PAVIORS LAING TRAVEL AWARD 2016 REPORTAndri Setiawan 3

Understanding the Punching Failure Phenomenon of Flat Slab Buildings in Earthquake-Prone Regions through Experimental Test

A. Background of the ResearchAs generally known, Indonesia is located within the ring of fire region and it has experienced approximately 35 major earthquakes with average magnitude of 7.2 SR since 2000 (Fig. 1). At the same time, Indonesia is currently the fourth biggest populated country in the world where the provision of sufficient housing is of major concern, especially in the big cities which are mostly concentrated in Java Island. Flat slab structural system which has proved to produce many practical advantages (e.g. short construction time, less required formwork, and simple reinforcement arrangement) may become one of the most potential solution. However, the performance of flat slab buildings under seismic events is not fully understood to date. Therefore, this research focuses on developing a better understanding in terms of punching shear of flat slab buildings subjected to seismic loading. It is believed that the results of this research would be beneficial, not only for Indonesia but for another country having the similar issues as well.Fig. 1-Indonesia seismicity map(Source: shipdetective.com)

B. Experimental Test Visit Funded by Paviors Laing Travel AwardWith the financial support provided by the Worshipful Company of Paviors, the author visited Prof. Pinho Ramos research group in Faculdade de Ciencias e Tecnologia, Unversidade Nova de Lisboa (FCT-UNL), Portugal from September to October 2016. Structural research group in FCT-UNL invented a novel testing frame which can be used to perform realistic reversed-cyclic loading test for 2/3 scale of flat slab specimens. The novelty of the test setup is the capability to simulate the shifting of the contra-flexure points which enables the redistribution between hogging and sagging moments. Besides, this novel system is also considered as a self-equilibrating system where the mechanism of balancing the external load occurs within the system itself. Thus, the gravity shear ratio at the connection can be kept constant during the test, even when cracking starts to take place at the column region. Therefore, more realistic simulation can be performed which resulting in more accurate and reliable test outcome. More detailed explanation about how the test setup works can be seen in the recent publication (Almeida et al., 2016). This testing frame has been used for the whole flat slabs project in the university that has been started in the last three years (Fig. 2).

Fig. 2-Experimental test setup for flat slabs subjected to reversed-cyclic loading in the laboratory of FCT-UNL, Portugal

In total, there were at least 15-20 slab specimens have been successfully tested with this testing frame and some of the test results have also been published into scientific journals. The main differences for these tested slabs were: the magnitude of the gravity loading; the presence of shear reinforcement, either as the form of conventional stirrups or post-installed shear bolts; the use of high strength concrete (HSC); and steel fibre reinforced concrete (SFRC) (Fig. 3).

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Fig. 3-Previously tested flat slabs in FCT-UNL, Portugal with: a) post-installed shear bolts and steel fibre reinforced concrete (SFRC); b) high-strength concrete (indicated with the dark grey region)

During the visit, there were two other flat slab specimens tested using the same test setup. Although both contained the same amount of transverse reinforcement as a form of shear studs and the same gravity shear ratio, the one that was tested earlier had only 3 radial layers whereas the later one had 5 radial layers. Therefore, the main interests of this test are to observe how the shear studs increase the performance of slabs against seismic loading which will be compared to the previously tested slab without shear reinforcement and slabs with conventional stirrups and to compare the influence of the extension length of the shear-reinforced zone to the failure mode.The first slab specimen with three layers of studs was tested on 6th October 2016. Prior to the application of the lateral loading, uniform gravity load was applied through eight steel plates (Fig. 2) to represent the constant gravity shear ratio of 0.5 at the slab-column connection. The first slab specimen reached its peak lateral load capacity at around 3.0% drift and the hysteresis behaviour was relatively stable without significant strength deterioration for the successive cycles. However, at the second cycle of 4.0% drift, an abrupt punching failure occurs where the column is punched through the slab and a distinct punching cone can be observed outside the shear-reinforced region (Fig. 4a).The second specimen with five layers of studs was tested on 18th October 2016. It consists the same amount of shear reinforcement and gravity shear ratio as the first slab specimen. By contrast, this specimen did not fail in the same punching drift with the first specimen and the loading can be continually applied until reaching 6.0% drift level. At this drift level, the test wasstopped since the measuring tools (LVDT) has reached their maximum measurement capacityand they cannot measure any further deformation. Therefore, thisdrift limit was assumed as the failure drift even before the slab is fully punched through. For the second specimen, no distinct punching cone can be observed (Fig. 4b) and the failure mode is more similar to the flexural failure mode rather than punching failure.

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Fig. 4-Top view of the slab specimens after failure: a) first specimen with 3 layers of shear studs; b) second specimen with 5 layers of shear studs

In conclusion, two important findings can be highlighted from the current test results:1. Shear studs proved to significantly increase the ductility of the slab-column connections subjected to seismic loading compared to the control specimen without shear reinforcement. Furthermore, it is also proved that shear studs perform better than conventional stirrups. Not only they may achieve higher drift limit, it also produces more stable hysteresis behavior, without significant loss of strength, whereas slab specimen with stirrups tend to experience more significant degradation before failure. This proves that better anchorage condition of the studs, provided by the head, produces superior seismic performance.2. Comparing the first and second slab specimen with shearstuds, it can be concluded that it is not only important to consider the amount of shear reinforcement ratio and successive spacing, but the extension length of the shear studs also needs to be considered carefully at design stage. With sufficient length of theshear-reinforced zone, failure outside thiszone can avoided which results in higher ductility of the specimen before punching occurs.

AcknowledgementsThe author would like to express his sincere gratitude for his supervisor, Dr. Robert Vollum and Dr. Lorenzo Macorini for their continuous help and support during this research project as well as Prof. Antonio Ramos for giving an opportunity to visit and observe the experimental test conducted in Faculdade de Ciencias e Tecnologia, Unversidade Nova de Lisboa (FCT-UNL), Portugal. The hospitality provided by the entire structural engineering research group (Brisid, Helisa, Florian, Miguel, Ricardo, Andre, Hugo, and Dinarte) in FCT-UNL is gratefully appreciated.

References1. Almeida, A. F. O., et al. (2016). "Punching behaviour of RC flat slabs under reversed horizontal cyclic loading." Engineering Structures 117: 204-219.2. Faria, D., et al. (2010). Punching of reinforced concrete slabs and experimental analysis and comparison with codes. Proceedings of IABSE-Fib Codes in Structural EngineeringDevelopments and Needs for International Practice, Cavtat, Dubrovnik, Crocia.3. Gouveia, N., et al. (2013). Punching of steel fibre reinforced concrete flat slabs. Proceedings of fib symposium Tel-Aviv.4. Gouveia, N. D., et al. (2014). "SFRC flat slabs punching behaviourExperimental research." Composites Part B: Engineering 63: 161-171.5. Incio, M. M. G., et al. (2015). "Punching of high strength concrete flat slabs without shear reinforcement." Engineering Structures 103: 275-284.6. Ramos, A., Duarte, I. and Lcio, V.; Strengthening of Flat Slabs with Transverse Reinforcement, Proceedings of CCC 2008 - Challenges for Civil Construction, FEUP, Porto, April, 2008.