EXPERIMENTAL AND NUMERICAL INVESTIGATIONS OF REINFORCED CONCRETE FRAMES WITH MASONRY INFILLS

UNDER COMBINED IN- AND OUT-OF-PLANE SEISMIC LOADING

Christoph BUTENWEG1, Marko MARINKOVIĆ2, Ekkehard FEHLING3, Thomas PFETZING4, Thomas KUBALSKI5

ABSTRACT It is well known that seismically loaded reinforced concrete frames with masonry infills exhibit an unfavourable behaviour which can lead to serious damages of the masonry infill panels and the surrounding reinforced concrete frame. Especially recent earthquakes clearly showed the low seismic resistance with serious consequences for the occupants of affected buildings. This is astonishing at first glance, since numerous experimental investigations and numerical simulations have been carried out to study the seismic behaviour of reinforced concrete frames with masonry infills. The results of the fundamental research served as a basis for the derivation of simplified seismic design approaches, which consider the seismic actions in-plane and out-of-plane as a separate design problem. Although the necessity of considering the interaction has already been pointed out by some authors, there is no applicable design concept for the engineering practise available. Within the framework of the joint European project INSYSME, comprehensive experimental test campaigns were carried out on reinforced concrete frames with masonry infills. The tests include separate in- and out-of-plane loading, sequential in- and out-of-plane loading and simultaneous in- and out-of-plane loading conditions. The experimental results lead to significantly lower resistances for sequentially applied loading and to rigorous reductions of the seismic resistance if the loads are applied simultaneously. The results show clearly, that the existing design concepts are insufficient and definitely not on the safe side. This article presents and discusses the main findings of the tests results and further simulation results. Keywords: RC frames; Masonry infill walls; Seismic load; Simultaneous loading; Combined loading; In-plane; Out-of-plane loading 1. INTRODUCTION Reinforced concrete frame structures with masonry infills represent a significant portion of the building stock in seismic active regions worldwide. Properly designed for seismic loading, they exhibit high seismic resistances with excellent deformation capabilities. In most of the reinforced concrete frame structures the subdivision inside the building and exterior closure is realized by means of masonry infills, which present an economical and durable solution. Furthermore masonry infills provide favourable characteristics regarding sound isolation, fire protection and thermal insulation. However, the combination of deformable reinforced concrete frames and stiff masonry infills requires a proper design considering interaction effects on the masonry infills as they are usually installed without leaving any gaps to the surrounding frame. The interaction of frame and infill in in-plane direction leads to a significant change of the global structural vibration behaviour and to a real structural contribution of the masonry infill forming a compression strut with high stressed corner

1 Director, SDA-engineering GmbH, Herzogenrath, Germany, butenweg@sda-engineering.de 2 Teaching assistant, University of Belgrade, Belgrade, Serbia, mmarinkovic@grf.bg.ac.rs 3 Professor, Fachgebiet Massivbau, Universität Kassel, Germany, fehling@uni-kassel.de 4 Teaching assistant, Fachgebiet Massivbau, Universität Kassel, Germany, pfetzing@uni-kassel.de 5 Teaching assistant, RWTH Aachen University, Germany, kubalski@lbb.rwth-aachen.de

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zones in contact to the frame. In addition, possible failures in out-of-plane direction accelerated by prior in-plane damage must be taken into account. However, in the everyday practise masonry infill walls are regarded as secondary structural elements and they are not considered in calculation models used for the structural design. The evidence of this fact is confirmed by the structural response of reinforced concrete frames with masonry infills in recent earthquakes. The unfavourable interactions are responsible for severe damages up to total collapses of buildings with loss of human lives. Based on field observations after earthquakes, such as in Duzce (Turkey, 1999), L’Aquila (Italy, 2009), Lorca (Spain, 2011), Christchurch (New Zealand, 2011), Emilia-Romagna (Italy, 2012), several examples of unsatisfactory structural responses have been reported (Figure 1).

