Base Shear Coefficient Research Articles

Response of steel buildings under near and far field earthquakes

In this study, the seismic behavior of steel framed buildings under the effect of near-field with pulse (NF-P), near-field without pulse (NF-NP), and far-field (FF) ground motions was evaluated in a comparative manner. For this, 8 and 12 story steel moment-resisting frames (SMRFs) having four equal bays and identical story height were taken into account and then subjected to a series of NF-P, NF-NP and FF earthquake motions based on the nonlinear dynamic analysis. Each earthquake series included 10 different acceleration records. They were scaled to match the target design spectrum. Numerical modeling and analysis of the SMRFs were performed through Seismostruct software. The response quantities utilized in the comparisons of the case study SMRFs were the inter-story drift ratio, roof drift ratio, base shear coefficients, and time history of displacement in different story level. These responses were examined comparatively.

Global Seismic Fragility Functions for Low‐Rise RC Frames with Construction Deficiencies

Shake table tests were performed on three two‐story 1 : 3 reduced scale RC frame structures, representative of buildings with construction deficiencies in Pakistan. The models were subjected to the natural accelerogram of the 1994 Northridge earthquake and tested under multiple scaled excitations until the models attained incipient collapse state. The models’ damage mechanisms were studied, and the seismic response in terms of floor acceleration and floor displacement was retrieved. A uniform damage scale was developed for global performance assessment of considered deficient RC frames, listing average estimate of limit states’ drifts and base shear coefficients, with measured uncertainty, and describing frame damage condition. Numerical frame models were prepared in finite element‐based program SeismoStruct and calibrated against experimental tests. The frames were analyzed through incremental dynamic analyses. Seismic fragility functions were derived using a probabilistic‐based methodology, which may be used for global damageability assessment of the considered frame building typology. Available building damage‐to‐loss factors were employed to transform structural damages to repair cost ratio, which were correlated with seismic intensity (repair cost ratio versus seismic intensity) to derive the seismic vulnerability curve, which may be used for the direct economic loss estimation of the considered frame building typology.

Base Shear Coefficient for Wooden Houses on Recent Observed Earthquake Motions in Japan

Base Shear Coefficient for Wooden Houses on Recent Observed Earthquake Motions in Japan

Fragility Functions and Loss Curves for Deficient and Haunch-Strengthened RC Frames

ABSTRACT Shake-table tests conducted on three as-built and six haunch-strengthened two-story RC frames were analyzed to develop damage scale; suggest roof drift and base shear coefficient for various performance levels of as-built and haunch-strengthened frames. Nonlinear numerical models were prepared in finite element-based software SeismoStruct, calibrated with the experimental tests. Incremental dynamic analysis of models was carried out using natural acceleration time histories. Seismic fragility functions were derived for both as-built and haunch-strengthened models using a probabilistic method. Damage-to-loss factors were employed to transform structural damages to repair cost ratio, which were correlated with seismic intensity to derive seismic loss curves.

Comparative Study of Seismic Design and Performance of OMRF Building Using Indian, British, and European Codes

In India, damage cause by some major earthquakes, such as India/Nepal 2015, Sikkim 2011, Kashmir 2005, Bhuj 2001, Latur 1993, and Uttarkashi 1991, have raised alarms to professionals. The probability of seismic risk is higher in more densely populated Indian cities, such as Bhuj, Kashmir, Sikkim, Uttarkashi, as they come under the highest seismicity zone in India. Therefore, our primary interest is to investigate the seismic performance evaluation of the buildings in these seismic prone areas. Significant research has been conducted on the seismic performance of existing buildings. However, investigations on the seismic performance of a building with different country codes for the same earthquake event has not been explored, which is crucial in providing a deeper knowledge of the seismic performance of buildings. This paper presents a comparative study of an Ordinary Moment Resistant Frame (OMRF) building designed using three major codes, Indian (IS: 456-2000, IS: 1893-2002), British (BS: 8110-1997) and European (EC-2, EC-8). Six typical building models considered with earthquake (WiEQ), and without earthquake (WoEQ), and their assessments were interpreted using non-linear static analysis for determining their seismic performance. Seismic performance is compared in terms of base shear coefficient (BSC) and drift ratio that shows WiEQ models, at the drift ratio of 1.5%, the BSC was as follows; 0.78, 0.88, and 0.96 for the models designed for British, Euro, and Indian codes, respectively. The results show that the building models, that have been designed for the Indian codal provisions for both cases, performed well as compared to the other country codes. Base shear and drift ratio are the vital parameters that vary considerably among the building models. This aspect of the Indian code makes it a safer design methodology with higher reserve strength and a reasonably good displacement capacity before reaching the Collapse Prevention (CP) performance level.

