11th INTERNATIONAL BRICKJBLOCK MASONRY CONFERENCE

TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

BEHA VIOUR OF MASONRY INFILLED W ALLS

C.K. Seahl, Y. Liul and J.L. Dawe2

1. ABSTRACT

The beneficial effect of masoruy infilled panels in framed structures has been well documented in research publications in the last four decades. Generally, the masoruy panels provide adequate stiffiless to an otherwise flexible frame system. In a reciprocal manner, the enclosing frame contains the brittle masoruy panels and provides ductility. Consequently, after ctacking, the masoruy panels are capable of sustaining displacernents and loads much higher than could be achieved without the frame.

Despite a reasonably large arnount of available information, there is no commonly accepted method for the design of masoruy infillcd 1hu!te systems. This paper summarises results of experimental and theoretical studies conducted to date and highlights some procedures that may be adopted by design engineers. Both in-plane and out-of-plane behaviour of infilled panels is considered.

2. INTRODUCTION

In addition to funcnoning as interior partitions and external cladding, panels infilled in frames may also serve, either intentionally or inadvertently, as bracing to provide lateral stability When building regulations were less stringent in the 1940's, many hollow clay tile masonry infilled frames were constructed in the United States. The frame was usually designed for

Keywords: Masonry infilled panels, Frame, Behaviour.

IGraduate student, Department of Civil Engineering, University of New Brunswick, Fredericton, N.B., E3B 5A3, Canada.

i>rofessor, Department of Civil Engineering, University ofNew Brunswick, Fredericton, N.B., E3B 5A3, Canada.

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gravity loading whilst the infill, whose thiekness was specified to meet tire and acoustie requirements, was presumed adequate in conjunction With the trame to resist horizontal loading. Laboratory tests performed over the years have confirmed that the masonry infill, with high in-plane rigidity, ean stiffen an otherwise flexible trame while the ductile surrounding trame confines the masonry after eraeking and allows the infill to resist loads and displacements mueh larger than those for a pane! without a trame. When subjected to out-of-plane loading, the confinement of the trame can result in arehing action of the infill thereby significantly inereasing its out-of-plane strength and post­craeking ductility.

TIris paper provides a brief summary of the research performed in the last forty years on infill trames and highlights some of the practical applications that may be adopted by design engineers.

3. HISTORICAL DEVELOPMENT

The first published researeh on infilled frames subjected to racking load was by Polyakov (1). The publication reported on a test program carried out from 1948 to 1953 whieh included testing of small specimens to evaluate the tensile and shear strength of masonry used as infills. To deternúne the racking strength of infilled frames, Polyakov perfonned 65 large-scale tests; thirty-two of these consisted of square frames with inside dimensions of 1200 mm while other test specimens consisted of rectangular frames with an inside width and height of 3000 mm and 2000 mm, respectively. Parameters investigated by Polyakov included effects ofthe type ofrnasonry units, mortar mixes, admixtures, methods ofload application (monotonie or cyclie), and walls with openings.

Polyakov described three stages of infill frame behaviour subjected to racking. In the first stage, the masonry infilling and the structural frame members behaved as a monolithic unit. TIris stage ended when Separation cracks between the infill and the frame began to develop. These separation cracks were noted around the perimeter of the infill-to-frame interface except for small regions where the compressive stresses were traTlSmitted trom the frame to the infill at two diagonally opposite comers. The second stage was characterised by a shortening of the compression diagonal and lengthening of the tension diagonal. TIris stage ended with cracking ofthe masonry infill along the compression diagonal. The cracks usually appeared in a step-wise manner through mortar head joints and bed joints.

In the third stage, the structural assemblage continued to resist an increasing load in spite of the diagonal crack. Existing diagonal craeks continued to widen and new eracks appeared . . This is considered to be the final stage since the system has no practical value once large craeks appear.

In a subsequent paper, Polyakov (2) described experiments performed on a three-bay, three­storey mode! steel frame infilled with masonry. The dimensions of the infills were 1200 mm x 1200 mm. Based on observations of the infill boundary separation, he suggested that the infilled frame system was equivalent to a frame with a compression diagonal replacing the filler pane!.

