Fire Design of Concrete Structures based on a Levels-of-Approximation Approach

Miguel Fernández Ruiz, Miguel Gomez
fib Symposium 2015, 2015
Conference paper

Abstract

Concrete structures, contrary to steel or timber structures, have not been traditionally considered as significantly sensitive to fire conditions. As a consequence, their performance in fire conditions has generally been assessed on the basis of quite simplified rules provided in codes of practice, mostly dealing with minimum geometric requirements in terms of thickness and concrete cover. Accidents under fire conditions have nevertheless revealed a certain level of vulnerability of concrete structures in fire and have thus shown that code regulations are potentially insufficient to cover this aspect of design. As a reaction to this level of simplicity, some codes are currently available with specific and very detailed provisions with reference to fire design in concrete structures, such as EN 1992-1-2, covering a wide number of simplified and advanced design procedures. Unfortunately, the hierarchy of these procedures and which one is most suitable for a given situation is sometimes unclear for designers. In addition, it is probably not necessary to perform rather complex and detailed analyses for all types of structures, but the level of refinement can be adapted to the sensitivity of the structure to fire conditions and to its significance for the society. In this paper, a possible approach for consistent design of concrete structures under fire conditions is proposed based on the Levels-of-Approximation philosophy. The Levels-of- Approximation approach has been successfully introduced into the new Model Code 2010 for a number of problems (such as shear, punching and 2nd order effects) and allows refining the accuracy of the analysis when necessary. This allows keeping simple rules for most cases but provides a general frame for assessing complex or sensitive structures. In addition, it does not only incorporate calculation methods, but specifies which ductility requirements are to be fulfilled in order to ensure a correct applicability of each method.

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Miguel Fernández Ruiz, Miguel Gomez
fib Symposium 2015, 2015
Conference paper

Abstract

Official source

https://infoscience.epfl.ch/record/208039?ln=en

About this result

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Revisiting Detailing Rules for Bending Bars, Anchorage and Minimum Shear Reinforcement in Concrete Structures

Frédéric Monney

Reinforcement detailing rules describe the shape, geometrical dimensions, and amount of steel to be placed in reinforced concrete structures. They allow for simple and fast designs, account for several effects neglected in the design, ensure a satisfactory behaviour under serviceability conditions, a sufficient robustness, and an adequate behaviour in case of unexpected actions. Most detailing rules provided in codes have not been updated to correspond to current manufacturing processes, material performances and scientific knowledge. They are often based on "rules of good practice" which, while satisfactory, lack a sound scientific basis and may not be needed. This is one reason why detailing rules may differ amongst countries and design codes. Some of these rules are overly conservative, in particular when evaluating existing structures, while others may neglect significant effects. Even though these rules play a major role in the economy and safety of concrete structures, little research has been performed in this domain. Several detailing rules have been identified as needing additional investigations to verify their adequacy to current practical needs and recent technological evolutions. This thesis presents a research programme on three main detailing rules: bend detailing and required mandrel diameter, anchorage of shear reinforcement with bends and hooks and minimum amount of shear reinforcement. This research aims at developing mechanical models, simplified formulas and detailing provisions, on the basis of experimental results. These were obtained using state-of-the-art measurement techniques such as Digital Image Correlation or Fibre-Optic Measurements.Bends and hooks of reinforcing bars are usually obtained by plastic bending of the bars against mandrels. Codes specify minimum mandrel diameters to avoid splitting or spalling failures that may limit the resistance of the detail. The thesis includes a research programme on the detailing of bends and the required mandrel diameter to avoid concrete failures leading to spalling of the concrete cover. A mechanical model for the design of bent reinforcement was developed. A simplifying standard bending procedure is proposed, in which details formerly requiring various bend diameters can be obtained by using a single mandrel, allowing for faster automated manufacturing of bent reinforcement.Bends and hooks at the end of the bars are efficient solutions for the anchorage of reinforcement. However, these details are relatively sensitive to the cracking state of the surrounding concrete. For shear reinforcement, spalling failure of the concrete cover for bars close to the concrete surface can occur. The thesis includes an investigation on the mechanical response and performance of bend and hook anchorages. A mechanical model and practical considerations on the activation of shear reinforcement in beams are presented.The required minimum amount of shear reinforcement in beams and slabs has been discussed for decades. It is crucial to ensure economic and safe new structures and to accurately assess existing ones. The investigation shows that the shear behaviour is strongly dependent on the amount and the post-yield response of the shear reinforcement (ductility class). For all investigated detailing rules, the implementation of these findings into codes is discussed, highlighting the consistency of the recent changes, particularly with the new generation of Eurocode 2.