Design and development of low-cost HDRBs seismic isolation of structures

This paper presents a simplified procedure for the preliminary seismic design of high damping rubber bearings (HDRBs) for seismic isolation of structures. The procedure is consistent with the ASCE 7-16 specified seismic design for isolated structure, and starts defining the seismic base shear coefficient (CS) demand for superstructure, reduced by an appropriate response modification factor (RI) for isolated structure. The target time period for isolated structure is identified from the (CS/RI) versus time period plot, corresponding the available base shear capacity of structure. The required lateral shear stiffness of HDRBs is calculated, assuming a reasonable height-to-length ratio of bearing. The preliminary design suggests the required dimensions (height, width and length) of bearing, required thickness of elastomers and number of steel shim plates. The proposed procedure was validated for the seismic isolation of three-story deficient RC frame structure, located in a high seismic zone with a design basis PGA of 0.44 g. Representative reduced scale bearings were locally produced and tested on shake table, using an acceleration time history record of 1994 Northridge earthquake, through multi-levels excitations ranging from 0.10 to 1.0 g. The hysteretic and shear force–deformation response curves were analysed to develop constitutive relationship and hysteretic model for HDRBs. The derived hysteretic model parameters were incorporated in SeismoStruct, using plastic and bi-linear hysteretic models, for testing and validation against the shake table tests. The numerically predicted to experimentally observed peak displacements/forces exhibited a little over-prediction using bi-linear hysteretic model while a little under-prediction using plastic hysteretic model. Combining results from both the hysteretic models, and averaging, minimized the average error to less than 0.10%. Comparative seismic analysis of both as-built and isolated frames was carried out for both design basis earthquakes DBE and maximum considered earthquake MCE (considered as 3/2 of DBE), showing promising behaviour of the simplified design procedure and locally produced HDRBs.

Effects of plan dimensions on gust wind loads for high-rise buildings

Scale-model wind tunnel pressure measurements were carried out for rectangular-plan high-rise buildings with plan ratios ranging from 0.11 to 9. Mean, fluctuating, and peak wall pressure coefficient distributions and area-averages were investigated. In addition, comparisons with the ASCE 7–16 provisions for the Main Wind Force-Resisting System (MWFRS) were made. The results show that the plan ratio has significant effects on pressure coefficients on the leeward and side walls for plan ratios less than about 4. The largest mean base shear coefficient occurs for plan ratio of about 0.67, with large values in the range of 0.5–1, but decreasing for larger or smaller plan ratios. ASCE 7–16 mean load coefficients tend to underestimate the data, particularly because of the values on leeward walls. ASCE 7–16 also underestimates the peak load coefficients due to the value of the gust effect factor. For rigid buildings, ASCE 7–16 has a gust effect factor of 0.85, while measurements indicate that it is closer to 1 for plan ratios between 0.67 and 2. Thus, the overall mismatch between the ASCE 7–16 MWFRS loads and the measured data is due to both the mean pressure coefficients and the gust effect factor. For plan ratios below 0.67, the decrease in plan ratio tends to be favorable for the MWFRS load coefficients; while for plan ratios above 4, the effects of plan ratios on the MWFRS load coefficients are limited.

Modeling, analysis and seismic design of structures using energy dissipators SLB

This paper initially describes aspects of the modeling of structures equipped with energy dissipators Shear Link Bozzo (SLB) and develops two iterative design procedures to select these devices. This methodology is applied to a precast 5-story reinforced concrete building. The SLB energy dissipation devices are initially stiff, but ductile with a range of yielding forces from 36 kN to 900 kN characterized by 52 + 52 standard devices. Moreover, these devices can be combined in parallel giving a very wide range of possibilities for selection and corresponding structural response. Therefore, to simplify its automatic selection, this article presents two procedures: (1) direct iteration and (2) inverse or fixed force iteration. Both procedures were implemented in an automatic application or “plugin” for the ETABS program that automates its selection for a specific structural system or architectural configuration of these elements. Using these devices, the energy introduced by an earthquake into the structure can be dissipated, protecting other structural elements that suffer damage. The SLB energy dissipation devices are affordable to get a significant performance improvement in the overall structural response. This work presents a five-story precast reinforced concrete building frame, called SLB Building, that provides 4 departments per level all with a diaphanous interior floor. The building is made up of 11 columns with a constant 40x40cm section and all its beams have hinges at the ends. This building was equipped with 120 small SLB devices showing its performance for the maximum earthquake of Peruvian seismic code without ductility reduction (R = 1) by means of nonlinear time history with ten seismic records compatible with the S1 soil spectrum. In this structure, all seismic energy dissipation was concentrated in these devices so there would be no structural damage. In addition, the levels of non-structural damage were controlled with initial stiffness of these devices since lateral displacements were reduced to levels below the Peruvian seismic code (or even immediate occupancy for devices greater than those provided in this example). At the same time, the levels of acceleration decrease in height to only 0.3g and the base shear coefficient is reduced from almost 1.2 to only 0.12-0.2 (this means an R factor between 6 and 10 without structural damage).

