Keywords

1 Introduction

The reinforced concrete frame structure with side corridor has good ventilation and lighting effects, and is widely used in densely populated building structures such as teaching and dormitory buildings in the south, once an earthquake disaster occurs, the collapse of such buildings will cause serious casualties and property damage. However, previous earthquake disasters have shown [1,2,3] that the seismic performance of the frame structure with side corridor under strong earthquakes is much lower than the design expectations, and the earthquake damage is very serious and even collapsed, such as the typical Xuankou Middle School teaching building that collapsed in the 5.12 Wenchuan earthquake, which caused over 50 teachers and students to lose their lives. Xuankou Middle School is located in Yingxiu Town, Wenchuan County, and the earthquake damage was very serious. As shown in Fig. 1, the main school buildings of the side corridor style RC frame structure on both north and south sides have completely collapsed, while the inner corridor style RC frame structure and masonry structure buildings located on the same site have achieved “extreme earthquake resistance”. From this, it can be known that the collapse reasons of side corridor frame structure system with typical and double unequal span in the school building are worthy of further studies and reflections, and the seismic performance of the corridor style frame structure system has obvious shortcomings.

The corridor style RC frame structure includes two types, single span corridor type and double span corridor type. The single span corridor type is clearly not recommended in the specifications, and the double span corridor type is still widely used. The biggest characteristic of the double span external corridor frame structure is the large size difference between the two spans and the uneven layout of the plan. Some studies [4,5,6] have shown that the seismic performance of the external corridor frame structure is far inferior to that of the internal corridor frame structure, and the layout of the structure plan and facade has a significant impact on the seismic performance of the external corridor frame structure, especially the arrangement of the infill wall cannot be ignored. At present, the impact of infill walls on the collapse performance of external corridor frame structures is mainly studied through vibration table tests and numerical simulation methods for the macroscopic performance of the structure, and there has not been in-depth and systematic research on collapse mechanism from plane to space, and from experiments to theories.

This article takes the side corridor RC frame structure as research object, the school building of Xuankou Middle School in Wenchuan as example. Through systemic seismic damage analysis, experiments and theoretical research, the earthquake collapse mechanism and reasons of the side corridor RC frame structure are explored. The influence of infilled walls on the seismic performance and collapse mechanism of the side corridor RC frame structure is summarized, and optimization design suggestions and seismic measures are proposed.

Fig. 1.
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Seismic damage to teaching buildings of Xuankou Middle School

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2 Earthquake Damage Analysis

Wenchuan Xuankou Middle School is located in Yingxiu Town, the epicenter of the earthquake. It consists of 14 buildings, including teaching buildings, faculty dormitories, student dormitories, and canteens. The school building is composed of eight independent frame structures separated by expansion joints, as shown in Fig. 1, forming a circular teaching building group. Buildings A, B, D, and E all collapsed in the earthquake. After investigation and analysis, it was found that the four completely collapsed teaching buildings were all side corridor style RC frame structure. The architectural plan of teaching building A is shown in Fig. 2, the transverse direction of the building is basically two spans. The classroom span is much larger than the corridor span. The collapse forms of the four teaching buildings are basically the same. The damage to the frame columns is significantly more serious than that to the frame beams, and the damage to the top of the bottom column is more serious than that to the bottom of the column. The failure of the first floor frame columns resulted in the overall collapse of the second and above floors towards the classroom side. Therefore, why did the corridor style frame structure suffer such serious seismic damage?

Fig. 2.
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Layout Plan of teaching building

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Fig. 3.
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Layout plan of frame units

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The RC frame structure includes three typical types of frame units in transverse axis, the pure frame, fully infilled wall frame in large-span, and fully infilled wall frame in two span. The frame structure has large door and window openings on the three longitudinal axes. The low infilled wall is arranged on the side corridor of the A-axis, the infilled wall with larger door and window openings is arranged on the B-axis, and the partial window wall is arranged on the classroom side of the C-axis, as shown in the Fig. 3. Through seismic damage analysis, it is found that due to the failure of the A-axis frame column on the bottom classroom side, the whole structure collapsed to the classroom side from the second and above floors in a cascading collapse mode. The frame column on the side corridor did not show significant damage, but the top of the bottom corridor column was broken due to the overall collapse towards the classroom side, as shown in Fig. 4.

