High Ductility Class Research Articles

Evaluation of the Failure Probability of a 2D RC Frame Subjected to Column Loss

Abstract: This study regards the evaluation of the failure probability of a symmetrical 2D reinforced concrete frame composed of 4 spans and 5 floors, in case of an accidental event which causes the central base column loss. The frame is an internal one of a typical building designed in a highly seismic area, characterised by a high ductility class. The frame is modelled in the non-linear finite elements software Atena 2D, accounting for both geometrical and material non-linearities. The uncertainties relevant to the problem are included by sampling both material and action variables, adopting the Latin Hypercube Sampling technique. To compute the failure probability associated to the accidental scenario, two sets of analyses are considered: the first set to compute the capacity of the structure against the column removal by means of displacement-controlled pushdown analysis; the second set to evaluate the demand in terms of external loads, properly combined within the accidental combination according to the codes. The external load is then amplified in order to include the dynamic effects characterising a scenario of a structural member loss. Finally, the probability of the demand exceeding the capacity is evaluated.

Critical discussion of DC2 design rules for MR-Frames in the framework of the prEN1998 draft

This work aims at analysing the design rules proposed for DC2 ductility class, introduced in the new prEN1998 draft, that is a new class intermediate between the low and hight ones. Simplified design rules aimed only at avoiding the so called soft-storey mechanism are proposed, because it is considered unnecessary to follow design rules that aim at the development of a global mechanism as occurs in the high ductility class. In particular, the design condition which is the statement of the DC2 design is based on an inequality that must be satisfied at all the storeys, which both does not consider the beam-column hierarchy criterion and does not possess any scientific background. This work aims to highlight the design rules proposed for MRFs in this new ductility class by carrying out a critical analysis. The attention is mainly focused on the soft-storey mitigation criterion and its application in a study case that represent the novelty of the paper.

Seismic Design of Elevated Silos and Tanks – a Study on Behaviour Factors

AbstractAccording to EN 1998‐4:2006 3.4 (4), elevated silos may be designed considering medium or high ductility class for the support structure, abbreviated DCM and DCH respectively, for which an overall behaviour factor q greater than 1,5 may be assumed.Undoubtedly, when a silo or tank supported by a sway frame or a vessel with integrated legs is considered, it is expected that plasticity will be induced due to the overall system's performance and thus, motion amplitudes will be reduced due to energy dissipation. However, the shell upper‐structure is a very thin‐walled class 4 cross section. Hence, it might fail due to buckling at much smaller motion amplitudes (corresponding to acceleration amplitudes) than those, which the support structure can follow without failing.Therefore, from the authors' point of view it is not sufficient to settle with a single behaviour factor for the overall structural system. Alternatively, a two‐step approach is recommended: With this two‐step approach it can be verified, that (sudden) buckling of the shell does not happen prior to the introduction of plasticity at the sub‐structure and in extension to the desired energy dissipation.The proposed two‐step approach is demonstrated by suitable examples with a different range of parameters. Therewith, it will be shown that the assumption of q = 1,5 for elevated silos or tanks can be unsafe. An improvement of the current EC8‐4 rules is proposed in order to enable a practical, safer and more efficient design procedure.

Collapse performance assessment of steel moment frames designed to Eurocode 8

This paper reports on a study performed on steel moment-resisting frame buildings with the aim of evaluating the safety margins against collapse of buildings designed according to Eurocode 8 (EC8) when subjected to maximum considered earthquake ground motions. To this end, the methodology proposed in FEMA P695 is employed. A set of 360 archetype buildings are designed according to EC8 using code-prescribed behaviour factors for medium and high ductility class structures. Additionally, behaviour factors determined according to an Improved Force-Based Design (IFBD) methodology are also adopted. IFBD allows for a more rational selection of the behaviour factor and is fully compliant with the EC8 prescriptions. The results from the seismic performance assessment of the structures allow for an improved understanding of the seismic behaviour of steel MRFs designed according to EC8 and highlight the efficiency that is achieved with the adoption of the IFBD methodology, both in terms of reductions of steel weight as well as in terms of the consistency between the design assumptions and the ductility demand that is explored from the buildings. Moreover, the results also indicate that moment-resisting frame buildings designed according to EC8 are characterized by acceptable safety margins against collapse.

