101 research outputs found
STRUCTURAL PERFORMANCES OF ARCH FRP FOOTBRIDGES
No full textThe paper presents a parametric analysis of arch pedestrian bridges entirely made by glass-fiber reinforced-plastic (GFRP) pultruded profiles. In order to understand the performances of this type of structure for different span lengths, 30-, 40- and 50- meter span footbridges are analyzed. The aim of the author is to establish the maximum span length that the proposed type of footbridge can reach without any problem of buckling, vibrations or rupture of joints. The self weight of the proposed footbridges is, respectively, 80, 110 and 150 kN for the 30-, 40- and 50-meter span structure. The ratio between live loads and dead loads lies in the range 3.5-4.5.
In the author’s opinion, the lightness of FRP materials can represent a great advantage, especially in presence of a soft soil foundation. Furthermore, the strong corrosion resistance of FRP materials permits to avoid frequent and expensive maintenances. The arches of the three proposed footbridges are achieved by connecting rectilinear profiles so as to realize a parabolic shape. The cross sections of main girders are made by two coupled channel profiles. Tension members are made by two coupled pultruded plates, whereas compression members by square tubes. These are inserted between the channel profiles of the main girders. In the paper some aspects concerning design and finite element analysis are explained. Due to arrangement of fibers, FRP profiles exhibit a pretty low shear stiffness. The value of the ratio between longitudinal Young modulus E and transverse elastic modulus G is about equal to 10 and, consequently, it is necessary to take shear deformation effects into account. The assumed kinematical model is based on Timoshenko’s bending theory and on Reissner’s torsion theory, allowing for the evaluation of shear strain effects due to both nonuniform bending and torsion. Numerical interpolation of the displacement field is based on “modified” Hermitian shape functions. Such polynomials contain parameters depending on both E/G ratio and slenderness, that go to zero when shear strain influence becomes negligible. A few years ago, Reddy demonstrated that these polynomials produce a locking-free element for Timoshenko beams. In a previous paper, the author demonstrated that similar polynomials can be used to interpolate torsional rotation and cross-section warping with excellent results. In order to evaluate the global buckling resistance of designed footbridges, second order terms are introduced in the displacement field, representing flexural-torsional coupling. By using the developed finite element program, the influence of shear deformability on deflection and buckling loads is consequently pointed out.
In order to understand the dynamic behaviour of the three compared footbridges under moving loads, several numerical analyses have been performed. The effects of both flexural and lateral pedestrian-induced vibrations on proposed structures have been evaluated. Remembering the highly publicized closure of the London Millennium Footbridge, due to excessive lateral vibration at its opening, the limit-situation of crowd density on bridge deck is taken into account in calculating the vibration frequencies. Due to the low stiffness of pultruded profiles, the design of most of the structural members is governed by the Serviceability Limit State or by interaction phenomena between global and local buckling modes. Hence, the design of the structural members is carried out by taking these possible interaction phenomena into account
Analisi dinamiche non lineari a sostegno delle ipotesi di intervento di miglioramento sismico del Laboratorio dell’INFN di Ferrara
No full textLa maggior parte dei capannoni che sorgono nel territorio emiliano investito dalla sequenza sismica del 2012 è stata costruita in un periodo precedente all’entrata in vigore di criteri di progetto specifici per la resistenza alle azioni sismiche. Come conseguenza, le ben note carenze dei collegamenti fra tegoli di copertura e travi, fra travi e pilastri e fra questi e i pannelli di rivestimento erano sistematicamente presenti in queste aree, e tale circostanza è stata la maggiore responsabile dei gravi danni subiti da tali edifici fino a distanze di 20 km dall'epicentro (Savoia et al. 2012, Minghini at al. 2014a, b). Nell’area urbana di Ferrara si è stimato che l’accelerazione risentita durante la sequenza sismica emiliana del 2012 non abbia superato il valore di 0.07g. Per tale motivo gli edifici prefabbricati presenti nelle aree periferiche della città a destinazione industriale-artigianale hanno subito al più danni modesti. L'edificio analizzato nel presente articolo, tipologicamente simile a