BEHA VIOR OF DAMAGED INFILL PANELS

Roger D. Flanagan1 and Richard M. Bennett2

ABSTRACT

Unreinforced masonry infill panels can add significant strength, stiffness, and energy absorption capability to otherwise laterally weak frames. However, after an earthquake, the infill panels are often damaged. To determine the behavior and remaining strength of damaged panels, several large scale steel frames with unreinforced structural clay tile infills were tested. Two infills were subjected to out­of-plane drift loadings, and one infill to out-of-plane inertial (air bag) loading to damage the panels. These infills were then tested in-plane until failure. Little strength or stiffness degradation was observed, with the panel behavior being similar to undamaged panels. Another infill was subjected to in-plane loading until there was significant diagonal cracking, and then was loaded out-of-plane with an air bag until failure. Although there was a decrease in out-of-plane capacity, significant arching action still developed. Two infills were tested in-plane to failure, alI cracked and failed tile removed, and the walls rebuilt and retested. Significantly cracked mortar joints were tuck pointed. One panel was reconstructed with similar structural clay tile, while the other had solid concrete masonry units in the previously damaged corners. Both infills had similar strength and stiffness behavior to' undamaged specimens. As a result af the testing program, it is concluded that damaged infill panels still provi de the positive attributes of infill construction. After an earthquake, only obviously damaged and failed masonry needs to be replaced to restore the lateral strength of the structure.

Keywords: InfiU, Damage, Seismic, Repair, Clay Tile

lSenior Engineer, Martin Marietta Energy Systems, Inc., Oak Ridge, TN, USA. 2 Associate Professor, Department of Civil Engineering, The U niversity of Tennessee, Knoxville, TN, USA .

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INTRODUCTION

Current seismic design philosophy embodies two goals for providing structural safety. For probable earthquakes during a structure' s lifetime, mini mal damage is the objective. For earthquakes of maximum credible magnitude, greater damage is typically accepted but stability and life safety functions must remain intact. The acceptance of some damage is a necessary risk when considering the fmanciaI requirements of damage free designs for extreme earthquake forces.

Following a large earthquake, damaged buildings need to be evaluated to determine structure condition including remaining capacity. Decisions about the amount of repair or retrofit need to be made. In the instance of severe damage, a replacement structure may have to be designed.

Masonry typically receives considerable attention after an earthquake. In particular, unreinforced masonry tends to crack readily and show much apparent damage. This is due to the brittle nature of the material and it's weakness in tension. In this instance the decision to completely replace the masonry may be reached prematurely. Significant loss of capacity may not accompany the apparent damage.

A typical example in which cracked or otherwise damaged masonry may still have adequate strength is that of infill construction. Many buildings are comprised of structural framing infilled with unreinforced masonry panels. Whether placed by desi~n or inadvertent construction, masonry infills significantly effect the response of the framed structure. Masonry infills add considerable stiffness to the framing, altering the distribution of lateral shear forces. The framing confines the masonry adding significant strength to the system. The infill adds lateral bracing, shortening the effective length of columns.

Experience has shown that unreinforced masonry infills displaying some damage are still efficient in adding to the lateral capacity of structures [1]. This re-emphasizes the fact that the onset of cracking does not necessarily coincide with overaIl structure degradation.

For masonry infills, seismic events generate both in-plane and out-of-plane forces in the structural system. In-plane racking forces or diagonal compression forces are created as the added stiffness and confinement of the panel attract story shear. Out­of-plane global drift results in imposed relative top-bottom displacements of infills. For a local panel, out-of-plane inertial forces are generated as the mass of the panel and any rigidly attached equipment respond dynamica1ly.

The in-plane behavior of a masonry infill is characterized by diagonal panel cracking and comer crushing [2]. Figure 1 shows a static hysteretic curve of a clay tile infill [3]. Here, diagonal mortar joint cracking is reached at less than half peak capacity while peak capacity is reached at the onset of comer crushing. Typical postpeak comer crushing is shown in Figure 2. The failure results from splitting of the tile faceshells in the upper loaded comers of the pane!.

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Out-of-plane drift behavior of a masonry infill is characterized by mortar joint (primarily bedjoint) cracking along the base and lower courses of the panel [4]. The forces generated in the panel are small and failure is a loss of stability. For clay tile panels snugly infilled in steel framing, acceptable out-of-plane drift displacements are typically larger than acceptable in-plane drift displacements of orthogonal walls.

