Numerical assessment of friction dampers under quasi-static and impact loading

Abstract

A construção metálicas tem sido vastamente aplicada para os mais diversos tipos de construções nos últimos anos. Estas estruturas são consideradas como umas das mais sustentáveis.As ligações de estruturas metálicas são vistas como um componente critico no que diz respeito à ductilidade e capacidade de deformação destas estruturas, sendo assim de extrema importância o conhecimento do comportamento das mesmas. O trabalho apresentado neste documento reproduz o estudo do comportamento de uma nova tipologia de ligação metálica quando sujeita a ações de impacto. A ligação em estudo é uma ligação viga-coluna em estudo no projeto Europeu FREEDAM- “Free from damage connections”. Esta ligação apresenta a particularidade de estar equipada com elementos de fricção, localizados num esquadro aparafusado ao banzo inferior da viga. Desta maneira, a dissipação da energia é obtida pelo escorregamento do banzo do esquadro nas placas de fricção, através da execução de furos alongados no banzo do esquadro. A resistência dos elementos de fricção é calibrada a partir do número e diâmetro dos parafusos e do pré-esforço introduzido nestes. Utilizando procedimentos numéricos para reproduzir o comportamento não-linear das ligações por fricção sujeitas a impacto, a resistência à fricção é calibrada, tendo em conta o nº de parafusos, o seu diâmetro e o pré-esforço aplicado.Por fim, um estudo paramétrico mostrou que ligações por fricção mais rígidas são menos propensas ao desenvolvimento de taxas de extensão elevadas e consequentemente o aumento de resistência é mais reduzido; por outro lado, a ductilidade reduz em ligações por fricção sujeitas a cargas de impacto.

Steel structures are being the most widely used form of construction in various types of applications around the globe in recent years. To make the built environment sustainable, steel construction is considered as the nodal intervention in the construction industry. Whereas, in steel frame structures joints are considered to play the most significant role in providing ductility, sufficient rotational capacity and adequate dissipation under extreme conditions. However, the accidental loadings such as blast, fire and impact, may instantaneously causes rupture of the elements that are located within the vicinity of the impact, in some situation leading to a disproportionate failure of structural components or even to the failure of complete structure. Therefore, to make the built environment safe and to ensure that buildings remain operational and more importantly do not collapse under extreme loading conditions, this work aims at investigating a new innovative design strategy of “FREEDAM” (Free from Damage Connections) connection under accidental conditions. This connection was initially design as a sustainable connection, able to withstand without any damage in the rotation demands due to seismic events. The response of this type of connections subject to transient dynamic loads is uncertain and yet absent in current design guidelines. Such innovative beam-to-column connections are equipped with friction dampers which are located at the bottom flange level of the connected beam to dissipate the input energy. Therefore, the study addresses the issues by developing a validated three-dimensional finite element model of such a connection under impact and quasi-static conditions. Exploring numerical procedures to simulate non-linear behaviour of friction damper subjected to impact loads, the friction resistance is calibrated by accounting the number of bolts, their diameter and tightening torque governing the preloading. The flexural resistance results from the product between the damper friction resistance and the lever arm. The friction damper numerical model is used to describe the behaviour of joint; i) “under quasi-static loading” and ii) “under impact conditions” presented in a beam-to-column moment resistant connection. The components of the friction damper are responsible in facilitating the dissipation of induced energy in joints and being able to provide sufficient ductility to a joint. Although, the friction damper model is a less intricate model, compared to the complete joint, yet its frictional behaviour incorporation to a joint’s dissipation capacity is quite arduous. Equipped with a failure criterion describing the softening phase of the materials, the FE model describes the failure modes observed experimentally in the displacing plate of the friction damper. Results exhibited that the transient loads application, induced elevated strain rate in the material, which enhanced the constitutive mechanical properties. Therefore, enabling the friction damper to resist the maximum load observed in quasi-static cases with reduced displacements. Parametric studies show that stiffer friction dampers are less prone to develop elevated strain rates and therefore less keen to strength enhancement; on the other hand, the ductility capacity is reduced for rather flexible dampers comparing the quasi-static with the impact response.