Showing papers on "Earthquake resistant structures published in 2011"

Damage-Based Design Earthquake Loads for Single-Degree-Of-Freedom Inelastic Structures

Abstract: This paper develops a new framework for modeling design earthquake loads for inelastic structures. Limited information on strong ground motions is assumed to be available only at the given site. The design earthquake acceleration is expressed as a Fourier series, with unknown amplitude and phase angle, modulated by an envelope function. The design ground acceleration is estimated by solving an inverse dynamic problem, using nonlinear programming techniques, so that the structure performance is minimized. At the same time, the design earthquake is constrained to the available information on past recorded ground motions. New measures of the structure performance based on energy concepts and damage indexes are introduced in this paper. Specifically, the structural performance is quantified in terms of Park and Ang damage indexes. Damage indexes imply that the structure is damaged by a combination of repeated stress reversals and high-stress excursions. Furthermore, the use of damage indexes provides a measure on the structure damage level, and making a decision on necessary repair possible. The material stress-strain relationship is modeled as either bilinear or elastic-plastic. The formulation is demonstrated by deriving the design earthquake loads for inelastic frame structures at a firm soil site. The damage spectra for the site are also established, to provide upper bounds of damage under possible future earthquakes. DOI: 10.1061/(ASCE)ST.1943-541X .0000074. © 2011 American Society of Civil Engineers. CE Database subject headings: Ductility; Damage; Optimization; Earthquake loads; Seismic design; Inelasticity; Earthquake resistant structures. Author keywords: Design earthquake loads; Input energy; Inelastic structures; Ductility ratio; Hysteretic energy; Damage indexes; Damage spectra; Nonlinear optimization.

Seismic Design and Behavior of Ductile Knee-Braced Moment Frames

Abstract: The design and behavior of a ductile structural system called a knee-braced moment frame (KBMF) are presented in this paper. The design of this structural system is based on a capacity-design concept that results in ductile behavior. For this system, the frames are designed so that the knee braces will yield and buckle under seismic loads; this is followed by plastic hinging of beams at the ends of the beam segments outside the knee portions. Through this concept, inelastic activities are confined to the designated elements. The knee braces also provide much less obstruction than the braces of conventional systems, making this structural system architecturally attractive. The design concept of this new structural system is addressed first. Results from an experimental study into the seismic behavior of the proposed system are then presented. Two approximately half-scale KBMF specimens were tested. The load-deformation characteristics obtained from the test indicate that the newly developed system can be a...

Effective Stiffness of Squat Structural Walls

Abstract: Reinforced concrete (RC) structural walls are the primary lateral-load carrying elements in many structures designed to resist earthquakes. A review of the technical literature shows considerable uncertainty with regards to the effective stiffness of these structures when subjected to seismic excitations, which many design practices currently deal with by employing a stiffness reduction factor. In an attempt to obtain additional information regarding the stiffness of these structures, an analytical approach, combining the flexure and shear components of deformation, is proposed to evaluate the effective stiffness of the RC walls tested. Based on this proposed analytical approach, a comprehensive parametric study comprising 180 combinations was carried out and a simple equation for assessing effective stiffness of RC squat structural walls then proposed, on the basis of these parametric case studies.

Time-dependent seismic fragility curves on optimal retrofitting of neutralised reinforced concrete bridges

Abstract: The neutralisation (carbonation) of concrete usually results in material deterioration of a reinforced concrete (RC) bridge, so that the seismic capacity of the structure tends to degrade over time. This paper determined the deteriorated plastic hinge properties of the neutralised RC bridge column and performed the pushover analysis to obtain the decayed seismic capacity curves. As a result, the time-dependent fragility curves with respect to some representative damage levels can be established and the possible seismic loss can be expressed as a function of service time. The S-surfaces representing retrofitting cost versus service time for a neutralised RC bridge subjected to different earthquake levels were determined quantitatively in a case study. Throughout the whole life-cycle of a bridge, critical service times corresponding to dramatically increased slopes in the S-surface associated with cost elevations can be identified to assist in the development of a financially optimised strategy for timely seismic retrofitting.

