Non-ductile Failure Research Articles

Effectiveness of external prestressing in enhancing the non-ductile hanger failure mechanism in reinforced concrete inverted T-beams

Recently, inverted T-beams have been used in reinforced concrete (RC) bridges to support transverse precast stringers. Inverted T-beams, contrary to practice with conventional beams, are loaded on the flanges upper surface. This loading configuration causes hanger failure due to the generation of vertical tensile stresses near the bottom of the web. The key purpose of this study is to investigate the efficiency of vertical external prestressing stainless-steel bars in mitigating non-ductile hanger failure in reinforced concrete inverted T-beams. An experimental study on six inverted-T beams, including two un-strengthened specimens, was carried out. The study showed that the value of the prestressing level had a considerable impact on the performance of hanger mechanism in relation to crack pattern, ultimate loads, cracking behavior, load–deflection, strains, and ductility. The experimental results indicated that the suggested method for strengthening inverted T-beams had efficacy in reducing the seriousness of the non-ductile hanger failure and resulted in a strength increase of up to 53% when compared to that of the un-strengthened specimen. Additionally, two analytical models for estimating the hanger capacity and the average crack width of the strengthened RC inverted T-beams were proposed. The models that were proposed exhibited a high degree of agreement with the experimental results.

Predictions and Evolution Characteristics of Failure Modes of Degenerate RC Piers

During the service process, piers are often in harsh chloride ion erosion environments. The failure mode evolution of reinforced concrete (RC) piers may occur under the action of continuous corrosion. Accurately identifying the failure mode types and evolution characteristics of corroded RC bridge piers is a prerequisite for the lifetime seismic performance evaluations of bridges. First, based on Fisher’s theory and 174 RC pier columns as the analysis samples, a two-stage discrimination formula for the pier failure modes was established and compared with the existing theoretical discrimination methods. Then, based on Fisher’s discriminant grouping, and combined with Bayes’ formula and chloride erosion theory, a failure mode discrimination method for corrosion-damaged bridge piers that considers probability was developed. Finally, taking a medium-span concrete bridge as an example, the failure modes of the corroded pier in different service periods were predicted, and the influences of the various parameters on the failure mode evolution process of the corroded pier were studied. The results show that the accuracy of the proposed discriminant model was significantly improved compared with those of previous theoretical studies. The development of the failure mode features depends on how the distinct RC pier material qualities degrade under the influence of chloride ions. The degradation of the stirrups and concrete accelerates the nonductile failure of RC bridge piers, while the degradation of the longitudinal reinforcements delays it.

Unbiased simplified seismic fragility estimation of non-ductile infilled RC structures

Infilled reinforced concrete (RC) buildings represent a prevalent taxonomy class in the Mediterranean region. Many were constructed before proper seismic provisions and detailing were enforced meaning they typically possess non-ductile failure mechanisms. Therefore, their simple but adequate seismic fragility estimation remains a challenge for the research community and practitioners. Moreover, their performance quantification can typically require characterisation via detailed numerical models that capture salient behaviour features, which involves extensive non-linear dynamic analyses with large computational burden. In this regard, an unbiased seismic fragility estimation methodology for the simplified assessment of infilled RC frame structures is described for both collapsing and non-collapsing scenarios. Its development is using extensive cloud analyses carried out with a large set of oscillators representative of the infilled RC frame's structural behaviour to permit the well-established pushover-based methods to be adopted in practice. The result is a novel set of empirical relationships relating the seismic behaviour of these typologies to their pushover curve parameters to allow practitioners to perform an accurate risk assessment and verification in an expedited manner. The choice of average spectral acceleration as the intensity measure used to characterise the fragility parameters for these relationships is shown to present notable advantages in reducing bias compared with other existing approaches. The results are validated via comparison with a detailed hazard-consistent assessment of case studies from a database of three-dimensional archetype building models. These were also developed here to capture the temporal evolution of building codes and architectural features of the building class in Italy.

