Moderate Seismic Regions Research Articles

New seismic design method to improve collapse performance of steel ordinary moment frames

Steel ordinary moment frames (OMFs) have been widely used as a seismic force resisting system in low to moderate seismic regions. In this study, the seismic collapse performance of steel OMFs was evaluated. For this purpose, 9-, 12-, 15-, and 18-story steel OMFs were designed in compliance with ASCE 7 (ASCE, 2017) and its reference standards. To improve the collapse performance of the OMFs, a new design method was proposed with a limiting value of column moment strength ratio. Unlike OMFs designed with ASCE 7 and its reference standards, all OMFs designed with the proposed design method satisfied the target collapse probabilities specified in ASCE 7. To investigate the effectiveness of the proposed design method, seismic loss estimation was also performed. The expected annual losses of OMFs were reduced more than 20% when the OMFs were designed with the proposed design method.

Simplified design approaches for panel zones in steel special moment frame buildings

Panel zones (PZs) can be proportioned to be weak, balanced or strong. Damage to steel special moment frames (SMFs) in past earthquakes has highlighted the limitations of weak PZ design, leading to its discontinuation from design practice. Balanced and strong PZ approaches remained acceptable, but the shear demand and capacity for which PZs need to be proportioned to achieve a balanced or strong PZ still remain ambiguous. In this work, three different PZ design approaches were investigated (named PZ-I, PZ-II and PZ-III) by incorporating them in seismic design of selected study buildings. The shearing behaviour of the PZs was evaluated using the results of non-linear analyses, which indicated that PZ-I, PZ-II and PZ-III led to weak, balanced and strong PZ behaviour, respectively. In particular, PZ-I design was found to lead to excessive shearing distortions in the PZ. Thus, in order to either limit or entirely eliminate inelasticity in the PZs of steel SMF buildings, it is appropriate to adopt PZ-II or PZ-III design. It is recommended that adopting the PZ-II design approach would suffice for moment frame buildings in regions of moderate seismicity.

Energy capacity and seismic resistance of loess cave structures under field ground motions

A shake table test studied the seismic response of a loess cave structure subjected to ground motions. The test specimen was a reduced-scale structure designed under current building codes and located in a region of moderate seismicity of the Asia area. The specimen was subjected to a sequence of increasing acceleration amplitude tests that represented frequent, design, rare, and very rare earthquakes at the site. The test structure performed well (basically in the elastic domain) under frequent and design earthquakes, approached the boundary between the performance levels of life safety and near collapse under the rare earthquake, and collapsed under the very rare earthquake. Damage concentrated covering at cave face and arch top at inner cave hole. The covering layer energy dissipated about 49.4% of the total energy that contributes to damage, and the rest was dissipated by the arch and cave leg layers. The total energy input on the structure until collapse under the very rare earthquake action was very close to the value obtained in a rare earthquake. The seismic capacity of the loess cave meets the seismic fortification index.

Component-based model for bolted brace connections in conventional concentrically braced frames

In low and moderate seismic regions, low-ductility concentrically braced frames (CBFs) are widely used as the seismic force-resisting system for steel structures. The capacity-based design method is not required for such systems, i.e. no individual component in the lateral load carrying path is explicitly designated to sustain plastic deformations under seismic loading. Such CBFs are referred to as conventional CBFs (CCBFs) in this paper. Prior studies have revealed that, in CCBFs, the brace-to-gusset connections are inherently weaker in tension than the adjoining braces and gusset plates. Therefore, the accurate numerical modelling of the brace connections is critical for the reliable seismic evaluation of CCBFs. However, few research publications address the inelastic bolted brace connection modelling necessary for the structural analyses of these braced frame systems. In this paper, an efficient inelastic numerical modelling method, comprising the component-based modelling concept, is proposed for bolted brace connections. The accuracy of the numerical model is validated through comparison with laboratory test results of full-scale I-shape brace connection specimens. Eight single-storey CCBFs with the symmetric diagonal bracing configuration were designed and modeled. The nonlinear static and dynamic analyses revealed that: 1) although the buckling of the middle column at small storey drifts resulted in substantial lateral strength deterioration, a secondary seismic mechanism provided stable resistance to prevent collapse; 2) when loaded in tension, the brace connections deformed more than the braces; 3) stronger brace connections resulted in higher structural lateral stiffness and triggered earlier buckling of the middle column; 4) stronger brace connections possessed higher frictional energy-dissipating capacity which reduced the maximum storey drift.