Figure 1. Damage of masonry infill walls after L’Aquila 2009 earthquake (Dazio et al. 2009) Studies related to the behaviour of infill walls under earthquake loading started many years ago with investigations of Polyakov (1960 and 1963), Thomas (1953) and Wood (1958). Historically, infilled frames have been mostly studied under in-plane loading conditions, both experimentally and numerically (Mehrabi et al. 1996; Al-Chaar et al. 2002; Stylianidis 2012). According to Paulay and Priestley (1992), Crisafulli (1997) and El-Dakhakhni et al. (2003) the failure modes of infill walls subjected to in-plane loading can be described by four failure modes depicted in Figure 2, left: Corner crushing (CC), sliding shear failure (SS), diagonal compression failure (DC) and diagonal cracking failure (DK). But just considering in-plane actions is not sufficient, as masonry infill walls are also exposed to out-of-plane actions due to the spatial nature of earthquakes, which can cause four types of out-of-plane failure modes as shown in Figure 2, right: the complete infill wall can fall out of the frame due to inadequate connections (Figure 2e), a one-way infill wall can fail in bending, if the resistance of the compression arch is exceeded (Figure 2f and Figure 2g), and a two-way infill wall can fail in bending after reaching the maximum bending capacity (Figure 2h).

(e) (f)

(g) (h)

Figure 2. In-plane (El-Dakhakhni et al. 2003) and out-of-plane failure modes

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Despite the great importance of preventing out-of-plane collapse, most of the reports and publications are focused on the in-plane behaviour. Out-of-plane investigations neglecting interactions effects were carried out by McDowell et al. (1956), Abrams et al. (1993) and Vougioukas (2012). Combined in- and out-of-plane loading was studied by Angel et al. (1994), Shapiro et al. (1994), Calvi and Bolognini (2001, 2004), Kadysiewski and Mosalam (2009) Pereira et al. (2012), da Porto et al. (2013) and Yuen and Kuang (2014). Most of these studies were focused on sequentially applied in-plane and out-of-plane loading schemes, as experimental investigations applying simultaneous in- and out-of-plane loading are quite complex. All these studies showed significant reductions of the out-of-plane resistance due to previous in-plane loading. Especially if prior damage in in-plane direction is present, the resistance in out-of-plane direction may be reduced significantly. Depending on the regarded study, reductions of the out-of-plane resistance up to about 80% were reported due to prior in-plane load application. However, the number of combined investigations is quite limited with respect to the variety of masonry types and installation conditions. Although the available publications considering the interaction of in- and out-of-plane loading predict a dramatic loss of seismic resistance, this fact is completely ignored in the engineering practise so far. In the following the results of experimental and numerical investigations on reinforced concrete frames infilled with modern vertically perforated clay bricks are presented for separate, sequential and simultaneous in- and out-of-plane loading conditions. The results are used to derive the reasons for the unfavourable interaction effects, which can be further used for specific developments for improving the seismic behaviour. 2. EXPERIMENTAL INVESTIGATIONS 2.1 Test set-up A test set-up has been specifically developed and constructed within the framework of the joint European project INSYSME (2016) to successfully carry out the in-plane and out-of-plane tests on reinforced concrete frames with masonry infills (Butenweg et al. 2014). The equipment and instrumentation of the test set-up is shown in Figure 3.

Figure 3. Test set-up for both in-plane and out-of-plane loading (Butenweg et al., 2016, 2017) The in-plane tests are performed through the application of increasing cyclic displacements on the RC frame top beam in horizontal direction with three cycles for each displacement level. For the out-of-plane tests the loading perpendicular to the wall plane has been applied using four pneumatic airbags with a capacity of 50 kN/m². Forces and deformations of the hydraulic cylinders are recorded by integrated displacement and force transducers. The force capacity of the two vertical hydraulic jacks amounts to ±400 kN each. The maximum stroke amounts to ±125 mm. For the application of the horizontal loading a cylinder with a maximum capacity of ± 320 kN and a stroke limit of ±150 mm was used. The forces and displacements of the hydraulic cylinders are measured with integrated