Behaviour Factor of Ductile Code-Designed Reinforced Concrete Frames

The current generation of seismic design codes is based on a linear elastic force-based approach that includes the nonlinear response of the structure implicitly through a response modification factor (named reduction factor R in American codes or behaviour factor q in European codes). However, the use of a prescribed behaviour factor that is constant for a given structural system may fail in providing structures with the same risk level. In this paper, the behaviour factor of reinforced concrete frame structures is estimated by means of nonlinear static (pushover) and nonlinear incremental dynamic analyses. For this purpose, regular reinforced concrete frames of three, five, seven, and nine storeys designed for high ductility class according to the European and Italian seismic codes are investigated, and realistic input ground motions are selected based on the design spectra. Verified analysis tools and refined structural models are used for nonlinear analysis. Overstrength, redundancy, and ductility response modification factors are estimated, and the effects of some parameters influencing the behaviour factor, including the number of bays and the number of storeys, are evaluated. The results are finally compared with those obtained from a previous paper for steel moment-resisting frames with the same geometry. According to the analysis results, the behaviour factors in the case of pushover analysis are significantly higher than those obtained in the case of nonlinear response history analysis. Thus, according to the pushover analysis, the behaviour factor provided by European and Italian standards seems highly conservative. On the contrary, the more refined nonlinear dynamic analysis shows that the code-prescribed value may be slightly nonconservative for middle-high-rise frame structures due to unfavourable premature collapse mechanisms based on column plastic hinging at the first storey. Thus, some modifications are desirable in local ductility criteria and/or structural detailing of high ductility columns to implicitly ensure that the recommended value of the behaviour factor is conservative.

Analiza oštetljivosti i robusnosti višekatne armiranobetonske zgrade

The robustness of a reinforced concrete (RC) five-storey building (frame system stiffened by walls) is analysed in the paper. A high ductility class structure is designed in accordance with structural Eurocodes. The response of the structure to eight different scenarios of the ground floor vertical element loss is analysed. Nonlinear Static Analysis (NSA) and Nonlinear Dynamic Analysis (NDA) methods are used for the robustness analysis. Fragility curves of the building are derived from statistical analysis of these results. The values obtained through NSA and NDA, damage limit states of the system, and fragility curves, are compared. The influence of the position of the removed element on robustness of the structure is also analysed.

Assessment of rigid steel frames: ductility versus damageability

ABSTRACT Modern seismic code relies on the capacity design approach to assure ductility. When the capacity design dictates, the frames are dimensioned for strength. Inadvertently, stiffness often presents itself as a dominating parameter in MRFs (Moment Resisting Frames), especially with strict drift limits. In this research, rigid steel moment-resisting frames have been designed as per Eurocodes provisions. Initially, the results obtained from modal analysis and nonlinear analyses are concisely elaborated in such a way to highlight the complexities of Eurocode 8 design procedures. Various parameters are assessed to evaluate their influences on high and medium ductility classes. A simplified predictive method is proposed through tables, graphs, and flowcharts. Several combinations are suggested where for an assumed ductility class, a specific drift limit is defined. The elastic overstrength, redundancy factor, and reserve overstrength factors are indicated with the confidence to allow an un-iterated design approach for steel moment-resisting frames designed with a pre-determined strategy for failure mechanisms by improving the design approach of Eurocode 8.

Seismic behavior of exterior RC beam-column joints without code-specified ties in the joint core

Beam-column joint design for reinforced concrete (RC) buildings is a critical issue for modern earthquake engineering, because for these structural elements, the current codes provide several geometrical constraints and reinforcement provisions, which may be very complex to implement during the construction phase. To investigate how a construction error might influence the joint behavior, two identical full-scale exterior beam-column joints specimens are built, according to Italian Building Code specifications for the high ductility class, but without the required horizontal ties inside the joint panel. This construction error could be plausible because, according to the design drawings, the panel was very crowded with reinforcement bars and the concrete casting and compaction was objectively difficult. The performance of the joints is tested under cyclic loads simulating seismic actions. Although the joints are designed for high ductility class, a 50% reduction of the horizontal reinforcement in the joint core, and 25% in the critical regions, led the specimens to shear failure, with crushing of the concrete in the joint core. A discussion of the failure mechanism is presented, in which shear strength obtained from the tests is compared to shear strengths calculated using the expressions provided for exterior joints by Eurocode 8, the ACI Code and the formula proposed by Pauletta et al.

