538

RACKING TESTS ON REINFORCED MVD PRESTRESSED llOLLOV CLAY MASONRY VALLS

ADRIAN W. PAGE Senior Lecturer, Department of Civil Engineering and Surveying

The University of Newcastle, N.S.W., 2308, Australia

and

ARIE nurzER Senior Instructor, Dep~rtment of Civil Engineering

The University of Calgary, Calgary, Alberta, Canada

ABSTRACT

An experimental study of the comparative ability of reinforced and prestressed hollow clay masonry walls to resist racking loads is described. Three walls, 3000 mm high x 2500 mm long x 200 mm thick, were constructed from hollow clay units and Type S mortar. One wall was vertically reinforced and fully grouted. The second and third walls were left ungrouted, but post- tens ioned wi th high strength Dywidag rods, one wall with prestress in the vertical direction only, the other with prestress applied both horizontally and vertically. Each wall was loaded monotonically with a horizontal racking force applied to a top corner of the wall. The overall response of each wall was moni tored as well as surface strains in criticaI locations and progressive cracking. The resulcs of the tests are reported, and the performance of the three wall types compared. It is shown that post- tensioning is potentially an extremely effective method of increasing the shear capacity of a wall.

INTRODUCTION

Masonry walls subjected to in-plane racking loads usually fail by diagonal cracking resulting from tensile stresses produced by the horizontal shear force. Tensile cracking also occurs at the heel of the wall. The ability of a shear wall to resist racking loads should therefore be enhanced if measures are taken to either reinforce or prestress the wall to increase its tensile capacity. This could be done either during construction or sometime later in the life of t he structure. Previous tests at the Uni versi ty of Calgary on post- tensioned hollow masonry walls constructed from both standard and special blocks have demonstrated the viabi 1 i ty of this concept (1-4) .

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539

In this paper, a further series of tests is described. Three walls constructed from hollow clay masonry with varying degrees of either prestressing or reinforcing were subjected to racking loads. One wall was vertically reinforced and fully grouted with bond beams top and bottom. The second and third walls were left ungrouted, but post- tensioned wi th high strength Dywidag rods, one wall with prestress in the vertical direct ion only, the other wi th prestress appl ied both horizontally and vertically. AlI the walls were loaded in shear and their performance compared .

EXPERIMENTAL PROGRAMME

Masonry:

The walls were constructed in running bond using 200 "Monarch" hollow clay units (nominal dimensions 400 mm long x 100 mm high x 200 mOI thick) and Type S mortar (1: 1/4:3 cement: lime: sand by volume). The mortar was pre-batched by the supplier and delivered in a dry state ensuring constant mix proportions. Water was added to achieve a flow of approximately 120%. The walls were buil t in the laboratory by an experienced mason using normal construction procedures. Face- shell bedding was used throughout. The masonry had a nominal prism strength of 23.7 MPa (see Table 1). Each wall was 29 courses high x 6 units long (approximately 3000 mm x 2500 mm).

Three walls (A, fi and C) were constructed as shown schemat ically in Figure 1. Walls A and B were ungrouted but constructed with 15 mm diameter high strength Dywidag rods inserted vertically down the cores of the hollow uni ts and embedded horizontally in small recesses cut in the units immediately below a bed joint. Wall C also contained Dywidag rods, but these were left unstressed to act as normal reinforcing. AlI cores of wall C were filled with a medium strength grout (mean cylinder strength 27.5 MPa - see Table 1). For ease of handling, each wall was constructed on a prefabricated concrete base with alI vertical rods being attached to starter bars us ing standard couplers. AlI walls were air cured in the laboratory.

*

TABLE 1 PROPERTIES DF MASONRY AND GROU!'

DESCRIPTION

2 High Masonry * Prisrn

75 rnm Grout Cylinder, 150 mm High

NO. DF SPECIMENS

3

6

MEAN STRENGTH

MPa

23.7

27.5

COEFFICIENT DF VARIATION

157.

