HERA VIOUR OF UNREINFORCED MAsONRY P ANELS INFILLING RC FRAMES: PRELIMINARY RESULTS

1. ABSTRACT

When masonry panel infilled frames are subjected to lateral in-plane forces, it is commonly known that the panels behave as if subjected to diagonal compression. Diagonally loaded, infilled panels are being used in a current experimental program to assess the behaviour of reinforced concrete frames infiUed with unreinforced brick masonry. Tests are carried out on one-third scale RC infilled frames in order to investigate the effect of their aspect ratio, H/ L , moment of inertia ratio, 1/ l e, on initial stiffness, diagonal crack load, ultimate capacity, and failure modes of the composite system.

Keywords: masonry, infilled panel, experimental, behaviour, reinforced concrete frame

lphD student, 2Professor of Civil Engineering, 3Research Engineer Department of Civil Engineering, University of N ew Brunswick, Fredericton, N.B. , CANADA, E3B 5A3, Fax: (506) 453 3568

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2. INTRODUCTION

Masonry panels infilling reinforced concrete or steel frames are frequently encountered in structural systems. Using masonry as a partition, the structural integrity is generally assessed by designing the frames to sustain the loads during the structure lifetime. The panels are considered to be structurally inactive and not taken into consideration in the design processo Early experiments investigated the contribution of infilled walls to the encasing frame strength when the structure was subjected to lateralloads. It was found that masonry panels had a significant effect on the stength and the stiffness of the composite system (Thomas 1953 and Ockleston 1955). It was considered important to explore beyond this qualitative information by testing different types of infills, with and without openings. Benjamin and Williams 1957 carried out tests on scale mo deis and formulated approximate reIationships based on experiments in order to predict their behaviour. Wood 1958 suggested that neglecting the composite action of infills in stiffening tall buildings would lead to difficulties in assessing the behaviour of the frames. In order to design the frames, he suggested the use of collapse design methods which take into account the contribution of infilling panels. Polyakov 1960 suggested that the analysis of the infilled frame system should be idealized as a frame with a diagonal strut which takes the place of infill. Taking the same approach, Rolmes 1961 developed a semi-empirical method to predict the strength of a frame with the wall as a diagonal brace. Ris tests were conducted on steel frames infilled either with brick masonry or reinforced concrete walls. Smith 1966, 1967 conducted tests on scale models of infilled steel frames and developed the concept of equivalent width based on the analogy of a beam on elastic foundation. Smith and Carter 1969 used that concept and proposed a procedure to assess the diagonal crack load and the ultimate strength of infilled frames. Mainstone 1971 conducted experiments on a wide range of one-seventh models of frames infilled with square and rectangular panels and used the equivalent strut concept to formulate expressions which predict the capacity of infilled frames. The preliminary results presented herein are part of a broad research program designed to investigate the behaviour and to assess the applicability ofthe aforementioned formulations in predicting the behaviour and the strength of reinforced concrete (RC) frames infilled with unreinforced masonry panels.

3. EXPERIMENTAL PROGRAM

The experimental program was designed in order to investigate the effect of the aspect ratio, H/L, and the ratio of beam-to-column moment of inertia, Ib/lo on the behaviour of infilled RC frames in which two leveIs of each parameter has been considered. In addition, bare RC frames (without infill panel) have been cast enable the assessment of the masonry paneI contribution to the overall behaviour of the composite structural system. The experiments have been arranged in factorial of 2 x 2 x 2 with 1 replicate as shown in Table 1. In order to accommodate existing testing facilities, tests were conducted on one-third scale specimens. Specimens, as summarised in Table 2, were cast and stored in a laboratory environment along with their

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companion cylinders.

Table 1. Parameters under investigation

Aspect Ratio Moment of Opening Size .

H/L Inertia ratio I/ Ic

0.5 1 O

1 5 1

• O: Solid infill 1: Bare Frame

Mter ten days, an experienced mason built masonry panels infilling the RC frames. At the same time, masonry prisms and mortar specimens were taken for material mechanical qualification.

