prokázáno, ze vlv podélnych dutn je znacny a ze je nutno k nemu prhlízet prl sestavování vztahu pro vypocet zdenych sten. Predkládá se téz jednoduchy matematcky model porusení jednovrstvé chelné steny pr zatízení tlakem. SUMMARY The compressve behavour of sngle wythe brck walls, from 365 to 400 rnrn thck, bult from vertcally cored brcks / rnrn/, both wth the longtudnal holes n horzontal /bed/ jonts and wth full jonts have been studed. The effect of longtudnal holes on compressve behavour should be consdered n the desgn formulas of brck walls. A smple mathematcal model of brck wall falure s proposed. 1. NTRODUCTON New types of brck walls have been worked out n Czechoslovaka n order to meet recently revsed requrements [1] relatng the thermal nsulaton parameters of exteror walls. The desgn outsde wnter temperature s C, the oblgatory exte- 2-1 ror wall thermal resstance s 0,95 m KW. 677

2 240 Fg 1 Brck CD-NA Sngle wythe brck walls, from 365 to 400 mm thck, bult from vertcally cored brcks CD-NA /Fg 1/ n header bond, belong to these new structures. rfhe term "sngle wythe" denotes that one brck dmenson s equal to the wall thckness. The horzontal /bed/ mortar jonts are provded wth two longtudnal holes, 50 mm wde each, parallel to wall faces. The holes are placed closer to the exteror wall face than to the nteror one /Fg 2, pllars PN/. The brck densty should be p = 1100 kg m- 3 The brcks are lad close to each other. Due to the groove n one lateral face of each brck, vertc al jonts perpendcular 678

3 to wall faces are flled wth mortar only n the nner part of wall. Masonry walls wth such modfed jonts are supposed to be loadbearng. The necessty of approprate structural research n ths feld has been obvous. 2. LOADNG TESTS ON MASONRY SPECMENS Masonry test specmens /pllars, Fg 2/ were made from the vertcally cored brcks CD-NA /Fg 1/. The mean dmensons / length, wdth, heghtj of brcks were mm. The mean -3 densty of dred brcks was p = 1055 kgm, the mean compressv e strength was R un = 7,60 MPa, var y ng from 5,52 to 9,20 MPa. c The horzontal secton area of sold clay materal n the brck s equal to mm 2, e.. 58 % from the total horzontal brck area / mm 2 /. Table 1 Man test results Test Eccentrcty e Compressve Loadbearng seres about centre strength of capacty of lne of total mortar Rm-r> pllar N b c o s secton /mm/ /MPa/ /kn/ PN-O O 4,3 752, 630, 850 PN-l 80 3,4 556, 606, 606 PN ,0 516, 483, 483 PL-O O 4,1 950,1090, 694 PL-l 80 3,0 468, 606, 450 PL , 1 256, 370, 266 PSA-O O 3,5 575, 470, 675 PSB-O O 3,1 420, 375,

4 PN-O (1,2,3) N PN-1 e=tt PN - 2 e=ot (4,5,6 ) (7, S, 9) l- T T p5 165 T 1_ PL -O (10,11,12) ~ N PL -1 (13,14,15) e=1t r ~ -r- t t PL-2 e=n (16,17, 18) l- T A ; T A PSA -O (19,20,21 ) PSB -O (22,23,24) A-A :) \, \, 1 J Fg 2 Test specrnens 680