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4. RESEARCH IN THE 1960's and 1970's

Subsequent to Polyakov's investigations, a great deal of experimental work of tlús kind was carried out both in fuIl-scale and mode1 tests by Hohnes (3), Stafford-Smith [(4), (5), (6), (7)], Mainstone (8), and Liauw and l..ee (9). AlI these works advocate the use ofthe diagonal strut approach for predicting the behaviour of infiIled multi-storey frames subjected to in­plane Iateralloads.

Investigations by BerJjamin and Wtlliams (10) were aimed at predicting the behaviour of one­storey brick and concrete shear waIIs. They concluded that both concrete and masomy are too variabIe to mode1 mathernatically. A1tematively, they based their stiffuess prediction on a strength of materials approach by considering shear and bending deformations.

Holmes (3) proposed a method for predicting the deformation and strength of an infiIled frame based on the equivalent diagonal strut concept. He assumed that the diagonal strut was the same thlckness and had the same elastic modulus as the infiIl with a width equal to one­third the diagonal length. The deformation was then predicted by an elastic analysis of the equivalent braced frame structure. Later, Hohnes (11) proposed semi-empirical methods to predict the behaviour of infiIled frames subjected to lateral and gravity loadings. Tests on model steel frames with concrete infiIling were described.

Stafford-Smith's investigation into infiIl frame behaviour was also based on the equivalent diagonal strut concept. He performed a series of tests on infiIled frames and found that the length of contact, which extends from the loaded comers, was related to the relative stiffuess ofthe frame and the infiIl. The length ofbearing of each story-height column on the adjacent infill was govemed by the relative stiffuess of the column· and the infiIl. As a crude approximation, an analogy was drawn with the beam on e1astic foundation theory (Hetenyi, 12) from which the length ofbearing, is estimated by:

where

1t IX =-

2Ã

_ ( E",t ) v. Ã - --

4Elh

. in whic.'t E", and t are the elastic modulus and thlckness of the infiIl, respectively. EI and h are, respectively, the columnrigidity and column height.

Later, Stafford-Smith and Carter (13) investigated various modes ofinfill and frame failure and proposed a method to analyse the infiIled frame. They concluded that the lateral in -plane stiffuess of an infiIled frame may be obtained by statically analysing the equivalent pin-jointed frame in which the infiIl was replaced by an equivalent diagonal strut. They also found that the effective width of an infiIl acting as a diagonal strut was influenced by a) the relative stiffuess of the infiII and the frame, b) the length-to-height ratio of the infiIl, c) the stress-strain relationship ofthe infiIl material, and d) the magnitude ofthe diagonalload on the infiIl. As a

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refinement to HoImes' method, they presented a series of charts to detennine the effective width ofthe equivalent diagonal based on the above parameter.

5. EV ALUATION OF INFILLED FRAME BEHA VIOUR USING FINITE ELEMENT METHOD

Karamanski (14) used the technique offinite element anaIysis to solve the problem of inflUed frames. Ris method gives better results than the other because it satisfied the boundary conditions exactly. However, he adopted the assumption that the frame carries only axial stresses and is infinitely flexible in the direction perpendicular to the frame member axes. This, to some extent, undermined the advantage of bis technique. In addition, bis method cannot allow separation of infill and frame wbich further limits the validity ofbis technique to integral inflUed frames only.

The advancement in the finite element technique for structural analysis has prompted many researchers to use tbis method to examine the complex behaviour of infiUed frames. Quite notable among these were Mallick and Sevem (15), Riddington and Stafford-Smith (16), King and Pandey (17), and Liauw and Kwan (18). Mallick and Sevem (15) introduced an iterative technique whereby- the points of separation between the frame and inflU, as well as the stress distribution along the length of contact between frame and infill, were obtained as an integral part of the solution. Slip between the frame and infiU was also taken into account. Standard beam elements were used to model the frame while plane stress rectangular elements were used for the infiU. The contact problem was solved by initially assuming nodes on the infill and the frame to have the same displacement. Having determined the nodal displacement, the load along the periphery of the infiU was determined and checked for tension. If a tension force was found, separation was assumed to occur and the corresponding nodes on the frame and the infill were allowed to move independently in the next iteration. TQis procedure is repeated until a prescribed tolerance for convergence is acbieved. The effect of slip and interface frietion was considered by introducing shear forces along the length of contact. However, the authors ignored the axial deformation of columns in their formulation.

