In flat-slab frames, which are typically designed as secondary seismic structures, the shear failure of the slab around the column (punching failure) is typically the governing failure mode which limits the deformation capacity and can potentially lead to a progressive collapse of the structure. Existing rules to predict the capacity of flat slab frames to resist imposed lateral displacements without losing the capability to bear gravity loads have been derived empirically from the results of cyclic tests on thin members. These rules account explicitly only for the ratio between acting gravity loads and resistance against concentric punching shear (so-called Gravity Shear Ratio). Recent rational models to estimate the deformation capacity of flat slabs show that other parameters can play a major role and predict a significant size effect (reduced deformation for thick slabs). In this paper, a closed-form expression to predict the deformation capacity of internal slab-column connections as a function of the main parameters is derived from the same model that has been used to develop the punching shear formulae for the second generation of Eurocode 2 for concrete structures. This expression is compared to an existing database of isolated internal slab-column connections showing fine accuracy and allowing to resolve the shortcomings of existing rules. In addition, the results of a testing programme on a full-scale flat-slab frame with two stories and 12 columns are described. The differences between measured interstorey drifts and local slab rotations influencing their capacity to resist shear forces are presented and discussed. With respect to the observed deformation capacities, similar values are obtained as in the isolated specimens and the predictions are confirmed for the internal columns, but significant differences are observed between internal, edge and corner slab-column connections. The effects of punching shear reinforcement and of integrity reinforcement (required according to Eurocode 2 to prevent progressive collapse after punching) are also discussed.
The SlabSTRESS project investigated the behaviour of a full-scale Reinforced Concrete flat slab building under both gravity loads and seismic lateral actions. The two-storey structure had 9.0 m × 14.5 m plan dimensions, three bays in the loading direction and two orthogonally. This layout enabled to test simultaneously three types of slab-column connections: interior, edge and corner. The specimen also accommodated several variants in the construction details: a different distribution of longitudinal reinforcements, with or without punching shear reinforcement. The mock-up was connected to the ELSA reaction wall by four hydraulic actuators of 500 kN each. Pseudo-dynamic tests have been performed first on the specimen treated as part of a building with two sub-structured (numerically simulated) ductile shear walls to study its seismic performance at the Serviceability and Ultimate Limit States. Quasi-static tests have been performed on the specimen – without the numerical shear walls – subjected to cyclic loading with increasing displacements and varying the magnitude of vertical loads. The setup was designed to measure internal actions in each column and to capture any local deformation of slabs and columns in real time. Special transducers able of measuring flexure, shear, and axial load were ad-hoc designed and installed at mid-height of each column, along with a network of transducers (inclinometers and displacement transducers), the latter installed on only eight of the twelve columns. Finally, a distributed data acquisition system converted the 192 measured signals routing them to a central database. This optimised setup was used for the first time on a specimen of such dimensions and complexity. It allowed the integration of global and local measurements and thus provided an in-depth understanding of the role of individual slab-column joints and their interaction with the overall response of the structure to different levels of lateral actions.
Full-scale testing of a two-storey flat slab structure is reported, undertaken in the SlabSTRESS research project; the construction and testing were planned and carried out at the ELSA laboratory of the European Commission's Joint Research Centre. The dimensions are three bays by two, spans 4.5 and 5 m, slab thickness 0.2 m, interstorey height 3.2 m. Two different longitudinal reinforcement details were considered; welded studs shear reinforcement was provided only in the second floor slab. The testing program included seismic tests for service and ultimate actions, using the pseudodynamic technique with virtual walls. To this aim a building structure was designed with primary walls and the flat slab frame as secondary element. Cyclic loading tests followed up to ultimate drift capacity of the structure. The sequence of tests included strengthening of a set of damaged connections using bolted bars in holes drilled through the slab, followed by cyclic testing to failure. The instrumentation was provided for the global response and the connections with local rotations in the columns and slab; cracking around the columns was measured with through-crack sensors; a measurement system for internal forces and moments was included within the columns. The results show the response with deformations and damage for the different loading conditions up to failure. The results obtained on a full-scale structure extend and confirm the knowledge in the literature, mainly based on isolated connections and/or small-scale samples.
This paper presents a critical review of the state of the art of experimental research concerning the seismic response of reinforced concrete flat slab frames. After a summary of tests on connections, the paper examines tests carried out on frames with gravity and cyclic lateral loading, and shake table tests; scaled specimens and one real scale study are included. A discussion of the results reached so far is provided focusing on the global response, the different load types and effects; the ultimate rotations at failure in relation to the gravity shear and a classification of different failure modes for different types of connections. Based on this analysis, the research needs are highlighted. An experimental program launched to address these open questions is described. Further open topics are highlighted.
Flat slab buildings for commercial, office and residential use are built in many countries. Yet, their behaviour under seismic and gravity actions is still not very well understood. Many studies have been carried out in North America but European research is lagging behind and currently Eurocode 8 does not cover the design of buildings with flat slab frames. The SlabSTRESS project (www.slabstress.org) was therefore undertaken at the ELSA Reaction Wall of the Joint Research Centre, within the Transnational Access activities of the SERA project (www.sera-eu.org). The test specimen is a full-scale two-storey flat slab structure with three bays in the direction of loading and two in the orthogonal one. This layout allows testing interior, edge and corner slab-column connections. The plan dimensions are 9.0 m ×14.5 m. The slab of the second storey has punching shear reinforcement, while the one of the first storey does not. In addition, half of the slab at each floor is constructed with uniformly distributed horizontal reinforcement and the other half with the same amount of horizontal reinforcement mostly concentrated close to the columns. Pseudo-dynamic tests have been performed first on the specimen treated as part of a building with two sub-structured (numerically simulated) ductile shear walls to study its seismic performance at the Serviceability and Ultimate Limit States. Quasi-static tests have been performed on the specimen-without the numerical shear walls-subjected to cyclic loading with increasing displacements and varying the magnitude of vertical loads. The aim is twofold: first to verify the seismic performance of flat slab frames in a structure with earthquake resistant ductile walls; secondly to study the performance of the system beyond the design displacements. The results of the project will ultimately provide the basis for a deformation-based design procedure and will allow the improvement of the Eurocode and fib Model Code punching shear models and reinforcement detailing rules to account for seismic loading.
A research entitled Slab STRESS - Slab STructuralRESponse for European Seismic Design, within the European project SERA - H2020-INFRAIA-2016-1, is currently in progress to study the response of flat slab floors under combined gravity and lateral loads. The project is being carried out by a group of European institutions, led by the Politecnico di Milano together with EPFL Lausanne, UNOVA Lisbon and UTCB Bucharest. A real scale flat slab building will be tested at the JRC ELSA reaction wall facility in Ispra (IT). The Seismic European Code ENV 1998 for does not cover flat slab buildings, that are intensively used because of the reduction in construction costs and time, the simplicity of the geometry and possibility to increase available volumes. The paper describes the research objectives, the specimen and features of the testing program.
Blind Competition Program A research program at Politecnico di Milano (D. Coronelli), EPFL Lausanne (A. Muttoni), UNOVA Lisbon (A. Ramos) and UTCB Bucharest (R. Pascu) is being developed within the SERA-TA program - H2020 https://sera-ta.eucentre.it/index.php/sera-ta-project-02/.