5 The mean compressve strength of mortar, u s ed n test spef rom 100 rnrn cubes, was R cmens, deter mned - mr = 3,44 MPa. c All 24 masonry test pllar s /8 seres, each ncludng 3 pllars/ were bult n header bonde The nomnal dmensons of pllars were rnrn. The pllars of the seres PL-O, PL-l and PL-2 /Fg 2/ were provded wth full horzontal /bed/ mortar jonts. The pllars belongng to the remanng 5 seres were bult wth mortar jonts, weakened by two longtudnal cores. The cores were arranged ether symrnetrcally /seres PSA-O and PSB-O, where the wdth of each core was 50 or 95 rnrn, respectvely/ or unsyrnrnetrcally /seres PN-O, PN-l and PN-2, where the wdth of earch core was 50 rnrn/, modellng the actual brck masonry. The fgures O; 1 and 2 n the notatons of test seres denote the eccentrctes e of test loads / e - O; 80 and 130 rnrn, respectvely/ related to the centre of gravty of the total horzontal cross secton area Atot. The pllars were loaded wth monotonously ncreasng vertcal compressve load N up to the loadbearng capacty N b /Table o s 1, Column 4/. The test equpment was provded wth two horzontal cylndrcal jonts /on the top and n the bottom of pllar/ and wth rgd steel dstrbuton plates, the wdth of whch was equal t f = t ' Z - 2e, where t z denotes the thckness of pllar. e p& p& Durng the load test, the changes of the crack pattern, as well as vertcal pllar strans, n the mddle thrd part of heght, were estmated carefully. 3. FALURE PROCESS OF MASONRY UNDER MONOTONOUSLY NCREASNG CONCENTRC COMPRESSON Expermental data obtaned from the loadng tests of pllars PL-O, PSA-O and PSB-O are an acceptable bass for studyng the falure process of masonry under monotonously ncreasng concent r c compresson. The pllars dffer from each other n the values of effectve area Ant ' gven by the equaton 681

6 /1/ where: Atot horzontal total cross secton area of a pllar, A horzontal cross secton area of holes n one mortar bed hz jont. The correlaton analyss [2] of the set of correspondng values /N b.; k A. = A t. : A tot / has proved that the relao s~~ ~~ n ~~ ton between the loadbearng capacty N of pllar and the u rato k wthn the nterval k < 0,48; 1,0 > A A s lnear. t s possble to accept the assumpton that the vertcal compressve stress a nt caused by the test load N s unformly dstrbuted over the area Ant wthn the entre nterval N: Nu < O; 1> Consequently, the stress a nt s determned as /2/ The compressve strength capacty N. u a u corresponds to the loadbearng t was estmated by the tests that wthn the nvestgated nterval k < 0,48; 1,0> there s a cornrnon defnte dependence A of the stress a nt on the vertcal compressve stran e: S /Fg 3/. Therefore, t s justfed to characterze the falure process of masonry by means of a n t. The falure process wthn the nterval a t < O; a > n u should be subdvded nto three sectons : /a/ elasto-plastc secton, /b/ crack development secton and /c/ total falure secton. Elasto-plastc secton s delmted by the stresses a = nt and a k = kta /where k t s wthn the nterval <0,42; 0,58>/. n t u The stresses a t wthn ths secton are almost unformly dstrn t buted over the area Ant. Under the stress a, the frst vsble nt cracks appear mostly on the lateral vertcal faces of a pllar. The mean values of related compressve strans e ms are from -3-3 Y -0, to -0,

7 4,0 3,0 C b 2,0 1,0 cru = 3,83 MPa o 0,1 0,2 0,3 0,4 o mean value (ms ,5 Q6 Fg 3 Stress - stran relaton Crack development secton s vlthn the nterval (7 t < (7tt ; (7 fd t ~ 0, 9 (7 > Prevously developed cracks are wn n n u denng and new cracks are appearng. Al cracks are vertcal. Most of them are 120 mm dstant from east or west face of a pllar. The plane of those cracks s dentcal wth the plane of t he weakened vertcal cross sectons of brcks, ndcated through 683

8 the encrcled fgures 111 n Fg 1 close to the borders of ther parto The crack development causes the ncreasng ununform dstrbuton of vertcal normal stresses. Total falure secton s wthn the short nterval fd G nt < (j nt; (ju >. At the begnnng of ths secton, a pllar s dvded nto two or three slender pllars whch are progressng towards total falure very fast. 4. FALURE PROCESS OF MASONRY UNDER MONOTONOUSLY NCREASNG ECCENTRC COMPRESSON Pllars wth full mortar jonts Falure process can be subdvded nto three sectons, dentcal wth masonry under concentrc compresson [2,3,4]. t Elasto-plastc secton s delm ted by the loads N = O and N, when the frst vsble cracks appear n the vertcal, concdng wth the secton 111 n brcks mentoned abovel n the more stressed zone of the masonry cross secton. The compressve ms t -3 strans e n ths vertcal at N are from -0,25.10 to -0, YldentcallY wth the concentrc compresson/. Wthn the crack development secton, there are appearng new cracks over the whole heght of the pllar. The mostly wdenng!'man" cracks are n the less stressed zone of the masonry cross secton. The total thckness of two lor threel partcular small pllars, wthstandng the test load, corresponds to the wdth t of loadng steel plates. ef Total falure secton dffers from the same secton under fd > concentrc compresson only n the va 1 ue N = O ~ 95 N b. o s Pllars wth longtudnal holes n mortat jonts Falure process s subdvded also nto three sectons. The poston of so called man crack or cracks strongly depends on the poston of longtudnal holes, rrespectve of vertcal sectons 111 n brcks. Consequently, the horzontal areas of 684