Seismic assessment of RC frame building designed using gross and cracked section as per Indian standards

This study presents the comparative seismic assessment of 2-, 4-, 8- and 12-storey RC frame buildings designed with gross/uncracked section (IS 1893-2002) and effective/cracked section (IS 1893-2016) properties of RC members using conventional force-based design approach. RC frames of similar height are designed using the same seismic base shear coefficient based on empirically obtained period of the building as recommended in IS 1893. All the considered frames are analyzed using nonlinear static pushover analysis and nonlinear time history analysis (NLTHA). The results of the pushover analysis and NLTHA show that the inter-storey drift at design load of RC frames designed using gross section property is well within the prescribed limit of maximum permissible inter-storey drift ratio. Whereas the inter-storey drift of RC frame designed using effective section properties of beams and columns for the force criteria alone is observed much higher as compared to RC frame designed using gross section and beyond the limit of maximum permissible inter-storey drift ratio. The RC frame buildings designed using the modified cracked section for the force as well as drift criteria of IS 1893 show the improved structural performance and its inter-story drift ratio observed is within the prescribed limit.

Estimating the cross-sectional area of inverted-V braces required for mitigating the progressive collapse of Steel Intermediate Moment Resisting Frames

The use of inverted-V braces is one of the retrofitting strategies engineers employ to mitigate progressive collapse in steel intermediate moment-resisting frames (SIMFs). However, the required cross-sectional area of the bracing member is not only dependent on the structural configurations but also on the seismicity of the region. In this study, a new equation is proposed to estimate the cross-sectional area of inverted-V braces required to mitigate failure progression in SIMFs that are subject to abrupt loss of an exterior column in the first storey. In the proposed equation, the area of beam element in the affected span and the seismicity of the region are considered. For this purpose, six SIMF buildings are designed, considering variation in the seismic base shear and the number of storeys. Then, the structural response of these SIMFs when subjected to an exterior column loss is studied. Using dynamic pushdown analyses, failure load factors of these frames are determined. Then, inverted-V braces of five different sizes are added to the studied frames. Results provide a mathematical correlation between the base shear coefficients, the size of structural elements and the associated retrofitting effect. This approximate equation is then validated by analysing two extra case study structures.

Financial risk evaluation of non-ductile reinforced concrete buildings in eastern and western Canada

Financial risks due to catastrophic earthquakes can be severe. This is particularly applicable to existing buildings that were constructed prior to modern seismic hazard maps and seismic design provisions. Pre-1970 non-ductile reinforced concrete (RC) frame buildings in Canada are of typical examples, where design base shear coefficients were essentially the same across the country, despite very diverse seismic hazards in the east and the west. This study implements a comprehensive performance-based earthquake engineering methodology to evaluate the financial risks of non-ductile RC frame buildings with different numbers of stories for seismicity in Montreal and Victoria. In addition to seismic loss curves that consider a comprehensive event set of regional seismicity, conditional loss distributions for critical scenarios are developed. The results of the financial risk evaluations for the buildings strongly indicate the effects of regional seismicity are significant and overall seismic risks for buildings in Victoria are greater than those in Montreal. Nevertheless, seismic vulnerability of these buildings remains to be high for both locations, meaning that should a major earthquake strike near the location, the buildings studied will suffer major damage. The results from the detailed risk assessments are also useful for developing effective financial risk mitigation strategies through earthquake insurance.

