Teses

In this Thesis a study about the behavior of steel structures on fire was carried out. Comments about fire models (temperature-time curves of hot gases) are made and the expressions to determine the thermal action on the structures and its effect, the temperature on steel, are derived. The influence of load fire, opening factor (ventilation) and section factor (massivity factor) is discussed. Stress-strain curves of steel, including the creep phenomena, under high temperature are presented. A comparison between the safety criteria for the accidental situation of structures in fire, based on Brazilian and European Standards, is made. A simplified method of steel structures fire design is recommended and the influence of geometrical and material non linearities and thermal deformation are analyzed.

Neste trabalho, foi deduzida a equação geral para a determinação do momento crítico à flambagem lateral de vigas com seção monossimétrica em regime elástico e analisados diversos casos particulares. Foi feita uma comparação às recomendações da Norma Brasileira (ABNT NBR 8800:1986 e normas norte-americanas AISC/LRFD/1986 e AISI/1986). Foram indicadas e comentadas as limitações no uso das expressões presentes nessas Normas. Foi incluído também um resumo dos principais resultados numéricos para uso no cálculo expedito do momento crítico em esquemas estruturais convencionais.

Reinforced concrete beams are submitted to thermal deformations when exposed to fire. The lengths of the spans elongate, a fact that triggers the horizontal displacement of their supports, and they begin to bend sharply, resulting in their rotation. If these deformations are hindered by the support conditions of the element or by surrounding structural elements, for instance, additional efforts will act on the beams in order to modify their performance when facing the action of fire. Studies have pointed out that the effects of such efforts may be beneficial to the fire resistance of the beams; however, in the few researches focused on the experimental analysis of this issue, the restraints were admitted only in an isolated way, i.e., the beams were either submitted to axial or to rotational restraints. Their coupled effect, more representative of what occurs in reality, and the consideration of different stiffness levels imposed on the deformations, were evaluated in numerical investigations, without suitable experimental data for validating the results, though. In this PhD Thesis, the performance of concrete beams was evaluated experimentally by performing bending tests on full-scale elements under different support conditions: unrestrained, only with axial restraints and with both axial and rotational restraints. Regarding the restrained elements, two levels of axial and rotational stiffness were analyzed, 0.02 and 0.04EA/l; 1 and 2EJ/l. There were also reference tests on simply supported beams at ambient temperature to check the load-bearing capacities and failure modes. The experimental data obtained for different beam static schemes still motivated the conception of numerical models that would be representative of their behavior. With the aid of the DIANA software, which is based on the finite element and displacement methods, beam models to represent beams tested at ambient temperature and in fire conditions were created. These models were implemented considering several properties that characterize the nonlinear behavior of the materials and led to good correlations when their results were compared to those obtained in the laboratory. The main conclusion of this experimental and numerical study was that the fire resistance of RC beams always increases when any type of restraint (axial or axial plus rotational) is introduced. In addition, by fixing the rotational stiffness, the beams with higher axial stiffness level presented higher fire resistance than those with the lower level. The same was observed by fixing the axial stiffness and varying the rotational stiffness. Beams in which the combined effect of the restraints was admitted led to higher resistances than those with only axial restraint. For most of the studied situations, the increases of the resistances showed to be significant when confronted with the ones for unrestrained beams. Thus, it was confirmed that the standard simplified methods that allow the non-consideration of these effects during the fire design of the RC beams lead to conservative results. The numerical and experimental results presented herein may aid in the conception of alternative tools that allow applying restraint effects to design.

Reinforced concrete columns are very important elements in structures in fire. Their collapse may affect very much the equilibrium and stability of the complete structure. In structural elements analysis is very important to consider the interaction with other elements of the structure. However, the behavior of the structural elements is usually considered as isolated, without considering any interaction. There are many studies of reinforced concrete columns in fire situation considering them as isolated. However, there are too few considering the interaction with beams and frames that are typical from buildings. This doctoral thesis studies reinforced concrete columns in fire situation considering the interaction with the structure. In the analysis, material non-linearity, concrete cracking, and mechanical properties of concrete and steel variation as a temperature function, e.g. strength and young’s module loss and the stress-strain curves varying with temperature are considered. In addition, variations of the thermal properties are considered, e.g. specific heat, thermal conductivity, and thermal elongation. Some structures configuration combined with the concepts given before are analyzed with specialized software, including some own codes.

