1,720,978 research outputs found
Collapse of prestressed reinforced concrete jetties: durabilityand faults analysis
Full text linkCatastrophic 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
EXCEPTIONAL ACTIONS: BLAST LOADS ON REINFORCED CONCRETE STRUCTURES
No full textThe paper deals with the flexural failure of Reinforced Concrete elements under
blast loads. The main topics and results of a PhD thesis are here summarized, whose
aim was to develop theoretical dynamic and energy models capable of evaluating the
dynamic response of R.C. elements under explosive load.
In the original work, models with different levels of complexity were presented, but
for sake of simplicity only the simplest Single Degree Of Freedom (SDOF) system has
been here discussed. Strain-rate effects are also accounted for.
A sensitivity analysis to determine the key parameters in beam response under
blast load has been developed by means of the SDOF model. Results of numerical
simulations obtained in terms of deflection and velocity have been fitted by proper
polynomial least-square interpolation.
Among the various interpolations considering several parameters (peak load,
positive phase duration, slenderness, span length, concrete strength, reinforcement
ratio etc.) slenderness (corresponding to stiffness) and peak load prove to be the most
important parameters, but span length (corresponding to mass) is also a key
parameter. Other variables such as concrete strength and reinforcement ratio do not
seem to have a high correlation with results.
Lastly some suggestions for blast-resistant bridge design are presented
INCREMENTO PRESTAZIONALE DI EDIFICI IN C.A. RESISTENTI AD ESPLOSIONE E RELATIVA VALUTAZIONE ECONOMICA
No full textIl lavoro ha come obiettivo quello di valutare l’incremento del costo complessivo di un edificio sismo-resistente affinché questo sia in grado di sopportare anche un carico da esplosione. Si sono valutati diversi scenari di carico in cui la struttura è costituita da un sistema intelaiato monopiano in C.A. (tipico dell’edilizia industriale) localizzato in zone con differente pericolosità sismica. Si è prima calcolato il costo della struttura sottoposta alle combinazioni di carico usuali (anche quelle sismiche) quindi quello della stessa costruzione sottoposta alla combinazione di carico eccezionale riferita ad un evento esplosivo. Dai risultati è possibile desumere semplicemente il costo per incrementare la robustezza strutturale necessaria per un dato livello di sicurezza. Si fornisce anche un’indicazione su quale deve essere la minima distanza di sicurezza dalla costruzione (distanza di stand-off) economicamente accettabile da garantire attraverso l’utilizzo di elementi di arredo urbano o opportuni dissuasori
Azioni Esplosive sulle strutture in C.A.
No full textThis paper deals with the effect of blast
loading on R.C. structures. After depicting load
and materials characteristics in blast event,
different alternative structural models are
proposed. The first approach idealizes the beam
as a single degree of freedom system, the second
models it by means of Euler-Bernoulli’s theory and
its elastic-plastic behaviour is expressed through a
nonlinear relationship between bending moment
and curvature. The effects of strain rate are taken
into consideration by introducing time-variable
coefficients into the equations of motion derived
from the two models. The comparison with some
experimental results assesses the reliability of the
approach, and a sensibility analysis of the
different involved parameters is then developed.
Slenderness and span length proved to be the key
parameters and let the Authors to find a
polynomial relation between them and the
maximum displacement/velocity of the beam
AZIONI ESPLOSIVE SU TRAVI IN ACCIAIO
No full textIn this paper, the structural responses of steel beams subjected to air-blast loading are investigated.
In order to assess the response of beams, two Single Degree of Freedom models are
developed. In the first model (denoted as “dynamic model”), the dynamic behaviour of the
beam is simulated by a simple spring-mass oscillator. The second model (named “energy
model”) is developed from the equation of energy balance. In both formulations, the strain
rate dependencies of the constitutive properties of the beams are considered. In fact, these effects
should not be ignored in problems concerning blast and impulsive loads, since the mechanical
properties of steel strongly depend on the rate of deformation. In this paper, strain
rate effects are considered in accordance with the espression provided by Cowper-Symonds
strain rate model. Finally, in order to test the validity of the two approaches, the theoretical
results produced by the latter ones are compared with some experimental data found in literature.