Barau and Mallick (19) used finite element analysis to analyse infill frames and with a technique similar to the method proposed by Sachanski (20) except that a finite element technique was used to determine stiflhess coefficients of the boundary nodes on the infill. Unlike Sachanski, Barau and MaIlick allowed for the separation between infill and frame and incIuded the effect of slip.

Riddington and Stafford-Smith (16) conducted an extensive series of plane stress finite element anaIyses and showed that criticaI stresses relating to shear failure and tension failure occur at the centre ofthe infill. Based on results oftheir anaIysis, empiricaI equations suitable for practicaI design were given. The applications of these equations in infill frame design taking into consideration the relevant allowable stresses given by current codes and standards are illustrated by Stafford-Smith and Coull (21).

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6. RESEARCH IN THE 1980's

Wrth a few exceptions, most experimental work described previously was based on scale models. These tests provided an insight into the behaviour of infiIled frame systems but the unanswered question of scale effect had prevented the widespread adoption of the assortmem of design recommendations resulting from those works. Large scale tests on stee1 frames infilled with concrete masonry infiIl were conducted by McBride (22), Yong (23), Amos (24), and Richardson (25) at the University ofNew Brunswick. Ofthe parameters investigated by these researchers, interface conditions between panel edges and frame wcre found to significantly affect the strength and behaviour of the system. Column-to-panel ties were found to be ineffective in increasing ultimate strength while initial stifihess was only marginally increased. A 20 mm gap between the upper edge ofthe panel and the roofheam was particularíy detrimental to the system in-plane shear capacity. Tests of specimens with panel openings have shown that, while openings may reduce initial stiffuess and first crack load, the same was not necessarily true for their flffects on ultimate strength. Placing reinforced bond beams at one-third and two-third panel heights was found to bring the major aack load close to the ultimate, which itself was only margina1ly increased. Strengthening the compression diagonaIs by grouting vertical reinforcing bars oflengths equal to the expected compression diagonal width into the cells ofthe concrete block panel resulted in only minor increases in stiffuess and ultimate strength. A detailed summary of the above work has been presented by Dawe and Seah (26).

Despite the wea1th ofinformation available, most codes and standards do not have provisions for the design 01' infilled frames. This has severely restricted their use as bracing. It has been more usual when designing an infiIled frame struc1.ure, to aminge the frame to resist gravity as well as latera110ading and to include the infiIl on the assumption that if precautions are taken to avoid load being transferred to them, they ~ not participate as part of the load resisting structure. A variety of construction techniques have resulted from this concept, the most notable of which is to tie the infill to the columns while specifYing a gap at the roof bearn-to-panel interface to allow the roofbeam to deform freely without imposing gravity load on the infiIl. Tests performed by Riddington (27) and Richardson (25) have confirmed that such an approach is not always valido While the gap results in reduction in stiffuess and strength, the infiIl still participates in resisting a portion of the applied horizontalload.

7. RECENT RESEARCH

In the wake of recent earthquake activity in the United States, studies on infiIled frames have once again been thrust into the forefront ofmasonry research. Following the October 1989 Loma Prieta earthquake, a survey of downtown Oakland revea1ed that most of the city's older unreffiforced masonry buildings survived with little damage (Langenbach, 28). Most of these buildings consisted of steel or reinforced concrete frames with hollow c1ay tile masonry infiIl. Following the earthquake" the city of Oakland enacted a Darnaged Building Repairs Ordinance which prevents building owners from repairing the darnage, Any building which has lost over l00!o of its pre-earthquake lateral strength must be upgraded to a slightly modified version ofthe 1988Uniform Building Code (29), The cost ofthe upgrade is often enormous. Additiona1ly, the lack of a universa1ly accepted design procedure for infiIled frame evaluation has resulted in enormous difficulty in assessing the pre-earthquake strength of these structures anei, if repaired, the capacity of the repaired structures. A special session on

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infilled frames was organised for the 1994 ASCE Structures Congress to gather recent researeh data on the system.