Prisms tested in face·shell bedding with fibre board strips as capping. Compressi ve strength calculated using a nominal face- shell bedded area

of 27300 mm2. Prism and grout tested at 28 days .

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Rond Reams:

To simulate common practice, bond beams were included in both the top and bottom of alI wall s. For "alI C the bond beams were formed by the inclusion of 2/15 mm diameter un stressed Dywidag rods in the first and tIJenty eighth courses before the grouting of the wall . For ",alls A and R, ungrouted bond beams ",ere formed using horizontal post-tensioned rods as shown in Figure 1.

õ u

o "' '" N

r 2425 UIl'.icoll

WALL A

(stressed 8: ungrouted)

l - - - - : stressed rods

_._. - : unstressed rods

65 130 130 65

~--,- ---t --f.J25 1 1"=;:=1 1 1 ,-:::r:::=c 1 I , I I 1

, ,I , , I' I I I' I I 'I , , " li- --I=--i --th~

J >i, '" "H ,l WALL B

(stressed 8 ungroated)

i~! i I i i . .

I I

WALL C

(unslressed a lull1 grouled)

'* Nominal initiol prestress i nQ force (KN)

Figure 1. Wall Details

Post-Tensioning:

líalls A and B were post- tensioned at an age of 3 months. The individual bar forces are shown in Figure 1. Prestressing losses ",ere monitored for the ensuing 7 months before testing by means of resistance strain gauges attached to each rod, with total losses ranging from 3% to 11%. The final average leveIs of prestress ",ere approximately 2 MPa in the vertical direction for walls A and R and 1 MPa in the horizontal direction for the central region of ",alI A (based on the face-she ll bedded area). Although these leveIs of prestress are not high, care must be taken in transferri.ng the prestressing fo rce to the hollo", masonry to avoid the possibility of local ",eb cracking. This mechanism has been discussed else",here (5). It is signifi cant that in stressing the end vertical bars in wall A, a load of approximately 130 kN ",as inadvertently applied, and t hen reduced to the fi nal l eve I of 65 kN. During this operation, several internaI cracks ",ere heard to occur, presumably in the webs of the uni ts near the load ing point. This had important implications for the subsequent racking test fo r this wall. ..

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Racking Tests:

Each wall was subjected to a racking load as shown in Figures 2 and 3. The load was applied to the top courses of the masonry using a spherically seated hydraulic jack acting on a steel bearing plate. The load was applied monotonically to failure and moni tored by means of a load cell located between the jack and the reaction frame. To prevent rotation of the wall and its supporting beam about the toe , the concrete beam was anchored to the reaction floor near the heel of the wall.

During the test the overall deformations of the wall as well as surface strains were monitored. The arrangement of the instrumentation is shown in Figure 2. Racking deflections were monitored by means of linearly varying displacement transformers (LVDT ' s) mounted on an independent frame in positions #1 to #4. Wall-base separation and lifting of the loaded corner were monitored by means of LVDT's in positions #5 and #6. Surface strains at the centre and heel of the wall were measured over long gauge lengths using linear potentiometric displacement transducers in corresponding positions on both sides of the wall. Long gauge lengths were used in an attempt to encompass potential cracking planes. The output from the instrumentation was fed directly into a data acquisition system allowing displacements to be plotted during the test and progressive cracking to be readily observed.

RESULTS

A summary of the wall- base separat ion load at the heel of the wall and ultimate load for each wall is given in Table 2. A plot of racking load versus racking deflection is shown in Figure 4.

TAllLE 2 Summary of Failure Loads

WALL WALL-llASE SEPARATION LOAD

kN

A 30

B 50

c o (zero prestress)

ULTIMATE LOAD

kN

146

175

115

FAILURE MO DE AND REMARKS

Premature local web splitting near point of load application

Diagonal tension failure

Diagonal tension failure

\ / \ /

.--~--. /\

/ \ • •

Shoin Rosette (lOOOmm Qouqt: length)

Concrete Base

542

..