Influence of Bearing Types and Design Code Advances on Seismic Vulnerability of Simply Supported Highway Bridges

ABSTRACT Multi-span simply supported (MSSS) highway bridges are one of the most common bridge types across the world and have several vulnerable components, including bearings, piers, and abutments. Although several past studies have evaluated the seismic response and fragility of common multi-span simply supported bridges, there is negligible research that explores the influence of bearing types, and non-ductile failure modes of bridge piers on the performance of older non-seismically designed simply supported bridges. Additionally, a better understanding of seismic risk to highway transportation systems necessitates the exploration of design code advances and subsequent impact on the seismic vulnerability of MSSS highway bridges. Consequently, this study also focuses on developing and comparing fragility curves for the older design era non-seismically designed, and modern era seismically designed MSSS bridges. High-fidelity three-dimensional (3-D) analytical models, which account for the nonlinear behavior of different components, including non-ductile failure modes of bridge piers, are developed. Deterministic seismic response assessment reveals a higher seismic demand on bridge pier for MSSS bridges with steel bearings as compared to those with elastomeric bearings. The bridge component and system fragility curves are developed with a suite of ground motions representative of the seismic hazard of the case-study region. Results reveal the significant vulnerability of older non-seismically designed MSSS bridge with steel bearings due to the non-ductile failure of the bridge pier. The vulnerability of the bridge is significantly reduced with the adoption of modern seismic design and detailing practices.

Seismic retrofit of existing school buildings in Italy: Performance evaluation and loss estimation

The seismic vulnerability and inadequate behaviour of existing school buildings observed during past earthquakes in Italy have raised awareness of the need to upgrade their performance. This paper examines different retrofit strategies for three case study school buildings, representing the main typologies found within the Italian school building stock. The three building typologies investigated in this study are representative of reinforced concrete (RC), precast concrete (PC) and unreinforced masonry (URM) school buildings. A seismic performance assessment was carried out using detailed numerical models that consider the main structural deficiencies documented for older buildings in Italy, generally designed and built before the 1970s. The retrofit interventions were specifically aimed at mitigating these main structural deficiencies in order to meet current building code limit state requirements. These requirements are set to limit the damage to non-structural elements and prevent non-ductile failure mechanisms in the structural system, following a typical building code and practitioner-oriented process. The retrofit alternatives were then evaluated through increasing ground shaking intensities to quantify risk-based decision variables, such as the expected annual loss and mean annual frequency of collapse. The results highlight the level of adequacy of each retrofit option in reducing both the economic losses and collapse vulnerability. To predict the economic feasibility of these interventions, a cost-benefit analysis was also conducted using estimated implementation costs of each retrofit alternative. Lastly, the results were also compared with the outcome of the seismic risk classification guidelines, recently proposed in Italy.

A model-based data-interpretation framework for post-earthquake building assessment with scarce measurement data

Recent earthquake events throughout the world have once again exposed the vulnerability of buildings with respect to earthquakes. It is unlikely and unsustainable to design and - especially in regions with low-to-moderate seismic hazard – to retrofit all buildings to remain within elastic displacement ranges during earthquakes with high return periods. Therefore, post-earthquake assessment plays a fundamental role in the resilience of cities, given the potential to reduce time between an earthquake event and the clearance for (renewed) occupancy of a building. In this paper, a framework for model-based data interpretation of measurements of earthquake-damaged structures is presented. The framework allows engineers to combine ambient-vibration measurements and visual inspection to reduce parametric uncertainty of a high-fidelity model using the error-domain model-falsification methodology. For building types that have limited stiffness contributions from non-structural elements (i.e. shear-wall buildings) and for which non-ductile failure modes (such as out-of-plane failure) can be excluded, reduction in natural frequency and damage grades derived from visual inspection provide global measurement sources for structural identification. The application of the proposed methodology to a shear-resisting building tested on a shake table illustrates that vulnerability-curve predictions provide accurate damage estimates for subsequent earthquakes with probabilities between 50% and 100% for five measured scenarios. In complete absence of baseline information regarding the initial building state and the earthquake signal, parametric uncertainty is reduced by up to 76%. This study thus demonstrates usefulness for certain building types to enhance post-seismic vulnerability predictions.