Table 2. Specimen Description

Specimen Aspect Ratio Inertia Type Concrete Mix Ratio Ir

MR1B101 0.5 1 Bare 1 MR5B102 0.5 5 Bare 2 MS1B103 1 1 Bare 3 MS5Blü4 1 5 Bare 4 MR1P104 0.5 1 InfIlled 4 MR5P103 0.5 5 InfIlled 3 MS1P102 1 1 InfIlled 2 MS5P101 1 5 InfIlled 1

In addition, masonry panels with the same aspect ratios as infilled panels were built in order to assess the diagonal tensile strength of the infills. These results are summarized in Table 3. On the day of testing, specimens were moved from the curing area to the testing room and set in a universal testing machine with a capacity of 200 kips as shown in Figure 1. Special care was taken during handling and to reduce stress concentration during testing, masonite pads were intercalated between the machine head and the specimen. The configuration included one-storey one-bay RC infilled frames for which eight specimens were constructed according to the experimental design outlined above. The reinforced concrete frames were built according to CAN-3 A-23-3 M84 and ACI 318-83 as applicable. Rebars were #10M and #15M while stirrups were made of galvanized steel wire 9WG of 3.9mm diameter. Concrete was modelled by microconcrete whose mix proportion was obtained after a series of trial mixes.

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Table 3. Material Properties

Compressive Tensile Diagonal Material Size Strength Strength Tensile

MPa MPa Strength

50x100 mm B1: 25.40 Microconcrete Cylinder B2: 18.80 - -

B3: 20.15 B4: 16.47

Mortar 50x50x50mm 15.53 -Tension Coupon - 1.99 -

Masonry LxHxt Prism 191x187x57mm 69.6 - -

Masonry LxHxt Panel

Square 565x592x57mm - - 0.83 Rectangular 670x380x57mm - - 0.63

Bx deslgnates MIX nr.

4. TEST SETUP AND INSTRUMENTATION

In order to simulate the behaviour of infilled frames subjected to in-plane forces, specimens were tested in diagonal fashion as illustrated in Figure 1. This procedure has been successfully used by numerous investigators induding Esteva 1966. The load was increased monotonically up to failure while monitoring a series of parameters including the composite system and masonry panel deformations by providing linear strain convertors (LSC) along two or three directions as shown in Figure 1. At each load increment, readings were recorded by a data acquisition system. Lengths of contact along the beam and the column were also measured and progressive cracking was traced at each load increment and reported on a chart. Specimens were loaded up to their ultimate capacity at which point instruments were removed and loading continued until they underwent substantial damage (corner crushing or complete failure ofthe masonry panel) or distortion. Typicalload-displacement curves are provided in Figs. 2 and 3 for specimen MR1P104 and MS5P101, respectively.

5. TEST RESULTS AND DISCUSSION

5.1 GENERAL BEHAVIOUR

The load-diagonal displacement curves of infilled frame specimens show that their overall behaviour can be subdivided into three distinct phases. Initially, the infilled frames behave monolithically until the occurrence of the panel-to­frame separation. The first phase was characterized by a linear load displacement curve which extended up to the occurrence of the diagonal crack in the masonry infill and was followed by a noticeable drop in loading and

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eventuaUy by an audible snapping sound.

Fig. 1. Views of General Test Setup.

A second phase led to a diagonal crack initiated at the infiU center and propagated towards loaded corners. Beyond this point, the stiffness of the composite system decreased and subsquent loading was translated into progressive cracking of the infiU. Observed cracks were roughly parallel to the diagonal direction of the panel and moved towards unloaded corners. This induced a non linear behaviour of the composite system due to crushing of mortar joints and loaded corners. In addition, it was observed that the diagonal crack widened up to 40.0 mm at the center of the masonry panel.

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70 n

60 w Vl <: I a.

50

N 40 w

~

Z Vl

;;:. <: I

"O 30 a.

o o

...J

20 w Vl

10 <: I a.

O

-10 -50 O 50 100 150 200

Displocement (mm)

Fig. 2. Load vs Diagonal Displacement of Specimen MRIPI04

~

z .;,:

"O o o -'

100

80 N

W Vl <: I

60 a.

40

w Ul <:

20 I a.

O --- -----

_20L--L __ ~~~-L __ ~~ __ -L---'

-10 o 10 20 30 40 50 60 70

Displacement (mm)

Fig. 3. Load vs Displacement Curve of Specimen MS5PIOl

At the end of this phase, a local maximum load was reached and subsquent loading induced opening of the loaded corners and increased the width of the

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diagonal crack substantially. In the third phase, the ultimate capacity of the composite system was reached. The characteristics of this phase depended on the goodness of the interlocking effect between the panel and the deformed frame. It was observed that, during this phase, the brick units ofthe degraded panel underwent substantial slip and rotation in order to fit the deformed shape of the RC frame. This phenomenon is accompanied by stress redistribution within the masonry panel due to high leveI of normal contact forces and joint opening. Similar behaviour has been reported by Chappell 1975 as one of the mechanical characteristics of discontinuous systems. Tests performed on bare frames showed that their behaviour was alrnost linear up to the ultimate capacity. Beyond that point, they were subjected to extensive deformation which induced cracking of the unloaded corners and widening of loaded corner joints. No major flexural cracks were observed. Figures 4 and 5 illustrate the overall behaviour of RC square and rectangular infilled frames respectively with respect to that of bare frames.