Factors in the Relationship Between Optimal CO2 Emission and Optimal Cost of the RC Frames

Nowadays, reduction of greenhouse gases emissions from the construction industry is seriously under investigation. The aim of this study is to investigate the various effective factors on the relationship between optimal cost and optimal carbon dioxide emissions of the reinforced concrete structures with nonlinear structural behavior. A four-story reinforced concrete frame is designed for various peak ground accelerations (PGAs) and all ductility classes according to Iran’s seismic resistant design-2800 code, as well as for different concrete classes. The frames are optimally designed according to ACI 318-08 and FEMA codes. The results of optimal designs show that the design of structures with medium and high ductility class produces less cost and CO2 emissions than the low ductility class. On the other hand, the relationship between cost and CO2 emissions shows that in the low ductility class, increasing the percentage of the optimal cost can greatly reduce the amount of CO2 emissions. PGA design has a significant effect on reducing optimal cost and CO2 emissions. Especially in the low ductility class, reducing this parameter can greatly decrease the amount of the objective functions. Also, the use of concrete with low class can reduce the cost and CO2 emissions but the effect of this parameter in the objective is very small.

Earthquake loss assessment of steel moment-resisting frames designed according to Eurocode 8

Current design and assessment guidelines define several seismic performance levels, aiming to ensure that structures exhibit adequate behaviour at different seismic intensity levels. Typically, the acceptance criteria at different performance levels are met by ensuring that local deformation demands are lower than pre-defined capacities. Despite the proven effectiveness of such an approach, it provides an ambiguous measure of the performance of the building, which, in most cases, is neither meaningful nor appropriate for building owners, stakeholders or decision-makers. The main objective of the research presented in this paper is to evaluate the expected direct economic seismic losses of steel moment-resisting frame structures designed according to Eurocode 8 (EC8). A set of 120 archetype buildings, representative of the current building stock in Portugal, were designed according to Part 1 of EC8 using three different behaviour factors, q: a) code-prescribed upper bound limits for medium and high ductility classes; b) behaviour factor defined according to an Improved Force-Based Design (IFBD) procedure. The PEER-PBEE methodology with the improvements proposed by Ramirez and Miranda [1] was employed for the estimation of expected seismic losses evaluated for the seismic intensity levels considered in Part 3 of EC8. The results obtained indicate that the buildings designed in accordance with EC8 comply with the non-collapse criteria. However, the level of damage could imply significant repair costs. Importantly, the results also highlight that a rational selection of the behaviour factor can result in a reduction of steel weight but still ensuring acceptable levels of expected annual losses.

The influence of moment resisting beam-to-column connections on seismic behavior of chevron concentrically braced frames

Chevron concentrically braced frames (C-CBFs) are very popular lateral-resisting systems, often opted for steel buildings due to their improved architectural functionality and reduced cost of fabrication and erection as X bracings. According to EN 1998-1 these systems are expected to provide limited ductility, thus penalizing the design of C-CBFs with smaller values of the behavior factor (namely q = 2 and q = 2.5 for medium “DCM” and high ductility class “DCH”, respectively). To improve the ductility and the energy dissipation capacity of chevron bracings, the influence of the secondary frame effect provided by moment resisting beam-to-column connections belonging to the braced bays is investigated in this paper. To this aim, parametric non-linear analyses are carried out on a set of structural archetypes representative of low, medium and high-rise multi-story buildings. The results show that fully restrained joints can have beneficial effects providing an additional reserve of strength, stiffness and ductility.

Seismic assessment of a heavy-timber frame structure with ring-doweled moment-resisting connections

The performance of heavy-timber structures in earthquakes depends strongly on the inelastic behavior of the mechanical connections. Nevertheless, the nonlinear behavior of timber structures is only considered in the design phase indirectly through the use of an R-factor or a q-factor, which reduces the seismic elastic response spectrum. To improve the estimation of this, the seismic performance of a three-story building designed with ring-doweled moment resisting connections is analyzed here. Connections and members were designed to fulfill the seismic detailing requirements present in Eurocode 5 and Eurocode 8 for high ductility class structures. The performance of the structure is evaluated through a probabilistic approach, which accounts for uncertainties in mechanical properties of members and connections. Nonlinear static analyses and multi-record incremental dynamic analyses were performed to characterize the q-factor and develop fragility curves for different damage levels. The results indicate that the detailing requirements of Eurocode 5 and Eurocode 8 are sufficient to achieve the required performance, even though they also indicate that these requirements may be optimized to achieve more cost-effective connections and members. From the obtained fragility curves, it was verified that neglecting modeling uncertainties may lead to overestimation of the collapse capacity.