Evaluation of Seismic Demand for Substandard Reinforced Concrete Structures

Background: Reinforced Concrete (RC) buildings with no seismic design exhibit degrading behaviour under severe seismic loading due to non-ductile brittle failure modes. The seismic performance of such substandard structures can be predicted using existing capacity demand diagram methods through the idealization of the non-linear capacity curve of the degrading system, and its comparison with a reduced earthquake demand spectrum. Objective: Modern non-linear static methods for derivation of capacity curves incorporate idealization assumptions that are too simplistic and do not apply for sub-standard buildings. The conventional idealisation procedures cannot maintain the true strength degradation behaviour of such structures in the post-peak part, and thus may lead to significant errors in seismic performance prediction especially in the cases of brittle failure modes dominating the response. Method: In order to increase the accuracy of the prediction, an alternative idealisation procedure using equivalent elastic perfectly plastic systems is proposed herein that can be used in conjunction with any capacity demand diagram method. Results: Moreover, the performance of this improved equivalent linearization procedure in predicting the response of an RC frame is assessed herein. Conclusion: This improved idealization procedure has been proven to reduce the error in the seismic performance prediction as compared to seismic shaking table test results [1] and will be further investigated probabilistically herein.

Creep Crack Growth Behavior of Alloys 617 and 800H in Air and Impure Helium Environments at High Temperatures

The environmental degradation of intermediate heat exchanger (IHX) materials in impure helium has been identified as an area with major ramifications on the design of very high-temperature reactors (VHTR). It has been reported that in some helium environments, non-ductile failure is a significant failure mode for Alloy 617 with long-term elevated-temperature service. Non-ductile failure of intermediate exchangers can result in catastrophic consequences; unfortunately, the knowledge of creep crack initiation and creep crack growth (CCG) in candidate alloys is limited. Current codes and code cases for the candidate alloys do not provide specific guidelines for effects of impure helium on the high-temperature behavior. The work reported here explores creep crack growth characterization of Alloy 617 and Alloy 800H at elevated temperatures in air and in impure helium environments, providing information on the reliability of these alloys in VHTR for long-term service. Alloy 617 was found to exhibit superior CCG resistance compared to Alloy 800H. For Alloy 617 tested at 973 K (700 °C), a notable increase in the resistance to crack growth was measured in air compared to that measured in the helium environment; CCG results for Alloy 800H suggest that air and helium environments produce similar behavior. Testing of grain boundary-engineered (GBE) Alloy 617 samples revealed that, although the technique produces superior mechanical properties in many respects, the GBE samples exhibited inferior resistance to creep crack growth compared to the other Alloy 617 samples tested under similar conditions. Grain size is noted as a confounding factor in creep crack growth resistance.

Experimental tests of unreinforced exterior beam–column joints with plain bars

Reinforced concrete buildings designed according to obsolete seismic codes or for gravity loads only are widespread in Italian and Mediterranean building stock. Past earthquakes showed that, for these buildings, shear failure in beam–column joints can lead to structural collapse mainly due to the inadequate joint confinement. The construction details used in these structural elements are commonly recognized as the likely cause of non-ductile structural failure modes under seismic actions, due to the lack of capacity design principles.A significant amount of research on seismic performance of unreinforced beam–column joints has been carried out in last years, but a very few portion of them handled with specimens unreinforced in the joint region and with plain hook-ended longitudinal bars. A higher number of tests is needed to deeper understand joint seismic response in order to validate the (few) existing models or calibrate new ones.This study aims at improving the understanding of seismic performance of exterior non-conforming joints in existing RC buildings. Two experimental tests on joints without transverse reinforcement are designed and carried out. The specimens are different for beam longitudinal reinforcement ratio and they are both reinforced with plain longitudinal bars. Two different kinds of joint failure are expected, with or without yielding of the adjacent beam. Design criteria, adopted setup and main experimental results are described. Strain gauges located on beam bars and displacement transducers on the joint panel allow the complete definition of both the deformability contributions of fixed-end-rotation and shear strain of joint panel. Finally, a comparison between joint shear strength models existing in literature or codes and experimental results (considering a wider experimental database) is carried out.