At present, the seismic performance and the collapse reasons of Xuankou Middle Teaching building have been analyzed by many scholars [6, 7], including the structure's low bearing capacity reserve, poor deformation capacity, poor energy dissipation capacity, low redundancy and poor integrity, etc. These are analyzed only from a macroscopic seismic performance perspective without detailed analysis and suggestions. In summary, all the reasons of the collapse are mainly attributed to the failure mechanism of “strong beam and weak column” in the structure, which is contrary to the design expectation. The root cause is related to the neglect of “super” influence of cast-in-place floor and the associated failure mechanism caused by the setting of filled wall [8,9,10,11]. In the following, through the further analysis of pseudo-static test and shaking table test results, from the two aspects of the impact of floor and filled wall on structure, the serious earthquake damage of the corridor frame structure is traced.

Fig. 4.
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Collapse of teaching building [13]

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3 Research on the Plane Frame Units of Side Corridor Frame Infilled Walls

As we all know, there is a complex interaction between masonry filled walls and RC frame columns, and the unreasonable arrangement of the infilled walls can easily cause the “column hinge” failure mechanism of the frame structure that is beyond expectation [12,13,14]. In order to further our Understanding of the effect of masonry infills on side corridor RC frames, based on Xuankou middle school building, three representative frame units are selected, and quasi-static testson three 1/2-scale masonry-infilled RC frame specimens have been conducted under in-plane reversal cyclic load [15, 16]. The strength, stiffness, energy dissipation capacity and failure mechanisms of one story, two-bay RC frames with aerated block infills were investigated. The parameters of the test specimens are summarized in Table 1, Speimen 1 is the reference bare frame, specimen 2 is fully infilled with aerated block walls in the larger span, and Specimen 3 was infilled with half height aerated block walls. Geometrical dimensions and reinforcements of the two specimens were selected to be the same. The experimental setup consisted of rigid floor, reaction wall, loading equipment, instrumentation, lateral bracing and data acquisition system, as is shown in Fig. 5. All of specimens were tested under reversed cyclic lateral loading simulating seismic action, the specimens were loaded in-plane displacement control, and the loading history is shown in Fig. 6.

Table 1. Parameters of test specimens
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Fig. 5.
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Test setup

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Fig. 6.
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Loading history

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Through analysis of experimental phenomenon, the following conclusions can be drawn. Specimen 1 demonstrates a typical “strong-beam and weak-column” flexure failure mechanism, the top of the column yields earlier than the bottom of the column, and plastic hinges appeared first in columns compared to beams. The whole failure mode of specimen 1 is shown in Fig. 7. The failure mode of specimen 2 is completely different from specimen 1, due to the restraint effect on the frames of masonry infill panels, the diagonal shear failure was extremely serious in the RC frame fully in-filled panels. The whole failure mode of specimen 2 is shown in Fig. 8. Specimen 3 did not demonstrate a typical the “short column” shear failure mechanism under the effect of aerated block walls, flexural/shear crack induced the structure destruction, as shown in Fig. 9. Thus, the failure mechanism of test models were consistent with the actual damage of prototype structure in Wenchuan earthquake.

Through comparative analysis of experimental results, the initial stiffness of the large-span filled wall frame is 2.43 times that of the pure frame, its bearing capacity is 1.77 times that of the pure frame, and its energy dissipation capacity is 1.5–3 times that of the pure frame. However, the initial stiffness of partially-filled wall frame is 1.56 times that of the pure frame, and its bearing capacity is 1.26 times that of the pure frame, but its energy dissipation capacity is lower than that of the pure frame and the fully-filled wall frame, which indicates that the contribution of the partial-filled wall to the earthquake resistance of the structure is not obvious. When the strength of the filled wall material is too high, it is easy to cause the short column effect, and then the energy dissipation capacity of the frame structure is reduced [17,18,19]. Therefore opening windows through the walls is unfavorable to the seismic resistance of the frame structure.