Derivation of energy-based base shear force coefficient considering hysteretic behavior and P-delta effects

A modified energy-balance equation accounting for P-delta effects and hysteretic behavior of reinforced concrete members is derived. Reduced hysteretic properties of structural components due to combined stiffness and strength degradation and pinching effects, and hysteretic damping are taken into account in a simple manner by utilizing plastic energy and seismic input energy modification factors. Having a pre-selected yield mechanism, energy balance of structure in inelastic range is considered. P-delta effects are included in derived equation by adding the external work of gravity loads to the work of equivalent inertia forces and equating the total external work to the modified plastic energy. Earthquake energy input to multi degree of freedom (MDOF) system is approximated by using the modal energy-decomposition. Energy-based base shear coefficients are verified by means of both pushover analysis and nonlinear time history (NLTH) analysis of several RC frames having different number of stories. NLTH analyses of frames are performed by using the time histories of ten scaled ground motions compatible with elastic design acceleration spectrum and fulfilling duration/amplitude related requirements of Turkish Seismic Design Code. The observed correlation between energy-based base shear force coefficients and the average base shear force coefficients of NLTH analyses provides a reasonable confidence in estimation of nonlinear base shear force capacity of frames by using the derived equation.

STRUCTURAL CHARACTERISTICS OF TRADITIONAL THATCHED HOUSES IN OLD POST TOWNS

We report the progress of our research on seismic performance estimation of a traditional thatched house in Ouchi-Juku, Fukushima prefecture, registered as an IPDGHB in 1981, based on field surveys. First, we carried out a field survey of two existing thatched houses to understand their structural properties and conducted microtremor measurements of these houses and examined their ground to analyze their vibrational characteristics. Based on vibration mode of the house, it was found that the amplitude was large on the front side, which was used as often as a souvenir shop and Japanese restaurant, because the front of the house had less walls. Finally, we calculated the yield base shear coefficient as an indicator of seismic performance and compared it with the regression functions of traditional town houses in Kyoto. It is found that the thatched house in Ouchi-Juku had a higher shear force than a wooden house in Kyoto.

Ambient Vibration Measurements and Seismic Evaluation of Historical Japanese-style Wooden Offices in Taiwan

In order to conserve or reuse historic buildings in Taiwan which are located in an earthquake-prone area, it is critical to ensure their seismic capacity. The historical Japanese-style wooden houses are those of concern in this study. Three historical offices of this kind which were damaged in the 2010 Kaohsiung earthquake were selected. First, ambient vibration measurements were taken to investigate their natural frequency and vibration mode. Performance-based assessment for wooden buildings in Japan was adopted and modified to evaluate the seismic capacity of the offices. By comparing the evaluation results and the damage observed in the earthquake, applicability of the method was verified. Accordingly, a seismic evaluation method for the Japanese-style wooden buildings in Taiwan was established. In addition, the relationship between measured natural frequency and calculated base shear coefficient was investigated and compared to the empirical formula for wooden houses in Japan.

Evidence of significant forward rupture directivity aggravated by soil response in an Mw6 earthquake and the effects on monuments

SummaryWhile strong directivity effects have been mostly recognized in Mw > 6.5 earthquakes, the paper investigates the case of a strong such effect in a relatively small‐magnitude event on 3 February 2014 in the island of Cephalonia, Greece. The second of two events (both of Mw ≈ 6) produced a pernicious accelerogram in the region's main town, Lixouri. The paper provides evidence from geology, interferometry, and seismology to convince that the motion was the result of constructive interference in front of the direction of rupture of the obliquely‐strike‐slip fault. The nature of the record is explored to demonstrate that its frequency content, its high velocity pulse, and its strong fault‐normal (FN) favorable polarity are associated with directivity. Moreover, the broad spectral acceleration peak (of 1.7 g) of the FN motion, centered at a period (T ≈ 1.4 s) which almost coincides with the period of the velocity pulse, is shown to have also been affected by soil amplification, in quantitative agreement with Bray et al. (2009). Such a directivity‐and‐soil‐affected motion explains much of the profound damage to monuments, slopes, and harbor quaywalls. In particular, toppling (as well as excessive rotation and sliding) of nearly‐all the tombstones in Lixouri cemetery are shown to correlate well with characteristics of the FN component of motion. By contrast, the excellent performance of the building stock — despite the destructive shaking that prevailed — is persuasively attributed to conservatively‐robust construction practices of the past and the high base shear coefficient of the strict latest (2000) seismic code. Copyright © 2017 John Wiley & Sons, Ltd.

北関東の市街地に現存する歴史的建造物の構造特性評価

In this research, first of all, we clarified the tendency of the structural characteristics (i.e. building scale and weight, dimension of columns, dimension of walls) of the existing historic buildings in Tochigi City, based on actual survey data of the past. And the vibration characteristics of historic buildings in the northern Kanto region were evaluated by the vibration measurements. Then we evaluated the maximum horizontal proof strength and this base shear coefficient of the warehouse structures. Finally, we proposed the method for estimation of the maximum base shear coefficient from the natural period.