100

MS5Pl0l

80

60

Z ~

" 40

o o ....

20

o

-20L-~--~~--~--L--L--~~

-1 0 o 10 20 30 40 50 60 70

Oisplocement (mm )

Fig. 4. Load vs Diagonal Displacement Curves for Square Specimens.

5.2 STIFFNESS DETERIORATION

As loading progressed, alI infilled frame specimens exhibited a progressive stiffness degradation. The stiffness decreased dramatically beyond the system ultimate strength. It was observed that the deterioration rate increased after the occurrence of the diagonal crack. This phenomenon may be explained by progressive cracking of the infill along with increasing deformations induced by the loading to the panel and the RC frame. The stiffnesses corresponding to the aforementioned three phases are summarised in Table 4 along with those of bare frames. The increase in initial stiffness of the composite system is in a range of 15 and 45 times that of corresponding bare frames. The lowest value was observed from a rectangular system with an inertia ratio of 1 while the highest value was derived from a square specimen with inertia ratio of 5.

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100

80

60

I "

40 o o --'

20

O MR5Bl02

-20 - 20 O 20 40 60 80

Oisplocement (mm )

Fig. 5. Load vs Diagonal Displacement Curves for Rectangular Specimens.

Table 4. Mechanical Characteristics of Bare and Infilled Frames

Diagonal Stiffness Stage Aspect

Inertia Ratio: 1 Inertia Ratio: 5 of Ratio Loading H/L Bare Infilled Bare Infilled

kN/mm kN/mm kN/mm kN/mm

1 1.27 32.97 2.75 39.86 PHASE 1 0.5 1.98 88.17 4.83 74.6

PHASE 2 1 - 1.93 - 3.34 0.5 - 3.97 - 11.54

PHASE 3 1 0.63 - 1.01 -0.5 1.76 1.30

Diagonal Crack Load 1 - 44.10 - 57.87

kN 0.5 - 26.64 - 32.14

Ultimate Load 1 14.70 83.60 27.60 87.30

kN 0.5 22.05 62.46 24.81 81.75

The presence of infill significantly affects the initial diagonal stiffness of the composite system. The inertia ratio affects the stiffness much less significantly. These observations corroborate the findings made by other investigators including Mainstone 1971.

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5.3 STRENGTH OF INFILLED FRAMES

The ultimate strength was defined as the maximum diagonal load at which the composite system underwent subtantial distortion. For most specimens, the presence of masonry inflll increased the ultimate strength of RC frames at least by three times except for a rectangular specimen with an inertia ratio of 1 for which the ultimate capacity was increased by almost six times. This beneficial effect is constistent with that observed by Smith 1967 and Mainstone 1971. Observed ultimate strengths were compared to values obtained by using formulations of Mainstone 1971 and Smith and Carter 1969. It was found that these two appoaches overestimated both diagonal crack load and ultimate strength ofthe test units. Similar findings have been pointed out by Leuchars and Scrivener 1976. This discrepancy between the predicted and the actual experimental strengths may be due to the fact that these formulations were derived from results of scale models (less than one-sixth scale) of steel frames infilled with microconcrete or plaster. In addition, contrary to conclusions drawn by Smith 1967, diagonal crack load and ultimate capacity of square specimens (aspect ratio of H/L = 1) have been consistently greater than those of rectangular infilled frames (aspect ratio H/L = 0.5). The main reason of this discrepancy is imputed to be the orientation of the resultant force acting on the masonry infill with respect to the masonry bed joints (Daou and Hobbs 1991). In fact, reducing the angle of inclination by decreasing the aspect ratio (H/ L) yields a lower diagonal tensile strength. In other words, the departure of the bed joints from the horizontal orientation reduces the diagonal capacity of the infill.