EfJ .,..

•

..

+

Figure 3.

Figure 2 . Schematic Testing Arrangement

Testing Arrangement

180

150

z '" 100 .. ~ o ....

co co

:;;: · U o

50 o::

o 5

-

10

Racking

543

15

DefleClion

20

(mm)

25 30 35

Figure 4. Racking Load -vs- Racking Deflection at Position 4

Figure S. Failure of Wall A in the Region of the Racking Load

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Wall A (Vertically and Horizontally Prestressed) :

As the racking load was increased, rotation of the wall occurred about its toe, and wall- base separation occurred at position #6 at a load of 30 kN once the induced tensile stress exceeded the level of vertical prestress . This was followed by horizontal tensile cracking in the heel region at a load of approximately 90 kN. At a load of 146 kN sudden local failure occurred in the region adjacent to the racking load, caused by splitting of the web of the hollow units as shown in Figure 5. This mode of failure is typical for face-shell bedded hollow masonry subjected to concentrated loads (5), and in this case was caused by the local effects of the vertical and horizontal prestressing combined with the horizontal racking load. As mentioned previously, it was also likely that webs in this region had already been cracked by initial local overstressing during post-tensioning. To avoid problems of this nature for wall B, the size of the bearing plate beneath the racking load was increased.

It is apparent from Figure 4 that the performance of wall A up until its premature failure was very similar to that of wall B, with the shear stiffness of the two walls being almost identical. This is not surprising since there was no evidence of diagonal tension cracking in the wall before its premature failure. It is therefore reasonable to assume that if premature local failure had not occurred, the racking behaviour of wall A would have been similar to wall B, with a marked change in the load- deflection curve with the onset of diagonal cracking. The load at which this change would have occurred can be estimated if the masonry in wall A is assumed to have the same diagonal tensile strength to that in wall B (as calculated in Appendix A) . If allowance is made for the influence of the additional prestress, diagonal cracking would have been expected to occur at a racking load in the order of 330 kN, a significant increase on the ult imate load for wall B. I t appears therefore that considerable benefits can be gained by the use of horizontal as well as vertical prestressing.

Wall B (Vertically Prestressed):

In this case wall-base separation at the heel occurred at a racking load of 50 kN. This was followed by horizontal tensile cracking in the heel region at a load of 100 kN. No further evidence of distress was observed up to a load of 170 kN when diagonal cracking occurred in the central region of the wall. Upon further loading, the cracking gradually propagated down the diagonal and eventually extended into the toe region as shown in Figure 6. Diagonal cracking was accompanied by large increases in racking deflections as can be seen from Figure 4. The failure was ductile in nature with no sudden drop in the racking load. The maximum load observed was 175 kN.

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Figure 6.

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•

Diagonal Cracking for Wall B (Wall C similar)

Wall C (Reinforced and Grouted):

The performance of the reinforced wall was different in several respects to the post-tensioned walls. Due to the absence of prestress, wall-base separation occurred at the heel as soon as the racking load was applied. This was almost immediately followed by horizontal tensile cracking of the masonry in the heel region at a load of 20 kN. Diagonal cracking commenced in the central re~ion of the wall at relatively low loads in the order of 50 kN (this is In marked contrast to wall B when the f irst diagonal crack occurred at 977. of the ultimate load) . Final failure occurred as the result of extensile diagonal cracking at an ultimate load of 115 kN in a mode similar to that shown for wall B in Figure 6. The influence of the substantial progressive cracking can be seen in the load-deflection curve shown in Figure 4. Once cracking in the heel region occurs at low loads, the shear stiffness of the reinforced wall is approximately 30% lower than i ts prestressed counterpart. As for the prestressed wall, the reinforced wall exhibited ductile behaviour, with much smoother post-cracking behaviour due to the differing nature of the cracking for the grouted wall. It is obvious that prestressing can

546

substantia11y increase the racking capaci ty of a shear wa11. In this case, moderate amounts of prestress increased the shear capacity by 52% for wa11 B and by an estimated 190% for wa11 A when both horizontal and vertical prestress were present.

