High Seismic Area Research Articles

Seismic design and performance of multi-tiered steel braced frames including the contribution from gravity columns under in-plane seismic demand

This paper examines the possibility of mobilizing gravity columns in the resistance of in-plane bending moments imposed on the columns of multi-tiered steel braced frames subjected to seismic loading. This would be the case when horizontal struts are used to connect gravity columns to braced frames at every tier level, as is often seen along exterior walls. A seismic design strategy based on the AISC Seismic Provisions is presented for four-tiered prototype steel concentrically braced frames. Three different approaches are proposed for the design of the braced frame and gravity columns. A set of 12 four-tiered X-braced frames, ranging from 15 to 30m in height and located in a high seismic area were designed based on the proposed design approaches. The seismic behavior of the frames is evaluated using nonlinear response history analysis. The results show that the seismic performance of the braced frames is improved as nonlinear seismic demand on the bracing members is reduced when mobilizing the gravity columns for lateral resistance. Furthermore, gravity columns bending moment demands are distributed between braced frame and gravity columns in proportion of their relative flexural stiffness. Adequate seismic performance and cost-effective design can be achieved when columns of both types are designed to resist their respective share of the flexural demand. Alternatively, satisfactory response was achieved when the gravity columns are verified for the seismic induced bending moments acting together with concomitant axial loads.

Collapse analysis of slender masonry barrel vaults

Significant part of the world cultural heritage is represented by masonry buildings. Many of them are characterised by a complex architecture like as the religious buildings and need to be preserved, from a structural point of view, especially in high seismic risk areas. The recent earthquakes showed that the Historical and Monumental buildings are often characterised by high seismic vulnerability. The lowest seismic performances are associated with the presence of thrusting elements like as arches and vaults. In this background, the numerical analyses and experimental simulations provide important information about the structural behaviour of such elements. The structural behaviour of curved elements can be complex to simulate and to predict exclusively by numerical analyses. Indeed, the experimental tests can provide an efficient contribution to the calibration and interpretation of the numerical models.The present work focuses on a particular typology of vaults generally used as roofs in religious buildings. These vaults typically do not include any backfill and are slender. These typologies of masonry vaults cannot be analysed with classical approaches where no-tension is assumed. In fact, the tensile strength must be included in order to assess the seismic capacity of these masonry elements. A simplified analytical model, which includes the tensile strength, was proposed. Validation of the analytical model is provided by comparing predictions of the load capacity and the failure mode with those obtained from previous shaking table test series on two full scale masonry vaults. The proposed method represents a useful modelling tool in order to design dynamic tests on masonry vaults and to assess their vulnerability.

Enhanced stress and changes to regional seismicity due to the 2015 Mw 7.8 Gorkha, Nepal, earthquake on the neighbouring segments of the Main Himalayan Thrust

In this study we evaluate stress evolution and change in seismic hazard after the 2015 Gorkha earthquake sequence. We take a methodology usually used in areas with well-established seismic monitoring and apply it to an area with a sparse dataset and a limited time observation window. Our goal is to validate this approach as a rapid response tool for seismic forecasting after large earthquakes. We propose a long-term seismic forecasting model of the Main Himalayan Thrust using the historical earthquake catalogue and regional paleo-seismicity. Through application of the rate-and-state friction model, we evaluate short-term rate evolution after the Gorkha earthquake. The long elapsed time since the last megathrust event and the mainshock coseismic stress increase on the Main Himalayan Thrust suggest high seismic potential in the Lalitpur and Lamjung areas along the fault system. We also calculate the stress change on optimally oriented planes in the region and model the regional seismicity rate using a smoothing kernel method and seismicity since 1921. The location of the consequent earthquakes coincides with areas of high background seismicity rate and areas where stress was enhanced by the Mw 7.8 mainshock and Mw 7.3 aftershock. We model the change of seismic rate over time and project a fast decrease, due to the short aftershock duration assumption we use.

Fragility functions for eccentrically braced steel frame structures

Eccentrically braced frames (EBFs) represent an attractive lateral load resisting steel system to be used in areas of high seismicity. In order to assess the likely damage for a given intensity of ground shaking, fragility functions can be used to identify the probability of exceeding a certain damage limit-state, given a certain response of a structure. This paper focuses on developing a set of fragility functions for EBF structures, considering that damage can be directly linked to the interstorey drift demand at each storey. This is done by performing a Monte Carlo Simulation of an analytical expression for the drift capacity of an EBF, where each term of the expression relies on either experimental testing results or mechanics-based reasoning. The analysis provides a set of fragility functions that can be used for three damage limit-states: concrete slab repair, damage requiring heat straightening of the link and damage requiring link replacement. Depending on the level of detail known about the EBF structure, in terms of its link section size, link length and storey number within a structure, the resulting fragility function can be refined and its associated dispersion reduced. This is done by using an analytical expression to estimate the median value of interstorey drift, which can be used in conjunction with an informed assumption of dispersion, or alternatively by using a MATLAB based tool that calculates the median and dispersion for each damage limit-state for a given set of user specified inputs about the EBF. However, a set of general fragility functions is also provided to enable quick assessment of the seismic performance of EBF structures at a regional scale.