The use of cold-formed steel profile in construction has increased because of its high efficiency, expressed as the ratio between load capacity and weight, and ease of manufacturing, characterized by the possibility of production of elements with different cross sections. Due the high slenderness ratio of the sections elements, the design of these profiles, either at room temperature or in case of fire, is determined by local, distortional and global buckling phenomena. The aim of this Thesis is to develop computational tools that allow the assessment of the structural behavior of cold-formed steel columns in fire. For this purpose, two softwares are developed. The first one, called ATERM, allows determining the temperature field under transient analysis of two-dimensional structures formed by any material, subjected to any time-temperature fire curve, and is based on the finite element method. This software interacts with another program, ATERM-DIM, used for plastic design of lateral restrained beams steel in fire. Results of ATERM are compared to those obtained from the Swedish software Super Tempcalc. The second program, named INSTAB, can perform linear and nonlinear stability studies of cold formed profiles, taking into account the local, distortional and global buckling, by means of the splines finite strip method for elastofragile material, considering the reduction of mechanical properties caused by the increase in temperature. Resistant values obtained by the INSTAB software are compared with results from the commercial finite element program ANSYS, which considers the plastic behavior of the material and also with results obtained by means of a simplified method for design of cold-formed profiles in fire, proposed by the author for the Brazilian fire standard. This comparison allows analyzing the effect of elastoplasticity in columns of cold-formed steel. Another objective of this study is to provide background for the development of procedures for the structural analysis of cold formed profiles in fire.

Steelworks outside a building in fire situation are exposed to radiation that come from the windows in the façade and from flames, as well as the convection of hot gases. The absorption of the heat for the element depends on its placement in relation to the openings. Margaret Law, based on many experimental data about fire in buildings, estimated the external heat transfer to steel elements and the maximum temperature value reached by them. The Eurocode 1, part 1.2 (2002) and Eurocode 3, part 1.2 (2003) have adopted the method previously mentioned with small modifications. The Brazilian standard ABNT NBR 14323:1999 - “Dimensionamento de estruturas de aço de edifícios em situação de incêndio – Procedimento” (Steel structures fire design – Procedure) allows employment proposed by Eurocodes 1 and 3 without detailing it. A software, ExteelFire, to determine the maximum temperature on external steel structures of buildings in fire, based on Margaret Law’s method was elaborated in this work. Results from ExteelFire and the values determined by numerical analysis, with software CFD (CFD - Computational Fluid Dynamics) Smartfire and Super Tempcalc (thermal analysis) have been compared. Furthermore, results from ExteelFire and values determined in tests in fire compartment, in natural scale (Dalmarnock experiment and Mittal Steel Ostrava experiment) have also been contrasted. In this work it was also applied ExteelFire to commonly used situations in civil construction, showing conditions where the thermal protection can be excluded.

The thermal and mechanical properties of building materials are reduced at high temperatures, and the structural resistance of reinforced concrete buildings, as well. If the means of active protection are not efficient the fire will develop and the consequential increase in temperature can take an important role on the local failure of a single member or the progressive collapse of the building. The structural design must take into account the possibility of a fire happening as an accidental action during the lifetime of the building, aiming mainly at the protection of the users lives. This doctoral thesis aims to contribute to the development of the technical references in Portuguese about the fire design of reinforced concrete structures, to stimulate further researches and afterwards standard reviews related to the structural design in fire of reinforced concrete buildings. The work reviews the heat effects on the thermal and mechanical properties of the materials and the consequential impact on the structural behaviour of reinforced concrete buildings, the calculation methods available in the international technical reference for the fire design of reinforced concrete structures and presents a proposal of an optimized simplified calculation method for the members under simple bending or composed axial-moment load, considering the geometric and concrete characteristics very usual in Brazil.

Methods for designing a building with adequate fire safety are poorly studied and applied in Brazil. This lack of study is even more present in the compartmentation subject, much-undeveloped topic and not yet standardized by ABNT. The compartmentation, especially the vertical, is critical to life safety, since it minimizes the spread of fire between building floors, and to validate the standardized procedures for structures in fire situation, because an important hypothesis of design methods is the vertical compartmentation. Currently the requirements of compartmentation are only present in the Technical Instructions of the States Fire Department, varying from state to state. This paper presents a study on compartmentation, its influence on the fire safety of buildings and a comparison of the vision of some countries of four different continents on this subject. After a comparative study of the vision and standardization between countries such as Brazil, Portugal, England, Hong Kong and the United States, computer simulation based on the theory of the finite volumes and computational fluid dynamics were carried out in order to try to check thoroughly the influence of compartmentation requirements in a real fire spread. Ultimately, the main focus of this work is to demonstrate that the Computational Fluid Dynamics (CFD) can be used in order to assist the study of fire, in order to allow a more in-depth analysis of the subject. Keywords: fire, compartmentation, standardization, fire resistance, computational fluid dynamics, FDS, computational modeling.

