Rectification of “restrained vs unrestrained”

For furnace testing of fire‐resistant floor and roof assemblies in the United States, the ASTM E 119 standard (and similarly the UL 263 standard) permits two classifications for boundary conditions: “restrained” and “unrestrained.” When incorporating tested assemblies into an actual structural system, the designer, oftentimes a fire protection or structural engineer, must judge whether a “restrained” or “unrestrained” classification is appropriate for the application. It is critical that this assumption be carefully considered and understood, as many qualified listings permit a lesser thickness of applied fire protection for steel structures (or less concrete cover for concrete structures) to achieve a certain fire resistance rating if a “restrained” classification is confirmed, as compared with an “unrestrained” classification. The emerging standardization of structural fire engineering practice in the United States will disrupt century‐long norms in the manner to which structural behavior in fire is addressed. For instance, the current edition of the ASCE/SEI 7 standard will greatly impact how designers consider restraint. Accordingly, this paper serves as an exposé of the “restrained vs unrestrained” paradigm in terms of its paradoxical nature and its controversial impact on the industry. More importantly, potential solutions toward industry rectification are provided for the first time in a contemporary study of this paradigm

Minimum cost performance-based seismic design of reinforced concrete frames with pushover and nonlinear response-history analysis

Previous studies compare results of pushover and nonlinear response-history analysis of predesigned reinforced concrete frames. The present study employs nonlinear response-history analysis and pushover analysis with the N2 method in a computational framework for the optimum performancebased seismic design of reinforced concrete frames according to the fib Model Code 2010 methodology and compares their obtained design solutions in terms of cost and structural performance. It is found that the minimum costs of pushover-based designs are similar to the costs from response-history analysis for regular frames but the pushover-based designs can be more expensive for irregular frames. Furthermore, the pushover-based designs are not guaranteed to satisfy performance objectives when subjected to response-history analysis even when more than one lateral load distributions are applied

Performance-Driven Measurement System Design for Structural Identification

Much progress has been achieved in the research field of structural identification, which is attributable to a better understanding of uncertainties, improvement in sensor technologies, and cost reductions. However, data interpretation remains a bottleneck. Too often, too much data are acquired, which hinders interpretation. In this paper, the writers describe a methodology that explicitly indicates when instrumentation can hinder the ability to interpret data. The approach includes uncertainties and dependencies that may affect model predictions. The writers use two performance indices to optimize measurement system designs, i.e.,monitoring costs and expected identification performance. A case study shows that the approach is able to justify a reduction in monitoring costs of 50% compared with an initial measurement configuration

Optimum Lateral Load Distribution for Seismic Design of Nonlinear Shear-Buildings Considering Soil-Structure Interaction

The lateral load distributions specified by seismic design provisions are primarily based on elastic behaviour of fixed-base structures without considering the effects of soil-structure-interaction (SSI). Consequently, such load patterns may not be suitable for seismic design of non-linear flexible-base structures. In this paper, a practical optimization technique is introduced to obtain optimum seismic design loads for non-linear shear-buildings on soft soils based on the concept of uniform damage distribution. SSI effects are taken into account by using the cone model. Over 30,000 optimum load patterns are obtained for 21 earthquake excitations recorded on soft soils to investigate the effects of fundamental period of the structure, number of stories, ductility demand, earthquake excitation, damping ratio, damping model, structural post yield behaviour, soil flexibility and structural aspect ratio on the optimum load patterns. The results indicate that the proposed optimum load patterns can significantly improve the seismic performance of flexible-base buildings on soft soils

Existing prefab R/C industrial buildings: Seismic assessment and supplemental damping-based retrofit

A research study on prefab reinforced concrete buildings designed with older Technical Standards is presented in this paper, where attention is focused on hall-type industrial structures. A representative case study, which includes the main sources of seismic vulnerability, is examined in detail. The possible rigid rotation of the bottom end zone of columns, which are encased in smooth socket-type foundations, and the frictional contact between the neoprene pads situated on top of the columns and the terminal zone of the roof girders are modelled in time-history assessment analyses. The latter are initially carried out by assuming an elastic behaviour of columns, highlighting unsafe response conditions under seismic action scaled at the basic design earthquake level, and near-collapse at the maximum considered earthquake level, which is caused by the loss of support of several girders from the neoprene pads. A second step of the analyses, where plastic behaviour of columns is investigated by incorporating fiber-type plastic hinges at their bottom end sections, assesses a remarkable ductility demand, as well as potential collapse induced by the complete loss of support of girders. The high lateral displacements of columns may also cause failure of the fastenings of the connected cladding panels, likely to results in their overturning-induced collapse. Based on these data, a supplemental damping-based retrofit hypothesis is proposed, consisting in the installation of dissipative braces equipped with pressurized fluid viscous spring-dampers. The protective system allows attaining a completely undamaged response of structural and non-structural members, and therefore meeting the requirements of the Immediate Occupancy limit state, up to the maximum considered earthquake level