SEISMIC ASSESSMENT OF NON-DUCTILE REINFORCED CONCRETE C-SHAPED WALLS

Buildings that rely on reinforced concrete walls and cores as their primary lateral loading system are prevalent in much of Australia’s building stock. Capacity design principles do not have to be adhered to in most low-to-moderate seismic regions, such as Australia. Consequentially, the level of detailing typically provided in accordance with the current and past concrete material standards, AS 3600 and AS 1480, is regarded as non-ductile from the seismic design point of view. These non-ductile reinforced concrete elements have been known to perform poorly when subjected to large lateral loads, as observed in the Christchurch earthquake in 2011. This paper presents an investigation into the seismic performance of C-shaped reinforced concrete walls acting as a core of a Mid-Rise building using current and past building codes in Australia. The displacement capacity of the building was calculated using a displacement-based assessment. A shear capacity model, which is a function of the curvature ductility of the walls, was also considered in the assessment. The results indicate that the older building is likely to fail in shear in the event of a 1000-year return period earthquake event. The building designed to current standards is vulnerable to a non-ductile failure from premature fracturing of the longitudinal reinforcing steel bars.

Strain Rate Effect on Development Length of Steel Reinforcement

Accidental or premeditated explosions have detrimental effects on the infrastructure near the center of explosion and pose major threats to human life. Thus, research is currently underway to study the effects of explosions on infrastructure systems with the ultimate goal of minimizing infrastructure damage and saving lives. Because reinforced concrete is the most common building material used in blast-resistant infrastructure design and construction, understanding the effect of blast loads on reinforced concrete components is essential to reaching this goal. The prevailing design philosophy for blast-resistant structures is energy dissipation through reinforcement yielding (ductility) and large bending deformations without the incidence of nonductile failure modes such as shear and bond. However, information regarding the bond behavior and strength of steel reinforcement–concrete bonds under blast loads is rather scant; therefore, this paper reports on an experimental program designed to investigate the strain rate effect on steel reinforcement–concrete bond. Reinforced concrete beams longitudinally reinforced with 15M, 20M, or 25M were tested in a shock tube under simulated blast loading. The test results show that high strain rate increases the steel reinforcement–concrete bond strength and thus, that the static load development lengths of these bars are adequate for developing their dynamic yield strengths at high strain rate. The dynamic increase factor for bond stress is determined to be 1.11 for 15M, 2.24 for 20M, and 3.68 for 25M bar.

Cladding in RPV Integrity and Lifetime Evaluation

According to all design codes (ASME, RCC-M, KTA, JARC, PNAEG,…) austenitic cladding on inner surface of RPVs is usually taken only as a protection against corrosion. Its tensile properties as well as its existence is not taken into account in design of the RPVs, i.e. in the determination of their dimensions as well as during stress analysis and comparison with allowable stresses/stress intensities.Austenitic cladding due to its manufacturing history – mostly by strip welding without subsequent austenitization – has not very high toughness that can be important in evaluation of some regimes like during emergency cooling when “PTS – pressurized thermal shock” is present.Existence of cladding, its properties, defectness and system of in-service NDT inspection determine the type, size and shape of so-called “postulated defect” in evaluation of RPV resistance against non-ductile failure. According to some codes (e.g. IAEA VERLIFE etc.) fracture properties must be taken into account in these calculations. Unfortunately, each of cladding layers has different properties and their fracture properties are changing during operation – some embrittlement can be found as a result of neutron irradiation.This paper will describe problems connected with the effect of cladding on RPV resistance against non-ductile failure during regimes of PTS type and also gives some examples and criteria for their evaluation.