Damage Modeling and Damage Limit State Criterion for Wood-Frame Buildings Subjected to Seismic Loads

Abstract: Current research trends in residential construction focus on the development of performance-based design methodologies for wood structures. As one key prerequisite to the implementation of such design concept, the performance objectives and their corresponding limit state criteria must be properly defined first. So far, the displacement-based limit state criterion, such as the one proposed in ASCE 41, is the only type of standard available for wood structures. In this paper, damage index-based limit state criterion is proposed for wood-frame buildings. The Park-Ang damage model is used to estimate damage to the wood structures caused by seismic loads. An incremental dynamic analysis-based approach is developed to calibrate the damage model parameter for wood shear wall. To predict the potential damage of wood-frame buildings under future earthquakes, the relationship between calculated damage index and observed building damage is established and validated by experimental test data. The results of this res...

Post-earthquake Reconnaissance Report on Transportation Infrastructure Impact of the February 27, 2010, Offshore Maule Earthquake in Chile

Abstract: This report documents the findings and lessons learned from the February 27, 2010, M8.8 offshore Maule earthquake in Chile. Fewer than 0.15 percent of the bridges in Chile’s inventory, most built after 1995, collapsed or suffered damage that rendered them useless. Many spans of precast prestressed discontinuous girder bridges with continuous decks fell off their supports, probably due to significant in-plane rotation of the superstructure as a result of severe shaking. Lateral steel stoppers used to provide both vertical and lateral restraints on girders were largely unsuccessful due to their inadequate connection detail to cap beams and abutments. Reinforced concrete shear keys performed well as fuses limiting the transfer of excessive seismic loads from the superstructure to the foundation of bridges even though they could be optimized for maximum energy dissipation as part of the lateral restraint system at the bottom flange of girders. Vertical seismic bars were widely used to restrain the vertical motion of decks, and they also performed well. Bridge substructures (foundation, column, and cap beam) generally behaved satisfactorily except for two columns that suffered shear failure due to ground settlement and lateral spreading. All mechanically stabilized earth walls exceeded the expected performance.

Cyclic Testing of Large-Scale Rectangular Bridge Columns under Bidirectional Earthquake Components

Abstract: Bidirectional cyclic testing was performed on four half-scale reinforced-concrete rectangular bridge column specimens to examine the need to account for bidirectional seismic loading in design for earthquakes expected in eastern and western regions of North America. The prototype structures are common two-span, skewed bridge structures designed according to the seismic provisions of Canadian Standards Association (CSA)-S6-06. The column specimens are 1.2×0.6 m in cross section and 3.0 m tall and assumed to carry a gravity load of 6% Agfc′. Two specimens were designed for Montreal, Quebec, Canada (east site), using 0 and 30% combination rules, resulting in longitudinal steel ratios of 0.41 and 0.57%. Two specimens represented the column part of bridges located in Vancouver, British Columbia, Canada (west site), with longitudinal steel ratios of 0.97 and 1.72% resulting from the application of 0 and 40% combination rules. Site-specific cyclic displacement test protocols were developed from time-history ana...

ASCE 7-10 Wind Loads

Abstract: ASCE 7-10 Minimum Design Loads for Buildings and Other Structures contains several changes regarding wind loads. The major editorial change is a complete reorganization to a multiple-chapter format as done previously for seismic loads with the objective being to make the provisions easier to follow. Technical changes include the introduction of new wind speed maps to be used with a 1.0 load factor for LRFD and a 0.6 load factor for ASD, the reintroduction of Exposure D for water surfaces in hurricane-prone regions, and revised wind-borne debris regions. A new simplified procedure for buildings up to 160 ft has been added based on the provisions for buildings of all heights.

Analysis of Flanged Shear Wall Using Ansys Concrete Model

Abstract: The frequent occurrence of the major earthquakes in the Indian subcontinent, and construction of tall buildings, especially over the last two decades demands for the construction of earthquake resistant buildings. Many of the tall buildings had collapsed in recent earthquakes and the reasons attributed were poor design and construction practices. The objective of this work is to discuss the possibilities of modeling reinforcement detailing of reinforced concrete models in practical use. To carry out the analytical investigations, the structure is modeled in a Finite Element software ANSYS. The specimens are modeled as (i) discrete model and (ii) smeared model. It reports the results of the analysis of the flanged shear wall with two different types of modeling under cyclic loading. The consequences of small changes in modeling are discussed and it is shown that satisfactory results are obtained from the two models.

