This thesis work arises from the need to investigate and verify what are the possible resistant mechanisms that can be activated, in a mechanical connection used in wooden structures as a result of static and cyclic actions beyond the elastic limit. For this reason it has been realized a campaign tests, at the Material Testing Laboratory of the Department of Structural Engineering, Politecnico di Milano, which investigate the behavior of angle brackets connections, connected to wood beams by means of cylindrical metal connectors (in the cases of study have been employed wood screws), loaded by static and cyclic actions beyond the elastic limit. The campaign tests therefore provided both static ductility and cyclic ductility tests. During the internship held at the Politecnico di Milano activity it was also checked the option to apply or not nonlinear models proposed by current standards that should characterize the response nonlinear of these connections as a result of static and cyclic loads that exceed the elastic limit.The verification of the possibility of applying these theoretical models is essential to forecast the correct response of a structure which has these connections thanks to the type of nonlinear structural analysis (pushover analysis or nonlinear dynamic analysis). The experimental evidence obtained by the campaign on the angle brackets showed that the ductility of the connection, namely the ability to deform beyond the elastic limit, are mainly related to: - the ability to deform the angle brackets for bending actions perpendicular to the plane of the plates of the angle bracket; - the ability of the connectors that work in shear to deform plastically (bending deformation with formation of one or more plastic hinges).The presence of the cylindrical metal connectors in almost all connections of wooden structures, the need to qualify all products for building applications (hence also the singleconnector) via standardized tests and the experimental evidence of the campaign tests on the angle brackets connections (ie take advantage of the ductility of the connectors to both static and cyclic actions) it has led to the creation of the second experimental campaign developed in the course of this thesis. This experimental campaign has focused on the design of a test apparatus from scratch and the realization of a test procedure that were able to verify whether or not (via a standardized method and easily repeatable experimentally) the ductility resources of the cylindrical metal connectors (pins, bolts, nails and screws) employed for shear actions. This ability, is verified and evaluated by the ability or not of the connector to be able to deform plastically, a capacity that is expressed in experimental terms from possibility or not to form "plastic" hinges along the connector as a result of bending deformations. The plastic hinges are in fact zones of deformed material which may continue to deform plastically without having a substantial reduction of resistance.Since the static ductility is defined as the ratio between the last strain limit (scrolling) and the elastic strain limit (scrolling) of a material (or a system), the test apparatus designed has been specially designed to verify precisely the the ability to deform the connector. This capacity was evaluated, for static actions, such as the relationship between the bending angle that the connector reaches before breaking (or before losing substantially its resistance) and the bending angle that the connector assumes when it reaches the elastic limit. To define instead ductility to cyclical actions has been performed following the existing standards dealing with systems equipped with ductility to cyclical actions that have been designed at a predetermined ductility.
Il presente elaborato di tesi nasce dalla necessità di indagare e verificare quali siano i meccanismi resistenti possibili che si possono attivare, in una connessione meccanica impiegata in strutture in legno a seguito di azioni statiche e cicliche oltre il limite elastico. Per questo motivo è stata realizzata una campagna prove, presso il Laboratorio Prove Materiali del Dipartimento di Ingegneria Strutturale del Politecnico di Milano, che indagasse il comportamento di connessioni angolari, connesse a travi di legno per mezzo di connettori metallici a gambo cilindrico (nei casi di studio sono state impiegate viti da legno), caricate da azioni statiche e cicliche oltre il limite elastico. La campagna prove ha previsto quindi sia prove di duttilità statica che duttilità ciclica. Nel corso della attività di tirocinio svolta presso il Politecnico di Milano è stata verificata inoltre la possibilità di applicare o meno i modelli non lineari proposti dalle normative vigenti che dovrebbero caratterizzare la risposta non lineare di queste connessioni a seguito di azioni statiche e cicliche che eccedono il limite elastico. La verifica della possibilità di applicare tali modelli teorici è di fondamentale importanza per prevedere correttamente la risposta reale di una struttura che presenta queste connessioni grazie alle analisi strutturali di tipo non lineare (analisi pushover o analisi dinamiche non lineari).Le evidenze sperimentali ottenute dalla campagna sugli angolari hanno dimostrato che la duttilità della connessione, ovvero la capacità di deformarsi oltre il limite elastico, sono principalmente legate a: - la capacità dell’angolare di deformarsi per azioni flettenti perpendicolari al piano delle piastre che compongono l’angolare; - la capacità dei connettori che lavorano a taglio di deformarsi plasticamente (deformazione a flessione con formazione di una o più cerniere plastiche).La presenza di connettori metallici a gambo cilindrico in quasi tutte le connessioni delle strutture in legno, unita alla necessità di qualificare tutti i prodotti per impieghi edilizi (quindi anche il singolo connettore) tramite procedure standardizzate e