Evaluation of P-T Limit Curves According to Alternative Fracture Toughness Requirements

During the heat-up and cool-down processes of nuclear power plants, temperature and pressure histories are to be maintained below the P-T limit curve to prevent the non-ductile failure of the RPV(Reactor Pressure Vessel). The ASME Code Sec. XI, App. G describe the detailed procedure for generating the P-T limit curve. The evaluation procedure is containing the evaluation methods of RTNDT using 10CFR50.61. However, recently, Alternative fracture toughness requirements were released 10CFR50.61a. Therefore, in this study, RTNDT of RPV according to the 10CFR50.61a was calculated and used for evaluation of P-T limit curve of a typical RPV under cool-down condition. As a result, it was proven that the P-T curve obtained from 10CFR50.61 is conservative because RTNDT value obtained from the alternative fracture toughness requirements are significantly low.

HSS-to-HSS seismic moment connection performance and design

Abstract Hollow structural sections (HSS) provide an alternative to typical wide flange sections in low-rise seismic moment resisting frames. Their beneficial strength-to-weight ratio as well as bending, compression, and torsional resistance increases the versatility of moment frames and potentially improves performance under earthquake loads. With an understanding of current design requirements for seismic moment connections and static HSS-to-HSS connections, 39 fully welded unreinforced HSS-to-HSS connections are explored through finite element analyses. However, performance is limited due to column face plastification and inability to develop the plastic moment capacity of the beam. To rectify this problem, 24 through plate and 24 external diaphragm plate HSS-to-HSS moment connections are analyzed. Several important geometric parameters, such as the beam width–column width ratio, beam thickness–column thickness ratio, plate length, and plate thickness are considered to understand their effect on the connection moment capacity and sources of inelastic rotation. The through plate and external diaphragm reinforcement greatly improve the connection behavior moving yielding away from the column face and into the beam member. The results from the reinforced connections are used in combination with current seismic design provisions to develop a design procedure that optimizes the performance of these connections. The reinforced connections can be detailed to develop plastic hinging in the HSS beam member while minimizing the likelihood of a non-ductile weld failure.

Steel-plate composite (SC) walls for safety related nuclear facilities: Design for in-plane forces and out-of-plane moments

Steel-concrete (SC) composite walls being considered and used as an alternative to conventional reinforced concrete (RC) walls in safety-related nuclear facilities due to their construction economy and structural efficiency. However, there is a lack of standardized codes for SC structures, and design guidelines and approaches are still being developed. This paper presents the development and verification of: (a) mechanics based model, and (b) detailed nonlinear finite element model for predicting the behavior and failure of SC wall panels subjected to combinations of in-plane forces. The models are verified using existing test results, and the verified models are used to explore the behavior of SC walls subjected to combinations of in-plane forces and moments. The results from these investigations are used to develop an interaction surface in principle force (Sp1–Sp2) space that can be used to design or check the adequacy of SC wall panels. The interaction surface is easy to develop since it consists of straight line segments connecting anchor points defined by the SC wall section strengths in axial tension, in-plane shear, and compression. Both models and the interaction surface (for design) developed in this paper are recommended for future work. However, in order to use these approaches, the SC wall section should be detailed with adequate shear connector and tie bar strength and spacing to prevent non-ductile failure modes.

Failure Initiation of Reinforced-Concrete Beam-Column Connections — Binomial Logistic Regression Based Probabilistic Model

The inelastic mechanisms leading to failure (characterized by strength degradation) initiation in a weak-beam-strong-column reinforced concrete beam-column (RCBC) frame connection subjected to seismic loading are either non-ductile joint failure prior to beam yielding or a ductile mechanism of beam yielding followed by joint failure. Using an extensive assembled database of interior and exterior beam-column joints, binomial logistic regression models are developed to provide a probabilistic estimate of qualitative inelastic mechanisms based on quantitative design parameters obtained from specimen material and geometry. The relative importance of these quantitative parameters is also quantified in this research. The models provide opportunity to an engineer designing new connection to modify geometrical and/or material parameters to obtain ductile mode of response; an engineer retrofitting damaged connection to obtain an understanding of factors influencing initiation of strength degradation and thereby develop suitable retrofit measures for similar connections at eminent risk of failure.