Out-of-plane inertial behavior of a masonry infill is characterized by panel cracking and the development of an arching mechanism as segments of the panel rotate about the fracture lines and panel boundaries [5]. Figure 3 shows static air bag load­unload cycles of a clay tile infill [6]. The panel is snugly fitted against the webs of the wide flange columns with no out-of-plane bearing against the flanges.

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(300(80) (60) (40) (20) o 20 40 60 80 Displacement (mm)

Fig. 1 Static In-Plane Hysteresis

Fig. 2 Comer Crushing Failure

Horizontal bedjoint cracking develops near the center of the panel and the infill arches vertically until failure of the top and bottom course tiles. Stability of the panel is maintained with some loss of capacity as the panel arches horizontally. Further tile failure along the column boundary and in the center of the panel degrade the strength and overall stability of the infill. Figure 4 shows the infill after completion of air bag testing. The observed failure of the upper and lower course tiles is a combination of shear and compression in the tile.

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EXPERIMENTAL PROGRAM 30~~r--'---'---c~-'~-'---'---'

To assess the remaining capacity of an infill following a damaging earthquake and the effectiveness of various repair techniques, a series of large-scale tests were performed. Various combinations of sequential in-plane and out-of-plane loadings were used . The effect of potential damage on each of the infill failure mechanisms is addressed.

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5 10 15 20 25 30 35

Midpanel Displacement (mm)

Fig. 3 Load-Unload Air Bag Hysteresis

The infill structures tested consisted of steel framing with unreinforced structural clay tile panels. A portion of these tests relating to the behavior of damaged infills is presented in Table 1. The infilled frames had nominal dimensions of 2.5 meters height by either 2.5 meters or 3 meters length. The masonry panels were constructed of 200 mm nominal thickness tile with running bond and side construction.

Out-of-Plane Drift Damage

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The effect of out-of-plane drift on the in-plane capacity of structural clay tile infilled frames was evaluated with test specimens 11 and 13. The specimens were first subjected to cyclic out-of-plane displacements imposed either at the top or at midheight of the infill. Both specimens were then loaded in-plane to failure . The results were compared to specimen 2 which was an identical specimen that was only loaded in-plane to failure .

Fig. 4 Air Bag Loading Failure

Out-of-Plane Inertial Damage

The effect of damage from out-of-plane inertialloads on the in-plane capacity of clay tile infills was evaluated by first loading specimen 20 with an air bago Static load­unload cycles were applied at nominal pressure increments up to 75 % of ultimate

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capacity as determined in control specimen 18. Specimen 20 was then loaded in-plane until failure and the results correlated with the in-plane control, specimen 2.

Table I Description of Damaged Infill Testing

Test Size (m) Description HxL

2 2.5x2.5 In-Plane Loading, Control

lOa 2.5x3.0 In-Plane Loading, Column/Panel Gap, Control

lOb 2.5x3.0 In-Plane Loading, Column/Panel Gap, Retrofit With CMU

11 2.5x2.5 Out-of-Plane Drift Loading, Followed By In-Plane Loading

13 2.5x2.5 Out-of-Plane Drift Loading, Followed By In-Plane Loading

18 2.5x2.5 Air Bag Loading, Control

19 2.5x2.5 In-Plane Loading, Followed By Air Bag Loading

20 2.5x2.5 Air Bag Loading, Followed By In-Plane Loading

21a 2.5x3.0 In-Plane Loading, Control

21b 2.5x3.0 In-Plane Loading, Repaired (Replace Tile & Tuck Point)

In-Plane Damage

Five additional specimens were tested to evaluate in-plane damage and the effectiveness of two repair schemes. Specimen 19 was first tested in-plane to 75 % of ultimate capacity as defined by control specimen 2. Specimen 19 was then tested with an air bag until failure. Increasing static load-unload cycles were used and the results were correlated with control specimen 18.

Specimen 21a was a another control specimen tested in-plane to failure. After testing in-plane well beyond peak capacity, the specimen was repaired by replacing ali damaged tile in the upper two courses. Large cracks were tuck pointed and the specimen was retested under similar cyclic in-plane loading. The repaired specimen was numbered 21b.

Specimen lOa was identical to specimen 21a with the added feature of a one inch gap between the columns and the masonry pane!. Specimen lOa was an in-plane control specimen for determining the behavior of the gapped boundary condition and for evaluating a solid masonry retrofit scheme. Since the strength of specimen 21a and lOa was limited by crushing in the upper comers of the panel, a comer retrofit scheme was applied to specimen 10a. The retrofit consisted of replacing the damaged upper comer tiles with solid concrete masonry units and a higher strength mortar. The rest of the specimen was repaired similar to specimen 21b. This new specimen, IOb, was then tested in-plane until failure.