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displacements and force transducers. The horizontal and vertical displacements of the infill wall are recorded by up to nine inductive displacement transducers, which are fixed to the test frame to be independent from the infill wall. Two potentiometers measure the displacements in the diagonal directions of the wall. The deformations of the infill itself are recorded by an optical measuring system, the measurements being carried out for a glued-on grid of measuring points. The optical system records the deformations in- and out-of-plane simultaneously. The tests were carried out by Kassel University in close cooperation with SDA engineering GmbH and Arbeitsgemeinsachaft Mauerziegel im Bundesverband der Deutschen Ziegelindustrie e.V. 2.2 Experimental test campaign and loading protocols In the following the results of experimental tests on traditional reinforced concrete frames with masonry infills are presented. Table 1 presents the test programme involving sequential in-plane and out-of-plane loading, out-of-plane loading and combined in-plane and out-of-plane loading.

Table 1. Test program with systems A and B.

Specimen In-plane - Out-of-plane Out-of-plane In plane + Out-of-plane A

Bare frame

B

Traditional

BI

BO

BIO

Specimen A is a RC frame without infill, which was tested to determine the frame contribution. The specimens BI, BO and BIO represent traditional masonry infills in which the masonry completely fills out the frame with direct contact to the columns and the beam. Specimen BI was tested for successively in- and out-of-plane loading. At the beginning in-plane loading was applied up to a level of 1.25% of drift, followed by out-of-plane loading up to maximum displacement of 27 mm perpendicular to the wall. Finally, increasing in-plane loading cycles up to the maximum capacity were applied. Specimen BO was loaded with the pressure of the airbags in out-of-plane direction up to failure. In order to account for unfavourable effects due to openings (e.g. doors or windows) BO is just supported along three sides, realized by an air gap between one of the columns and the masonry infill. The infill of the BIO specimen is supported along all sides with direct contact of the masonry infill to the columns and the beam. The BIO specimen was loaded with an out-of-plane pressure of 5 kN/m2, which was kept constant during simultaneous application of increasing in-plane displacement cycles up to a drift of 1 %. The loading protocols for the specimens BI and BIO are depicted in Figure 4.

Figure 4. Loading protocol of the specimens BI (left) and BIO (right)

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2.3 Reinforced concrete frame and masonry infill The reinforced concrete frames have been designed according to DIN EN 1992-1-1 (2011) and DIN EN 1998-1 (2010) for ductility class DCL. The design was carried out for the loading conditions of a five storey office building with regular geometry and stiffness distribution. The total dimensions of the frame are L = 3.27 m and H = 2.88 m, the infill area is 2.77 x 2.52 m, the dimensions of the quadratic columns are 25 x 25 cm and the top beam was executed with a cross section of 26 x 45 cm. For all specimens the same dimensions for the frame were used. The infill was installed using the perforated hollow clay brick MZ 70 produced by Lücking (2017) with thin layer mortar and non-mortared head joints. The dimensions of the brick are B/H/L = 365/248/249 mm. 3. NUMERICAL MODELS Two numerical models on micro- and macro level are used for the numerical simulations. On micro level each brick is modelled and connected with contact interfaces to the neighbouring bricks and to the reinforced concrete frame. The reinforced concrete frame is idealized by volume elements with embedded reinforcement represented by beam elements. The model was developed in Abaqus (2014) using the “Concrete Damage Plasticity Model” for concrete and masonry. On macro level the diagonal strut model according to Crisafulli (1997) is used in combination with a nonlinear fibre model for the reinforced concrete frame within the framework of the Software SeismoStruct (2016). A more detailed description of the models is given in Kubalski et al. (2016 and 2017) and Butenweg et al. (2017). 4. EXPERIMENTAL AND SIMULATION RESULTS 4.1 Bare frame under in-plane loading (Specimen A) In order to identify the contribution of the reinforced concrete frame, a reference test and simulations have been performed on the bare frame. Figure 5, left shows the results for cyclic loading and Figure 5, right depicts the envelopes in positive and negative directions. The load-displacement curves show a really good agreement of experimental and simulation results. The maximum base shear was obtained with 120 kN at a drift of about 2.0 % (55 mm). At the maximum applied drift of 3.5 % (96.25 mm) the horizontal load decreased to about 95 kN. The degradation of stiffness is caused by cracking of concrete and later on by yielding of the reinforcement, concentrated in the frame corners and the bottom of the columns. The behaviour along the load path is stable and characterized by a high deformation capacity.

