Influência dos efeitos globais de segunda ordem em manifestações patológicas em um edifício de concreto

Structural accidents of high-rise buildings due to instability has occurred with some frequency in Brazil. Generally, building instability implies in a series of pathologies in structural and non-structural elements. The Brazilian code regulation suggests two simplified procedures, named as instability parameter α and γz coefficient. In terms of design, the main difference between such simplified procedures is that, differently from the instability parameter α, the γz coefficient can be used to estimate the second order effects and it is applied to buildings that do not present double symmetry. In the light of the foregoing, this paper addresses to the evolution process of pathological manifestations due to structural instability in an actual high-rise building. As soon as the building was occupied, cracks nucleated and propagated in few days. The observed cracking pattern indicated that the building could break in two pieces as a way to compensate the second order effects. Thereby, the structural instability and its consequences were evaluated by means of a commercial design computer software

Floors number influence on the instability parameter of reinforced concrete frame-braced buildings

This work aims to investigate the floors number influence on the instability parameter limit α1 of reinforced concrete frame-braced buildings; it succeeds another work in this field of knowledge, in which the same question was investigated for wall- and core-braced buildings. Initially, it is showed how the ABNT NBR 6118:2014 (Brazilian code for concrete structures design) defines when a second order analysis is needed. Topics concerning to physical nonlinearity consideration and to the lateral deflection components of frames are also presented. It follows an analytical study that led to the derivation of a method for determining the limit α1 as a function of the floors number and the relation between bending and shear stiffness. Finally, some examples are presented and their results are used for checking the method accuracy.

ANÁLISE DE ESTABILIDADE GLOBAL EM EDIFÍCIOS ALTOS

RESUMO: O projeto de edifícios esbeltos em concreto armado normalmente tem como principal desafio a busca por uma solução estrutural que viabilize sua estabilização horizontal, sem comprometer a segurança, nem os aspectos arquitetônicos ou a economia. Para formar o sistema de contraventamento desses edifícios, normalmente têm-se disponíveis a associação de vigas e pilares formando pórticos, ou a adoção de núcleos rígidos quando a estrutura de tais edifícios exige grande rigidez frente a ações horizontais. Neste trabalho é apresentado o estudo da estabilidade de uma edificação em concreto armado composta por laje nervurada, com ordem de esbeltez de um para quatro para três tipologias de sistemas de contraventamento: pórticos, núcleos rígidos em formato (“U” e pilar-parede). Em cada sistema, foram considerados dois diferentes tipos de ligações (rígidas e semi-rígidas) para avaliar não só a participação das vigas no contraventamento da estrutura como também, a influência de tais modelos e critérios nos deslocamentos laterais e na estabilidade global da edificação, a partir do parâmetro de instabilidade z previsto na norma brasileira NBR 6118 (ABNT, 2014). Para as modelagens foi utilizado o programa comercial Altoqi Eberick V9, que considera as premissas e as prescrições de dimensionamento de estruturas de concreto previstas nas normas brasileiras. Os valores encontrados pelo Software foram analisados e reforçam a importância da utilização de Pilar -Parede ou núcleos rígidos em formato “U” para edifícios altos em concreto armado. ABSTRACT: The Project of slender buildings in reinforced concret, normally has as main challenge the search for a lasting solution that allow its horizontal stabilization, without compromising the safety, neither the architectural aspects or the economy. To form the bracing system of these buildings, normally has been available the association of beams and pillars forming the frameworks, or the adoption of rigid cores when the structure of these buildings requires great rigidity against horizontal actions. In this research aimed to study the overall stability of a building in reinforced concrete consisting of waffle slab, with slenderness approximately one to four, for three types of bracing systems, frameworks and rigid core into two types of format ("U" and pillars in which the lengths are signifi cantly larger than their widths). In each system were considered two different types of connections ( rigid and semi-rigid ) to evaluate not only the participation of the beams in the bracing structure, even as the influence of such models and criteria in the lateral displacement and the overall stability of the building, using instability parameter z fixed on Brazilian standard NBR 6118 (ABNT, 2014). For the modeling, was used the commercial software Altoqi Eberick V9, which considers the assumptions and the sizing requirements of concrete structures described in Brazilian standards. The values that were found by the software, they were analysed, and reinforce the importance of using pillars in which the lengths are signifi cantly larger than their widths or rigid core "U" for tall buildings in reinforced concrete.

Dynamical Bifurcation of the Generalized Swift–Hohenberg Equation

In this paper, we prove that the generalized Swift–Hohenberg equation bifurcates from the trivial states to an attractor as the control parameter α passes through critical points. The bifurcation is divided into two groups according to the dimension of the center manifolds. We show that the bifurcated attractor is homeomorphic to S1 or S3 and it contains invariant circles of static solutions. We provide a criterion on the quadratic instability parameter μ which determines the bifurcation to be supercritical or subcritical.