alle evidenze sperimentali della campagna prove sugli angolari (ovvero poter sfruttare la duttilità dei connettori sia alle azioni statiche che cicliche) ha portato alla realizzazione della seconda campagna sperimentale sviluppata nel corso della presente tesi. Questa campagna sperimentale ha riguardato il progetto di un apparato di prova da zero e la realizzazione di una procedura di prova che fossero in grado di verificare o meno (tramite un metodo standardizzato e facilmente ripetibile sperimentalmente) le risorse duttili dei connettori metallici a gambo cilindrico (spinotti, bulloni, chiodi e viti) impegnati a taglio. Questa capacità, viene verificata e valutata tramite la capacità o meno del connettore di potersi deformare plasticamente, capacità che viene tradotta in termini sperimentali dalla possibililità o meno di formare cerniere “plastiche” lungo lo sviluppo del connettore a seguito di deformazioni di tipo flessionale. Le cerniere plastiche sono infatti zone di materiale deformato che possono continuare a deformarsi plasticamente senza avere una sostanziale riduzione di resistenza.Poiché la duttilità statica è definità come rapporto tra il limite di deformazione (scorrimento) ultimo e il limite si deformazione (scorrimento) elastico di un materiale (o di un sistema), l’apparato di prova progettato è stato concepito appositamente per verificare proprio la capacità del connettore di deformarsi. Tale capacità è stata valutata, per azioni statiche, come il rapporto tra l’angolo di piega che il connettore raggiunge prima di spezzarsi (o prima perdere in modo sostanziale la sua resistenza) e l’angolo di piega che il connettore assume quando raggiunge il limite elastico. Per definire invece la duttilità alle azioni cicliche si sono seguite le norme vigenti che trattano i sistemi dotati di duttilità alle azioni cicliche che sono stati progettati a duttilità prefissata.
Comportamento strutturale di connessioni metalliche e connettori metallici a gambo cilindrico per strutture in legno sottoposti ad azioni monotone e cicliche
MAZZOLA, PAOLO FRANCESCO
2016/2017
Abstract
This thesis work arises from the need to investigate and verify what are the possible resistant mechanisms that can be activated, in a mechanical connection used in wooden structures as a result of static and cyclic actions beyond the elastic limit. For this reason it has been realized a campaign tests, at the Material Testing Laboratory of the Department of Structural Engineering, Politecnico di Milano, which investigate the behavior of angle brackets connections, connected to wood beams by means of cylindrical metal connectors (in the cases of study have been employed wood screws), loaded by static and cyclic actions beyond the elastic limit. The campaign tests therefore provided both static ductility and cyclic ductility tests. During the internship held at the Politecnico di Milano activity it was also checked the option to apply or not nonlinear models proposed by current standards that should characterize the response nonlinear of these connections as a result of static and cyclic loads that exceed the elastic limit.The verification of the possibility of applying these theoretical models is essential to forecast the correct response of a structure which has these connections thanks to the type of nonlinear structural analysis (pushover analysis or nonlinear dynamic analysis). The experimental evidence obtained by the campaign on the angle brackets showed that the ductility of the connection, namely the ability to deform beyond the elastic limit, are mainly related to: - the ability to deform the angle brackets for bending actions perpendicular to the plane of the plates of the angle bracket; - the ability of the connectors that work in shear to deform plastically (bending deformation with formation of one or more plastic hinges).The presence of the cylindrical metal connectors in almost all connections of wooden structures, the need to qualify all products for building applications (hence also the singleconnector) via standardized tests and the experimental evidence of the campaign tests on the angle brackets connections (ie take advantage of the ductility of the connectors to both static and cyclic actions) it has led to the creation of the second experimental campaign developed in the course of this thesis. This experimental campaign has focused on the design of a test apparatus from scratch and the realization of a test procedure that were able to verify whether or not (via a standardized method and easily repeatable experimentally) the ductility resources of the cylindrical metal connectors (pins, bolts, nails and screws) employed for shear actions. This ability, is verified and evaluated by the ability or not of the connector to be able to deform plastically, a capacity that is expressed in experimental terms from possibility or not to form "plastic" hinges along the connector as a result of bending deformations. The plastic hinges are in fact zones of deformed material which may continue to deform plastically without having a substantial reduction of resistance.Since the static ductility is defined as the ratio between the last strain limit (scrolling) and the elastic strain limit (scrolling) of a material (or a system), the test apparatus designed has been specially designed to verify precisely the the ability to deform the connector. This capacity was evaluated, for static actions, such as the relationship between the bending angle that the connector reaches before breaking (or before losing substantially its resistance) and the bending angle that the connector assumes when it reaches the elastic limit. To define instead ductility to cyclical actions has been performed following the existing standards dealing with systems equipped with ductility to cyclical actions that have been designed at a predetermined ductility.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/134381