EPFL2022

Interaction sol-structure dans le domaine des tranchées couvertes

Sylvain Plumey

The present research on cut-and-cover tunnels gives a theoretical contribution to the understanding of the behaviour of these structures up to their ultimate limit state. A simplified method inspired on the convergence-confinement method is investigated and applied systematically to several practical cases. This method, based on the failure mode of the structure, gives an approximate solution of the equilibrium between the soil and the structure. It enables a better understanding of the complex soil-structure mechanisms, typical of these constructions, constituting in this way a design tool. The behaviour of the soil which interacts with the structure comprises typically two main phases. The elastic phase is followed by the progressive yielding of parts of the soil until a plastic mechanism is formed (plastic phase). Depending on the geometrical and the mechanical parameters, the structure takes more or less advantage of the soil contribution in carrying the loads. The main conclusion of this work is the demonstration of the existence of different modes of behaviour. Three main modes of practical interest, defined mainly by the phase of the soil governing the structural behaviour, are distinguished : elastic soil, elasto-plastic soil and completely plasticized soil. The identification of the mode then enables an efficient design and analysis of the structure. The theory of plasticity (upper bound) was applied to two soil-structure systems, a surface strip footing submitted to a centred load and a lateral wall of a frame-type cutand- cover tunnel under construction, aiming at studying their behaviour at the ultimate limit state. This study clearly emphasizes the favourable or unfavourable role played by the soil-structure interaction in the collapse of such structural systems. A proper consideration of the structure failure kinematics is thus fundamental to a proper representation of the ultimate limit state. A new safety format is proposed to define the ultimate limit state when the finite element method is used. This safety format is compatible with the new SIA codes of practice and clarifies the structural design procedure. The research also showed that the structure's ductility plays a major role in the development of the structural soil capacity. Several ductility limits are emphasized for cut-and-cover tunnels. For frame structures, the deformation capacity of the top slab is very small if no shear reinforcement is provided in the zones of significant shear forces. Stirrups are thus recommended in these structures. For arch structures, the spalling of the concrete cover can limit the deformation capacity of the structure. Experimental tests performed within this research showed that this phenomenon was affected negatively by plastic deformations of the steel reinforcement and by reinforcement splices and anchorages. SIA 262 (2003) code of practice design criterion is judged insufficient. The structural design of cut-and-cover tunnels with important plastic redistributions is thus possible only if certain ductility conditions are fulfilled.

EPFL2007

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The Triemli Personalhäuser are three equal 15-story buildings located on the Zürich Triemli Stadtspital campus and erected between 1964 and 1969. Cast-in-place reinforced concrete (RC) slabs and walls form the building cores and their surrounding corridors. The rooms are arranged around these cores. The load-bearing intermediate walls, made of prefabricated masonry, support thin precast slabs used as permanent forms. A RC layer is cast over these precast slabs and connects the room slabs with the cast-in-place slabs of the corridors and cores. The self-supporting facade consists of precast RC panels. The City of Zürich plans the deconstruction of these three buildings, thus making available a large amount of RC elements composing the structure and the facades of these buildings. Little-known and rarely implemented, the reuse of concrete elements from obsolete buildings in new projects is a sustainable approach that promotes a circular economy. When reusing, the components of obsolete buildings are carefully dismantled without being crushed. They are then cleaned, possibly repaired or trimmed, and reused without many transformations in a new project, maintaining their shapes, technologies, and mechanical properties. In addition to maintaining the embodied energy and history of the reused components, reuse allows the construction industry to reduce demolition waste, greenhouse gas emissions, and material consumption. This report is a preliminary resource assessment and aims at inventorying and assessing all structural elements of the Triemli Personalhäuser, focusing on their potential value for reuse. Both precast and cast-in-place RC elements are included. They are part of the load-bearing structure or are self-supporting such as the precast facade elements. The proposed methodology allows identifying all properties needed to evaluate the potential for reuse of an element: geometry, material properties, current condition, aesthetics, accessibility, resistance, future durability and environmental impacts. After reviewing available reports and drawings on the buildings, onsite visits are carried out to complete the information and visually inspect the structural elements. During the inspection, the elements are assessed with regards to their suitability for reuse and their condition is classified into a five-grade scale. The investigations are completed with destructive and non-destructive testing of the material properties. Together, Buildings A, B and C are made up of approximately 7 000 m3 of materials constituting their load-bearing system, with approximately 2 500 m3 of precast concrete, 3 400 m3 of cast-in-place concrete and 1 100 m3 of masonry. Of this total, approximately 4% of the volume is dropped from the analysis due to the bad condition of the elements, namely the balcony slabs, the roof slab, and the external stairs. The other elements are in a good or acceptable condition and ae inventoried and analysed in detail. The inventoried elements are divided into 5 categories: (1) facade elements; (2) slab elements; (3) wall elements; (4) column elements; and (5) staircases. Each of these categories are subdivided into a certain number of element types for which a complete factsheet is prepared, including pictures, drawings and useful information on their condition. The volume and weight of each element types are given, as well as their share of the total material volume. The embodied global warming potential (in kgCO2eq) for fabrication and demolition of the elements is also calculated. The results of the investigation on material properties confirm sufficient compressive strength for all elements. The carbonation depths measured on the cores are lower than the cover thickness of the reinforcement. Thus, the concrete is not carbonated in the reinforcement areas and the risk of corrosion is kept low, insuring a good durability of the elements. This document should serve as a base for designing and planning future reuse applications for the concrete elements extracted when deconstructing the Triemli Personalhäuser. The information presented here will help the planners to prioritize the reuse strategy on the elements in the best conditions, with the largest volume share and thus with the largest embodied global warming potential.

EPFL2022

Interaction sol-structure dans le domaine des tranchées couvertes

Sylvain Plumey

EPFL2007

Revisiting Detailing Rules for Bending Bars, Anchorage and Minimum Shear Reinforcement in Concrete Structures

Frédéric Monney

EPFL2022

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EPFL2022