Pilot Study for Investigating the Cyclic Response of the Recentering Bridge Bearing System Combined with the Friction Damper

The bridge bearing is one of the component members which provide resting supports between piers and decks. The bridge bearing is intended to control longitudinal movement caused by traffic flow and thermal expansion, thereby reducing stress concentration. In high seismicity area, the bridge bearing has been utilized as the base isolation system to mitigate acceleration transferred from the ground. Although the existing bridge bearing installed between superstructure and substructure provides extra flexibility to the base of the entire structure, considerable permanent deformation occurs due to lack of recentering capacity after earthquake. It is required to spend extra cost for repairing impaired parts. The bridge bearings integrated with superelastic shape memory alloy (SMA) devices used for upgrading the recentering effect into the friction damper are proposed in this study. The refined finite element (FE) analyses are introduced to reproduce the response of such new structures under cyclic loading condition. The bridge bearing systems that maintain uniform recentering capability are designed with various friction coefficients so as to examine energy dissipation and residual deformation through FE analyses. After observing FE analysis results, optimal design for the recentering bridge bearing system will be proposed to take advantage of energy dissipation and self-centering capacity.

Structural response of RC frame under seismic loading by varying inclination angle of cross bracing

Structural performance is greatly affected by the use of concrete bracing in high seismic zone areas. A study representing the structural performance of a five-storey RC braced frame structure, by varying the inclination angle of concrete bracing, has been carried out by the use of numerical modelling for an earthquake load relating to a PGA value of up to 0.24 g. To investigate the problem, hypothetical models have been generated and analysed in SAP 2000 software using 3D modelling. Three models with different angle of bracing (i.e., < 45 degree, 45 degree and > 45 degree) are studied and compared with frame structure. Design considerations as per 'UBC-97' and 'BCP-2007' were used and response spectrum analysis has been carried out to observe the structural behaviour under dynamic loading. The parameters considered in this study are lateral displacements, inter-storey drifts, time period, base shear forces and axial forces and bending moments in ground floor columns. It has been concluded that the bracing angle affects the overall performance of structure. RC braced frame model with bracing angle less than 45 degree gives better seismic performance in terms of lateral drift and forces in structural members.

FEM Modelling of the Seismic Behavior of a Tunnel-soil-Aboveground Building System: A Case History in Catania (Italy)

In the design of tunnels it is extremely important to assess the possible damage to the tunnel and to the aboveground structures in order to provide adequate mitigation measures, above all in high seismic hazard areas. Great attention has been devoted to separated tunnel-soil interaction analysis and soil-aboveground structure interaction analysis. Unfortunately, studies involving tunnel plus soil plus aboveground structures (full-coupled analysis) are still very rare. The present paper deals with the seismic response of a tunnel-soil-aboveground building system. The effects of the tunnel on the response of the soil and/or of the building and vice versa have been analysed by means of a full-coupled FEM modelling. A cross-section of the recently-built underground underground in Catania (Italy), including an aboveground building has been analysed and considering the expected scenario earthquake. Results are reported in terms of amplification ratios, frequency ratios, as well as dynamic bending moments in the tunnel. The main goal of the paper is to highlight the combined effect of tunnel and aboveground structure.

Use of explosives to investigate liquefaction resistance of aged sand deposits

Current methods of predicting liquefaction potential were largely developed using data from relatively young deposits or deposits that have been frequently disturbed (i.e., areas of high seismicity). While engineers recognize that these prediction methods are overly conservative for assessing liquefaction potential in geologically aged deposits, there is no widely accepted method for quantitatively accounting for age in these assessments. Because a major disturbance, such as an earthquake or explosion that causes liquefaction, resets a deposit's “geological age,” data from explosive compaction projects in aged deposits are used herein to provide information about both the aged and fresh deposits. A recent explosive compaction project performed in Griffin, IN, as well as four other explosive compaction projects, is used to develop an aging correction relationship for liquefaction resistance. Using a log-linear trend frequently proposed in previous studies, the method proposed herein predicts an approximate 20% gain in liquefaction resistance per time log cycle. The proposed relationship can be used directly if the time since deposition or last disturbance is known or in conjunction with the measured-to-estimated-velocity-ratio (MEVR) relationship proposed by Andrus et al. if the deposit's age is unknown.