Shear Wall and Frame Dual Systems Featuring Discontinuous Load Paths in Frame Elements in Low-to-Moderate Seismic Regions

ABSTRACT Reinforced concrete buildings constitute a significant portion of construction in Australia. Many existing buildings in Australia and other low to moderate seismic regions have been designed with little to no consideration for ductile detailing. The majority of these buildings also possess vertical irregularities, which can exacerbate their vulnerability in an earthquake event. The seismic performance of shear wall and frame dual systems possessing discontinuities in frame elements in the form of ductility and failure mechanism has been assessed. And design recommendations in terms of response modification factors and designation of appropriate methods of analysis have been provided.

Parametric Collapse Performance of Low-Ductility Concentrically Braced Frames with Reserve Capacity

AbstractIn moderate-seismic regions, cost-effective steel building design solutions such as the R=3 and ordinary concentrically braced frame (OCBF) systems do not consistently provide reliable life...

Biaxial Seismic Performance of a Two-Span Concrete Bridge Model with Six ABC Connections

Abstract A prerequisite to the implementation of accelerated bridge construction (ABC) in moderate and high seismic regions is reliable data proving that bridges constructed by assembling precast e...

Seismic response of precast bridge columns with composite non-proprietary UHPC filled ducts ABC connections

Accelerated bridge construction (ABC) utilizes advanced planning, new construction techniques and innovative detailing to expedite construction of new bridges or rehabilitation and replacement of old deteriorated bridges. ABC has been widely used in low seismic regions and mostly implemented in the superstructure elements but much less considered for substructure elements such as column-base connections, especially in moderate and high seismic regions. While robust seismic ABC connections have been developed using ultra-high performance concrete (UHPC), sole sourcing of robust commercial UHPC mixes is causing bidding issues and limiting the wide adoption of such seismic ABC connections. Thus, validating alternative solutions that use non-proprietary materials could help overcome such challenges. This paper presents experimental and analytical investigation of the structural and seismic behavior of precast bridge columns with non-proprietary UHPC-filled ducts column-to-footing connections. For this study, a recently developed non-proprietary UHPC mix at the University of Nevada, Reno (UNR) is used. Two large-scale column models, varied only in reinforcement debonding, were tested at UNR under combined axial and cyclic lateral loading to evaluate their seismic performance. The column models were compared with reference cast-in-place column models and another column model incorporating proprietary commercial UHPC-filled duct connection from previous studies. The test results showed that the column models were emulative of conventional connections. Analytical modeling for the two tested columns was also conducted and shown to successfully reproduce the local and global behaviors of column models with acceptable accuracy when bond-slip and bar debonding effects are properly considered.

An experimental study on the seismic behavior and replaceability of the replaceable steel shear links

The reinforced concrete columns and steel beams (RCS) composite frames have received popularity in low to moderate seismic regions due to advantages of the system. However, the poor repairable capacity and deteriorating pinched hysteretic behaviour of the traditional RCS framed structure limit the application in practice under earthquakes. This study presents a modified RCS framed structure with replaceable members aiming to enhance the energy dissipation and resilience capability. The structural configuration character and the general predetermined levels of lateral response of the proposed RCS framed structure with replaceable members is first introduced. Seven quasi-static loading tests were subsequently conducted to evaluate the seismic behavior and replaceability of the replaceable shear links. Parameters including the link length ratio, stiffener configuration and link-to-column connection were examined. The test results depicted that the specimens exhibited the shear dominated behavior. The major failure features of the shear links included web-to-stiffener weld tear, web buckling and web fracture. All tested specimen showed a stable bearing strength, large inelastic rotation capacity and excellent energy dissipation behavior. The same link length ratio of the specimens with different specialized connections presented the similar behavior. Noted that end plate and web stiffeners connection can alleviate the stress concentration on the fillet weld of the flange-to-end plate weld and optimize the performance of the shear link. The average overstrength factor and inelastic rotation capacity of the test specimens were 1.9 and 0.087 rad significantly exceeding 1.5 and 0.08 rad provided by the AISC 341-16 provisions for the EBF links, respectively. In addition, the replaceability of the specimens was evaluated underwent the entire loading process. Compared with the T-shaped and Π-shaped plate connections, replacement of the shear link with the end plate demanded the least effort and time.