Earthquake Resistant Buildings: Dynamic Analyses, Numerical Computations, Codified Methods, Case Studies and Examples

Abstract: Introduction into Earthquake and General Introduction into Structural Dynamics.- Geotechnics and Earthquakes.- Building Codes.- Design Analysis of Low and Medium Rise Buildings.- Testing.- Seismic Analysis and Design of Tall Buildings and Components.- New Technology: Preveting Damages of Buildings.

Experimental Evaluation of Pretopped Precast Diaphragm Critical Flexure Joint under Seismic Demands

Abstract: Precast concrete diaphragm seismic response is examined in experimental research integrating model-based simulation with physical testing. The experimental substructure is the diaphragm critical flexural region of a prototype precast parking structure. This region is expected to undergo significant inelastic flexural deformation, while potentially nonductile regions remain elastic on the basis of capacity design rules from an emerging design methodology. The physical test is conducted at half-scale. The test specimen is detailed using diaphragm reinforcement intended to meet deformability requirements. Predetermined displacement histories are applied to the test specimen on the basis of nonlinear transient dynamic analyses of the prototype structure. The loading history is applied by a test fixture capable of simultaneously providing shear, axial, and moment to the joint. Moment strength, stiffness, rotational deformation capacity, and progressive damage are examined under a sequence of increasing intensity earthquakes. Design recommendations are provided.

Seismic Retrofitting Effects of Reinforcing Over-track Buildings with Knee-brace Dampers

Abstract: The study presented in this paper verifies the seismic retrofitting effects of reinforcing over-track buildings with knee-brace dampers Firstly shaking table tests were carried out on large-scale test models, to simulate over-track buildings reinforced with knee-brace dampers Resulting responses or hysteresis loops, confirmed the seismic retrofitting effects of knee-brace dampers In addition, natural frequencies and damping factors of the models were estimated by ARX model Furthermore, analytical study of over-track building models simulating true structures confirmed the seismic retrofitting effects of knee-brace dampers

Seismic Behavior and Design of Segmental Precast Post-Tensioned Concrete Piers

Abstract: Segmental precast column construction is an economic environmental friendly solution to accelerate bridge construction in the United States. Also, concrete-filled fiber reinforced polymer tubes (CFFT) represents a potential economic solution for durability issues in the bridge industry. Combining the segmental precast and CFFT will result in a rapid durable construction system. The proposed research will build on recent work by the principal investigator (PI), where he experimentally investigated the seismic behavior of tens single-column and two-column bents constructed using precast post-tensioned CFFT. The columns were constructed by stacking precast CFFT segments one on top of the other and then post-tensioned using unbonded tendons. Two specimens had external energy dissipation devices and another two specimens had neoprene in the joints between the CFFT segments. The neoprene significantly reduced the seismic displacement demand. The columns re-centered upon the conclusion of the test resulting in minimal residual displacement, which represents, in the case of a real strong earthquake, a huge advantage since the post-earthquake repair measures will be minimal. 3-D finite element models were developed by the PI to predict the performance of the single-column under monotonic lateral loads. The main objective of this proposal is to improve and expand the capabilities of these finite element models to produce design recommendations. In particular, the models will be expanded to include dynamic loading, two-column bents, and the neoprene in the joints. Including dynamic loading in the model is essential to quantify the energy dissipation due to rocking of the columns segments. The output of this research will be recommendations on the optimum construction characteristics of the system including the segment height/column diameter ratio, neoprene thickness and hardness, external energy dissipater requirements, and post-tensioning force level. The proposed research will develop a durable environmental friendly rapid construction bridge system, which has low life-cycle costs, construction waste, noise, traffic disruption, and initial construction cost. In addition, the developed system will has high work zone safety, efficient use of construction material, a short construction time, and improved constructability. The proposed construction system will not have a leakage of wet concrete into waterways leading to pollution of water and harm migrating fish. Finally, when the proposed construction system is fully developed and implemented in construction, it will reduce the expense of bridge replacement, repair, and continuous operation interruption after earthquakes.

