Safety Margin Of Structure Research Articles

Towards Displacement-Based Seismic Assessment of Concrete Dams Using Non-linear Static and Dynamic Procedures

The current paper demonstrates the application of a displacement-based procedure in the seismic safety assessment of large concrete dams. Two dam structures, double arch and gravity, are used as case studies and analyzed through conventional force-based approaches for various seismic levels. Non-linear static analyses are used for the assessment of structural capacity and non-linear dynamic analyses—to obtain the maximum response for each seismic level. The safety of the dams is assessed through displacement-based damage indicators in the form of damage indexes (DIs) and safety factors (SFs). The DI accounts for the accumulated inelastic deformations in the structure and is related to pre-defined damage levels (DLs), while SF evaluates the structural safety margins in terms of seismic load intensity. In addition, the paper provides an assessment of the applicability of non-linear static procedures (NSP) for seismic assessment of large concrete dams. The results obtained from NSP for seismic level maximum credible earthquake (MCE) are compared with a set of ten dynamic non-linear analyses for the same seismic intensity. NSP results tend to give a more conservative prediction, but are still consistent with the results calculated via non-linear dynamic analyses.

A statistical approach to determine 2D optimal equivalent linear parameters with application to earthquake engineering

The equivalent linearization method approximates the maximum displacement response of nonlinear structures through the corresponding equivalent linear system. By using the particle swarm optimization technique, a new statistical approach is developed to determine the key parameters of such an equivalent linear system over a 2D space of period and damping ratio. The new optimization criterion realizes the consideration of the structural safety margin in the equivalent linearization method when applied to the performance-based seismic design/evaluation of engineering structures. As an application, equations for equivalent system parameters of both bilinear hysteretic and stiffness degrading single-degree-offreedom systems are deduced with the assumption of a constant ductility ratio. Error analyses are also performed to validate the proposed approach.

Analysis on Structural Property and Seismic Performance of the P’ai-Lou Gateways in Xi'an Great Mosque

Study on the mechanics performance of ancient Chinese timber structure is of great significance. The paper based on P’ai-lou of Xi'an Great Mosque as the research object, and used spring elements to simulate semi-rigid of mortise-tenon joints, just answered to the finite element numerical analysis. The stiffness values of mortise-tenon joints were got by nonlinear contact analysis; Under the vertical load, the inner force of structure is far less than its design value, and the safety margin of structure is above 60%; The differential motion equation of structure was given, the first six natural frequencies and the vibration performance of the P’ai-lou were got by the model analysis; Based on Spectrum analysis and time-history analysis , the results show that the ancient Chinese timber structure is soft, and has a better energy consumption performance as well as a "meet strong a strong" intelligent shock absorption characteristics. Analysis and calculation results provide the theory basis for maintenance and protection of ancient Chinese timber structure.

The Structural Safety Assessment of the Overloaded Members of Highway Bridges

This article presents a residual reliability index for assessing the load-carrying structures of existing highway bridges overloaded by unforeseen vehicular live loads. The authors discuss the primary and inspection structural safety margins of particular members of bridges. The primary and revised survival probabilities of intact particular members are analyzed. Target reliability indices of the structural members of road bridges are presented. The authors recommend a transformed Bayes theorem in the analysis of revised failure probabilities of overloaded members of highway bridges. The Bayesian approach is combined with the method of transformed conditional probabilities for series systems. Numerical examples are presented that demonstrate the simple calculation of revised reliability indices of particular members.

A reply

We thank Dr Mackenzie for his letter raising a number of issues which we are grateful for the opportunity to respond to. The system referred to by the author is the Callisto ‘No-Touch’. This system is specifically designed so that the tip of the blade is prevented from touching the barrel of the laryngoscope handle. This is to prevent possible episodes of cross-contamination whereby a clean blade can be contaminated, prior to insertion, from the barrel of a reusable handle against which the tip of a used blade may have rested [1]. The correct position prior to the blade being fully locked onto the handle is well shown in Fig. 2a of Dr Mackenzie’s letter. Figure 2b however, shows a ‘non-recommended’ position for the Callisto ‘No-Touch’. In our instructions for use leaflet, which accompanies all of our single-use laryngoscope blades, we state that, prior to use on the patient, the laryngoscope blade is correctly fitted to the handle and a check is made to ensure that the light is functioning. We also advise that spare blades and handles are available in case of failure and we note that this was carried out in the case described. We try to make it as clear as possible within our instructions for use, in both the line drawings and accompanying text that prior to actual use. Pressure is applied to the heel of the blade until it ‘clicks’ into position onto the bar. Immediately thereafter, the tip of the blade is lifted such that it ‘locks’ into a normal operating position on the laryngoscope handle, rendering the blade and handle ready for use. In Fig. 2c, the laryngoscope is also in a ‘non-recommended’ position and we suggest that it was this error that led to the incident described. However, in light of the issues raised by Dr Mackenzie, we are undertaking a review of our Callisto instructions for use to examine whether there is any way in which they can be made clearer. With respect to the physical strength of the Callisto system blade/handle interface, we wish to point out that due to failures of early single-use blades from some manufacturers, this blade is designed such that the metal of the blade continues fully into the hook structure. This prevents any possibility of a blade detaching from a plastic hook moulding. Furthermore the system has been formally tested by The Rubber and Plastics Research Association in February 2008. Briefly, a fixture attached to a tensile test machine was produced which would grip the handle of the laryngoscope and measure any applied force. A second fixture was then designed to apply an upward vertical load to the centre of the blade, simulating its normal orientation during use. On a series of blades, a load of 30 N was applied and maintained for 5 min. During this time the interface between blade and handle was inspected for distortion or detachment, none being seen. At the end of this period, the load was increased until failure occurred. This occurred over a range of 303–327 N, representing an upward force equivalent of 31–33 kg which we believe should be a sufficient structural safety margin for most patients.

