Damage avoidance design steel beam-column moment connection using high-force-to-volume dissipators

Existing welded steel moment frames are designed to tolerate substantial yielding and plastic rotation under earthquake loads. This sacrificial design approach can lead to permanent, and often irreparable damage when interstory drifts exceed 2%. The experimental seismic performance of a 50% full-scale damage avoidance designed structural steel beam-column connection is presented. The beam-column joint region consists of a top flange-hung beam connected to the column by an angle bracket. High-force-to-volume (HF2V) devices are attached from the column to the beam to provide joint rigidity and energy dissipation as the joint opens and closes. The HF2V devices are connected either below the beam flange or concealed above the beam's lower flange. Reversed cyclic lateral load tests are conducted with drift amplitudes up to 4%. No damage is observed in the principal beam and column structural elements. The need for stiff device connections to achieve optimal device performance is demonstrated, and potential design solutions presented. Stable hysteresis and repeatable energy dissipation for a large number of cycles up to the 4% drift level is observed. It is concluded that superior and repeatable energy dissipation without damage can be achieved for every dynamic motion cycle, in contrast to conventional sacrificially designed welded moment frame connections

Are stronger, stiffer buildings indeed costlier? Case study of RC frame buildings

Reinforced concrete (RC) buildings designed as per current seismic codes/standards are expected to satisfy “life safety” in a design level earthquake, but modern buildings have suffered significant damage in recent earthquakes, which were either irreparable or required lengthy and costly repair. Seismic codes/standards allow buildings to be designed for different lateral strength as long as the associated ductility and drift demands are catered for. The design seismic force is primarily controlled by a force reduction factor; a higher reduction factor leads to weaker/softer building but requires stringent confinement detailing to achieve a higher ductility. Buildings designed for higher reduction factor are more likely to suffer structural damage in moderate earthquakes. It is intuitively believed that stronger buildings, which respond elastically in a design level earthquake, are prohibitively costlier. However, cost of building skeleton, which governs its strength, is only a minor contributor to the total project cost. This paper conducts cost analysis on RC frame buildings designed with different reduction factors to investigate their effect on the construction cost. Based on the cost breakdown, it is found that the structural material cost of low-medium rise RC frame buildings is normally 25-30% of the total project cost, which varies within ±10% for the full range of force reduction factors allowed by modern building standards. The additional initial cost required to design RC frame buildings (with ductile detailing) that respond elastically in design-level shakings is insignificant when compared against the cumulative reduction in damage repair costs and financial losses due to business interruption over their lifetime. Hence, to achieve low-damage design through traditional measures, engineers should be encouraged to design stronger and stiffer buildings so that they remain in “operational” state after a DBE

Validation of a numerical model for prediction of out-of-p instability in ductile structural walls under concentric in-plane cyclic loading

Instability failure (also referred to as out-of-plane instability) has been observed in several experimental studies conducted on seismic performance of rectangular structural walls under in-plane loading. Observation of this failure pattern in some well-confined modern walls during the 2010 Chile and the 2011 Christchurch earthquakes has raised concerns about the reliability of current design code provisions. In this study, a numerical model composed of nonlinear shell-type finite elements was proposed and validated for seismic performance prediction and simulation of out-of-plane instability failure in rectangular walls. The plane sections are not enforced to remain plane in the planar direction in this type of model, and the in-plane axial-flexure-shear interaction can be simulated without requiring any empirical adjustment. The element used in the model (curved shell element) had integration points along the thickness unlike flat shell elements in which integration is performed in one plane only. This element was consequently able to capture the variation of strain along the thickness and simulate the deformation in the out-of-plane direction. Experimental results of cantilever wall specimens which failed in out-of-plane mode were used for verification of the adopted modeling and analysis approach. The numerical model was found to be able to predict the trend of initiation, increase, and recovery of out-of-plane deformation as well as the formation of out-of-plane instability that was observed during the tests. Development of this failure mechanism in the numerical model has been scrutinized using detailed response of the reinforcement and concrete elements positioned along the thickness of one of the specimens and at different stages of the failure mode. Also, the dependency of initiation and amount of the out-of-plane deformation on the maximum tensile strain developed in the longitudinal reinforcement during a specific loading cycle, and consequently all the parameters that influence its value (e.g., axial load, wall length, and cyclic loading protocol), as well as wall thickness has been confirmed by a set of parametric studies conducted on the models developed for one of the wall specimens

Blind prediction of in-plane and out-of-plane responses for a thin singly reinforced concrete flanged wall specimen

This paper describes the blind prediction carried out to simulate the response of a thin reinforced concrete wall tested under uni-directional (in-plane) quasi-static reverse cyclic loading. The specimen was a singly reinforced T-shaped wall panel with a shear-span ratio of 3.7. The response of the test specimen was simulated prior to the release of test results using a finite element model which had already been verified for its capabilities in capturing different failure patterns of rectangular walls, particularly out-of-plane instability. The numerical model predicted a flexural dominated response for the specimen accompanied by considerable out-of-plane deformations. The blind prediction report, submitted in advance to the principal investigator of the experimental campaign, included lateral load-top displacement response of the specimen, maximum out-of-plane deformation corresponding to each drift level, evolution of out-of-plane displacements throughout in-plane loading, response of the longitudinal reinforcement at the section exhibiting the maximum out-of-plane deformation, and von Mises as well as reinforcement stress distribution at some key points of the wall response. Furthermore, a parametric study was carried out addressing the effects of shear-span ratio, reinforcement eccentricity and axial load ratio on the wall response. Results of the numerical simulation that had been included in the blind prediction report have been compared with the experimental measurements indicating that the evolution of the out-of-plane deformation was well captured by the model