Fig. 7.
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Ultimate failure modes of specimens 1 [13]

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Fig. 8.
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Ultimate failure modes of specimens 2 [13]

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Fig. 9.
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Ultimate failure modes of specimens 3 [13]

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In addition, based on specimen 3, the influence of the height Hw of the infilled wall and the masonry material on the failure mechanism of the frame structure was studied through nonlinear finite element numerical analysis [20]. The infilled wall is equivalent to the diagonal support of the frame column. As the height of the infilled wall arrangement increases, the height of the upper free section column decreases, and the bending point of the frame column moves upward, as shown in Fig. 10, the internal force is redistributed. Through analysis, it can be seen that when the height of the infilled wall is big, the strength of the masonry material is high, and the shear span ratio of the upper free section column is reduced, the upper column is prone to short column shear failure. When the wall is fully infilled, due to cracking and damage at the corners of the infilled wall, short columns are formed in the upper section of the frame column. This results in shear failure at the top of the column, as shown in specimen 2. When the height of the infilled wall arrangement is relatively small, due to the upward displacement of the yield position and reverse bend point of the longitudinal reinforcement of the constrained column, it is easy to cause bending failure of the short column in the frame structure. For example, in specimen 3, although the bending failure of the short column is not as harmful as the shear failure of the short column, it will also reduce the seismic performance of the frame structure. From this, it can be known that the collapse of the side corridor style school building in Xuankou Middle School is closely related to the layout of the infilled wall. It is obviously unreasonable to ignore the effect of the infilled wall in the structural design process.

Fig. 10.
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Longitudinal reinforcement stress curves along the column height

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4 Research on the Spatial Framework Model of Side Corridor Frame Infilled Walls

In order to further explore the impact of infill walls on the failure mechanism of frame structures, analyze the collapse mechanism of side corridor frame structures, based on the teaching building of Xuankou Middle School as a prototype, a comparable 1:5 scale side corridor infilled wall frame test models were designed, as shown in Fig. 11. The experimental model is made of particle concrete and Q235 galvanized iron wire, and the infill walls are constructed with MU3.5 concrete hollow blocks and Mb5 mixed mortar, and the similarity relationship between the scaled model and the prototype is derived according to the consistent similarity rate of seismic simulation experiments [15]. The layout of the infilled panels is designed according to the prototype of the collapsed teaching building of Xuankou Middle School, and the seismic motion includes 15 input conditions of El Centro wave and Wolong wave.

Fig. 11.
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Basic overview of experimental model

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Through the analysis of the results of the model vibration table test, it was found that the infilled wall cracked firstly as the first seismic fortification, the top columns of frame yielded earlier than the bottom columns, and the frame column yielded earlier than the frame beam. The fully infilled walls caused oblique shear failure at the top of the column, and the partially infilled walls caused short column bending failure at both ends of the free section column. In conclusion, the arrangement of infilled walls has changed the internal force redistribution and failure mode of the frame structure, and the damage of columns is very serious, which is consistent with the results of the pseudo static test of the frame elements.

In addition, the experiment shows that there is a significant difference in the failure of the three frame units in transverse direction, as shown in Fig. 12. The side frame of axis ① and ③ is placed with infilled walls in large spans, and there is obvious cracking and damage at the top of the column. However, the middle frame of axis ② is not equipped with infilled walls, and the cracking and damage are not significant. From this, it can be seen that after being fully infilled with masonry walls, the stiffness of the two frames on both sides significantly increases, and the allocated seismic shear force also increases, resulting in severe shear failure at the top of columns in the axes ① and ③. The failure phenomenon is consistent with the pseudo static frame element test. The damage to the frame columns in the middle axis ② is not significant, especially in the early stage of loading when cracking is not obvious. With the failure of the infill wall, the overall stiffness of the structure decreases. The damage to the A-column in the second axis is more severe in the later stage.