Study of global stability of tall buildings with prestressed slabs

The use of prestressed concrete flat slabs in buildings has been increasing in recent years in the Brazilian market. Since the implementation of tall and slender buildings a trend in civil engineering and architecture fields, arises from the use of prestressed slabs a difficulty in ensuring the overall stability of a building without beams. In order to evaluate the efficiency of the main bracing systems used in this type of building, namely pillars in formed "U" in elevator shafts and stairs, and pillars in which the lengths are significantly larger than their widths, was elaborated a computational models of fictional buildings, which were processed and analyzed using the software CAD/TQS. From the variation of parameters such as: geometry of the pillars, thick slabs, characteristic strength of the concrete, reduceofthe coefficient of inertia for consideration of non-linearities of the physical elements, stiffness of the connections between slabs and pillars, among others, to analyze the influence of these variables on the overall stability of the building from the facing of instability parameter Gama Z, under Brazilian standard NBR 6118, in addition to performing the processing of building using the P-Delta iterative calculation method for the same purpose.

New Criterion for Prediction of Amorphous Alloy Compositions: A Сombination of Dense Packing of Spheres and the Lambda Criterion through the Coordination Number

This work presents the development of a new criterion which can indicate unknown compositions for amorphous metallic alloys. This new criterion was based on dense packing of spheres combined with the lambda criterion through the coordination number. A simplified mathematical development is presented, and software was developed with the purpose of indicating alloys with best glass forming ability (GFA). This software includes and concatenates several other criteria. For this purpose, we performed a mathematical analysis of the criteria, which included: (i) λ_min criterion which uses the minimum topological instability parameter and is used as an indication of phase competition during the solidification. (ii) the γ parameter, which reflects the relative GFA between bulk metallic glasses (BMG), and it is based on characteristic temperatures, such as: the glass transition temperature - Tg, the crystallization onset temperature - Tx and the liquidus temperature - Tl. (iii) the parameter Zc that is the critical thickness for bulk glass formation, which corresponds to the maximum dimension in which the molten can be formed without any crystals precipitation and (iv) the parameter Rc that is the critical cooling rate for glass formation, which decreases inversely to the Zc values. Through a mathematical formalism combining all the parameters, the software can decide the most suitable composition range to produce a new alloy. The results are presented and compared with the ones available in the literature. The results indicate good agreement with the literature since it is not a general theory, but intends to meet some specific cases with a reasonable convergence and which still need to be better studied.

Numerical study on unstable surfaces of oblique detonations

In this study, the onset of cellular structure on oblique detonation surfaces is investigated numerically using a one-step irreversible Arrhenius reaction kinetic model. Two types of oblique detonations are observed from the simulations. One is weakly unstable characterized by the existence of a planar surface, and the other is strongly unstable characterized by the immediate formation of the cellular structure. It is found that a high degree of overdrive suppresses the formation of cellular structures as confirmed by the results of many previous studies. However, the present investigation demonstrates that cellular structures also appear with degree of overdrive of 2.06 and 2.37, values much higher than ${\sim }$1.8 suggested previously in the literature for the critical value defining the instability boundary of oblique detonations. This contradiction could be explained by the use of differently shaped walls, a straight wall used in this study and a custom-designed curved wedge system so as to induce straight oblique detonations in previous studies. Another possible reason could be due to the low and possibly insufficient resolution used in previously published studies. Hence, simulations with different grid sizes are also performed to examine the effect of resolution on the numerical solutions. Using the present results, analysis also shows that although the characteristic lengths of unstable surfaces are different when the incident Mach number changes, these length scales are proportional to tangential velocities. Hence, the interior time determined by the overdrive degree is identified, and its limitation as the instability parameter is discussed.

A Method Optimizing the Forging Process of Titanium Alloy Blisk Based on the Processing Map

Isothermal compression tests were conducted at various temperature and strain rate combinations over the hot working range (880 to 1040°C and 0.01 to 10.0 s-1) of TC11 alloy. Based on the experimental data, the strain rate sensitivity factors m were calculated. Both the efficiency of power dissipation and the instability parameter were computed, and then processing maps incorporated instability maps were accordingly established at strains of 0.1, 0.3 and 0.6. A method to optimize the forging process of Titanium alloy blisk was proposed based on the processing map. The optimized process of TC11 alloy blisk was achieved using the proposed method and was verified by numerical simulation and experiment. The results showed that this optimal process is applicable to produce blisk with duplex structures.

Floors number influence on the instability parameter of reinforced concrete wall- or core-braced buildings

This work aims to investigate the floors number influence on the instability parameter limit α1 of buildings braced by reinforced concrete walls and/or cores. Initially, it is showed how the Beck and König discrete and continuous models are utilized in order to define when a second order analysis is needed. The treatment given to this subject by the Brazilian code for concrete structures design (NBR 6118) is also presented. It follows a detailed analytical study that led to the derivation of equations for the limit α1 as functions of the floors number; a series of examples is presented to check their accuracy. Results are analyzed, showing the precision degree achieved and topics for continuity of research in this field are indicated.