The safety of an Oakridge plant, wbieh was built in the J 94O's to house atoDÚe w:eapons research and development facilities, was investigated by a senatorial safety comnúttee. Buildings in this facility are primarily steel frame with hollow clay tile infilling. The Iack of information on this type of structure has led to a series of tests at the University of Tennessee. F1anagan, Bennett and Barelay (30) conducted a comprehensive test program to evaluate the resistance of steel frames infilled with hollow elay tile masonry.

8. ANAL YTICAL METHODS

Sachanski (20) performed tests on infilled models and prototype frames. Based on test results, he proposed an analytical model in wbich he analysed the contact forces between the frame and the infill by assuming their mutual bond to be replaced by thirty redundant reactions. These were than deternúned by fOrDÚng and solving the equations for the compattbility of displacement of frame and infill. Later, Karamanski (I4) used the technique offinite e1ements to solve the problem ofinfilled frames. Ris method gives better results than the other because it satisfies the boundary conditions exactly. However, he adopted the assumptions that the frame carries only axial stresses and is infinitely flexIble in the direction perpendicular to the frame member axes. This, to some extent, undernúned the advantage of bis technique. The advancement in the finite element technique for structural analysis has prompted many researchers to use this method to examine the complex behaviour of infilled frames. Quite notable among these were Mallick and Sevem (15), Riddington and Stafford­SDÚth (16), King and Pandey (17), and Liauw and Kwan (18). Many difficulties that have arisen in this type of analysis have been solved. Separation between frame and infill upon loading is a complex and variable phenomenon. Among other things involved are friction, bond strength, geometry and the effects of any components that tend to tie frame and infill together. Additionally, the variable material behaviour of the infill has made correlation between analysis and test results difficult.

Commercially available finite element software now has the capability of analysing infilled frame problems with these complex phenomena. However, the mesh size required to obtain satisfactory results has made this approach for solving practical problems probibitively expensive.

9. PLASTIC THEORY

Wood (31) presented a paper deaIing with plasticity, composite action, and collapse design of frames with unreinforced shear panels. He identified four distinct collapse modes based on observations cf full-scale as well as model tests. The first three modes were pred.icted by combining standard plastic theory for frames and an idealized plastic yield criterion for membranes, wbich either crush at a constant yield stress or crack at zero tensile stress. The forth collapse mode involves comer crushing of the infill and a more elaborate theory is needed to pred.ict the extent of crushing. Wood investigated each collapse mode and simplified bis results into a form suitable for practical designo He also suggested penalty

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factors to reduce material strengths based on results of full-sca1e and model tests by other researchers.

An extensive study on the behaviour ofinfilled frames was conducted by Liauw and Kwan [(18), (32), (33), (34)]. The most notable achlevement of their study is perhaps the development of a plastic theory whlch was based on findings from non-linear finite element anaIysis and experimentaI investigarions. Their findÚlgs had also been shown to compare favourably with experimentaI results given by many researchers on small-sca1e model tests. This investigation is on-going and largt>-sca1e model tests and plastic anaIysis of multibay infilled frames have been reported by Kwan and co-workers (35)

10. OUT -OF-PLANE BEHA VIOUR

Out-of-plane behaviour of masonry infilled panels enclosed withln a pin-connected steel frame W8s investigated experimentally and anaIyticaIly by Dawe and Seah (36). Nme large scaIe concrete masonry infilled panels were tested to destruction under uniform air pressure. Four stages ofload-deformation behaviour were identified. Stage I is characterized by linear elastic behaviour prior to initial cracking, while in stage n, propagation of initial cracks and development of a fracture line failure mechanism occurs. In stage m, archlng of the infill confined within the frame causes the load to increase to a maximum above that predicted by standard fracture line anaIysis. In stage N, the load drops off due to gradual crushlng of masonry at the fracture!ines anel, finally, u1timate collapse occurs. NumericaI techniques were developedto predict first crack and u1timate loads due to archlng action ofinfill panels and good correlation between test results and predicted behaviour was achleved.