Adverse Effects of Soil Grouting on Sandy Soils

Uncontrolled growing of construction industry in Turkey has caused a large number of buildings were constructed without the benefit of sufficient engineering supervision. These phenomena have contributed to the increase in the number of seismically weak buildings. After the Great Marmara Earthquake seismic performances of the existing structures have been inspected and some of them fortified. In this context, an existing building foundation, built on a problematic soil and located in Denizli where is in high seismic risk area, has been strengthened by cement injection. Grouting holes up to 7 m in depth from the basement level were used in 45 points. The injection has created a less permeable barrier, acts like an underground dam. This barrier has caused rising of the underground water table around the recharge area of the foundation. The raised underground water has increased hydraulic gradient and hydraulic pressure. Then, the basement floor has arched up to 12 cm and underground water has risen up to 30 cm high after one year of the injection practice. In this study, hydrogeological characteristics of the soil structural analysis have been evaluated and some solutions suggested remedying the problem. It should be kept in mind as injection practice is employed in aquifers.

EFFECT OF BASE ISOLATION IN MULTISTORIED RC IRREGULAR BUILDING USING TIME HISTORY ANALYSIS

Base isolation (BI) is a technique that has been used around the world to protect the building structures from the damaging effects of earthquake. The installation of isolator in building at base level significantly increases the time period of the structure, which means it reduces the possibility of resonance of the structure giving rise to better seismic performance of the building. The study is performed to compare the effectiveness of base isolation in plan irregular and vertical irregular multi-storied RC frame building. For this study, 15 storied R.C frame building is considered and Time History analysis is carried out using ETABS 2013 software. The Lead Rubber Bearing (LRB) is designed as per UBC 97 code and the same was used for analysis of base isolation system. The results obtained from the analysis were time period and base shear. Time period for the base isolated structures are higher than that of the fixed base structure. Due to the presence of isolator, Base shear is significantly reduced in each direction (X and Y direction) as compared to fixed base building. It has been found that when compared to plan irregular base isolated building the vertical irregular building gives better performance in high seismic prone area by using isolators at the base of the building.

Seismic Loss and Downtime Assessment of Existing Tall Steel-Framed Buildings and Strategies for Increased Resilience

AbstractIn areas of high seismicity in the United States, the design of many existing tall buildings followed guidelines that do not provide an explicit understanding of performance during major earthquakes. This paper presents an assessment of the seismic performance of existing tall buildings and strategies for increased resilience for a case study city, San Francisco, where an archetype tall building is designed based on an inventory of the existing tall building stock. A 40-story moment-resisting frame system is selected as a representative tall building. The archetype building is rectangular in plan and represents the state of design and construction practice from the mid-1970s to the mid-1980s. Nonlinear response history analysis (NLRHA) are conducted with ground motions representative of the design earthquake hazard level defined in current building codes, with explicit consideration of near-fault directivity effects. Mean transient interstory drifts and story accelerations under the 10% in 50-year...

Ground motion observations and simulation for local earthquakes in the Campi Flegrei volcanic area

Ground motion produced by low magnitude earthquakes can be used to predict peak values in high seismic risk areas where large earthquakes data are not available. In the present work 20 local earthquakes (MD∈[−0.3, 2.2]) occurred in the Campi Flegrei caldera during the last decade were analyzed. We followed this strategy: empirical relations were used to calibrate synthetic modeling, accounting for the source features and wave propagation effects. Once the source and path parameters of ground motion simulation were obtained from the reference data set, we extrapolated scenarios for stronger earthquakes for which real data are not available. The procedure is structured in two steps: (1) evaluation of ground motion prediction equation for Campi Flegrei area and assessment of input parameters for the source, path and site effects in order to use the finite fault stochastic approach (EXSIM code); (2) simulation of two moderate-to-large earthquake scenarios for which only historical data or partial information are available (Mw4.2 and Mw5.4). The results show that the investigated area is characterized by high attenuation of peak amplitude and not negligible site effects. The stochastic approach has revealed a good tool to calibrate source, path and site parameters on small earthquakes and to generate large earthquake scenario. The investigated magnitude range represents a lower limit to apply the stochastic method as a calibration tool, due to the small size of involved faults (fault length around 200/300 m).

