Steel Moment-resisting Buildings Research Articles

Post Fire Dynamic Analysis of Steel Buildings Subject to Wind Loadings in South of Iraq

Post-fire analysis of a six-story square steel building under the action of dynamic wind forces in the south of Iraq is presented. The nonlinear time-history analysis using direct integration method is accomplished by SAP 2000 V16 program while geometric nonlinear parameters are included. The post fire deformations values and their configurations along building are based on available literature that related to post fire deformations of steel buildings at 550oCwith reducing of yield stress and modulus of elasticity) by 10% due to fire. In the present study, two post fire scenarios are considered, with wind speed of 42 m/s, according to Iraqi meteorological and IQS.301 standards. The aim of this study is to investigate the post-fire dynamic analysis of a multi-story steel moment-resisting building subjected to dynamic along-wind loads. Variations of base shear, base moment, drift ratio, and displacement are considered for discussion and comparison. Its concluded that the presence of deformation in building after fire and the reduction in yield stress and modulus of elasticity increased the base shear, base moment and drift ratio as average by about 12.5, 20& 64%respectively under the effect wind load. The fire deformations may be critical and the structural decision for the building safety should be done via structural analysis of the building taking into consideration different parameters related to post fire effects.

Effect of stand-off distance on blast fragility of steel moment-resisting buildings

One of the most serious terrorist threats to the strategic structures is vehicle bombs. A blast-resistant design approach is to increase distance between a potential bomb and structure. The best way to do this is to provide the perimeter protection (fence, guards, and barriers) around the building. The aim of this study is to evaluate the effect of stand-off distance on the blast fragility of steel moment-resisting buildings. To this end, the perimeter protections have been considered at distances of 2, 6 and 10 m from building and the fragility curve has been developed for three steel moment-resisting buildings with different heights of 4, 8 and 12 stories. For comparison objectives, the probabilistic-based assessment of buildings without the perimeter protection has been also performed. The results show that providing the perimeter protection can be effective in improving the performance of low-rise buildings whereas for high-rise buildings, the perimeter protection not only cannot be effective, but also may make the high-rise buildings more vulnerable against explosion hazard.

Blast Fragility of Steel Moment-Resisting Buildings for Different Seismic Design Levels

Blast Fragility of Steel Moment-Resisting Buildings for Different Seismic Design Levels

A multi-hazard-based design approach for LRB isolation systemagainst explosion and earthquake

Base isolation system is originally known as one of the most efficient earthquake-resistance control systems. Isolated strategic structures may also experience terrorist attacks during their lifetime. In this paper, a design method is proposed for lead rubber bearing (LRB) isolation system under multi-hazard of explosion and seismic. This method is based on transforming the design problem into an optimization problem. The seismic response of structure has been defined as objective function while the constraints have been applied on its blast response. To validate the effectiveness of the proposed design method, LRB is designed for controlling a four-story steel moment-resisting building. For comparison objectives, this control system has been also designed under seismic hazard without paying attention to its blast performance. The results show that the earthquake-based optimally designed LRB has effective performance under seismic hazard whereas its blast performance is not as good as its seismic performance. Therefore, this control system cannot be considered as a well-designed control system for multi-hazard. The multi-hazard-based optimally designed LRB shows excellent performance under both blast and seismic loadings, so the proposed design method can be introduced as an effective design approach for LRB under multi-hazard of explosion and seismic.

