The nonlinear behaviour of connections between structural elements is critical to the performance of mass-timber structures under seismic loads. However, limited work has been developed in nonlinear modelling and fragility assessment of mass-timber structures. To improve the accuracy of this approach, in particular when considering structures with ring-doweled moment-resisting connections, a nonlinear modelling approach and fragility assessment are proposed and a prototype example of a three-story building is analysed herein as a case study. For the case study, connections and members were designed following the prescriptions in Eurocode 5 and Eurocode 8, considering a high ductility structure. The mechanical properties of the structure are modelled as random variables to evaluate the impact of uncertainty on the prediction of the structural performance, in particular, on the probability of occurrence of ductile and brittle failure modes. The structure is studied under both nonlinear static analysis and multi-record incremental dynamic analysis. From these, fragility curves for different damage levels are computed and a q-factor is proposed. Results indicate that the requirements of Eurocode 5 and Eurocode 8 are sufficient to guarantee adequate performance for this type of structure, albeit these may be overconservative. Moreover, it is shown that uncertainties in material properties have a significant impact on the collapse capacity of these structures.
In the seismic design of structures according to the dissipative structural behaviour, the connection ductility is crucial in order to ensure the desired level of energy dissipation of the overall structure. Therefore, in case of ductile zones composed of dowel-type fasteners arranged in series, it is important to ensure that all the fasteners can fully develop their energy dissipation capacity by plastic deformations. However, when different types of connections made of two symmetrical and serially arranged assemblies of dowel-type fasteners are tested, it often appears that only few fasteners fully work in the plastic region while most of the remaining ones exhibit very low yielding.
Looking at the causes of this dysfunction, a possible explanation is due to the fact that the rules for the seismic design of dissipative zones in timber structures given in international codes and used in common practice often make reference only to the steel quality of the dowel-type fasteners specifying a minimum tensile strength or sometime, like is the case of the current version of Eurocode 8, only to maximum values of the dowel-type fastener diameter and of the thickness of the connected timber or wood-based members. Also, the research conducted so far about the ductile behaviour of serially arranged connections was not focused on the post-elastic properties of steel. However, for the seismic design of ductile zones of other materials, such as for example is the case of reinforced concrete walls, post-elastic characteristics of steel are required for the reinforcing bars, in order to achieve the desired dissipative behaviour.
Inspired by this fact, timber connections composed of serially arranged dowels made of steel grades with different hardening ratio and elongation at maximum tensile stress were fabricated and tested. The purpose of this work is to understand if the use of steel with significant post-elastic properties may help to solve the problem of limited yielding in serially arranged dowel-type connections.
The tested specimens were composed of two symmetrical timber members made of Glulam and LVL connected to two 6 mm thick slotted-in steel plates by means of 9 steel dowels with a diameter of 6.0 mm, which were subjected to monotonic and cyclic tests carried out by implementing dowels made of steel with favourable post-elastic properties.
The results showed that the simultaneous yielding of two serially arranged dowelled assemblies is possible, although not fully. Moreover, assuming as reference the steel grade with the lowest post-elastic properties, the connection ductility and strength measured through monotonic and cyclic tests increased by about 30% for the steel grades with the highest hardening ratio and elongation at maximum tensile stress, whereas the displacement at maximum strength was about five times higher.
In addition, it was found that confinement of the timber members and shaping of holes were crucial in order to avoid undesired and premature brittle failures and to increase the connection strength and ductility.
The results obtained may be useful in order to bring a reassessment of the code requirements regarding the steel properties of ductile connections as well as of certain principles of dimensioning and detailing.
According to Eurocode 8 moderate to high dissipative behaviour of timber structures requires sufficient ductility of the critical regions. Earlier experiments on timber connections with slotted-in steel plates and laterally loaded common steel dowels rarely achieved high ductility values. Connections consisting of LVL-C members, dowel-type fasteners with different post-elastic steel properties, full confinement of the timber member and measures to restrain the notch effect were investigated by means of monotonic and cyclic tests with regards to the displacement ductility. The measures taken proved to be effective in enhancing the plastic deformation capacity of the steel dowels to a large extent. However, a new aspect was observed: the constriction of the dowels in the contact area with the plate. The research results provided a better understanding of the factors influencing the behaviour of slotted-in steel plate connection.
The performance of heavy-timber structures in earthquakes depends strongly on the inelastic behavior of the mechanical connections. Nevertheless, the nonlinear behavior of timber structures is only considered in the design phase indirectly through the use of an R-factor or a q-factor, which reduces the seismic elastic response spectrum. To improve the estimation of this, the seismic performance of a three-story building designed with ring-doweled moment resisting connections is analyzed here. Connections and members were designed to fulfill the seismic detailing requirements present in Eurocode 5 and Eurocode 8 for high ductility class structures. The performance of the structure is evaluated through a probabilistic approach, which accounts for uncertainties in mechanical properties of members and connections. Nonlinear static analyses and multi-record incremental dynamic analyses were performed to characterize the q-factor and develop fragility curves for different damage levels. The results indicate that the detailing requirements of Eurocode 5 and Eurocode 8 are sufficient to achieve the required performance, even though they also indicate that these requirements may be optimized to achieve more cost-effective connections and members. From the obtained fragility curves, it was verified that neglecting modeling uncertainties may lead to overestimation of the collapse capacity.