Seismic Performance of Nonductile Reinforced Concrete Frames with Masonry Infill Walls: I. Development of a Strut Model Enhanced by Finite Element Models

Reinforced concrete (RC) frames with masonry infill walls are prevalent in high-seismicity areas worldwide and have experienced significant damage in earthquakes. This paper proposes a finite element–enhanced strut model to simulate the in-plane seismic response of masonry-infilled RC frames through time-history analysis. The strut backbone defining the behavior of the wall is developed from the response extracted from the finite element (FE) model(s) for the infill and frame configuration of interest. These struts are combined with models capturing flexural and shear failures of beam-columns to simulate building response. The strut model takes advantage of the accuracy of the FE modeling results, yet is computationally efficient for use in nonlinear dynamic analysis. The robustness of the proposed strut model is examined through comparison with experimental results for frames with different failure modes. This modeling approach is used in the companion paper to simulate the collapse response of 1920s-era California frames.

Earthquake Protection of Colonial Bell Towers in Colima, Mexico with Externally Prestressed FRPs

ABSTRACTA methodology for the seismic vulnerability reduction of old masonry towers with external prestressing is presented. It is applied at the Colonial bell-towers of the Cathedral of Colima, Mexico, characterized for being a high seismic area (M>7.5). The 3D FE models are calibrated with experimental data and assessed through nonlinear static approaches including the seismic demand and an accurate validated masonry model. Based on an extensive parametric study on different configurations of old masonry towers, it is selected an optimal prestressing force and device. The Colonial towers are retrofitted with four prestressing devices of FRPs to convert them into a high energy-dissipative reinforced masonry. The external vertical prestressing is included at key points identified in the seismic vulnerability assessment. This technique is in compliance with the demand for architectural conservation and may be located without drilling and unbounded in order to be fully removable. The seismic performance is enhanced by increasing force, displacement, and internal confinement. It is observed an upgrading of 35% and 20% of displacement capacity. With these results it is corroborated that external vertical prestressing allows a substantial increment of ductility for seismic energy dissipation purposes.

Stressing of fault patch during seismic swarms in central Apennines, Italy

AbstractPersistent seismic swarms originate along the normal faulting system of central Apennines (Italy). In this study, we analyze the space‐time‐energy distribution of one of the longer and more intense of these swarms, active since August 2013 in the high seismic risk area of the Gubbio basin. Our aim is to verify if information relevant to constraint short‐term earthquake occurrence scenarios is hidden in seismic swarms. During the swarm, the seismic moment release first accelerated, with a rapid migration of seismicity along the fault system, and suddenly dropped. We observe a decrease of the b‐value, along the portion of the fault system where large magnitude events concentrated, possibly indicating that a fault patch was dynamically stressed. This finding suggests that the onset of seismic swarms might help the formation of critically stressed patches.

Development and urban-scale application of a simplified method for seismic fragility assessment of RC buildings

A simplified analytical method for the seismic fragility assessment of Reinforced Concrete buildings at large scale is presented. The proposed method is based on a simulated design procedure to define the structural model and on non-linear static analysis of a simplified structural model based on Shear-Type assumption to evaluate seismic capacity. Damage States are defined according to the observational-based Damage States provided by the European Macroseismic Scale (EMS-98). Presence of infills is considered, both taking into account their influence on the structural response and evaluating the damage to such non-structural elements. The method is applied to the Reinforced Concrete building stock data provided by the field survey carried out on a city in a high seismic area in Southern Italy, which are illustrated and compared with data from other sources. Uncertainties in seismic demand, material characteristics, and capacity models are taken into account through a Monte Carlo simulation technique. Fragility curves are obtained for each building, leading to the evaluation of annual failure probability at the assumed Damage States. The influence of key parameters in predicting seismic fragility, such as the number of storeys and the age of construction, is illustrated. The spatial distribution of annual failure probability at the different Damage States provides information on areas most prone to seismic risk within the city, depending both on building stock characteristics and on local amplification of seismic hazard due to soil conditions. A comparison with empirical-based fragility curves from literature is also illustrated.