Performance-Based Design through Implementation of FEMA P-58 Methodology in Developing Countries

This paper investigates a simple approach proposed towards performance-based earthquake engineering (PBEE) which has potential applications to the performance-based design (PBD) and performance-based assessment (PBA) fields. The simple method of PBEE encompasses three areas of seismic risk which include seismic hazard, structural analysis, and loss models. The aim of the PBEE process, entitled as FEMA P-58, is to present essential data needed to make a rational decision regarding predicted performance, where various sources of uncertainties are involved. In developing countries, the lack of suitable real ground motions corresponding to site characteristics and seismicity particularly for larger intensities and the scarcity of demands, which makes it hard to identify the seismic capacity of a structure, is the main our motivation of using the FEMA method. In this paper, the method of FEMA P-58 is investigated, in terms of available tools and required data, in such a way that it will be applicable for developing countries which are located in high seismic hazard zones. To achieve this goal, three steel moment-resisting buildings with low and high ductility, and three steel braced-frame buildings are selected as case studies. The mean annual loss is estimated by the available software, Performance Assessment Calculation Tool (PACT). The achieved results, i.e. the loss curves, will provide a simple means by which the engineers can quantify and communicate seismic performance to other stakeholders. In the case study buildings, the braced one has less annual losses in comparison with other investigated cases, and the structure with high ductility can be considered as the next ones. Execution cost of each building should be considered by contractors. Also, seismic fragility curves of structures for various limit states, as well, the corresponding loss models are identified as the most essential data towards application of the investigated PBEE process.

Three-dimensional response of steel moment-resisting buildings under seismic sequences

This study examines the response of three-dimensional (3-D) steel moment-resisting buildings having 3-, 9-, and 20-story height under bi-directional attack of real seismic sequences with different earthquake ground motion features. For this purpose, 3-D analytical models were developed for each building, which include or not include interior gravity frames. The fracture behavior in beams included in perimeter moment-resisting frames and the bending capacity of interior beams in gravity frames was taken into account in the modeling process. The seismic response of the 3-D building models, in terms of lateral interstory drift demands, was also compared with the response of their corresponding two-dimensional (2-D) bare frame model to highlight differences in modelling criteria. It is shown that differences among 2-D and 3-D building models depend on the type of seismic sequence. When differences arise, it is clearly illustrated that the 3-D building models experience different response than the 2-D model. Between the 3-D models, it is shown that the seismic response is different, with differences depending on the angle of incidence of the orthogonal components of the seismic sequences and the number of stories. This study demonstrates that the modelling approach in 3-D models has significant influence in seismic response of steel framed-buildings under seismic sequences.

Reliability-based optimal design of nonlinear viscous dampers for the seismic protection of structural systems

Viscous dampers are widely employed for enhancing the seismic performance of structural systems, and their design is often carried out using simplified approaches to account for the uncertainty in the seismic input. This paper introduces a novel and rigorous approach that allows to explicitly consider the variability of the intensity and characteristics of the seismic input in designing the optimal viscous constant and velocity exponent of the dampers based on performance-based criteria. The optimal solution permits controlling the probability of structural failure, while minimizing the damper cost, related to the sum of the damper forces. The solution to the optimization problem is efficiently sought via the constrained optimization by linear approximation (COBYLA) method, while Subset simulation together with auxiliary response method are employed for the performance assessment at each iteration of the optimization process. A 3-storey steel moment-resisting building frame is considered to illustrate the application of the proposed design methodology and to evaluate and compare the performances that can be achieved with different damper nonlinearity levels. Comparisons are also made with the results obtained by applying simplifying approaches, often employed in design practice, as those aiming to minimize the sum of the viscous damping constant and/or considering a single hazard level for the performance assessment.

Failure Sensitivity Analysis of Tall Moment-Resisting Structures Under Natural Fires

A series of sensitivity analyses of structural fires are performed, focusing mainly on the possibility of structural failure during the cooling phase. To investigate this, two tall ten-story steel moment-resisting buildings, with two different span lengths, 6000 and 7500 mm, but identical story height of 3500 mm, are designed for two different load combinations: gravity loads, and seismic loads. Assuming different opening ratios in a range between 0.02 and 0.2, different fire curves are created, using the EN 1991-1-2 procedure. The cases are then subjected to the fire curves to monitor their structural responses when interactions between the opening ratios, span lengths, and load combinations are considered. The results show that all of the cases meet their required FRR under all of the fire curves. The results, however, also show that, in the structures designed for gravity loads, while the span length increases, the structural vulnerability increases during the cooling phase. As well, opening ratios of more than 0.1 are seen to cause more vulnerability to the structures during the cooling phase. In the seismically designed structures, even though they are shown to be sensitive to opening ratios more than 0.1, no collapse is observed throughout the analyses.