capannoni più prossimi agli epicentri e rivelatisi molto vulnerabili, è stato reso fruibile a seguito di interventi locali effettuati ai sensi del D.L. 74/2012. Per tale edificio la presente nota illustra alcune proposte di intervento che consentono, sulla base di analisi numeriche in campo elastico lineare, di elevare il livello di sicurezza ad oltre il 60% della sicurezza richiesta agli edifici di nuova costruzione. Le proposte vengono poi validate tramite analisi dinamiche non lineari, che rappresentano lo strumento più sofisticato fra quelli oggi disponibili per la valutazione del rischio sismico delle costruzioni. Da tali analisi emerge come gli interventi proposti possano garantire il pieno adeguamento dell’edificio
Prova di pressoflessione ciclica su un pilastro prefabbricato collegato al plinto tramite ferri di ripresa inghisati
No full textThe results of a cyclic test on a precast RC column connected to the foundation through a grouted sleeve joint are presented. The column, having square cross-section with side length 0.5 m, was 3.7m heigh. It was subjected to an axial compression of 1700 kN using two externally post-tensioned 47mm-diameter Dywidag bars. A transverse displacement time-history was applied to the column cross-section 2.8m far from the column-foundation joint using a worm screw jack with nominal loading capacity of 500 kN. The test was stopped after 19 displacement cycles, when the complete spalling of the concrete cover took place at the base-joint. Correspondingly, a 15% degradation of the base-joint resistance was attained and the drift reached 5.25%. At this deformation level, the dowel action involving the connection rebars was significant, leading to a maximum slip within the joint of about 7 mm. The ultimate curvature was about 10 times the yielding curvature. Then, a behaviour factor of about 3.7 was estimated for use in spectral response analyses based on the Italian Standard
Stima dello scorrimento richiesto dalle connessioni di travi prefabbricate reticolari miste
No full textHybrid steel truss RC-beams are analyzed using nonlinear finite element analysis and two simple design methods assuming linear or cubic slip within the beam. Two different beams typologies with limited slip capacity are investigated, pointing out the inadequacy of the design methods
Grouted sleeve connections used in precast reinforced concrete construction - Experimental investigation of a column-to-column joint
No full textThe results of an experimental campaign concerning full-scale tests on precast reinforced concrete column-to-column connections made with grouted sleeve splices are presented. The precast column units had a square cross-section with the side of 500 mm. Eight 20 mm-diameter bars protruding from one unit were grouted into corrugated steel sleeves encased in the other unit. The column-to-column connections were subjected to four monotonic tests (axial tension, bending with and without axial compression, and shear) and to one cyclic bending test.
In the tension test failure took place far from the interface between the precast units and highlighted the effectiveness of the stress transfer along the splice region. In all other tests, damage developed at the interface between the two units. In the bending tests with and without axial compression significant over-strengths with respect to design resistances computed for equally-reinforced monolithic members were attained. Because of the reduced thickness of the interface between the precast units, the rotation that concentrated at the interface led to a moderate reduction of the global bending stiffness. In the shear test the pure shear capacity of the bars crossing the joint was achieved. The cyclic bending test showed a ductile and stable hysteretic behavior of the connection
Nonlinear analysis of composite beams with concrete-encased steel truss
No full textComposite beams constituted by a concrete-encased steel truss welded to a continuous steel plate are analyzed using a nonlinear finite element formulation based on Newmark's classical model. The web member of the steel truss is made by deformed or structural steel rebars and behaves like a deformable shear connection. In order to avoid slip locking, finite elements based on second-order interpolation of longitudinal displacements and flexural rotations are employed. Simply supported composite beams subjected to a uniformly distributed transverse load are considered. The bending capacity is evaluated for short up to long spans, taking the nonlinear behavior of concrete, steel and shear connection into account. The effects of the shear connection ductility are put in evidence, showing that, for short spans, the interfacial stress transfer resulting from the yielding of connection may be penalizing. In fact, the high slip gradient arising in sections near the supports may lead to a premature concrete failure. In this case, the exact solution to the linear elastic problem for steel–concrete composite beams can be used for design purposes