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EXPERIMENTAL RESULTS

Out-of-Plane Drift Damage

The effect of out-of-plane drift damage on subsequent out-of-plane drift is minimal. After mortar joint (primarily bedjoint) cracking in the lower courses of the panel early in the loading sequence, specimens 11 and 13 behaved somewhat linearly as additional cyclic drift loads were applied. Bond between the frame and infill as well as arching mechanisms allowed the panel to follow the movements of the framing over the range of displacements tested , greater than 1.5 % drift.

The effect of out-of-plane drift damage on subsequent out-of-plane inertial capacity is also mini mal. Observations from test 11 and 13 drift loadings indicate no significant damage of the interface along the panel boundary. Likewise, no loss of stability was observed for the drifts imposed. While cracking in the joints along the lower courses could affect the formation of fracture lines in the panel, the development of arching action should not be impeded.

Prior out-of-plane drift damage tends to reduce the in-plane diagonal cracking strength of infills. As the leveI of drift damage increases, subsequent in-plane panel cracking strength may be reduced to zero. However, correlations of the results of specimen 11 and 13 with specimen 2 indicate liUle effect of out-of-plane drift on in-plane stiffness and strength. This is expected because the in-plane capacity is dominated by comer crushing and there was no apparent damage in the upper comers of the panel. For larger infills, where diagonal cracking strength is higher, out-of-plane drift damage may cause a decrease in in-plane stiffness. In cases where the diagonal cracking strength is greater than the comer crushing strength, out-of-plane drift damage may limit the in-plane capacity to the comer crushing capacity.

Out-of-Plane Inertial Damage

Damage to clay tile infills from out-of-plane inertial loadings may be separated into two types. First, mortar joint cracking occurs at relatively low uniform panelloading. These cracks form the fracture lines of the panel segments as it arches out-of-plane. The crack pattem may vary based on the bond strength of the masonry and based on the boundary conditions and geometry of the panel. For the square panels tested , horizontal midpanel cracking occurred first. This is due to the relatively low bond strength of the masonry bedjoint and the use of running bond construction.

The second damage leveI of clay tile infills is tile failure along the panel boundaries. After significant damage occurs along multiple boundaries, capacity and overall stability of the panel is lost. For the square panels, failure in the top course occurred first, followed by failure in the bottom course, and finally failure along the columns and throughout the panel. This follows logically since the panel initially arched vertically.

The effect of out-of-plane inertial cracking damage on subsequent out-of-plane drift or out-of-plane inertial strength is minimal. The addition of midpanel horizontal cracking would not be expected to dramatically effect out-of-plane drift stability. The

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panel would still would still follow frame movements due to horizontal arching. In each of the air bag tests, load-unload cyc1es for increasing pressure increments behaved similarly until tile failure.

Conversely, out-of-plane inertial cracking may reduce in-plane panel cracking strength to zero. As with out-of-plane drift, inertial loads may eliminate the stiffness change associated with the in-plane panel cracking limit state. Correlating the results of specimens 2 and 20 show that prior out-of-plane inertialloads of 75 % of peak reduced the initial in-plane stiffness by approximately 50%. Reduction in in-plane capacity was negligible. However, high inertial forces coincident with in-plane forces may reduce in-plane strength. In-plane compression is induced from arching and biaxial compression effects in the comer regions of the panel are likely to be increased.

The effect of tile failure due to out-of-plane inertial loading is more significant. Tile failure along the boundaries may allow instability under out-of-plane drift loadings. Likewise, tile failure represents imminent loss of capacity in subsequent inertial loads. Finally, tile failure at any point along the diagonal of the panel would significantly reduce in-plane stiffness and strength.

In-Plane Damage

In-plane damage of c1ay tile infills may be separated in two categories, diagonal (stair step) mortar joint cracking and comer crushing or comer tile splitting. Panel diagonal cracking would not be expected to effect subsequent out-of-plane drift stability. Likewise, the presence of in-plane panel cracking should not significantly effect out­of-plane inertial capacity. Comparison of specimens 18 and 19 indicate that prior in­plane loading of 80 % of peak in-plane capacity, reduced ultimate out-of-plane inertial capacity by only 20%.