Applicability of the N2, extended N2 and modal pushover analysis methods for the seismic evaluation of base-isolated building frames with lead rubber bearings (LRBs)

In the recent investigations, the pushover analysis has been mainly used for the seismic evaluation of fixed-base structures, whereas a limited number of research investigations have focused on the applicability of pushover analysis to base-isolated buildings. Therefore, this study attempts to extend the modal pushover analysis (MPA) and the extended N2 (EN2) method to medium-rise base-isolated building frames to account for the effect of higher modes in predicting the seismic demands of these structures. Since the displacement at the isolation level and subsequently the effective stiffness of the isolation system are not predetermined at first, an iterative process was used to fulfill the MPA method for base-isolated frames. The original N2 method, which was recently extended to base-isolated buildings, was also implemented with three different lateral load distributions, i.e. the inverted triangular, the first mode and the PSC load patterns. For this purpose, two steel moment-resisting building frames including low-rise (3-storey) and medium-rise (12-storey) ones were considered. The structures were isolated with lead rubber bearing (LRB) isolation systems. Three types of isolators with different stiffnesses including the hard (H), normal (N) and soft (S) isolators were selected. It was observed that N2 method with the PSC load distribution, in most cases, gives better estimates of the seismic demands for low-rise base-isolated frames. Also, the MPA and the EN2 methods can result in accurate estimates of the seismic demands for medium-rise base-isolated frames with the hard type isolators, but their accuracy deteriorates with the increase in the damping and decrease in the stiffness of isolators.

A Study on the Estimation of Economic Loss Costs due to Seismic Damage of Low Rise Steel Structures

Economic losses due to seismic structural damage are growing up due to urbanization, civilization, etc. Recently, quantitative evaluation methods for direct and indirect seismic structural damage have been increasingly studied. A Hazus system developed by the FEMA provides a methodology that can predict direct and indirect economic seismic losses of various building types using probabilistic approaches. Based on the Hazus system, an Earthquake Disaster Response System (EDRS) has been developed in Korea considering the unique characteristics of domestic buildings and seismic environment of Korea Peninsula. This paper presents the methodology that can calculate direct economic losses resulting from structural damage due to earthquakes potentially occurring in Korea Peninsula. This study explains the process computing direct economic losses causing from structural seismic damage and then presents how to obtain data required for the process and how to manage the obtained data. Finally, low-rise steel moment-resisting buildings is selected as an example in order to specifically present a stepwise process for the calculation of direct economic losses due to structural seismic damage.

Risk evaluation of steel frames with welded connections under earthquake

Numerous failures in welded connections in steel moment-resisting building frames (SMRF) were observed when buildings were inspected after the 1994 Northridge Earthquake. These observations raised concerns about the effectiveness of such frames for resisting strong earthquake ground motions. The behavior of SMRFs during an earthquake must be assessed using nonlinear dynamic analysis, and such assessments must permit the deterioration in connection strength to capture the behavior of the frame. The uncertainties that underlie both structural and dynamic loading also need to be included in the analysis process. This paper describes the analysis of one of approximately 200 SMRFs that suffered damage to its welded beam-to-column connections from the Northridge Earthquake is evaluated. Nonlinear static and dynamic analysis of this SMRF in the time domain is performed using ground motions representing the Northridge Earthquake. Subsequently, a detailed uncertainty analysis is conducted for the building using an ensemble of earthquake ground motions. Probability distributions for deformation-related limit states, described in terms of maximum roof displacement or interstory drift, are constructed. Building fragilities that are useful for condition assessment of damaged building structures and for performance-based design are developed from these distributions.