Contribution to sustainable seismic design of reinforced concrete members through embodied CO2 emissions optimization

The embodied CO2 emissions of reinforced concrete (RC) structures can be significantly reduced by structural optimization that maximizes structural efficiency. Previous studies dealing with design of RC structures for minimum CO2 emissions do not address seismic design provisions. This is the case despite the fact that in many countries around the world, including most of the top 10 countries in CO2 emissions from cement production, RC structures have to be designed against earthquake hazard. To fill a part of this gap, this study, using exhaustive search, examines optimum designs of RC beam and column members for minimum embodied CO2 emissions according to Eurocode‐8 for all ductility classes and compares them with optimum designs based on material cost. It is shown that seismic designs for minimum CO2 footprint lead to less CO2 emissions but are more expensive than minimum cost designs. Their differences strongly depend on the assumed values of the environmental impact of reinforcing steel and concrete materials. Furthermore, it is concluded that seismic design for high ductility classes can drive to significant reductions in embodied CO2 emissions.

An evaluation of the GSA Nonlinear Static Progressive Collapse Methodology applied to EC 8 Seismically Designed RC Moment Frames

This study aims at evaluating the progressive collapse potential of RC framed structures designed to Eurocode 8 with varying design ground accelerations and ductility classes. Four typical 11-storey RC framed structures were designed with ground accelerations of 0.10g and 0.25g at low, moderate and high ductility classes. Analysis of the performance levels and progressive collapse vulnerability of these structures was carried out considering the four column loss scenarios and acceptance criteria specified by the General Service Administration (GSA) document. It was observed that high design ground acceleration and low design ductility classes increase the progressive collapse resistance of EC 8 designed RC framed structures. This observation is counter-intuitive to the principles underpinning the EC 8 design code as higher design ductility class is expected to enhance the energy absorption capacity or ductility of buildings with same design seismic action but varying ductility classes. It is concluded that the use of GSA to assess buildings designed to EC 8 must be modified to include the effects of both design action and design ductility class of a structure. The location of the lost column was also found to be very influential on the progressive collapse resistance of the structure.

Linear Time-History Analysis for EC8 design of CBF structures

Increased availability of strong motions, improved computational capabilities and availability of open-source software for Eurocode 8-compliant record selection make Linear Time History Analysis (LTHA) a potential more rigorous analysis approach for the design of structures. This methodology is not explicitly codified in the Eurocode and it requires some “code-friendly” recommendations for its implementation in practice. Herein, LTHA is employed for the first time as design method for a five-storey archetype high ductility class concentric brace frame (CBF). A new-optimized design method for braces is presented for the smooth implementation of both Response Spectrum Analysis (RSA) and LTHA. In fact, the tension-only design approach for CBF in Eurocode 8 has some criticalities arising even when a routine RSA is employed. The design process is rather smooth only in the case of linear static approach classically implemented for regular, mid-rise structures.Results of this study provide insights for validation of LTHA as design method for CBFs, and for its code implementation as routine design method in EC8, valuable in near source conditions or for irregular structural configurations.

Numerical modelling of reinforced‐concrete structures under seismic loading based on the finite element method with discrete inter‐element cracks

SummaryThe behaviour of the earthquake excitated reinforced concrete (RC) structures up to the collapse, previously designed according to the prescriptions of Eurocode 8 cannot be fully predetermined because of the diversity of the nature of the earthquake. Thus, one needs to make a judicious choice between the two ductility classes balancing between the desired response of the structure, dimension of structural elements, costs and complexity of the design and construction. Powerful numerical models stand as significant tools to obtain more insight into the peculiarity of each structure. This paper presents a numerical model for dynamic analysis and prediction of the collapse of RC structures under the seismic loading based on finite discrete element method. The model includes several numerical algorithms which are essential for realistic description of highly non‐linear effects of these structures: cyclic crack opening‐closing mechanism based on discrete cracks, interaction between the reinforcement and concrete taken by steel strain‐slip relation, the influence of adjacent cracks to the slip of reinforcing bar, local slip of reinforcing bar because of a high plastic deformation under reversed cyclic loading, the influence of the curvature of reinforcing bar to yield stress reduction of the steel and cyclic behaviour of concrete and steel. Performance of the model was demonstrated through incremental dynamic analysis of RC wall, previously designed according to the Eurocode 8 for medium and high ductility classes. The presented results demonstrated the ability of the model for simulation of the crack patterns and collapse of these structures. Copyright © 2016 John Wiley & Sons, Ltd.