Studies ofrnasonry infiIls confined within reinforced concrete frames and subjected to out-of­plane loading are currently being undertaken by AbranlS and Angel (37).

11 . CONCLUSION

This 'paper summarises the available experimental and theoreticaI research relating to infilled frames. In some cases, practicaI design applications are available. In general, research into the behaviour ofinfilled frames is incomplete. This is particularly true ofmulti-storey, multi-bay infilled frames. There is a lack of both analyticaI and experimental knowledge and understanding in thls area.

12. REFERENCES

1. Polyakov, S.Y., "Masonry inframed buildings (Godsudarstvenoe Isdatel' stvo Literatury Po Stroidal stvui Archltecture. Moscow, 1956)", Translated by G. L. Caims. National Lending Library for Science and Technology, Boston Spa, Yorkshire, u.K., 1963.

2. Polyakov, S.V. "On the interaction between masonry filler walls and enclosing frame when loaded in the plane of the wall", Earthquake Engineering. Earthquake Engineering Research Institute, San Francisco, CA, 1990, pp. 36-42.

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3. Holmes, M ., " Steel frames with brickwork and concrete infilling", Proceedings, the Institution ofCivil Engineers, Vol. 19, 1961, pp. 473-478.

4. Stafford-Smith, B., "Lateral stiffuess of infilled frames". ASCE Iourna1 of Structural Division, 88(ST6): 183-199, 1962.

5. -ibid. "Behaviour of square infi11ed frames", ASCE Ioumal of the Structural Division, 92 (STl): 381-403, 1966.

6. -ibid. "Methods of predicting the lateral stiffuess and strength of multi-storey infi11ed frames", Building Science, 2 : 247-257, 1967.

7. -ibid. "A method of analysis for infi11ed frames", Proceedings, the Institution of Civil Engineers, 1969, Vol. 44, pp. 31-48.

8. Mainstone, RJ., "On the stiffuess and strengths ofinfilled frames", Proceedings of the Institution ofCivil Engineers, Supplement IV, 1971, pp 57-90.

9. Liauw, T.C. and Lee, S.W., "On the behaviour and the analysis of multi-storey infilled frames subject to lateralloading". Proceedings of the Institution of Civil Engineers. Part 2,1977, Vol. 63, 641-657.

10. Benjamin, J.R, and Williams, H.A. ; "The behaviour of one-storey brick shear walls" . ASCE Ioumal ofthe Structural Division, 1958, Vol. 84, pp. 1723.1 -1723.30

11 . Holmes, M., " Combined loading on infilled frames" . Proceedings, the Institution ofCivil Engineers, 1963, Vol. 25, pp. 31-38.

12. Hetenyi, M., "Beams on elastic foundations" . Vol. XVI. University ofMichigan Studies, Scientific Series, 1946.

13. Stafford-Smith, B. and Carter, C., "A method of analysis for infilled frames". Proceedings, the Institution ofCivil Engineers, 1969, Vol. 44, pp. 69-102.

14. Karamanski, T., "Calculating infilled frames by the method offinite element", Tall Buildings, Pergamon Press, Oxford, 1967, pp. 455-463

15. Mallick, D .V. and Sevem, R.T. , "The behaviour of infilled trames under static loading". Proceedings of the Institution of Civil Engineers, 1967, Vol. 38, pp 639-656.

16. Riddington, I .R and Stafford-Smith, B., "Analysis ofinfilled frames subjected to racking with design recommendations". Structural Engineer, 55(3): 183-199,1977.

17. King, G.J.W. and Pandley, P.C., "The analysis of infilled frames using finite elements". Proceedings, the Institution ofCivil Engineers, 1978, Part 2, Vol. 65, pp. 749-760.