Seismic base shear coefficients of long-period structures subjected to far-field ground motions

Seismic base shear coefficients of long-period structures subjected to far-field ground motions

既存壁式プレキャスト鉄筋コンクリート構造集合住宅建物の基礎と地盤を考慮した地震応答解析

Existing wall-type precast reinforced concrete residential buildings maintain high structural quality, although they were constructed widely in Japan more than 40 years ago. A large number of the residential units in these existing buildings are standardized and typically small. Consequently, they do not suit for modern living uses. In order to utilize these buildings, seismic behavior was evaluated by static pushover analyses in past research. It was concluded that the failure mechanisms are rocking of the walls in the transverse direction and beam yielding in the longitudinal direction. In this research behavior of the foundations was not studied. Past large earthquakes such as Hyogoken-Nambu earthquake in 1995 showed that the WPC buildings maintained high structural seismic resisting performance and the foundations including piles were relatively weak. This fact implies need of analytical study of the seismic behavior of the buildings considering their foundation and soil. It is more true on the evaluation of their performance in the transverse direction, where the behavior of the foundation is more influential to the overall structural behavior than the longitudinal direction. In this research, time-history seismic response analyses of the WPC buildings under large earthquakes with 500 year of the recurrence period are conducted. Two cases of combinations of the soil condition and foundation system are studied. One is continuous footing on hard soil and the other is pile in soft soil. 2-D sway-rocking models were created for the two cases with corresponding sway and rocking soil springs. The superstructure is the model which is identical in the cases and composed of inelastic joint springs and elastic precast concrete panels. Pile stresses are evaluated by applying the seismic deformation method. Findings in this research are shown below: (1) In the study case for the building with the continuous foundation, uplift of the building by the overturning moment (OTM) is observed. It is referred that the failure mechanism is rocking of shear walls associated with failure of the vertical joints in past study of pushover analyses with the fixed-base model. In the time-history analyses, the maximum base shear coefficient (CQ1MAX), which is defined as the ratio of lateral force to the building weight, is 0.73. (2) In the study case with the pile foundation, the piles can fail in shear before the collapse of the superstructure. While CQ1MAX reaches 0.81 with the elastic pile model, it would be lower in studies considering inelastic shear failure of piles. (3) In the study case for the building with the continuous foundation, shear stress of shear walls is no more than 1.8N/mm2, which is lower than probable ultimate shear stress (2.0-2.7N/mm2). Although, in the study case with pile foundation, the shear stress is at most 2.1N/mm2, this is the lower limit of ultimate shear stress and would decrease considering shear failure of piles. Therefore, shear walls would not fail under the large earthquakes, while foundation may be damaged. This analytical finding agrees with the past seismic damage of this type of buildings.

宮崎県の重要伝統的建造物群保存地区（日向市美々津）の伝統木造住宅構造調査

Mimitsu in Miyazaki prefecture is selected to grasp the seismic performance of traditional wooden houses. Seismic performance of 8 houses is evaluated based on microtremor measurements and structural investigation. As results, there are 2 types of traditional houses. One is the type with rooms in a line, and the other is with rooms in 2 lines. The traditional houses have skip floor structure, and the vibration mode is complicated. Yield base shear coefficients of houses with rooms in 2 lines are lower than those of houses with rooms in a line.

Base shear capping buildings with graphite‐lubricated bases for collapse prevention in extreme earthquakes

SummaryDamage or collapse of buildings vulnerable to seismic forces may cause human casualties, and seismic upgrading of such structures is a practical solution to this deficiency. The study presented here proposes a simple approach to prevent structural collapse by separating the superstructure from its foundation to let the superstructure slide during extreme ground shaking. The sliding mechanism contributes to cap the horizontal force exerted on the superstructure. In such approach, the key is to maintain the friction force between the superstructure and the foundation sufficiently low and stable. This research proposes to realize a controlled sliding mechanism, which acts as a structural fuse, by means of carbon powder lubrication at the bases of the structure's columns. The fundamental behaviour of the proposed structural system, named the base shear capping building, is investigated by shaking table tests and numerical simulation. Both experimental and numerical results showed that graphite lubrication is an efficient and robust lubrication material, maintaining the friction coefficient between the steel column bases and mortar foundation at around 0.16. The sliding at the bases significantly reduced the acceleration transmitted to the superstructure, keeping the base shear coefficient not greater than about 0.40. Copyright © 2016 John Wiley & Sons, Ltd.