Fig. 12.
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Destructive behavior of model A [13]

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Through comparative analysis of the results of longitudinal frame unit tests, it can be seen that both the A-axis and C-axis are arranged with partial infilled walls, resulting in a short column constraint effect. There are obvious horizontal cracks at the top of the frame columns and walls, as shown in Fig. 12. The transverse spacing between AB columns is relatively large and fully infilled walls, while the longitudinal A-axis is arranged with partial infilled walls, resulting in significant damage of A-axis frame columns comparing to other axis columns. The spacing between BC columns is relatively small, and although the longitudinal C-axis frame columns are arranged with partial infilled walls, the damage is not significant.

Fig. 13.
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Final failure modes of model A and model B [13]

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The model has two spans in both transverse and longitudinal directions, and it has fewer arrangements of infilled wall and weaker stiffness in the longitudinal directions than the transverse directions. Therefore, before collapse, the A-axis frame column on the first floor failed and tilted along the longitudinal direction. Finally, due to the large self-weight on the side of the large-span classroom, the structure collapsed layeredly towards the classroom side, as shown in Fig. 13. In conclusion, the collapse mode of the test model is basically consistent with the collapse mode of the school building at Xuankou Middle School. Therefore, opening larger doors and windows in the infilled wall causes uneven stiffness of the structure and column hinge failure, and reduces the energy dissipation performance and anti-collapse ability of the structure, which is the main reason for the collapse of the Xuankou Middle School teaching building [21].

5 Conclusions

Through experimental research and related numerical analysis of RC frame units and spatial frame models with masonry infilled walls, the collapse mechanism and reasons of the corridor style frame structure school building were deeply explored, and the following conclusions were drawn:

Firstly, based on the characteristics of the teaching building of Xuankou Middle School, which is a typical side corridor frame structure, the difference of transverse span is large. When the masonry infilled walls are not arranged reasonably, it is easy to cause uneven stiffness of the whole structure, and local frame columns fail, thus cause the “column hinge” failure mechanism. From the analysis, it can be seen that the frame columns on the long span classroom side are severely damaged, while the damage on the short span corridor side is not severe. Therefore, it is recommended to increase the bearing capacity of the large span frame columns on the classroom side, it is necessary to increase the cross-sectional size or reinforcement during design.

Secondly, masonry infilled walls can have stiffness and constraint effects on the frame structure, with both advantages and disadvantages. However, unreasonable layout of infill panels can turn the advantages into disadvantages. The layout of the infilled walls in the side corridor style school building of Xuankou Middle School is obviously unreasonable. The infilled walls frame in the longitudinal direction has larger door and window openings, while the transverse frame has a large span and is infilled with infilled walls. The stiffness ratio of the longitudinal and transverse frames is imbalanced. In addition, the partial infilled walls in the longitudinal frame did not achieve the purpose of the first seismic fortification, and at the same time it had a negative impact on the energy consumption of the frame structure. Therefore, it is recommended that the longitudinal infilled wall frame at the side of classroom should not arrange thorough windows. To improve the stiffness and energy dissipation capacity of the structure, and avoid the “short column” effect, the measures of setting up a wall between windows or replacing masonry walls with lightweight infill wall panels are recommended.

Finally, the arrangement of infilled walls increases the stiffness of the frame structure and enhances its load-bearing capacity. However, the increase of the load-bearing capacity of the frame columns cannot offset the increased seismic force caused by the increase in stiffness and the failure of column end correlation caused by constraints. From this case, it can be known that the influence of stiffness on the anti-collapse ability of building structures is crucial. It's not the fact that the higher the stiffness, the better the seismic performance of the structure. It is necessary to achieve the best balance between stiffness and ductility, which requires further quantitative exploration.