Figure 5. Specimen A: Experimental and simulated in-plane hystereses (left) and envelopes (right) 4.2 Traditional system under sequential in-plane and out-of-plane loading (Specimen BI) The resulting experimental and simulated in-plane hystereses curves for the entire loading protocol in Figure 4, left are illustrated in Figure 6, left and the corresponding envelope curves are depicted in Figure 6, right. The simulation results show a satisfactory agreement with the experimental results. The first phase with in-plane loading up to a drift level of 1.25 % (34.4 mm) leads to a maximum base

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shear of 220 kN. At the beginning, the base shear rapidly increases to 110 kN without any stiffness reduction. The subsequent decrease of the stiffness seems to be in relation with first cracks on the backside of the masonry infill. At drifts between 0.3 % (8.25 mm) and 0.4 % (11 mm) the bed-joints start to open and close which leads to stiffness reduction and drop down of the base shear. Afterwards, the base shear increases steadily with a substantially decreased stiffness up to 220 kN. At this point stepwise cracks in diagonal direction of the infill wall were observed. After reaching 1.25 % (34.4 mm) of in-plane drift, the in-plane loading was stopped and frame was returned to the zero position.

Figure 6. Specimen BI: Experimental and simulated in-plane hystereses (left) and envelopes (right) The second phase with out-of-plane loading was executed using the airbags. The pressure in the airbags was increased and decreased in cycles until reaching the out-of-plane capacity of 21 kN which corresponds to a distributed load of 3 kN/m². The displacements at the centre of the wall increased to 29 mm (Figure 7, left), while the restoring force increased continuously with each load cycle. Furthermore it is important to point out that the out-of-plane deformations did not reverse after each cycle, so that significant residual deformations remained. Much more important is the evaluation of the displacements at the end of the BI test along different sections over the wall height as shown in Figure 7, right. The displacements show clearly a movement of the whole infill wall out of the frame accompanied with a tilting of the wall. At the top the displacements increased up to 100 mm, while at the bottom out-of-plane displacements ranging from 10 to 30 mm were measured.

Figure 7. Specimen BI: Out-of-plane hystereses (left) and final out-of-plane displacements (right) This behaviour is somewhat surprising since it was expected that the infill wall is able to generate a stable compression arch with the clay bricks providing a sufficient thickness of 365 mm. The reason for the tilting movement of the wall as a rigid body is an insufficient support along the upper beam of the concrete frame. The inspection of the test specimen after the test clarified that the mortar joint between the uppermost bricks and the reinforced concrete frame was not able to provide a reliable support for the generation of the arching effect for two reasons: first of all, a complete filling of the gap at the top is quite difficult and especially in case of vertically perforated bricks with a percentage of about 60% of voids is nearly impossible. There is always the danger that the mortar on top of the brick drops into the voids, which are filled with highly deformable insulation material. Second of all, a

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damage of the mortar joint took place during the in-plane movement. The damage is even more pronounced for a low quality mortar joint as in the present case. Figure 8, left shows the insufficient mortar joint after the finalized BI test. Although the gap was filled carefully under laboratory conditions, just the gap without any mortar is visible. Another unfavourable effect triggered by the tilting movement is the splitting of the bricks of the uppermost row, which further increases the weakness of the contact area.

Figure 8. Mortar joint (left) and spitted brick (right) at the top after the finalized test BI After removing the out-of-plane load, the second in-plane loading phase started. The load displacement curves in Figure 6 already include the second in-plane loading phase and show a maximum base shear of 230 kN at a drift level of 1.5 % (41.25 mm). During the second in-plane loading phase, the failure is characterized by stair-case like cracks, through bed-joints and head-joints as well as brick failures. The bricks mainly failed on the backside of the specimen, since the brick with a width of 365 mm and the columns with a thickness of 250 mm are just in full contact on the backside, corresponding to the interior side in a real building. Finally it is important to mention, that the out-of-plane displacements increased further during the second phase of in-plane loading and the wall started to move completely out of the reinforced concrete frame. Therefore it was necessary to stop the test procedure to ensure the safety of the equipment, although the in-plane capacity was not reached. 4.3 Traditional system under out-of-plane loading (Specimen BO) The out-of-plane loading was applied in four cycles with increasing pressure of the airbags. The maximum load was reached at 170 kN, which conforms to a uniformly distributed load of 24 kN/m² (Figure 9, left). The corresponding out-of-plane displacement of 4 mm was measured in the centre of the wall. The obtained maximum capacity is quite high and sufficient for accelerations in high seismic hazard regions considering a self-weight of the infill wall of about 1.4 t.