Seismic response and design of steel multi-tiered concentrically braced frames of the conventional construction category in moderate seismic regions of Eastern Canada

This study examines the seismic behaviour of steel multi-tiered concentrically braced frames (MT-CBFs) of the conventional construction category (Type CC) in moderate seismic regions of eastern Canada and proposes a new seismic design method for such structures. A total of 32 prototype MT-CBFs part of a single-storey building are selected and designed in accordance with the 2019 Canadian steel design standard (CSA S16-19) provisions for Type CC structures, excluding the design requirements prescribed for frames taller than 15 m, namely the amplification of seismic forces varying by the frame height and the amplification of the column factored seismic force. Nonlinear response history analyses are then performed to examine their seismic response. The results confirmed that tiers experience limited lateral deformation, intermediate horizontal struts are required between tiers, and columns are subjected to combined axial force and bi-axial bending, which in some cases led to column buckling. The results are used to propose a new seismic design method in the framework of CSA S16.

Performance of reinforced concrete columns confined with spiral reinforcement

SummaryThe study proposes a practical strengthening method for improving the seismic behavior of reinforced concrete square columns using spiral as lateral reinforcement in moderate seismic zone regions. Two specimens one without ductile detailing and the other with ductile detailing designed as per Indian standard specifications and six specimens with spiral ties (in one direction and cross directions) at 25°, 35°, and 45° of inclination were tested under constant axial load and cyclic lateral loads to study the feasibility of the provision of spiral ties as a seismic‐strengthening method. The important variable used was the angle of inclination of the spiral bars. Test results revealed that the column specimen strengthened with cross spirals at 25° had almost similar performance and ductility behavior when compared with the control specimen with ductile detailing and specimen strengthened with cross spirals at 35° had better performance and ductility behavior when compared with the control specimen without ductile detailing and can be adopted in moderate seismic regions. Considering the enhanced behavior, such as the strength, stiffness, and ductility, along with the construction easiness, for better performance of columns in moderate seismic zones, angle of inclination of 35° is sufficient.

An experimental study of I beam-RHS column demountable joints with welded studs

This paper presents the results of an extensive experimental investigation on the behaviour of a new type of joint between open structural steel beam and rectangular hollow section (RHS) column. The proposed joint is demountable: threaded studs are welded to the front face of the hollow section column and angle cleat legs are then bolted to the column by means of the welded studs and to the beam flanges by ordinary bolts. Twenty-one full-scale single side or double side bolted beam-column joints combining different sections have been tested under monotonic and cyclic loads. The test results confirm that these joints behave as semi rigid and partial strength ones which possess sufficient stiffness and moment resistance for use in the construction of low to medium-rise steel buildings as an alternative to other traditional solutions. The proposed joints also exhibit a moderate energy dissipation capacity under cyclic loads, suitable for construction in moderate seismic regions. This research work demonstrates that the proposed joint is easy to execute on site without specialized manpower or expensive facilities.

Numerical investigation into I-shape brace connections of conventional concentrically braced frames

In many low and moderate seismic regions, a low-ductility concentrically braced frame (CBF) is used as the seismic force resisting system for steel structures. The design of such CBFs is straightforward: all members and connections are designed based on the seismic force demand obtained through linear elastic structural analysis; capacity-based design and additional seismic detailing are not required. There is no designated energy-dissipating fuse in the lateral load-carrying path. Such CBFs are referred to as Conventional CBFs (CCBFs) in this study. In CCBFs, the brace-to-gusset connections are inherently weaker in tension than the adjoining gusset plates and braces. This occurs because both the gusset plates and the braces are selected on the basis of their respective compressive buckling resistances, and hence, typically have a much greater resistance in tension. Described herein is a numerical study of the popular flange plate bolted I-shape brace connection configuration. A high-fidelity finite element (FE) simulation procedure was developed and validated against laboratory test results. The resulting numerical models were created with the objective of improving our understanding of the inelastic response of these brace connections. The force transfer mechanism within the two branches of the connection was characterized. Significant nonuniform shear distribution was found to exist within the bolt group in the flange branch, which may be detrimental to the safe functioning of these bolts in the seismic design context. The loading eccentricity on the weld group was quantified. Recommendations on how to avoid brittle bolt shear rupture and premature weld fracture are proposed.

