The findings of a recent survey on the steel roof structure of an early-twentieth-century building located in Sofia, Bulgaria are reported. The steel roof structure is supported by solid masonry walls around the building perimeter and on slender cast-iron columns along two interior longitudinal lines. The south part of the building was severely damaged during the bombing raids over Sofia in WWII. The building underwent a major refurbishment within the 1995-2000 period accompanied by detailed survey and condition assessment of the steel roof structure. In 2020 the authors carried out a survey for condition assessment of the steel roof structure including checks of riveted connections, analysis of chemical composition by OES metal analyser and inspection of the roof cladding components by endoscopic device.

The paper presents results from several condition surveys of existing steel crane-supporting structures with a focus on the performance of the links of the runway beams to the supporting columns. These links require careful detailing in order to safely transfer the crane lateral forces to the columns and accommodate the beam end rotations. Different types of industrial buildings have been inspected and the major factors contributing to the observed damages are identified. An overview of several typical connections between the runway beams and supporting columns representative of Bulgarian design practice is included along with newly proposed detailing concepts. The analysis of survey findings shows that the amount of the observed damage and failures can be directly related to link detailing. Recommendations are made for improving the long-term performance of the links connecting the steel crane runway beams to the supporting structures via better detailing.

The paper addresses results of an international effort in further developing a novel friction damper device for a reduction of induced vibrations in structures. A description of the device, results of small-scale experiments, and results of full-scale shaking table tests are presented. The result of the investigations shows that the effectiveness of the device is determined not only by the friction material but also by the location as well as the way of its installation. The devices have a stable energy dissipating behaviour. They are flexible in their application, since they only need limited space. The device can be installed easily and readjusted after installation. The damping capacity of the device can be increased by simply adding friction layers. The friction damper device proves to be an efficient and economical device for a reduction of dynamic response of structures.

The existing steel structures designed to older generation seismic codes typically are exposed to high seismic risk. The limited local ductility capacity and the lack of member hierarchy may undermine the global ductility capacity which in turn could lead to unacceptable seismic damage and vulnerability to the aftershocks. The paper describes a new technique for passive seismic protection of existing steel structures based on the linked column concept. The conventional linked columns (LCs) are made up of two closely spaced vertical piers connected throughout their height using short horizontal ductile links which in this study were replaced by rotational friction dampers (RFDs). The added LCs with RFDs are meant to act as a primary lateral force resisting system while the existing structure is mainly involved in resisting gravity loads. Under ground motion, the relative deformations of the piers of the LCs activate the dampers which provide a stable source of energy dissipation while the piers are expected to remain elastic and together with the existing frames to maintain the self-centering of the structure. In this study, numerical studies of several steel structures have been carried out using SAP2000 software (C&S, Inc. 2015) in order to investigate the protective capability of the proposed system. The added LCs with RFDs were arranged symmetrically on both sides of pre-selected columns of the existing structures. The results obtained from the analyses are presented and summarized. They indicate that the friction-damped linked columns could be a promising technique for passive seismic protection of existing steel structures.

ABSTRACTThe existing steel moment‐resisting frames (MRFs) designed to seismic codes of older generation typically have limited local ductility capacity and may not perform to the desired “strong‐column‐weak‐beam” concept due to the lack of capacity design provisions. All these deficiencies require modifications for reducing their seismic vulnerability.The paper proposes a seismic retrofit technique for steel moment frames based on the linked column concept. The linked columns are dual piers connected throughout their height with short horizontal replaceable ductile link elements. Developed in the last decade for new seismic‐resistant steel structures, the linked column frame structural system combines two sub‐systems – a primary lateral force resisting system composed of linked columns and secondary moment resisting frames mainly involved in resisting gravity loads.During a seismic event, the relative deformations of the closely spaced piers activate the links which are designed to yield in shear or bending for seismic energy dissipation, but they also help in controlling interstorey drifts and limiting the forces transferred to the surrounding structural members.The authors present a new innovative design of the linked columns. Instead of short steel beams, rotational friction dampers (RFDs) are used as seismic links in order to avoid the damage expected in conventional link elements. The RFDs are supplied by a Danish company and have been used for seismic protection in many projects for the last decade. They have proven energy‐dissipation capacity and can be delivered in different shapes and slip capacities at relatively low cost.In this study three‐, six‐ and nine‐storey four‐bay steel MRFs have been analyzed using the capacity spectrum method. The added linked columns are arranged symmetrically on both sides of the first interior columns of the MRFs.The results obtained for three different arrangements of the seismic links are presented and summarized. They indicate that the seismic retrofit using linked columns with rotational friction dampers is a promising technique for seismic upgrade of existing steel moment frames.