CONCLUSIONS

This investigation has demonstrated the potential benefits that can be gained by the use of post-tensioning for walls subjected to racking loads. Comparison of the performance of the prestressed and reinforced wa11s showed that substantial increases in racking strength and shear stiffness were obtained by the use of prestressing. The potential benefits appear to be even more substantial if horizontal as well as vertical prestressing is used to suppress the formation of diagonal tensile cracks in the central region of the wall.

The technique poses no significant practical disadvantages, as the post- tensioning rods can be easily incorporated in the ho11ow masonry during construction a110wing both horizontal and vertical prestressing, and avoiding the need for infill grout. Only moderate levels of prestressing are required, but even with relatively 101.' jacking forces, consideration must be given to the transfer of load to the ho11ow masonry in the region of the anchorage to avoid possible splitting of the web of the masonry units. \~ork on the use of prestressing in shear wa11s is continuing.

ACKNOHLEDGEMENTS

The masonry units for this project were supplied by IXL Industries Ltd. , Medicine Hat, Alberta. The assistance of the technical staff of the Department of Civil Engineering, University of Calgary, particularly Mr. D. Ti11eman, is gratefu11y acknowledged. The financial support of the Natural Sciences and Engineering Research Council of Canada is also acknowledged.

REFERENCES

1. Huizer, A. and Loov, R.E., "The Strength of Stack-Bonded Concrete Block Masonry Panels Post-Tensioned in Both the Vertical and Horizontal Directions", Cln lnternational Council for Building Research, Studies and Documentation, Symposium on Hall Structures, Warsaw, June, 1984.

2. Huizer, A. and Shr i ve, N. G., "Loss of Prestress in a Post- Tensioned Masonry Wa11" , Internat ional Seminar on Reinforced and Prestressed Masonry, Edinburgh, August, 1984, pp. 381- 391.

3. Huizer, A. and Loov, R. E. , "Post- Tens ioning of a S ingle- Wythe Clay Brick Masonry Wa11s" , Proc. of 7th IBMAC, Melbourne, February, 1985, pp. 993-1000.

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547

4. Huizer, A. and Shrive, N.G ., "Performance of a Post-Tensioned, Single-Wythe Clay Ilrick Masonry lIa11 Tested in Shear", Proc. of 4th Canadian Masonry SVIll))osium, Fredericton, June, 1986, pp. 612-621.

5. Page, A.W., Shri ve, N.G. and Jessop, E.L., "Concentrated Loads on Ho11ol, Masonry - A Pilot Study", Masonrv International, Vol. 1, No. 2, 1987, pp. 58-61.

APPENDIX A ESTIMATE OF RACKING LOAD TO PRODUCE DI AGONAL CRACKING

IN lvALL A

Consider wa11 Il:

o y

average vertical prestress at mid-height 2.14 MPa

T average shear stress at mid-height

f ace- sheIl area (2425 x 35 x 2) P (kN) a' 2.14

~2õ2':-----'- Y l

Consider element at mid-height:

--~tO~ .. -_ax • O

t T' O.0059P

T = 0.0059 P MPa

o + O X Y ±

2 [

O - O 2 " (X y) 2] + T

(1)

For a diagonal cracking load (P) of 170 kN, 0 1 0.40 MPa

Consider wall A:

Assume principal tensile stress, 0 1 = 0.40 MPa

O average vertical prestress at mid-height = 2.24 MPa y

0x average horizontal prestress at mid-height = 1.01 MPa

T 0.0059 P MPa

Substitute in equation (1) and solve for the estimated racking load P:

P = 329 kN