Experimental Study on the Behavior of Existing Reinforced Concrete Multi-Column Piers under Earthquake Loading

When a seismic force acts on bridges, the pier can be damaged by the horizontal inertia force of the superstructure. To prevent this failure, criteria for seismic reinforcement details have been developed in many design codes. However, in moderate seismicity regions, many existing bridges were constructed without considering seismic detail because the detailed seismic design code was only applied recently. These existing structures should be retrofitted by evaluating their seismic performance. Even if the seismic design criteria are not applied, it cannot be concluded that the structure does not have adequate seismic performance. In particular, the performance of a lap-spliced reinforcement bar at a construction joint applied by past practices cannot be easily evaluated analytically. Therefore, experimental tests on the bridge piers considering a non-seismic detail of existing structures need to be performed to evaluate the seismic performance. For this reason, six small scale specimens according to existing bridge piers were constructed and seismic performances were evaluated experimentally. The three types of reinforcement detail were adjusted, including a lap-splice for construction joints. Quasi-static loading tests were performed for three types of scale model with two-column piers in both the longitudinal and transverse directions. From the test results, the effect on the failure mechanism of the lap-splice and transverse reinforcement ratio were investigated. The difference in failure characteristics according to the loading direction was investigated by the location of plastic hinges. Finally, the seismic capacity related to the displacement ductility factor and the absorbed energy by hysteresis behavior for each test were obtained and discussed.

Damaged Speleothems and Collapsed Karst Chambers Indicate Paleoseismicity of the NE Bohemian Massif (Niedźwiedzia Cave, Poland)

AbstractMultiphase speleothem damage and passage collapse in Niedźwiedzia Cave (NE Bohemian Massif, Poland) were dated with U‐series methods, revealing five events: (1) 320–306 ka, (2) 253–236 ka, (3) 162–158 ka, (4) 132–135 ka, and (5) >21 ka. Events 1, 3 and 4 are robustly constrained, and events 2 and 5 are less certain. Although we cannot unambiguously exclude other agents (frost or gravity collapses), the most likely trigger of damage in the cave was an earthquake, which is supported by timing (the damage occurred independently from climatic conditions in cold and warm periods) and deformation style (damage to the ceiling and walls as well as the passage floor). We applied ground motion models to determine the probable seismic source size, which is most likely the Sudetic Marginal Fault ‐ one of the most pronounced tectonic structures in Central Europe. Located <20 km from the cave and with documented earthquakes of M > 6, the Sudetic Marginal Fault can produce peak ground acceleration values high enough to break speleothems. The other plausible seismic sources are faults in the Upper Nysa Kłodzka Graben located to the east and the Trzebieszowice‐Biela Fault. Although there are sparse historical data that would allow estimating linked seismic hazards, the <8 km distance between the cave and faults should suffice to destroy the speleothems. Niedźwiedzia Cave shielded environmental earthquake effects from erosion. This study shows the advantage of employing speleoseismology in moderate seismic regions, where earthquake effects are rarely preserved in the geological record.

A simplified seismic design method for limited-ductility steel multi-tiered concentrically braced frames in moderate seismic regions

Steel multi-tiered concentrically braced frames (MT-CBFs) represent a bracing configuration where two or more concentric bracing panels are stacked between the ground and roof levels in tall single-storey buildings. A large proportion of MT-CBFs in Canada are located in low-to-moderate seismic regions (seismic category 0–3) where limited ductility CBFs are often preferred in design. Nevertheless, brace tensile yielding may not occur in all tiers of such frames. Additionally, the analysis and design procedure adopted by the 2019 Canadian steel design standard (CSA S16-19) can become tedious in tall frames with multiple panels. In this paper, the seismic behaviour of limited ductility MT-CBFs in moderate seismic regions of Canada is examined to propose a simplified design method. A set of 16 prototype MT-CBFs is designed in accordance with CSA S16-19, excluding the design requirements prescribed for MT-CBF columns. Nonlinear dynamic analyses are then performed to examine their seismic response. Finally, an efficient design method is proposed in the framework of CSA S16-19.

