Collapse of prestressed reinforced concrete jetties: durabilityand faults analysis

Catastrophic collapses often are caused by minor details and lack of adequate precautionary measures. Such a case is presented in this paper, where the authors investigate the failure and collapse of jetties in a marina (built in 1974). The inquiry begins with collecting the original design documents and with the examination of the structural elements recovered from water. The original calculation report and drawings have been analysed in the light of the former and the current structural codes. Particular attention is drawn to the conceptual approach of the original designer. Back in the seventies, it was customary that a fully prestressed structural element did not have any ordinary reinforcement for bending. Therefore, the resistance depended only on the prestressing forces, and so the ratio between resistant bending moment and design bending moment MR/ME varied along with the positions of the strands. The calculations performed by the authors using up to date theories, have shown that the service life was almost adequate. The main cause of the failure was hidden behind a small detail: the external section of the strands, where bond between steel and concrete was prevented by a PVC duct, did not have an adequate protection. Hence, the sea water could penetrate the core of the element, causing severe corrosion

Theoretical models to predict the flexural failure of reinforced concrete beams under blast loads

This paper presents two alternative approaches for the study of reinforced concrete beams under blast loads. In the first approach, the beam is modeled by means of Euler–Bernoulli’s theory and its elastic–plastic behavior is expressed through a new nonlinear relationship between bending moment and curvature. In the second approach, instead, the beam is idealized as a single degree of freedom system. The effects of strain rate, which are of paramount relevance in blast problems, are taken into consideration by introducing time-variable coefficients into the equations of motion derived from the two models. The latter are employed to assess the time-history of the maximum deflection of a simply supported beam subjected to a uniformly distributed blast load. By comparing the theoretical results with some experimental findings available in literature and with the solution obtained from a commercial finite element software, it is found that the first approach is capable of accurately evaluating the maximum deflection of the beam at failure; on the other hand, the second approach provides a less precise prediction, however it is simpler to implement in practice because it requires less computational effort

Flexural models of reinforced concrete beams under blast load

Nowadays, the issue of structural safety under blast loading has become a dramatic problem. The tragic news of the terrorist attacks of recent years (9/11/2001, New York; 7/7/2005, London; 7/23/2005, Sharm El Sheik; 1/24/2011, Moscow; etc), raise important, urgent questions regarding the real safety and reliability of our buildings. Extreme loads such as impacts, explosions, etc., can occur in everyday life with unexpectedly high frequency. Actually, the problem of terrorist attacks, so important for strategic and military building design, has been linked to residential and industrial building explosion accidents. The present thesis deals with the flexural failure of Reinforced Concrete beams under blast loads. The main aim is firstly to develop dynamic and energy models capable of evaluating the response of R.C. under explosive load. Then a sensitivity analysis is obtained by means of the above mentioned models in order to determine what are the key parameters in the beam response. In this way it is possible to attain simple predictive polynomial formulations and, finally, simple table for early structural assessment of beams under blast load during the design procedure. The thesis is composed of three sections. Various dynamic models are developed in Section 1, taking into account the strain-rate sensitivity of both steel and concrete, as well as other nonlinearities in structural behaviour. Models with different levels of complexity are presented, from the simplest Single Degree Of Freedom (SDOF) system to Continuous Beam and Finite Element models. The characteristics, advantages and disadvantages of each approach are stated and discussed. The author compares his models with some experimental findings available in the scientific literature. The principal innovation that can be inferred from this section is the hypothesis of distributed plasticity along the beam (in the continuous model), represented by a non-linear smooth relationship between bending moment and curvature. This relationship, in addition to the Euler-Bernoulli beam equation, yields a non-linear Partial Differential Equation solved by means of the Finite Difference Method. In Section 2, the same problem is solved in a different way: starting from the principle of energy conservation, the innovative procedure, developed by the author, can calculate the displacement field of a R.C. beam under blast load. This procedure, less accurate than the previous one, produces very good results regarding midspan displacement, especially as it requires less computation time. In order to better understand the phenomenon, with the aim of identifying the key parameters in structural response, a sensitivity analysis is developed in Section 3. To this end, the author has performed a numerical investigation referring to the SDOF model presented in Section 1. Several numerical simulations are performed, with random variation of beam and load characteristics. Results are expressed in terms of maximum deflection and maximum velocity for each case. Then a least-squares interpolation has produced various polynomial curves and surfaces representing both a simplified tool to estimate structural response and a sensitivity analysis of the key parameters involved. One of the possible developments of this useful work is represented by simple tables that provide the response of the beam under blast load for early assessment in design procedures

