Benno Hoffmeister

Influence of variability of material mechanical properties on seismic performance of steel and steel–concrete composite structures

Modern standards for constructions in seismic zones allow the construction of buildings able to dissipate the energy of the seismic input through an appropriate location of cyclic plastic deformations involving the largest possible number of structural elements, forming thus a global collapse mechanisms without failure and instability phenomena both at local and global level. The key instrument for this purpose is the capacity design approach, which requires an appropriate selection of the design forces and an accurate definition of structural details within the plastic hinges zones, prescribing at the same time the oversizing of non-dissipative elements that shall remain in the elastic field during the earthquake. However, the localization of plastic hinges and the development of the global collapse mechanism is strongly influenced by the mechanical properties of materials, which are characterized by an inherent randomness. This variability can alter the final structural behaviour not matching the expected performance. In the present paper, the influence of the variability of material mechanical properties on the structural behaviour of steel and steel/concrete composite buildings is analyzed, evaluating the efficiency of the capacity design approach as proposed by Eurocode 8 and the possibility of introducing an upper limitation to the nominal yielding strength adopted in the design.

Fragility of Steel Frame Buildings under Blast Load

AbstractEffects of different structural configurations, member orientations and standoff distances on blast-induced failure of steel frame buildings are investigated through development of fragilit...

PERFORMANCE ASSESSMENT OF SEISMIC RETROFITTING MEASURES ON SILO STRUCTURES USING INNOVATIVE SEISMIC PROTECTION SYSTEMS

Abstract. This paper deals with a real-world silo structure and the assessment of its seismic performance and possible retrofitting measures by using seismic protection devices. Firstly, the silo structure is shortly described followed by a simplified numerical modelling approach with the aim to save computational effort as much as possible but still achieve a reliable response of demands and failure modes. A comparison between the complex and simplified model is done, showing a very good match in terms of its dynamic properties, which are Eigenfrequencies, modal shapes and effective modal masses. With this simplified model incremental dynamic analyses are conducted to assess the current performance of the structure as-built. Strengths and weaknesses are highlighted by the simulation outcomes. Following, two possible retrofitting strategies using torsion-based hysteretic dampers are discussed. Hereby it is shown, how the seismic device can be modelled in any finite element software by using rheological models. In one strategy of seismic retrofit an innovative torsional device is inserted at single bays of the structural frame. In the second strategy the steel torsional hysteretic damper is applied at the base of the steel columns, such that the structure is separated from the ground. The first retrofit approach proves to be effective in prohibiting a severe sides-way collapse which occurred for some earthquakes. The second shows to be greatly effective in preventing failure and also decreases deformation demands on the structure efficiently.

Floor Response Spectra Considering Influence of Higher Modes and Dissipative Behaviour

Seismic design forces of nonstructural components are commonly obtained by application of floor response spectra. This method is usually applied using estimated modal shapes and periods of the main structure; it allows for a separated design of components and their anchorages by the producers of equipment. Simplified formulas for determination of floor response spectra are provided by current codes such as Eurocode 8. All of them follow the assumption of the first fundamental elastic mode governing the acceleration values at the floors. These approaches do not take into account effects of higher modes, topology, ground response spectrum and plastification of supporting structures. Floor response spectra of four different building frames, one typical for an industrial 5-storey steel supporting structure and other three representing 5-, 10- and 15-storey regular steel buildings, were investigated using nonlinear incremental dynamic analyses. The results were compared to current code provisions revealing large discrepancies which have impact on safety as well as on economy of the design. Three aspects were identified and qualified: Application of ground response spectrum values instead of peak ground acceleration as basic input variable; Importance of higher modes; Impact of plastification of the main structure and the components. It could be shown that all three parameters have a significant influence on the acceleration values, on the dimensioning of the anchorages and on the ductility demand for components designed to dissipate energy.

Guidelines for Seismic Design and Analysis of Pressure Vessels

An adequate design of pressure vessels, also for seismic actions, is crucial in particular if due to hazardous content the consequences of failure may become very severe. While very detailed and specific seismic design rules for structural buildings are provided by several codes, such rules are missing for pressure vessels. This paper describes the results of a study on seismic performance and applicability of existing European and American codes to pressure vessels with cylindrical and spherical shapes and provides a comparison of design outcomes according to these codes. The investigations were performed for different case studies of existing pressure vessels which were selected to be representative for the current practice. The studies comprised numerical investigations as well as simplified models for the estimation of the dynamic properties of the vessel structures. The applicability of behaviour factors was discussed based on proposals made by European and American codes, so that finally recommendations for the behaviour factor of pressure vessels with different shapes and types of supports have been developed based on pushover analyses and non-linear incremental dynamic analyses.Copyright

