Behaviour of Flat Slabs under Cyclic and seismic Actions
Civil and Mine Engineering
Fundação da Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa (FFCT/FCT/UNL)
Laboratório Nacional de Engenharia Civil (LNEC)
Centro de Investigação em Estruturas e Construção (UNIC/DEC/FCT/UNL)
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 present research project intends to contribute for the knowledge and understanding of the behaviour of flat slab buildings in sensible aspects, such as:
1) Punching behaviour and resistance under seismic actions.
During an earthquake, the horizontal movement of the ground induces large horizontal inertia forces and lateral drifts in the buildings. The inter-store drift makes the flat slab-column connection rotate and produces moments in that connection. The punching failure mechanism results from the superposition of shear and flexural stresses near the column, and it is characterised by a brittle failure. The punching resistance of flat slabs under seismic actions is not quantified by extensive experimental research. The existent formulations for the quantification of the punching resistance were developed for quasi-static loading tests, and do not take into consideration the degradation of the cyclic loading that happens during an earthquake. It is the aim of this team to analyse experimentally the behaviour and the resistance to punching of the column-slab connection under seismic actions.
2) Punching resistance in slabs made with HPC and UHPC.
High Performance Concrete (HPC) and Ultra High Performance Concrete (UHPC) are recent materials that present higher durability and mechanical resistance when compared with normal concrete. These high performance materials give the opportunity to design very thin, slender, lighter and elegant structures. Another innovative aspect in the use of these materials, associated with the slenderer structures, is the use of lesser amounts of raw materials in its manufacture, with lesser environment and economical impact. However, insufficient established design formulas have been obstacles to a larger application of HPC and UHPC in structural design. Our objective is to research on the punching and shear behaviour of slabs made with HPC and UHPC that will allow a rational use of this type of material.
3) Numerical modelling of the column-slab connection behaviour.
Given its associated lower cost and preparation time, it is possible to analyse a higher number of cases with numerical analysis than with experimental tests. The existing numerical models still do not present satisfactory results when dealing with the punching phenomenon. It is the purpose of this task to implement a discrete element model, based on the work developed by a team member for 2D problems, in order to model 3D punching problems. For this purpose, the numerical tool will be validated against recent experimental work of the project team and against the experimental work produced within the project.
4) Strengthening and retrofitting of flat slab structures.
Retrofitting or/and strengthening works are frequently needed in flat slabs manly due to changes of use and occupancy, increase of applied loads, construction or design errors, material damage or deterioration, etc. The strengthening of flat slabs is usually used for bending, punching and for excessive deflections. Joining the experience of the team members on research on punching, and strengthening of reinforced concrete flat slab structures, it is proposed to develop research on strengthening of flat slabs using post introduced vertical or inclined steel bolts near the column. The proposed solution was already tested by the research team for vertical monotonic loads. It was obtained increments in the punching resistance, but more relevant, higher ductility of the slab-column connection. We already made two experimental tests with cyclic loading, but we need to do more cyclic tests to evaluate the attainable gains in ductility and resistance.
The research team has developed large experience on research in subjects as punching, structural retrofitting and numerical analysis of reinforced concrete structures. 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 research proposed in this project aims to increase the knowledge on the behaviour and resistance of flat slab structures, particularly on the punching phenomena, to develop new technologies for punching resistance and strengthening of flat slab structures, and suggest design recommendations for safer flat slab 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.