Figure 9. Specimen BO: Out-of-plane hystereses (left) and final out-of-plane displacements (right) The reason for the high capacity is the stable formation of a compression arch in vertical direction. However, the third loading cycle showed the sensitivity of the compression arch with respect to

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changes of the supporting conditions at the top and bottom of the wall. A sudden contact failure occurred, caused by a splitting of the highly stressed bricks at the top and bottom (Figure 10b+c) combined with a sliding failure within the mortar joints. Due to the three sided support, the wall started to move out on the right side with the free edge, while the displacements on the left side were almost negligible (Figure 9, right). The final damage pattern of the specimen BO shows an out-of-plane movement of the masonry infill (Figure 10a) and completely destroyed joint and connecting bricks (Figure 10b+c).

(a) (b) (c)

Figure 10. Damage pattern (a), splitted bricks at top right (b) and bottom right (c) 4.4 Traditional system under combined in- and out-of-plane loading (Specimen BIO) The traditional system was initially loaded with an out-of-plane pressure of 5 kN/m2, which was kept constant during the application of in-plane loading (Figure 4, right). Figure 11, left shows the in-plane hystereses curves for the entire loading protocol and Figure 11, right depicts the corresponding envelopes in positive and negative direction.

Figure 11. Specimen BIO: In-plane hystereses (left) and envelopes in positive and negative direction (right)

The maximum base shear of 250 kN is reached at a drift of 0.65 % (17.88 mm). In the following cycles a substantial reduction of the base shear accompanied with a tilting movement of the masonry infill in out-of-plane direction was obtained (Figure 13c). During these cycles the pressure in the pneumatic airbags decreased and after recovering the pressure to the target value of 5 kN/m2 the infill wall continued to move in out-of-plane direction. The evaluation of the measurement data shows that notable out-of-plane displacements already started to appear at in-plane drifts of about 0.3 % (8.25 mm). At these lower drift values the maximum out-of-plane displacements occurred in centre of the wall. With increasing drift levels in the subsequent cycles the displacements at the top highly increased. The same effect happened at the bottom, but with lower displacement amplitudes. The out-of-plane displacements increased steadily until the end of the test (Figure 12, left). The displacements over the wall height along different sections at the end of the BIO testing protocol are shown in Figure 12, right. The load displacement curves confirm the movement of the wall with maximum displacements at the top of the wall of 120 mm. The increasing out-of-plane displacements were the

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reason to stop the test BIO. The combined application of in- and out-of-plane loading lead to a significant reduction of the maximum attainable drift, to an uncontrollable increase of the out-of-plane displacements at higher drifts and to a substantial decrease of the out-of-plane capacity.

Figure 12. Specimen BIO: Out-of-plane hystereses (left) and final out-of-plane displacements (right) The reasons for the highly unfavourable behaviour under combined in- and out-of-plane loading are more complex in comparison to the sequential loading protocol. The basic problem is again the loss of a stable compression arching effect. As already described in Section 4.2 for the test BI with sequential loading the following reasons can be specified: (a) the low quality of the mortar joint at the top due to the perforated bricks and the small gap at the top, (b) the damage of the mortar joint during the in-plane movement and (c) the splitting of the bricks in the uppermost row caused by exceeding of the transverse tensile strength (Figure 13b).

a) b) c)