Shake table test program of cold-formed steel in-plane partition walls

Cold-formed steel (CFS) partition walls accounting for a significant portion of the nonstructural inventory have been vulnerable to seismic events. The seismic damage to CFS partition walls may occur at even low-intensity ground shaking leading to negligible structural damage and, in turn, results in significant losses. Regarding this, current seismic design provisions require that CFS partition walls on egress paths should be designed to accommodate interstory drifts of a structure. However, there is little information on the seismic performance of CFS partition walls built at low- and moderate seismicity regions although previous studies emphasize that they could be damaged at even low-intensity ground shaking. In order to evaluate the seismic performance of CFS partition walls constructed in low-and moderate seismicity regions, shake table test with CFS partition walls was carried out in this study. The interior partition wall specimens with different stud spacing were built according to the Korean common construction practices and were planned to suffer in-plane deformations. Physical and measured responses of the CFS partition walls observed during the shake table test demonstrated that decreasing the stud spacing can improve the seismic performance of CFS partition walls subjected to in-plane deformation. From the statistical investigation characterizing the damage states of the CFS partition walls and their correlation with engineering quantities, the construction practice of CFS partition walls built in low- and moderate seismicity regions might not provide acceptable seismic performance for life safety even though the structure installing them was assumed to be design to meet the codified limitation related to its engineering quantity.

Shear-flexure interaction in RAC columns under simulated seismic actions

Inelastic flexural deformations adversely affect shear resistance of columns, and may cause building damage/collapse during earthquakes. In this paper, shear-flexure interaction on seismic performance of reinforced concrete columns constructed with recycled aggregate concrete is experimentally investigated. For this aim, reinforced concrete columns with low shear-span-to-depth ratio (2.3) were designed and constructed so that they reach their flexural lateral load capacities in accordance with capacity design principles, and the effect of shear deformations can be observed only after yielding of longitudinal reinforcing bars due to cyclic degradation. The shear-flexure interaction is characterized based on the amount of transverse reinforcement, and formulated within the framework of conventional reinforced concrete design theory during the design of the columns. These columns are then tested under the combined effects of axial and reversed cyclic lateral loads to simulate seismic loading. The recycled concrete aggregate is sourced from waste of low strength concrete, and replaced with 50% of natural coarse aggregate for the concrete mix. Comparison with columns constructed of natural aggregate is also presented in the study. The test results showed that the reinforced concrete columns either made of natural aggregate concrete or recycled aggregate concrete exhibited similar seismic performances for different interactions of shear and flexure. As the ratio of transverse reinforcement is reduced the deformation capability was reduced due to more pronounced effect of shear deformations. In addition, the load–displacement relationships determined through theoretical analysis were found to be compatible with the experimental data. Furthermore, a discussion on the minimum transverse reinforcement provisions of EN 1998-1, ACI 318-19 and TBEC-18 is also presented in order to investigate whether it is possible to reduce the required ratio of minimum transverse reinforcement per technical documents for the columns located in low to moderate seismicity regions.

An approach to derive seismic fragility curves

Seismic risk assessment is needed for the evaluation of disaster management, emergency preparedness and prior retrofitting. Based on feasible collapse mechanisms and calculation of their associated seismic parameter Peak Ground Displacement, the failure analysis of the structure is identified. Further, the outputs are manipulated to produce a measure vulnerability obtained from failure analysis. In regions of moderate and high seismicity, Zone – III Fragility analysis is used. An investigation is made on low rise Unreinforced masonry (URM) structure located in Zone – III, particularly in Chennai. To obtain capacity curve and bi-linear capacity curve the structure is modelled as Equivalent Frame Model (EFM), and non – linear static pushover analysis (NPOA).The structure damages, under various level of seismic excitation was developed by an analytical seismic fragility curve. For prioritizing retrofit, pre-earthquake planning, and loss estimation tools such as cost-benefit analysis this curve is utilized. The damage states considered for determining probability of exceeding a specified damage state involved in fragility curve generation are slight, moderate, extreme and collapse.

Data Set for Cyclic Tests of Eleven Lightly Reinforced Concrete Walls

A series of reinforced concrete wall tests were conducted at the University of Auckland to address the lack of experimental data on flexure-dominant lightly reinforced concrete walls that are common in multistorey buildings in regions of low or moderate seismicity. The experimental program comprised 11 rectangular reinforced concrete walls that were subjected to pseudo-static cyclic loading. The tests were used to investigate minimum vertical reinforcement provisions for reinforced concrete walls and formed the basis for revisions to the New Zealand Concrete Structures Standard NZS 3101 and the US Building Code Requirements for Structural Concrete ACI 318. The recorded documentation and data collected throughout the test program have provided a high-quality dataset that is a valuable resource to researchers investigating the seismic behavior of reinforced concrete walls. The dataset is published and publicly available on DesignSafe-CI with the project number PRJ-1648. The methodology of the data collection is described, and a road map for navigating the dataset is presented to support future use of the archived dataset.