Comparative performance of isolated and fixed-base reinforced concrete structures

The earthquake transmits to the structure a large quantity of energy that causes damage to structures. The seismic isolation technique can absorb a large quantity of the seismic energy. The seismic isolation concept is a new technique in earthquake engineering, its principle is quite simple, and it consists to create a discontinuity between the foundation and the superstructure, so that seismic energy cannot be completely transmitted into the structure. Therefore, this article includes a numerical application of the nonlinear static method, the capacity spectrum method (CSM), on two types of structures, fixed base structure and isolated base structure. The CSM is one of the methods used for the evaluation of seismic performance. Its principle consists in superimposing a curve which represents the capacity of the structure originated from a non-linear static analysis (Pushover), with a curve representing the solicitation brought by the earthquake. The intersection of these two curves represents the point performance, which evaluates the maximum displacement of the structure in the plastic domain. Algeria, the organization of technical control of construction of Chlef (CTC-Chlef) was the first to introduce this technique through the construction of the seat of its agency in Ain Defla, located in an area of high seismicity. The project of the Great Mosque of Algiers, which is under construction, will be built by using this technique. The conventional earthquake resistant designs rely on the strength and ductility of structural elements to resist seismic induced forces and to dissipate the seismic energy, which prevents the collapse of structures in case of earthquake. On the other hand, the approach of base isolation reduces the damages due to the seismic horizontal forces transmitted to the structure (Koh and Kelly, 1985). The objective of this study was to evaluate the seismic response of two structures, a fixed base structure (conventional structure), and an isolated base structure, the method of analysis used in this study is the method of spectrum capacity (CSM). A comparison of the results obtained was made for the two structures, in terms of maximum displacement in the plastic domain (Freeman, 2004). 2. The seismic isolation

Embodied Carbon Based Integrated Optimal Seismic Design for Super Tall Buildings with Viscoelastic Coupling Dampers

With the development of urban construction and building technology, more and more super tall buildings have been built. Due to its huge material and energy assumption, super tall buildings exert great impact on the environment. Embodied carbon is an important tool to measure the environmental impacts of super tall buildings, including the carbon emissions in the process of raw materials processing, structural member manufacturing and transportation. The embodied carbonof super tall buildings could be optimized by integrating the energy dissipation devices in the structural system. Viscoelastic coupling dampers(VCDs) is a kind of efficient energy dissipation devices. By replacing coupling beams in structural configurations, VCDs can effectively increase the level of inherent damping of structures, and thus reduce the wind-induced and earthquake-induced dynamic vibrations. Since the internal forces of structural members subject to lateral loads can also be reduced due to additional damping introduced by VCDs, optimization for the sectional dimension of structural components is made possible, accompanied by reductions to embodied carbon. Embodied carbon based integrated structural design method is introduced in this paper to minimize the embodied carbon of structures by integrating the VCDs. A super tall building located in high seismicity area is presented as an example to illustrate the proposed integrated optimal design method. The design case analysis results for a real super tall building project show that the proposed method is reasonable and can effectively reduce embodied carbon and total cost of super tall buildings.

Seismic performance of precast hollow bridge piers with different construction details

Currently the design scheme of precast hollow concrete bridge piers will be adopted in bridge design in China, but there is no code including specific design details of precast segmental piers in high seismic risk area. For comparative study of seismic performance of the hollow bridge piers which had different design details, six specimens of hollow section bridge pier were designed and tested. The specimens consist of the monolithic cast-in-place concrete bridge pier, precast segmental prestressed pier with cast-in-place joint and precast segmental concrete bridge pier with dry joints. Results show that all specimens have good displacement capacity. The bridge pier with bonded prestressed strands exhibits better energy dissipation capacity and higher strength. The un-bonded prestressed strand bridge pier displays less residual plastic displacement and energy dissipation capacity. The bridge pier with both bonded prestressed strands at the edge of the section and un-bonded in the center of the section not only exhibits more ductility capacity and less residual plastic displacement, but also shows better energy dissipation capacity. Compared with experimental results of prestressed bridge columns, analytical result demonstrates the developed numerical analysis model would provide the reasonable and accurate results.

Near-Field Explosion Effects on the Behaviour of Reinforced Concrete Columns: A Numerical Investigation

The high incidence of global terrorism and accidental explosions have highlighted the vulnerability of built infrastructure to blast loading. Thus, owners of iconic structures are looking for ways to protect their critical assets from the effects of explosions. Many of the research efforts, in this light, have been devoted to the effects of far-field explosions on structural elements. The effects of near-field explosions on structural elements, especially columns, have not been as widely investigated.This paper presents the results of a comprehensive research program designed to investigate the effects of near-field explosions on the behaviour of reinforced concrete columns designed and detailed for regions of different levels of seismicity. The numerical study shows that reinforced concrete columns subjected to blast loading from higher explosive charge masses have higher deflections than those subjected to blast loading from lower charge masses, but at the same scaled distance. Also, the effect of closely spaced transverse reinforcement is significant at smaller scaled distances. Thus, reinforced concrete columns designed and detailed for areas of high seismicity have an inherently high blast resistance.