Both approaches yield reliable results, even though they are based on some approximations.
Being also very simple to implement, they may serve as an useful tool in practical applications
ANALISI DELLA SICUREZZA DI TRAVI IN CA SOGGETTE A D AZIONE DI ESPLOSIONE
No full textIl carico da esplosione presenta grandi incertezze per quanto riguarda la posizione, l’intensità e la possibilità di accadimento. Sommando a queste incertezze quelle relative ai materiali, alla geometria e i sovraccarichi, si rende necessario un’analisi statistica finalizzata ad una più realistica stima della risposta strutturale. L'articolo presenta i risultati ottenuti da una indagine parametrica sulla risposta di travi inflesse in calcestruzzo armato soggette a carichi di tipo esplosivo. Mediante analisi dinamica non lineare di un sistema equivalente ad un grado di libertà sono state esaminate le risposte probabilistiche, in termini di massimo spostamento e di fattore di duttilità, di elementi inflessi in c.a. a seguito di azioni esplosive
On the conceptual design of R/C footbridges with the deck supported by shells of minimal surface
No full textDesigning footbridges (pedestrian bridges) with the deck supported by R/C shells is an interesting approach, that is illustrated in this paper. The advantage of this structural solution is that R/C shells can be shaped in accordance with the “minimal surface” criterion. In so doing, the shells are mainly subjected to biaxial compression with membrane stresses.
The shape of the minimal surface shells is first studied by suitably choosing the position of the boundary restraints and of the applied forces. Then, through a form-finding process containing an
optimisation algorithm, different R/C footbridges with architecturally-valuable shapes are worked out. It is also shown how Maillart’s bridges can be re-shaped in order to avoid the well known undesired bending effects, even though some tensile stresses may occur.
In the case of footbridges having a straight deck, it is shown how: (a) the choice of the position of the deck supports, (b) the shape of the shell close to the abutments, (c) the arrangement of the
shell edges along the abutments, and (d) the stiffening beams (if any) along the free edges can
minimize the bending effects and improve the behaviour of this type of structures
The design of RC bridges with shell of minimal area made of fibre reinforced concrete: an application to a pedestrian bridge
No full textTecnica della scorta di sicurezza virtuale: applicazione al caso dei grandi elettrodomestici nella grande distribuzione organizzata
No full textLa scorta ricopre un ruolo strategico all’interno della gestione
dei sistemi produttivi, in primo luogo poiché è una variabile cri-
tica su cui si può agire per complementare la carenza di tempo
e capacità produttiva, in secondo luogo perché al mantenimen-
to di materiali a scorta è associato un costo, che auspicabilmen-
te dovrebbe essere ridotto. In contesti con grande aleatorietà
della domanda, gli alti livelli di scorta di sicurezza comportano
spesso un problema, che può essere efficacemente affrontato e
risolto con la tecnica della virtualizzazione della scorta di sicu-
rezza. In questo articolo viene presentata una applicazione
della tecnica della scorta di sicurezza virtuale a un caso nella
grande distribuzione organizzata
The Church of Saint Domenico in Cagliari: Conceptual Analysis and Structural Checks
No full textThe church of St. Domenico in Cagliari is a reinforced-concrete structure built on an ancient medieval temple heavily damaged during the 2nd World War. The coexistence of two structures, obviously with many technical differences, strongly influenced the design of the new church. Characteristic features are the curved beams that intersect each other and create a sort of coffered ceiling, while the large windows at the base of the dome, between two successive beams, give the impression that the dome itself is suspended in the air.
In the first phase of this project, the original technical documentation was found out and a critical analysis was performed on the structural scheme adopted by the designer, who could not rely on modern computer-assisted methods of calculation and had to limit himself to a rather simplified analysis. In the second part, the authors compare the original results with those obtained by modelling the whole structure via finite elements. The aim is to assess the reliability level of the original calculations, and to carry out the structural checks (not reported in this paper), according to the Italian Design Code.
The original design turned out to be safe, in spite of its crude assumptions. Only in few cases there are significant differences between the original results and those obtained via FE analysis; safety, however, is never impaired
- …