Seismic Design of Steel Frames with Fuseis Beam Link Energy Dissipation Systems

Within the EU funded project INNOSEIS different seismic devices developed in earlier projects are worked up with the aim to provide information sheets and design guidelines for an easy practical implementation of these systems. One of these seismic protection systems is the FUSEIS beam link concept. The main idea is to use two closely spaced strong columns rigidly connected by several beams, resulting in a Vierendeel girder in upright position. Energy dissipation is originated by plastification of the connecting girders, similar as in seismically designed moment resisting frames. Forming the lateral load resisting system but not taking part intentionally in the vertical load transfer, the girders can be inspected and replaced much more easily after a strong earthquake. In this paper, first the FUSEIS beam link system is described in detail supported by results of experimental tests. Behaviour and main mechanical principles are outlined. Further on, three case studies consisting of steel office buildings of different heights are designed for seismic loads using the FUSEIS beam link solution. 878 Available online at www.eccomasproceedia.org Eccomas Proceedia COMPDYN (2017) 878-890

Prediction of Ductile Damage in Case of Seismic Action using Innovative Damage mechanics

Abstract. An approach to investigate the true behaviour of dissipative elements utilising numerical simulations is presented. By way of example dissipative members in terms of beamcolumn connections of moment-resisting frames are considered. Material failure in terms of crack formation is taken into account by incorporating a damage mechanics model into the numerical analyses. By means of a phenomenological damage mechanics model, which is based on critical strains depending on the stress state, an efficient simulation of large scale components is enabled. To validate this approach, an extensive testing program has been performed. Subsequently, numerical simulations of the experimental investigations have been conducted.

PERFORMANCE OF STEEL COLUMNS SUBJECTED TO VEHICLE IMPACT – EXPERIMENTAL AND NUMERICAL STUDIES

Hazard scenarios regarding exceptional actions often include the possibility of a vehicle impact against a building. Here the impact against columns is of particular interest as they are the most exposed members. At the same time consequences are vast, since their failure can result in the collapse of the building. To evaluate the dynamic response of an impacted member, six slightly downscaled vehicle impact tests have been performed at RWTH Aachen University. Three different boundary conditions for the column have been examined, including a simply supported column (ideally pinned) with and without a concentrated mass attached to the column head as well as a column with realistic boundary conditions and connections at column base and column head. By varying the boundary conditions and the attached mass the influence of various types of interaction between the column and the surrounding structure has been assessed experimentally. Also the velocity of the impact body has been varied to achieve different degrees of impairment and residual strengths of the column. In a second step state of the art commercial software (LS-Dyna) has been used to perform impact simulations with refined finite element models to augment the experimental results. Different member configurations have been investigated to further evaluate the influence of several parameters like impact velocity, mass and stiffness distribution of the member. The numerical models together with the implemented fundamental physical laws were validated by recalculations of the experimental tests.

SLENDER PROFILES FOR MOMENT RESISTING FRAMES IN MODERATE SEISMICITY REGIONS - NUMERICAL AND EXPERIMENTAL INVESTIGATIONS

Abstract. Moment resisting steel frames offer an excellent ductility and energy dissipation and can be designed using high behaviour factors. The condition however is, that compact cross-sections are used allowing for large rotations in the plastic hinges. Portal frames, as commonly designed and built for storage or industrial applications, are usually aiming at light weight solutions in order to be competitive to prefabricated concrete halls. Class 3 or 4 profiles are preferably used, this prevents however the application of any significant behavior factor. In particular in low and moderate seismicity regions a behaviour factor in the range of 2-3 would be fully sufficient in order to achieve an economically optimum design.

Seismic Design of Spherical Pressure Vessels

Spherical pressure vessels are globally used for storage of pressurized liquids or gases of different hazard classes. An adequate seismic design of these structures must consider their particular structural behaviour and consequences of possible damage or failure. A study of the current standard situation for seismic design of pressure vessels revealed significant gaps and missing design rules, in particular for spherical pressure vessels. Within the European Research Project INDUSE the seismic performance and applicability of existing European and American codes to pressure vessels with cylindrical and spherical shape were investigated. This paper describes the results of a study on different examples of spherical pressure vessels which were selected to be representative for the current practice. The study comprised numerical investigations as well as simplified models for the estimation of the dynamic properties of the vessel structures. It is shown, which failure modes and stress concentrations areas are crucial in the event of an earthquake. In addition engineering calculation methods to determine fundamental periods and internal forces for braced and non-braced spherical pressure vessels were developed and compared to results of numerical simulations. The applicability of behaviour factors is discussed based on proposals made by European and American codes in comparison to own results. Recommendations for the behaviour factor of spherical pressure vessels with different dimensions were developed based on push over analyses and non-linear incremental dynamic analyses. Furthermore the influence of sloshing effects in spherical vessels, for which no specific rules are given in the codes, was investigated according to the current state of the art.