Ductility of wide-beam RC frames as lateral resisting system

Some Mediterranean seismic codes consider wide-beam reinforced concrete moment resisting frames (WBF) as horizontal load carrying systems that cannot guarantee high ductility performances. Conversely, Eurocode 8 allows High Ductility Class (DCH) design for such structural systems. Code prescriptions related to WBF are systematically investigated. In particular, lesson learnt for previous earthquakes, historical reasons, and experimental and numerical studies underpinning specific prescriptions on wide beams in worldwide seismic codes are discussed. Local and global ductility of WBF are then analytically investigated through (1) a parametric study on chord rotations of wide beams with respect to that of deep beams, and (2) a spectral-based comparison of WBF with conventional reinforced concrete moment resisting frames (i.e. with deep beams). Results show that the set of prescriptions given by modern seismic codes provides sufficient ductility to WBF designed in DCH. In fact, global capacity of WBF relies more on the lateral stiffness of the frames and on the overstrength of columns rather than on the local ductility of wide beams, which is systematically lower with respect to that of deep beams.

Evaluation of new spiral shear reinforcement pattern for reinforced concrete joints subjected to cyclic loading

Using continuous spiral reinforcements can greatly improve the seismic performance of reinforced concrete columns, in terms of ductility and energy dissipation capacity. The simultaneous incorporation of this method in beams and columns might influence the behaviour of beam–column connections as wide spectrums of brittle failure were observed in this region. A new proposed beam–column connection introduced as ‘twisted opposing rectangular spiral’ was investigated in this research in both experimental and numerical manners along with comparing its seismic performance with both normal rectangular spiral and conventional shear reinforcement systems. The design of three full-scale beam–column connections was performed according to Eurocode (EC8-04) for high ductility classes, and the quasi-static cyclic loading recommended by American Concrete Institute Building Code (ACI 318-08) was hired to conduct the seismic tests. Finally, the experimental results were validated by numerical results obtained from the finite element analysis of the three specimens. The results revealed improved ultimate lateral resistant, energy dissipation capacity and ductility for the new proposed connection.

Assessment of the Mechanical Behavior in Dual-Phase Steel B400C, B450C, and B500B in a Marine Environment

AbstractThis experimental study examined the mechanical behavior in three types of reinforced concrete steel. An evaluation of the mechanical behavior in dual-phase steels B400C, B450C, and B500B of high and medium ductility class, with a 16 mm nominal diameter, was conducted before and after exposure in a laboratory salt spray environment. The results of all mechanical tensile tests were compared and combined with the results of scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis and consequently led to the following findings: The reduction in strength properties in the precorroded specimens was almost equivalent to the mass loss, which meets the demands set by Eurocode 2 (EC2) for safe structures. However, the drop in ductility properties, expressed as the deformation at maximum strength, is below the minimum limits. The findings also suggest that the degradation in the mechanical performance of steel cannot be attributed to a specific mechanism, but it appears to be the result...

Concrete-Plated Wooden Shear Walls: Structural Details, Testing, and Seismic Characterization

AbstractThis paper discusses the structural characterization of a novel hybrid shear-wall system formed by coupling standard platform-frame panels with an external reinforced concrete shelter formed of precast slabs screwed to the wooden frames. The external RC skin is intended as a supplementary bracing system, increasing strength and dissipative capacity of the bare timber frame. The structural performance of such hybrid shear wall under monotonic and cyclic loading was first theorized analytically on the basis of code provisions and then confirmed via experimental tests. The novel shear walls demonstrated to fulfill the requirements prescribed by Eurocode 8. In particular, the analyzed system belongs to high ductility class (HDC). Finally the seismic response of a reference building realized with the innovative hybrid shear walls was simulated by means of a numerical model validated on experimental tests; the suitable behavior factor for the building was estimated.