The paper presents an overview on nonlinear modeling and numerical evaluation of the seismic response of two steel single-storey concentrically braced frames with X-brace configuration previously tested by other researchers. The numerical simulations included series of nonlinear time history analyses using OpenSees computational framework. The brace members were modeled with fiber frame finite elements with distributed plasticity taking into account their initial imperfections. The material model accounted for the low-cycle fatigue effects based on the approach proposed by several other research works on the topic. The maximum response displacements of the investigated specimens were also evaluated following the procedure given in Annex B of EN 1998–1 (Eurocode 8). However, the previously conducted shake table testing and the herein presented numerical results indicated that this procedure may significantly underestimate the displacements of single-storey dissipative short-period CBFs designed with high values of the behaviour factor. It was also found that the numerical estimation of the maximum and residual lateral response displacements of the analyzed CBFs based on nonlinear time history analysis is very sensitive to small variations of the material properties and assumed damping model (damping matrix definition). Therefore the experimentally-obtained response displacements can be realistically predicted only if the parameter variations are taken into account in the numerical analyses. The authors believe that the use of high values for the behaviour factor in the design of low-rise CBFs may result in excessive residual displacements and low-cycle failure of the brace members. It is suggested the further revisions of Eurocode 8 impose lower reference values of the behaviour factor for the short-period steel CBFs with slender braces in order to reduce their ductility demand and related damage in strong earthquakes.

Insulated sandwich panel walls are typically screw-connected to their supporting substructure and therefore enforced to follow the displacements of the primary structure imposed by wind and seismic actions. The paper presents an investigation of the façade envelope of a warehouse building which was slightly damaged by a recent earthquake and underwent partial failure during a wind storm a few years later. Of particular concern are the corner zones of tall single-storey buildings with stiffness irregularity in plan where the interstorey drifts produced by relatively weak earthquakes could trigger deterioration of the panel fastenings and increase their vulnerability to subsequent wind suction effects. When concealed (hidden) fastenings are used the designer should take measures either to reduce the shear and tension demands on the fastenings or to increase their capacity, e.g. through application of load distributing plates.

A scaled specimen representing single-storey partially-composite steel plate shear wall was tested under cyclic loading. The slender steel web panel had a single-sided reinforced concrete encasement attached with headed shear studs in order to prevent the shear buckling and tension-field action of the slender web when subjected to lateral seismic forces. A specific feature of the tested specimen was the use of semi-rigid beam-to-column connections and purposely made gaps between the boundary steel frame members and the concrete encasement. The testing was conducted using the recommended testing procedure of ECCS and showed that the specimen has stable hysteretic behaviour and dissipative capacity superior to that of its pure steel counterpart tested under the same conditions. Parallel numerical simulations based on simplified and more refined nonlinear models were carried out in order to provide insight into the complex interaction between the shear wall components. Both experimental testing and numerical simulation revealed that the cyclic response of the specimen was affected by the formation of local tension-field action in-between the shear stud rows, which is an indication that the seismic codes shall improve their provisions for the design of reinforced-concrete encasement and its shear stud connection to the slender steel web panel.

A study on the dynamic response of single-storey steel frames equipped with a novel friction damper device (FDD) is presented. Extensive testing was carried out for assessing the friction pad material, damper unit performance and scaled model frame response to lateral harmonic excitation. Numerical simulations based on non-linear time history analysis were used to evaluate the seismic behaviour of steel frames with inserted FDD. The governing parameters were identified and their influence was traced and summarised along with implications for practical design. The application of the new FDD presents a feasible alternative to the conventional ductility-based earthquake-resistant design both for new construction and for upgrading existing structures.