Buckling control of cast modular ductile bracing system for seismic-resistant steel frames

The buckling behavior of a new ductile bracing concept for steel structures is examined. The system makes use of cast components introduced at the ends and the center of the brace to produce a special bracing detail with reliable strength, stiffness and deformation capacity. The system takes advantage of the versatility in geometry offered by the casting process to create configurations that eliminate non-ductile failure modes in favor of stable inelastic deformation capacity. This paper presents analytical research performed to determine the buckling strength and buckling direction of the bracing element based on the geometries of the cast components. Limiting geometries are determined for the cast components to control the buckling direction. Design formulas for buckling strength are proposed.

Deformation in metals after low-temperature irradiation: Part II – Irradiation hardening, strain hardening, and stress ratios

Effects of irradiation at temperatures ⩽200 °C on tensile stress parameters are analyzed for dozens of body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close packed (hcp) pure metals and alloys, focusing on irradiation hardening, strain hardening, and relationships between the true stress parameters. Similar irradiation-hardening rates are observed for all the metals irrespective of crystal type. Typically, irradiation-hardening rates are large, in the range 100–1000 GPa/dpa, at the lowest dose of <0.0001 dpa and decrease with dose to a few tens of MPa/dpa or less at about 10 dpa. However, average irradiation-hardening rates over the dose range of 0 dpa− D C (the dose to plastic instability at yield) are considerably lower for stainless steels due to their high uniform ductility. It is shown that whereas low-temperature irradiation increases the yield stress, it does not significantly change the strain-hardening rate of metallic materials; it decreases the fracture stress only when non-ductile failure occurs. Such dose independence in strain-hardening behavior results in strong linear relationships between the true stress parameters. Average ratios of plastic instability stress to unirradiated yield stress are about 1.4, 3.9, and 1.3 for bcc metals (and precipitation hardened IN718 alloy), annealed fcc metals (and pure Zr), and Zr-4 alloy, respectively. Ratios of fracture stress to plastic instability stress are calculated to be 2.2, 1.7, and 2.1, respectively. Comparison of these values confirms that the annealed fcc metals and other soft metals have larger uniform ductility but smaller necking ductility when compared to other materials.

Appropriate Overstrength of Shear Reinforcement in Precast Concrete Diaphragms

Current precast concrete diaphragm design may not necessarily protect the diaphragm from a nonductile shear failure in an overload situation. As evidence exists that diaphragms can attract large inertial loads during strong ground motion, emerging design methodologies are aiming to develop the diaphragm flexural strength. However, the performance target desirable or achievable in design may depend on a number of other factors. An analytical study examines the performance of precast diaphragms with different shear strength relative to design (flexural) strength, termed here “shear reinforcement overstrength.” The objective of the study is to determine the required shear reinforcement overstrength in the precast diaphragm to produce certain performance targets. Appropriate shear reinforcement overstrength design factors are proposed in terms of a number of key parameters related to diaphragm geometry and the properties of the diaphragm reinforcing details.

Rehabilitation of RC inverted-T girders using anchored CFRP sheets

Existing infrastructure includes significant number of reinforced concrete (RC) bridges that are old and designed based on outdated design codes. Inverted-T beams used as bentcap girders that support the bridge’s longitudinal precast beams provided a popular structural system of many existing bridges. The structural behaviour of inverted-T beam is different than that of conventional top-loaded beams, due to the fact that the loads are introduced into the bottom flange rather than the top of the beam. The objective of the present study is to experimentally examine the effectiveness of new rehabilitation techniques using anchored carbon fibre-reinforced polymer (CFRP) sheets to eliminate non-ductile failure mechanisms in hanger, web, and flange zones of RC inverted-T girders. The experimental program of this study included conducting eight tests on four simply supported RC inverted-T girders before and after rehabilitation. The girders represent a 1/3rd scale model of a prototype bentcap girder. The experimental results showed that the proposed new rehabilitation schemes proved to eliminate the non-ductile mechanisms in hanger, web, and flange zones. The rehabilitated girders were able to reach displacement ductility levels of up to 4 and showed an increase in strength up to 20% compared to the control girders.