Incremental fire analysis (IFA) for probabilistic fire risk assessment

In this paper, the concept of a probabilistic fire risk analysis method is presented in line with the seismic risk assessment approach. The probabilistic fire risk assessment approach runs through a series of probabilistic interrelationships between different variables representing fire severity (called the Intensity Measure IM), structural response (called the Engineering Damage Parameter EDP) and damage/loss (called the Damage Measure DM). The paper explains the development of a probabilistic interrelationship between IM and EDP through incremental fire analysis (IFA). For this purpose, a series of SAFIR analysis of a simple two span reinforced concrete beam subjected to different fire profiles have been conducted and are used to illustrate the required approach. Although a single EDP (maximum deflection) is considered in this investigation, two different IMs (maximum temperature and total radiant energy) are used. It is found that the radiant energy is more efficient than the maximum temperature in representing fire severity

Performance of firestopping systems: State-of-the-art and research needs in earthquake-prone regions

For maintaining an effective fire resistance for in-wall service penetrations (e.g. pipe and cable systems) under fire conditions, firestop seals are used to be applied to thoroughly fill the installation gap between perimeter of penetrations and inner surface of wall/floor openings. Consistent with other building elements installed for fire resistive purposes, through-penetration firestopping systems are not specifically designed with any earthquake-damage considerations. However, evidence from global earthquakes have revealed a varying extent of damage could occur around where the firestopping material sits, caused by differential movements between penetrations and supporting assemblies. If the associated damage is not identified and remedied, the building is left vulnerable to fire attack in the rest of its service life. This issue is particularly important for seismically active countries, such as New Zealand, Japan and the USA. As part of new research to establish interdependencies between the performance of firestopping systems under earthquakes and in fires, this paper conducts a review of the literature on identifying the deficiencies in current knowledge of through-penetration firestopping systems as well as suggesting specific research needs to assess the quake-affected residual fire resistance of these systems

Fully Floating Suspended Ceiling System: Experimental Evaluation of Structural Feasibility and Challenges

Full scale shake table testing is conducted on a novel type of suspended ceiling called fully floating ceiling system, which is freely hung from the floor above lacking any lateral bracing and connections with the perimeters. A gap is provided on all sides of the ceiling. Throughout different tests, a satisfactory agreement between the fully floating ceiling response and simple pendulum theory was demonstrated. When subjected to input motions with significant energy in the floating ceiling's resonant frequency, the horizontal displacement of the ceiling exceeded the gap width resulting in pounding induced large acceleration pulses. To mitigate such effects, the perimeter gap fillers of a compressible material are proposed. The addition of perimeter isolation was found effective in inducing extra damping and protecting the ceiling from pounding impact; resulting in much reduced ceiling displacements and accelerations. Throughout the tests a single panel dislodgement was observed as the only instance of damage

Seismic behavior of unitized glazed curtain walls in a low-damage structural steel building

In reconnaissance reports on some recent large earthquakes, it is reported that economic losses due to non-structural elements considerably exceed those related to earthquake-induced structural damage. Unitized Glazed Curtain Walls (UGCWs) which have been increasingly used for facades in modern buildings play an important role. Research on seismic performance evaluation of UGCWs is limited. There is possibility of cosmetic and weather tightness failures but not life-threatening for seismic risk of UGCWs. Deformation behavior of UGCWs depends on factors such as the width of clearance between glass panels and aluminium frames, gasket friction and degradation of structural glazing tapes. These factors should be calibrated through dynamic tests. Specifically, corners of UGCWs are more vulnerable under seismic excitations, while limited studies have been conducted to evaluate seismic behavior of corners of UGCWs, which is in lack of information. This study aims to investigate in-plane and out-of-plane deformation behaviour of UGCWs employing low-damage connection details. The overall deformation behavior of UGCW system is characterized, and specifically the seismic behavior at the low-damage corner condition is discussed. Bidirectional shaking table tests will be conducted at the International joint research Laboratory of Earthquake Engineering (ILEE) facilities, Shanghai, this year. The specimen is a 9 m tall three-storey steel building equipped with UGCWs on the top two floors at the opposite two corners. The UGCWs configuration includes three side-by-side primary panels in the longitudinal direction and two adjacent panels in the transverse direction at the corner of each floor. Displacement transducers such as Linear Variable Differential Transformers LVDTs as well as high-resolution cameras will be employed to record or capture deformation behavior of UGCWs within stack joints and hooks. It is theoretically shown that the connection details of UGCWs can provide low-damage behavior under the maximum allowable 3.4% inter-storey drift of the steel building. Additionally, simplified calculation model of UGCWs without and with shear keys are introduced in this paper, and a method is suggested for prediction of ultimate drift capacity of full-size UGCWs based on some assumptions. This calculation method of the deformation of UGCWs can help improve decision making regarding resilient seismic retrofit options of UGCWs