Methods to Assess the Seismic Collapse Capacity of Building Structures: State of the Art

The collapses of building structures during recent earthquakes have raised many questions regarding the adequacy of current seismic provisions to prevent a partial or total collapse. They have also brought up questions as to how to determine the collapse safety margin of structures, what is the inherent collapse safety margin in code-designed structures, and how to strengthen structures to effectively augment such margin. The purpose of this paper is to present a comprehensive review of the analytical methods that are currently available to assess the capacity of building structures to resist an earthquake collapse, point out the limitations of these methods, describe past experimental work in which specimens are tested to collapse, and identify what is required for an accurate evaluation of the seismic collapse capacity of a structure and the safety margin against such a collapse. It is contended that further research is needed before the collapse capacities of structures and their safety margin against collapse may be evaluated with confidence.

An alternative view of R curve testing

The R curve plotted in terms of G or K has been an accepted part of fracture mechanics methodology since the early 1960s. It is shown here that the central assumptions of R curve theory are incorrect. Toughness does not increase with crack extension, and G is not a correct measure of the energy absorbed during crack growth, even under LEFM conditions. Energy dissipation rate theory is used to provide an alternative explanation of stable and unstable crack growth. It is shown that stable growth is possible even though the dissipation rate, D, does not increase with crack growth. The instability predictions from energy dissipation rate theory and R curve theory are very similar. Energy dissipation rate theory is to be preferred for the following reasons: it is physically realistic; it clarifies structural safety margins; and a structurally relevant value of D can be deduced from a small specimen.

Failure assessment diagram (FAD) for I–II mixed-mode crack structures under biaxial loading

In this paper the results of fracture experiments under biaxial loading and theoretical analysis are used to study the fracture behaviour of a cruciform specimen, made of Chinese industrial standard 16 MnR steel, with a central penetrated I–II mixed-mode crack under biaxial loading. The failure assessment diagram (FAD) for evaluating the safety margin of structures containing I–II mixed-mode cracks under biaxial loading is explored. The fracture parameter J-integral of I–II mixed-mode cracks is calculated by the elastoplastic finite-element method and the instantaneous loads for the initiation of I–II mixed-mode cracks are detected by DC electric potential measurement. Modified methods for the failure assessment diagram of CEGB R6 and EPRI engineering approach for biaxial loading and inclined crack angle have been proposed in the present work. The experimental results reveal that the J-integrals based on the measured instantaneous load of initiation of growth of I–II mixed-mode crack, J i, are almost equal under different ratio of biaxial load. The instantaneous load of initiation predicted by mixed mode J-integral fracture criterion shows reasonable agreement with the experimental results. It is however found that the modified failure assessment diagram proposed EPRI method for biaxial loading is more in agreement with the experimental results.

引張りと熱衝撃の複合荷重による不安定延性破壊

In nuclear power plants, it is an important problem to estimate the structural safety margin of a reactor vessel under a pressurized thermal shock, which may occur in an accident such as small break loss of coolant. Therefore, from the view point of fracture mechanics, this paper deals both experimentally and numerically with the behavior of cracks existing in the A508 class 3 pressure vessel steel plate under the combined load of thermal shock and tension, which simulates the above mentioned phenomenon in an actual vessel.In order to estimate simply the structural integrity of the nozzle corner as a special case, the initial cracks located at the edge of the hole in the specimen were considered. To take into account the influence of the compliance of the pressure vessel itself and pressurized water, the pre-load was mechanically applied with a spring which was set in series with the specimen.By holding the pre-load at the initial elevated temperature, the specimen was quenched with cold water rapidly. The experimental results showed that the initial cracks extended unstably due to the thermal shock loading after a stable extension of a few milimeters.