Ripristino della fruibilità post-sismica di edifici prefabbricati dell’Università di Ferrara
No full textLa memoria riporta una panoramica degli interventi realizzati dopo la sequenza sismica emiliana del maggio 2012 negli edifici a struttura prefabbricata in c.a. ubicati presso il Polo Scientifico-Tecnologico dell’Università di Ferrara. Gli edifici sono sorti tra il 1996 e il 2002, quando Ferrara non rientrava in zona sismica. Essi hanno pertanto i tegoli di copertura e le travi in semplice appoggio rispettivamente su travi e pilastri, senza alcun dispositivo di ritegno. Non essendo lecito affidare unicamente alla forza d’attrito la resistenza nei confronti delle azioni sismiche, gli edifici sono stati dichiarati inagibili subito dopo la sequenza, pur non avendo riportato Danni significativi. Allo scopo di consentirne una rapida riapertura sono state progettate opportune connessioni metalliche. Le forze di progetto da affidare alle connessioni sono state determinate tramite modellazioni agli elementi finiti degli edifici e analisi dinamiche con spettro di risposta
Valutazione sperimentale della duttilità di un pilastro prefabbricato inghisato
No full textVengono riportati i risultati di una prova eseguita presso il laboratorio del Dipartimento di Ingegneria dell’Università di Ferrara. Il pilastro, di sezione 50×50 cm e altezza totale pari a 3.7 m, è stato collegato al proprio plinto di fondazione, prefabbricato anch’esso, tramite l’inghisaggio di 8 barre ∅20 di acciaio B450C, ancorate nel getto della fondazione. Il collegamento è stato effettuato iniettando in pressione malta ad alta resistenza nei condotti in PVC per l’alloggiamento delle barre. Lo sforzo normale di compressione, pari a 1700 kN, è stato applicato al pilastro per mezzo di un opportuno sistema di precompressione esterna. Spostamenti e deformazioni sono stati monitorati tramite 19 trasduttori lineari di spostamento. Uno di questi, posizionato in corrispondenza del giunto di base, ha permesso di valutare lo scorrimento dovuto alla deformazione da taglio delle barre di inghisaggio. Le condizioni ultime sono state individuate dopo 19 cicli di spostamento in corrispondenza di una riduzione del 15% della resistenza massima. Per scorrimento e drift sono stati raggiunti i valori massimi di 7 mm e 5.25%. La capacità deformativa del conglomerato compresso nella zona di inghisaggio si è rivelata maggiore di quella tradizionalmente adottata nei calcoli, fornendo una duttilità sezionale circa pari a 10
Elastic buckling analysis of pultruded FRP portal frames having semi-rigid connections
No full textPresented in this paper are results for the elastic instability of Pultruded Fibre Reinforced Polymer (PFRP) portal frames with semi-rigid connections. The kinematical model used is based on a second-order displacement field accounting for the shear strain influence on both non-uniform bending and torsion. A two-node locking-free finite element with seven degrees of freedom per node is adopted. Joint flexibility at member ends is included by means of a simple manipulation of the stiffness matrix to the finite element, such that the influence of joint behaviour on membrane, shear, bending and torsion deformations, as well as to cross-section warping, can easily be taken into account. Numerical examples are reported and discussed to illustrate the influence on elastic buckling loads of in-plane and out-of-plane moment-rotation behaviour of joints, as well as the role played by the base warping restraint and deformable lateral bracings
Postbuckling failure analysis of pultruded FRP beams under uniform bending
No full textThe postbuckling behaviour of Pultruded Fibre-Reinforced Plastic (PFRP) beams in uniform major-axis bending is analyzed through a nonlinear finite element (FE) analysis including initial out-of-straightness. Two wide-flange shapes by two pultruders are analyzed for six values of the beam slenderness, involving 24 FE models. The limiting imperfection amplitudes reported by the pultruders are shown to yield extremely low ultimate bending moments, associated with unrealistic failure mechanisms and strong interaction between local and flexural-torsional buckling modes. Conversely, reduced imperfection amplitudes reported in the literature are found to lead to well documented failure mechanisms and to a small influence of the buckling mode interaction. The influence of the warping restraints at the beam end sections is also discussed
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