A study will be carried out on the behavior of simply supported, full interaction composite steel and concrete beams composed by compact profiles in fire situation. Despite being designed as simply supported, the main idea is to consider, in the fire situation analysis, the moment resistance capacity reserve on the beam supports, usually neglected during room temperature design, due to the upper longitudinal reinforcement present in the concrete slab, being possible to dispense fireproof coating in these elements. First, room temperature design procedures of composite beams will be approached in order to explain the structural behavior of this type of elements, providing a basis for subsequent thermal analysis. The thermal analysis will be carried out, in a first stage, by simplified methods according to design procedures and adopting simplifying hypotheses in which indirect stresses caused by thermal expansion and thermal gradient are neglected, being possible to apply those concepts in structural design offices. In a second step, thermal stress analyses were performed with aid of numerical models to study the structural behavior and collapse time of the beam. The results were sufficient to validate the fire resistance time values found by the simplified analysis following design methods. As a conclusion, the structural capacity increase of the beam, provided by the adoption of the composite connection at the support in case of lighter steel profiles usually chosen to be used as buildings floor beams, was sufficient for situations of standard fire resistance requirements between 15 min and 21 min. Analyzes have shown that it is not possible to justify the absence of fire resistant coating for standard fire resistance requirements of 30 min or higher. Lower times can be adopted according to the so-called equivalent time method, limited to 15 min, which are more common for small buildings. Key-words: fire; steel structures; composite steel and concrete structures; thermal structure analysis; semicontinuous beams.

The issue of this Master Dissertation is the design of steel beams and columns in fire situation. The formulation for the design in fire was presented, explaining its origin. The main objective is the development of a computational tool, which was called AçoInc, able to calculate the resistances of steel elements at room temperature according to ABNT NBR 8800:2008 and in case of fire according to ABNT NBR 14323:2013. The temperature of the steel element in fire is also obtained through the AçoInc, following the heat transfer model of ABNT NBR 14323:2013 and basing on fire curves. These curves are the standard fire curve associated with the required fire resistance time and parametric curves for natural fire model of Eurocode 1 (2002). With the support of spreadsheets, some studies were performed. The first one is probabilistic study of failure of column and beam in fire situation considering the parametric fire and by means of the Monte Carlo method. Other study is parametric sensitivity analyses, which were performed to verify how the resistances are influenced by the dimensions of cross section, the variables of the natural fire and the properties associated with heat transfer. For this study, in addition to spreadsheets, we used the Software Statgraphics Centurion XV (2007), which contains resources of statistical analysis. Applying the spreadsheets, the equivalence between parametric curves and fire standard curve in the obtention of the steel temperature in the fire situation was verified. For the use of parametric fire, partial factors, based on the equivalent time method detailed on ABNT NBR 14323:2013 were proposed. At last, with the assistance of spreadsheets, simple non-computational tools were created for determining the critical temperatures for columns and beams in some constructive situations of practical interest. Some conclusions from these studies are that the steel temperature is the most important parameter in determining the resistance of steel structures in a fire situation, that the current formulation for parametric fire modeling leads to a variation of the fire temperature in function of the opening factor with undesirable discontinuity and that the data found in the international literature about probability of fire and flashover lead to divergent values of structural failure probability.

The Brazilian standard ABNT NBR 14323:2013 presents a method for the composite slabs fire design. However, it doesnt include the effect of the slab behaving as a membrane. Using the theory of plates for large displacements, the research establishes that high temperatures can induce a considerable tensile membrane action, assisting in the safety of the slab in fire. In developing a tool for the analytical method composite slabs fire design, some studies were made to analyze the behavior of tensile membrane at high temperatures. The most popular analytical method is the Baileys method. In this study, analysis of the Baileys methods, the Brazilian standard and the Vulcan and MACS+ software were performed and the results were compared. As expected, the results were similar. Still, in this work, there have been some numerical studies employing the finite element method, using the computer program Vulcan, and results were compared between the Baileys method and Vulcan. An investigation about possible simplifications of the formulation presented by Brazilian standard were also developed.

Steel structures strength is reduced at high temperatures. The aim of this work is to analyze the behavior of supported and fully fixed end beams at high temperature, considering: several load levels, uniform temperature on the beam and thermal gradient through the cross section. The displacements, loads and reaction efforts in accordance to the temperature variations will be presented, taking into account the resistance decrease and the strain amplification at elevated temperatures. The analysis has been carried out by means the computer program ANSYS® v. 6, using the “beam-24” fine element. It was considered the non-linear effects of the geometry and the material of the structure, besides the stress-strain diagram in high temperature to many thermal levels. The thermal properties of the steel adopted in the structural modeling of the steel beams are given in the Brazilian Standard NBR 14323 (1999) – “Dimensionamento de estrutura de aço de edifícios em situação de incêndio” (“Steel structures design of buildings in fire conditions”).