Increasing cyc1ic in-plane loadings cause pinching hysteresis loops due to interface degradation and plastic deformation in the infill. Thus, the stiffness of the panel under subsequent in-plane loads would be decreased, particularly at lower loads. However, the effect of in-plane cracking damage on subsequent in-plane strength is negligible prior to reaching peak in-plane capacity. Monotonic and cyc1ic ultimate strengths are similar. Stiffness and strength degradation are more affected by prior loading in the postpeak range.

In-plane comer crushing damage may leave a discontinuous interface along the column/infill boundary. However, the remaining portions of the panel that are in contact with the framing will allow arching to develop. Thus, the remaining portions of the panel will retain their stability under drift and inertialloadings. A test in which simultaneous out-of-plane air bag loads were applied while holding in-plane drift deformations showed remarkable panel stability, even at high in-plane deformations. After almost half of the infill tiles failed, the remaining portion maintained stability.

Repair and Retrofit

Repair of c1ay tile infills cracked by out-of-plane drift or inertial forces is primarily an architectural and functional concem. Noticeably cracked masonry joints should be

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tuck pointed and the waterproofing of any exterior panels checked for moisture penetration.

For infills with unit tile failures due to out-of-plane inertial forces, repair may be more difficult. Split tiles around the boundary of the infill need to be replaced and significant cracks repaired. If the boundary of the infill has faiIed, the masonry panel may be significantIy displaced out-of-plane. Permanent deformations may also exist near the midpanel. In these instances, there may be little choice but to rebuild the entire wall.

The results of specimens 21a and 21b indicate that a normal repair procedure of replacing obviously damaged tile and tuck pointing large cracks will restore the clay tile panel to the same approximate in-plane strength. Initial in-plane stiffness is reduced by approximately 50% as the frame/panel boundary is not totaIly repaired with this technique. For specimen 21b most of the upper two courses were replaced as part of the masonry repair.

The results of specimens lOa and 10b indicate that a retrofit scheme incorporating stronger upper comers where crushing develops, does not increase the panel strength beyond its original capacity. The replacement of obviously damaged tile and solid concrete masonry units in the upper comers of specimen lOb restored the in-plane strength to approximately 80% of its original capacity. Failure was shifted from the upper comers to the middle regions of the pane!.

As expected, the comer retrofit of specimen lOb increased the initial in-plane stiffness beyond that of the original gapped boundary specimen lOa. The in-plane hysteresis of specimen lOb was similar to that of specimen 21a prior to ultimate. This tends to validate compressive strut theories which suggest that comer bearing with a diagonal strut is the in-plane load resisting mechanism for infills well past panel cracking.

CONCLUSIONS

Unreinforced masonry is often completely removed after a damaging earthquake and replaced with either reinforced masonry or another building material. This may be an overreaction, particularly for infills. Damaged infill panels have been shown to maintain out-of-plane stability. In-plane strength can be restored by replacing obviously damaged tile and tuck pointing mortir cracks. Thus, the resistance to the maximum credible design earthquake may be restored. Although an unlikely second maximum credible earthquake will redamage the structure, the structure will still maintain its life safety functions. Strengthening the comer regions of the paneI did not increase the infill strength. Therefore, to increase the lateral resistance, a different panel strengthening mechanism is necessary.

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A CKNOWLEDGEMENTS

This work was sponsored by the Center for Natural Phenomena Engineering, Martin Marietta Energy Systems, Inc., under contract with the U.S . Department of Energy .

REFERENCES

1. Klingner, R.E., Beiner, R., and Amrhein, J. (1987). "Performance of Masonry Structures in the Mexican Earthquake of September 19, 1985," Proceedings of the Fourth North American Masonry Conference, 70: 1-70: 14.

2. Dawe, J.L., and Seah, C.K. (1989). "Behavior of Masonry Infilled Steel Frames." Canadian Journal of Civil Engineering, 16, 865-876.

3. Flanagan, R.D. , Barclay, G.A. , and Bennett, R.M. (1993) . "In-Plane Behavior and Strength of Structural Clay Tile Infilled Frames," Sixth North American Masonry Conference, Philadelphia, PA.

4. Flanagan, R.D. , and Bennett, R.M. (1992). "Inter-Story Drift Effects on the In-Plane Capacity of Infilled Frames." Proceedings of the Third International Masonry Conference, London , England.

5. Dawe, J.L. , and Seah, C.K. , (1989) . "Out-of-plane Resistance of Concrete Masonry Infilled Panels," Canadian Journal ofCivil Engineering, 16, 854-864.

6. Flanagan , R.D., and Bennett, R.M. (1994) . "Uniform Lateral Load Capacity of Infilled Frames," American Society of Civil Engineering Structures Congress, Atlanta , GA.

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