18. Liauw, T.C. and Kwan, K. H. , ''Non-linear analysis of multi-storey infilled frames." Proceedings, the Institution of Civil Engineers, 1982, Part 2, Vol. 73, pp. 441-454.

19. Barau; H.K. and Mallick, S.K., "Behaviour ofmortar infilled steel frames under lateralload." Building and Environrneot, Pergamon Press, UK, Vol. 12, pp.263-272.

20. Sachanski, S., "Analysis ofthe earthquake resistance offrame buildings taking into consideration the carrying capacity of the filling masonry". Proceediogs of the Second World Conference 00 Earthquake Engineering, 1960, Vol. 3, Tokyo, pp. 2127-2141.

21. Stafford-Smith, B. and Coull, A. , "Tall building structures - Analysis and Desigo". lohn Wiley and Sons Inc., A Wiley-Interscience Publicatioo, 1991, Vol. 91 , pp. 168.

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22. M(,d,oide, RT., "The behaviour of rnasonry infilled steel trames subjected to racking", M.Sc. thesis, Department of Civil Engineering, University of New Brunswick, Fredericton, N.B., 1984.

23. Yong, T.C., "Shear strength of rnasonry panels in steel trames". M.Sc. thesis, Departmem ofCivil Engineering, University ofN.B., 1984.

24. Amos, KA., "The shêãr strength ofrnasonry infilled steel trames". M.Sc. thesis. Department of Civil Engineering, University of New Brunswick, Fredericton, N.B., 1985.

25. Richardson, 1, "The behaviour of rnasonry infilled steel trames". M.Sc. thesis, Department of Civil Engineering, University of New Brunswick, Fredericton, N.B., 1986.

26. Dawe, lL. and Seah, C.K., "Behaviour of rnasonry infilled steel trames". Canadián Journal ofCivil Engineering, 1989, Vol. 16. No. 6. pp. 865-876.

27. Riddington, lR, "The influence of initial gaps on infilled trame behaviour". Proceedings, The Institution ofCivil Engineers, Part 2, 1984, Vol. 77, pp. 295-310.

28. Langenbach, R, "Earthquak:es: A new look at cracked masonry". Civil Engineering, 1992, Vol. 62, No.ll pp. 56-58.

29. Uniform Building Code. 1988, International Conference of Building Officials, . Whittier, California.

30. Flanagan, RD., Bennett, RM. and Barc1ay, G.A., "Experimental testing ofhollow clay tile infilled trames". Proceedings 6th. Canadian Masonry Symposium, Saskatoon, Saskatchewan, 1992.

31. Wood, RH., "Plasticity, Composite Action and Collapse Design ofUnreinforced Shear Wall Panels in Frames", Proceedings ofthe Institution ofCivil Engineers, Part 2, 1978, Vol. 65, pp. 381-411.

32. Liauw, T.C. and Kwan, KH., "Plastic theory of non-integral infilled trames". Proceedings, the Institution ofCivil Engineers, Part 2, 1983, Vol. 75, pp. 379-396.

33. -ibid, "Plastic theory of infilled trames with finite interface shear strength". Proceedings, the Institution of Civil Engineers, Part 2, 1983, Vol. 75, pp. 707-723.

34. -ibid, "Plastic design of infilled trames". Proceedings of the Institution of Civil Engineers. Part2, 1984, Vol. 77, pp. 367-377.

35. Kwan, KH., Lo, C.Q. and Liauw, T.C., "Large - scale model tests and plastic analysis of multibay infilled trames" . Proceedings, The Institution of Civil Engineers, Part 2, 1990, Vol. 89, pp. 261-277.

36. Dawe, lL. and Seah, C.K, "Out-of-plane resistance of concrete masonry infilled panels" . Canadian Journal of Civil Engineering, 1989, Vol. 16. No. 6. pp. 854-864.

37. Abrams, D.P., Angel, R., "Lateral strengthofcracked URMInfills" . Proceedings ofthe International Workshop on Unreinforced Hollow Clay Tile, Martin Marietta Energy Systems, Inc. Oak: Ridge, Tennessee, 1992.

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