On the whole spectrum of Timoshenko beams. Part I: a theoretical revisitation

The problem of free vibrations of the Timoshenko beam model is here addressed. A careful analysis of the governing equations allows identifying that the vibration spectrum consists of two parts, separated by a transition frequency, which, depending on the applied boundary conditions, might be itself part of the spectrum. For both parts of the spectrum, the values of natural frequencies are computed and the expressions of eigenmodes are provided. This allows to acknowledge that the nature of vibration modes changes when moving across the transition frequency. Among all possible combination of end constraints which can be applied to single-span beams, the case of a simply supported beam is considered. These theoretical results can be used as benchmarks for assessing the correctness of the numerical values provided by several numerical techniques, e.g. traditional Lagrangian-based finite element models or the newly developed isogeometric approach

An analytical assessment of finite element and isogeometric analyses of the whole spectrum of Timoshenko beams

The theoretical results relevant to the vibration modes of Timoshenko beams are here used as benchmarks for assessing the correctness of the numerical values provided by several finite element models, based on either the traditional Lagrangian interpolation or on the recently developed isogeometric approach. Comparison of results is performed on both spectrum error (in terms of the detected natural frequencies) and on the l2 relative error (in terms of the computed eigenmodes): this double check allows detecting for each finite element model, and for a discretization based on the same number of degrees-of-freedom, N, the frequency threshold above which some prescribed accuracy level is lost, and results become more and more unreliable. Hence a quantitative way of measuring the finite element performance in modeling a Timoshenko beam is proposed. The use of Fast Fourier Transform is finally employed, for a selected set of vibration modes, to explain the reasons of the accuracy decay, mostly linked to a poor separation of the natural frequencies in the spectrum, which is responsible of some aliasing of modes

Integrated approach for post-fire reinforced concrete structures assessment

In order to assess decay in the mechanical characteristics of re-exposed Reinforced Concrete (RC), it is crucial to recon- struct the temperature time history and the evolution of strain and stress elds. In this paper, the state of the art of assessment methods is presented and applied to a real structure damaged by re. It is a prestressed RC industrial warehouse located in the outskirts of the city of Cagliari (Italy). The collected data of several assessment methods are presented in order to produce the owchart of an integrated approach for post- re investi- gation. Among the various techniques, the authors highlight a thorough laser scanner geometric survey and destructive and non-destructive testing. In addition, the temperature distribu- tion and its time history has been reconstructed by means of optical and Scanning Electron Microscopy, X-ray diffractom- etry, Thermogravimetric Differential Thermo-Analysis and calibrated Colorimetry. Actually, refurbishment is needed, but the structure withstood the re very well. Central columns displayed the most impor- tant damage, and several beams presented important de ec- tions having lost the prestressing actions of the tendons

Terrestrial laser scanner for monitoring the deformations and the damages of buildings

The paper presents the use of the terrestrial laser scanner for the study and the assessment of damaged buildings. The terrestrial laser scanner provides the ability to detect the geometric 3D model of a building without any physical contact with the structure. Knowledge of the 3D model will give the opportunity to study the deformation and quantify the damages. Three case studies are presented relating to damaged and/or unsafe buildings: Sivillier Castle (Villasor - Sardinia- Italy), the Bell Tower of Mores (Sardinia-Italy) and industrial building (Cagliari - Italy). The first two cases concern buildings of historical and architectural importance that present a state of compromised conservation; the last, an industrial building compromised by fire. In all cases, a laser scanner survey was carried out that not only provided valuable information but also highlighted structural metric deformation and degradation

International Journal of Structural Glass and Advanced Materials Research: a new open platform for materials science

The International Journal of Structural Glass and Advanced Materials Research (IJSGAMR) is a new peer-reviewed, open access journal, which covers all aspects of theoretical and practical research of materials science. The journal aims to promote international exchange of knowledge and broad discussion on advancements, outcomes and recent developments in materials research for engineering applications