Figure 13. Specimen BIO: Damage pattern (a), brick failure in the upper right corner (b) and wall movement (c) However, in case of combined loading another important mechanism is responsible for the reduced resistance in out-of-plane direction. It is quite obvious that the cyclic in-plane displacements of the reinforced concrete frame disturb the load transfer of the out-of-plane loading by generating a compression arch in vertical direction. At higher drift levels the contact area between frame and infill is reduced to the contact length of the compression strut and the formation of an arching effect in vertical and horizontal direction is not-longer possible. The boundary condition provided by the frame is dynamically changing because of the reverse loading character of seismic actions. The detachment areas as well as the compression strut are continuously changing, so that the out-of-plane loading must be transferred with continuously changing support conditions. These unstable boundary conditions are responsible for the strong out-of-plane movement of the wall (Figure 13a). It has to be pointed out, that the out-of-plane movement at the bottom of the wall is much more pronounced under combined loading conditions than under sequential loading conditions as shown for specimen BI. The findings are fully in line with Paulay and Priestley (1992). They explained based on theoretical thoughts that the development of an effective compression arch is extremely unlikely during in-plane loading.

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5. COMPARISON OF THE RESULTS Figure 14 illustrates the comparison for separate in- and out-of-plane loading (A, BI, BO), sequential in- and out-of-plane loading (BI) and simultaneous in- and out-of-plane loading (BIO). The summary of the in-plane load capacities in Figure 14, left clarifies, that the capacities of the traditional infilled frames are about two times higher compared to the bare frame (A: 120 kN, BI: 230 kN, BIO: 250 kN), but the drifts at the ultimate limit state are significantly smaller: A: 3.5 % (96.25 mm), BI: 1.9 % (52.5 mm), BIO: 1.0 % (27.5 mm). Furthermore it is important to emphasize that the tests BI and BIO were stopped because of uncontrolled and steadily increasing out-of-plane displacements. For this reason it is not sufficient to consider just the in-plane behaviour. Figure 14, right shows the out-of-plane hystereses with a really high resistance of 170 kN for the test BO. As the out-of-plane loading for the tests BI and BIO was applied in combination with in-plane loading and not increased up to failure, a direct comparison with the pure out-of-plane capacity of test BO should not be done without considering this.

Figure 14. Comparison: In-plane hystereses (left) Out-of-plane hystereses (right)

Figure 15. Masonry infill with compression arch and changing boundary conditions due to in-plane loading The main reason for the much faster and uncontrolled failure in out-of-plane direction is the deletion of the vertical arching effect, which is necessary for the transfer of the out-of-plane loading to the frame (Figure 15). The arching effect is reduced by a low quality of the mortar joint at the top, the

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damage of the mortar joint at the top caused by in-plane movements and the damage of the bricks in the uppermost row. Another reason for the disturbance of the arching effect in case of simultaneous in- and out-of-plane loading is the detachment between the infill and the frame, due to the in-plane deformation of the frame, which will reduce the contact length and make corners of the infill wall vulnerable places for out-of-plane loading. This effect is even more pronounced if the corners are previously damaged due to in-plane loading. For all these reasons it seems to be too optimistic to count on development of effective compression arch action under simultaneous loading. Figure 15 shows the arching effect for the configuration without in plane loading and out-of-plane movement of the infill wall with the detached areas under simultaneous in- and out-of-plane loading. 6. CONCLUSION Experimental tests and simulations on reinforced concrete frames infilled with modern vertically perforated clay bricks were executed within the joint European project INSYSME. The investigations lead to significantly lower resistances for sequentially or simultaneously applied in- and out-of-plane loading in comparison to tests, carried out with separately applied in- and out-of-plane actions. It can be concluded that either the complex interaction must be taken into account or reliable decoupling measures providing sufficient boundary conditions must be placed in between frame and masonry infill to reach adequate seismic safety levels. 7. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Arbeitsgemeinsachaft Mauerziegel im Bundesverband der Deutschen Ziegelindustrie e.V. In addition, we wish to thank Dr. Udo Meyer and Dr. Thomas Fehlhaber for their valuable suggestions and support during the execution of the European project INSYSME. 8. REFERENCES ABAQUS (2014). ABAQUS Documentation, Dassault Systèmes, Providence, RI, USA.

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