5.4 FAILURE MODES

Failure modes were mainly due to substantial cracking of the infill panel which took place after the occurrence of a diagonal crack. The load­displacement curves given in Fig. 4 and 5 suggest that subsequent loading causes more load to be taken by the reinforced concrete frame. Therefore, cracks developed in frame members and the beam-to-column joints at loaded corners initiated their opening. Due to the arching action developed in Phase 3, RC frame columns underwent extensive cracking at their base which suggested a shear failure mode. This was limited to the columns in the case of specimens with a stronger beam (inertia ratio of 5) while both beam and column were affected in the case of members with similar flexural stiffness (inertia ratio of 1). The extent of cracking was limited to about 25.0 em with respect to the loaded corners along the column in the frrst case while it reached 50.0 cm along the frame members in the second case. It was only after extensive panel deterioration and frame distortion that further loading induced initiation of plastic hinges due to bending of the frame members.

6. CONCLUSIONS

Although it would be inappropriate to draw defini tive conclusions from these preliminary results, some trends can be pointed out. As has been shown by other investigators, the presence of infill has been found to be beneficial to

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the composite initial stiffness as well as the ultimate strength. Even if the inertia ratio does not affect the initial stiffness, it is found to significantly influence the diagonal crack load and the ultimate strength along with the aspect ratio. As the aspect ratio, H/L, decreases, the ultimate strength of the composite system reduces contrary to widely accepted concIusions. This suggests that the actual formulations might not be appropriate in assessing the strength of RC infilled frames. An extensive program is currently being undertaken and its results will be subsequently reported.

7. ACKNOWLEDGEMENTS

The authors wish to express their sincere gratitude to L.E. Shaw Ltd. and the Atlantic Masonry Association for their financial support.

8. BIBLIOGRAPHY

1. Thomas, F.G. 1953, "The Strength of the Brickwork", The Stuctural Engineer, VoI. 31, pp. 35-41.

2. Ockleston, AJ. 1955," Load Tests on a Three-storey Reinforced Concrete Building in Johannesburg", The Structural Engineer, VoI. 33, No. 10, pp. 304-322.

3. Benjamin, J.R and Williams, 1957, "The Behaviour of one-storey Reinforced Concrete Frame with Brick Masonry Infill under Lateral Loads", Journal of Structural Division, ASCE, VoI. 3, pp. 81-88.

4. Wood, R H., 1958, "The Stability of Tall Buildings", Proc. of I.C.E., London, VoI. 11, pp. 62-102.

5. Polyakov, SV 1960, "On the Interaction between Masonry Filler Walls and EncIosing Frame when Loaded in the Plane of the Wall", Translation in Earthquake Engineering, EERI, San Fransisco.

6. Holmes, M. 1961, "Steel Frames with Brickwork and Concrete Infilling", Proc. of I.C.E., VoI. 19, pp. 473-478.

7. Smith, B. S., 1966, "Behaviour of Square Infilled Frames", Proc. of Structural Div. Journ., ASCE, ST-6, VoI. 92, pp. 381-403.

8. Smith, B. S., 1967, "Methods for Predicting the Lateral Stiffness and Strength of Multi-Storey Infilled Frames", Building Science, VoI. 2, pp. 247-257.

9. Smith, B. S. and Carter, C., 1969, "A Method of Analysis for Infilled Frames", Proc. of I.e.E., VoI. 46, pp. 31-48.

10. Mainstone, RJ. 1971. On the Stiffness and Strength of Infilled Frames. Supplement of Proc. of ICE, VoI. 48, pp. 57-90.

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11. CSA CAN-3 A23-3 M84, 1984. Design of Concrete Structures for Buildings, Canadian Standards Association, Rexdale, Ontario.

12. ACI 318-83,1983. Building Co de Requirements for Reinforced Concrete. American Concrete Institute, Detroit.

13. Esteva, L. 1966, "Behaviour under Alternating Loads of Masonry Diaphragms Framed by Reinforced Concrete Members" Proc. of Int. Symp. on Repeated Loading of Materials and Structures, Rilem, VoI. 5, Mexico, sec.13-6.

14. Chappell, B. A., 1975, "The Effects ofConstraints on the Deformational Response of Slip along Planar Joints", Int. J. Rock Mech. Min. Sci. and Geomech. Abstr., VoI. 12, pp.265-270.

15. Leuchars, J. M. and Scrivener, J. C. 1976. Masonry Infill Panel Subjected to Cyclic In-plane Loading, BulI. of New Zealand National Society for Earth. Eng., voI. 9, no. 2, pp. 122-131.

16. Daou, Y. and Hobbs, B. 1991, "Strength of Brickwork Loaded in Different Orientations", Proc. of9th IBBMaC, Berlin, VoI. 1, pp. 157-163.

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