HiCON

Abstract

Flat slab buildings for commercial, office and residential use are a common solution in Portugal, as well as in many countries. Nevertheless, its behaviour under gravity and seismic actions is still not very well understood by the scientific community. The European Code EN1998.1, prescribes in 5.1.1(2)P: “Concrete buildings with flat slab frames used as primary seismic elements in accordance with 4.2.2 are not fully covered” by the EC8 section on concrete structures. This statement results from the lack of knowledge about the stiffness of the slab-column connection and the brittleness of the punching failure. It is also the consequence of insufficient knowledge about the behaviour of flat slabs under seismic actions and the need for further research on the punching behaviour and resistance under cyclic and dynamic horizontal loading.

High strength concrete (HSC) is a relatively recent material that presents higher durability and mechanical resistance when compared with normal strength concrete (NSC). This high performance material gives the opportunity to design slender, lighter, elegant and efficient structures. Another innovative aspect on the use of this material, associated with slenderer structures, is the use of fewer amounts of raw materials in its manufacture, with smaller environment impact. However, insufficient established design formulas have been obstacles to a larger application of HSC in structural design.

The research team already developed a HSC (with a compressive strength around 130 MPa), that was used in some punching tests of flat slab-column connections made entirely of HSC. But being a more costly material, when compared with the NSC, it is imperative to study its rational structural use. One of our objectives is the use of HSC in localized areas at the slab-column connection, being the rest of the slab cast with NSC. This way we can minimize the economic impact of using the HSC and enhance its competitiveness. It will be studied cast in situ and pre-fabricated solutions.

In a preliminary phase some punching test of flat slab-column connections under vertical monotonic loading will be performed, to access the potential benefits of different geometries layouts, with the localized and rational use of HSC. Afterwards, there will be some more tests under reversed cyclic horizontal loading until failure, to better understand the behaviour of this kind of structures under earthquake induced deformations. This subject is especially relevant, because the existent formulations for the quantification of the punching capacity were developed mostly for quasi-static loading tests, and do not take into consideration the degradation due to the cyclic loading that happens during an earthquake.

Another aim of the research team is to perform non-linear numerical modelling of the column-slab connections, and afterwards a parametric analysis. Experimental testing of reinforced concrete structures is an expensive and time consuming process. It requires specimens manufacture, well-equipped laboratory and specialized technical and academic staff to run the tests and analyze its results. Finite Element Analysis (FEA) has lower costs and preparation time, so it is possible to analyze a higher number of cases with FEA than with experimental tests. In this stage it will be used Non-linear FEA, validated by the experimental results obtained in the previous stages, and extend its study to other test parameters.

During the experimental punching tests that will be carried out along this project, it will be used modal identification techniques to access the damage and its evolution during testing, accessing its application to damage detection in flat slab structures. The research team wants to gain some knowledge about this subject, in order to be able to apply it in the monitoring of existing flat slab structures.

The research team has developed large experience on research in subjects as punching, structural retrofitting and FEA of reinforced concrete structures, including works where high performance concrete was used. The research produced by this team has already influenced European code recommendations, as is the example of clauses 6.4.3 and 9.4.3 of the EN1992.1.1. The team Principal Investigator is a member of fib WP 4.2.3 “Punching and Shear in Slabs” and was invited to belong to CEN/TC 250/SC 2/WG 1/TG 4 “shear, punching and torsion”, where the results of this research program will be presented in order to influence future fib publications, national and international codes.

The research proposed in this project aims to increase the knowledge on the behaviour of HSC structures. It also has the purpose to suggest design recommendations for safer structures, particularly under seismic actions. The research is strong-minded in the safety of people and in the economy of the construction industry. These objectives will be reached with experimental and numerical research. Flat slab buildings for commercial, office and residential use are a common solution in Portugal, as well as in many countries. Nevertheless, its behaviour under gravity and seismic actions is still not very well understood by the scientific community. The European Code EN1998.1, prescribes in 5.1.1(2)P: “Concrete buildings with flat slab frames used as primary seismic elements in accordance with 4.2.2 are not fully covered” by the EC8 section on concrete structures. This statement results from the lack of knowledge about the stiffness of the slab-column connection and the brittleness of the punching failure. It is also the consequence of insufficient knowledge about the behaviour of flat slabs under seismic actions and the need for further research on the punching behaviour and resistance under cyclic and dynamic horizontal loading.

High strength concrete (HSC) is a relatively recent material that presents higher durability and mechanical resistance when compared with normal strength concrete (NSC). This high performance material gives the opportunity to design slender, lighter, elegant and efficient structures. Another innovative aspect on the use of this material, associated with slenderer structures, is the use of fewer amounts of raw materials in its manufacture, with smaller environment impact. However, insufficient established design formulas have been obstacles to a larger application of HSC in structural design.

The research team already developed a HSC (with a compressive strength around 130 MPa), that was used in some punching tests of flat slab-column connections made entirely of HSC. But being a more costly material, when compared with the NSC, it is imperative to study its rational structural use. One of our objectives is the use of HSC in localized areas at the slab-column connection, being the rest of the slab cast with NSC. This way we can minimize the economic impact of using the HSC and enhance its competitiveness. It will be studied cast in situ and pre-fabricated solutions.

In a preliminary phase some punching test of flat slab-column connections under vertical monotonic loading will be performed, to access the potential benefits of different geometries layouts, with the localized and rational use of HSC. Afterwards, there will be some more tests under reversed cyclic horizontal loading until failure, to better understand the behaviour of this kind of structures under earthquake induced deformations. This subject is especially relevant, because the existent formulations for the quantification of the punching capacity were developed mostly for quasi-static loading tests, and do not take into consideration the degradation due to the cyclic loading that happens during an earthquake.

Another aim of the research team is to perform non-linear numerical modelling of the column-slab connections, and afterwards a parametric analysis. Experimental testing of reinforced concrete structures is an expensive and time consuming process. It requires specimens manufacture, well-equipped laboratory and specialized technical and academic staff to run the tests and analyze its results. Finite Element Analysis (FEA) has lower costs and preparation time, so it is possible to analyze a higher number of cases with FEA than with experimental tests. In this stage it will be used Non-linear FEA, validated by the experimental results obtained in the previous stages, and extend its study to other test parameters.

During the experimental punching tests that will be carried out along this project, it will be used modal identification techniques to access the damage and its evolution during testing, accessing its application to damage detection in flat slab structures. The research team wants to gain some knowledge about this subject, in order to be able to apply it in the monitoring of existing flat slab structures.

The research team has developed large experience on research in subjects as punching, structural retrofitting and FEA of reinforced concrete structures, including works where high performance concrete was used. The research produced by this team has already influenced European code recommendations, as is the example of clauses 6.4.3 and 9.4.3 of the EN1992.1.1. The team Principal Investigator is a member of fib WP 4.2.3 “Punching and Shear in Slabs” and was invited to belong to CEN/TC 250/SC 2/WG 1/TG 4 “shear, punching and torsion”, where the results of this research program will be presented in order to influence future fib publications, national and international codes.

The research proposed in this project aims to increase the knowledge on the behaviour of HSC structures. It also has the purpose to suggest design recommendations for safer structures, particularly under seismic actions. The research is strong-minded in the safety of people and in the economy of the construction industry. These objectives will be reached with experimental and numerical research.