9 small pllars wthstandng the loads N ~ N f d are ether larger or smaller than the correspondng areas n pllars wth full jonts. Ths feature nfluences the values N b t should be o s taken nto account n the desgn formulas of masonry wth longtudnal holes n mortar jonts [2,3,4] 5. MATHEMATCAL MODEL OF SNGLE v-ly THE BRCK WALL FALURE UNDER VERTCAL COMPRESSON The proposed mathematcal model of sngle wythe brck wall falure under vertcal compresson s based on the assumptons concernng both the behavour of actual masonry specmens paragraphs a, b, c, lsted belowl and the stress-stran relatons of materals used ~] lal Vertcal cracks on both lateral sdes of a pllar ln wall smlarly, to 01 start to extend over ts total heght, when the vertcal normal stress attans the value a ~~. These cracks, called as "ma n", are the man characterstc of the falure. Ther presence ndcates that the tensle strength of brcks n the horzontal drecton has been reached. bl The preceedng expermental nvestgatons showed that the horzontal dstance between the captal vertcal crack and the front surface of a pllar s equal to 0,25 tll 0,4 multple of the wall thckness brck length/. Ths one occurs even when brcks are prov ded wth a central hole the dmensons of whch are a pprox mately mm. The captal crack s located n the plane that s parallel wth the pllar walll front surface and corresponds to the most weakened sectons of brcks. n the brcks under consderaton, that secton s ndcated by the encrcled fgure 111. cl The horzontal normal tensle stresses n the brcks consst of two components, the frst one owng to the stress state of brcks wthout any bond to the horzontal mortar layers, the second one owng to the compatblty between brcks and mortar bed jonts. The actual stran condtons can be explaned 685

10 n the followng way. lf n masonry walls under axal compresson no bond frcton exsted n the nterfaces between brcks and mortar bed jonts, the unr2straned lateral stran E mr of the x mortar bed jonts would exceed that of brck EC. As a consex quence of the exstng bond and frcton, addtonal lateral cn mrn tensle stran E n brcks and compressve stran E n morx x tar bed jonts are arsng. crack ~4~ At EC E CN E;rN x x o.. E~r - x Fg 4 Compatblty condton The compatblty condton of horzontal /lateral/ strans, based on fve assumptons mentoned above, at the pont A n the contact nterface between brck and mortar bed jont /Fg 4/ can be wrtten as mr E X + E X /3/ More detaled explanaton ncludng numercal examples s presented n [2,3,4]. mrn ACKNOWLEDGEMENT The author expresses hs thanks to Research lnsttute for Constructon n Veselí nad Luzncí for the conducton of tests and to the General Drectory of Czechoslovak Brck lndustry for the knd support of ths nvestgaton. CONCLUSlONS The effect of longtudnal holes /ther secton area and 686

11 poston/ n bed jonts on sngle wythe brck masonry falure mechansm s consderable. t should be taken nto account n the desgn formulas concernng the load bearng capacty of masonry pllars and walls. The obtaned expermental results have allowed to propose a smple mathematcal model of brck wall falure. REFERENCES /1/ CSN Thermo-techncal propertes of engneerng structure and buldngs. Nomenclature. Requrements and crterons, /n Czech/. Czechoslovak Code. prague 1977 /2/ Pume, D.: Mathematcal model on compressve falure mechansm of brck masonry /n Czech/. Stavební vyzkum, 1983,4 /3/ Pume, D.: Falure mechansm of masonry bult from the new types of vertcally cored brcks under eccentrc compresson /n Czech/. Stavební vyzkum, 1983,5 /4/ Pume, D.: Compressve load bearng capacty of masonry structures bult from vertcally cored brcks wth unsyrnrnetr~ cal longtudnal holes n bed jonts /n Czech/, Stavební vyzkum, 1983,6 687

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