Seismic response of Cfs strap-braced stud walls: Theoretical study

The use of cold-formed steel (CFS) profiles in low-rise residential buildings has increased in European construction sector. The reason of this interest is related to potentialities offered by this constructive system, which are the high structural performance, lightness, short construction time, durability and eco-efficiency. Nevertheless, the current structural codes, such as Eurocodes, do not provide enough information about the seismic design of this structural typology. In an effort to investigate the seismic response of CFS structures, a theoretical and experimental research has been carried out at University of Naples Federico II, with the main aim to support the spreading of these systems in seismic areas. This study focuses on an “all-steel design” solution in which strap-braced stud walls are the main lateral resisting system. In the present paper the outcomes of theoretical phase are shown with the aim of defining the criteria for the seismic design of such structures. In particular, a critical analysis of the requirements for CFS systems provided by the American code AISI S213 has been carried out by comparing it with those given by Eurocodes for traditional braced steel frames

Vulnerability assessment and feasibility analysis of seismic strengthening of school buildings

The majority of structures in seismic-prone areas worldwide are structures that have been designed either without seismic design considerations, or using codes of practice that are seriously inadequate in the light of current seismic design principles. In Cyprus, after a series of earthquakes that occurred between 1995 and 1999, it was decided to carry out an unprecedented internationally seismic retrofitting of all school buildings, taking into account the sensitivity of the society towards these structures. In this paper representative school buildings are analysed in both their pristine condition and after applying retrofitting schemes typical of those implemented in the aforementioned large-scale strengthening programme. Non-linear analysis is conducted on calibrated analytical models of the selected buildings and fragility curves are derived for typical reinforced concrete and unreinforced masonry structures. These curves are then used to carry out a feasibility study, including both benefit-cost and life-cycle analysis, and evaluate the effectiveness of the strengthening programme

Statistical evaluation of a new resistance model for cold-formed stainless steel cross-sections subjected to web crippling

This paper presents a statistical evaluation according to Annex D of EN 1990 (2002) of a new resistance function for web crippling design of cold-formed stainless steel cross-sections. This resistance function was derived by Bock et al. (2013) through the use of carefully validated numerical models with the aim to propose a design expression for stainless steel sections, which are currently designed following the provisions for cold-formed carbon steel sections given in EN 1993-1-3 (2006). Although it was shown that the proposed design equation is appropriate for application to various stainless steels, the statistical uncertainties in material properties that the different types of stainless steels exhibit require an assessment of various partial safety factors. The statistical assessment showed that the proposed resistance function by Bock et al. (2013) requires adjustment to satisfy the safety level set out in EN 1993-1-4 (2006); A recalibration is performed herein. The web crippling design provisions given in EN 1993-1-3 (2006) and SEI/ASCE 8-02 (2002) American standard for application to stainless steel are also statistically evaluated herein. Comparison with test and numerical data showed that the predictions of the recalibrated resistance function are better suited and consistent than existing design provisionsResearch Fund for Coal and Stee

Experimental seismic performance evaluation of modular lightweight steel buildings within the ELISSA project

Lightweight steel buildings made up by cold‐formed steel (CFS) members as main structural components are growing in popularity in the most industrialized countries. Cold‐formed steel buildings start to be adopted also in seismic regions thanks to their efficient fabrication, reduced site work, short time of construction, and good structural performance. However, the dynamic properties and full seismic performance of CFS buildings completed with finishing is an open question. This paper attempts to provide a contribution to this research question, by a full experimental campaign aiming at investigating the dynamic properties of a modular building developed within the “Energy efficient LIghtweight Sustainable SAfe steel construction” (ELISSA) research project. The work shows the results of an international collaboration between universities and industrial partners aimed at developing a CFS prefabricated dry construction system with improved antiseismic properties and energy performance. This work will discuss the design of the modular building named ELISSA house, the experimental investigation going from small‐scale tests of components, to static tests of shear walls, up to shake table tests of a 2‐storey mock‐up building. It will analyze the dynamic properties of the structural system compared to the building completed with all the finishing, focusing on fundamental period of vibration, damping ratio, building drift, and observed damage