Design, Analysis, and Seismic Performance of a Hypothetical Seismically Isolated Bridge on Legacy Highway

Abstract: The need to maintain the functionality of critical transportation lifelines after a large seismic event motivates the strategy to design certain bridges for performance standards beyond the minimum required by bridge design codes. To design a bridge to remain operational, one may stiffen and strengthen the load carrying members to increase the capacity, or alternatively use response modification devices such as seismic isolators to shift the dynamic characteristics of the bridge, henceforth reducing the seismic demands. Seismic isolation systems are attractive because they are directly conducive to accelerated bridge construction techniques. The two strategies are compared for a typical Utah highway bridge, using a three-span, pre-stressed concrete girder bridge that crosses Legacy Highway as a case study example. The existing Legacy Bridge, which was designed as a Standard bridge for a 2500-year return period earthquake, is evaluated as an Essential bridge for a 1000-year return period earthquake. Subsequently, this bridge is redesigned and evaluated as a seismically isolated bridge. Configuration changes needed to accommodate a seismic isolation system are discussed, and reductions to column and foundation elements are proposed. Example seismic isolator designs are provided for several different types of isolation systems commonly used in the United States. Inspection and maintenance practices for seismically isolated bridges are discussed.

Seismic performance assessment based on damage of structures, Part 1: Theory

Abstract: The paper presents methodology for safety assessment and design of earthquake resistant structures based on application of damage spectra. The damage spectrum can be used for seismic evaluation of vulnerability of structures with given properties and can provide information of damage potential of the recorded ground motions. Damage spectrum represents a variation of a damage index versus structural period for a single-degree-of-freedom system subjected to an earthquake ground motion. The improved damage index, based on plastic deformation and hysteretic energy dissipation, is applied. It depends on maximal plastic deformation, ductility capacity and function including cumulative damage effects. This function, besides the parameter including influence of deterioration, depends on the history of cyclic deformations and on both cyclic and accumulative ductility.

FEMA P-58-Next-Generation Performance Assessment of Buildings

Abstract: In 2001, the Applied Technology Council (ATC) initiated the ATC-58 Project under contract to the Federal Emergency Management Agency (FEMA). Under this project, ATC is developing next-generation performance-based seismic design criteria for new and existing buildings that will be published as FEMA P-58. Unlike present-generation performance-based design criteria, which characterize structural and nonstructural performance in terms of standardized damage states, the P-58 criteria characterize structural performance in terms of the consequences of earthquake damage including: casualties; repair and reconstruction costs, expressed both in monetary and energy/carbon expenditures; and occupancy interruption time. The performance of nonstructural building components including architectural, electrical and mechanical components and systems is integrally considered with that of the structure. Following a methodology originally developed by the Pacific Earthquake Engineering Research Center, the P-58 procedures revolve around a building performance model that includes fragility and consequence functions for each of the damageable components and systems of significance. Monte Carlo simulation procedures are used to assess potential losses for: individual earthquake shaking intensities; scenario earthquake events, considering uncertainty in ground motion intensity; and, over a period of time, considering uncertainty in the magnitude and distances of scenario events that will occur. Project completion is scheduled for late 2011.

Collapse simulation and analysis of infill walls for earthquake-resistant structures

Abstract: To investigate out-of-plane behavior of infilled frame due to different construction measures,collapse simulation of the structure is carried out by LS-DYNA software.The influences of construction measures of infilled frame and grades of mortar are researched,respectively.The results show that the collapse prevention capacity of infill walls is effectively improved by increasing the grade of mortar and setting steel tie bars.The space of tie bars is advice to reduce when infilled structures are beyond the eighth seismic precautionary intensity.Other reasonable measures can be also adopted to protect infill walls from collapse and ensure the safety of people's life and possessions.

EPS Seismic Buffers for Earthquake Load Attenuation against Rigid Retaining Walls

Abstract: Expanded polystyrene (EPS) geofoam is an ideal material to construct seismic buffers to reduce earthquake-induced loads against rigid retaining wall structures. The paper provides a synthesis of published work by the writers on both experimental and numerical simulation work on EPS seismic buffers. Proof of concept is first demonstrated using a series of 1 m-high models placed on a shaking table. The tests showed that for the geofoam materials employed, a reduction in peak dynamic loads as great as 40% was possible compared to the rigid wall case. Next, a FLAC computer model was developed to simulate the experimental results. The validated model was then used to carry out a parametric analysis to investigate the influence of wall height, EPS geofoam type, thickness, stiffness, and excitation record on seismic buffer performance. One of the important outcomes of this research is that the practical quantity of interest to attenuate dynamic loads using a seismic buffer is the buffer stiffness defined as K = E/t (E = elastic modulus, t = thickness). For the range of parameters investigated in this study, K ≤ 50 MN/m 3 was observed to be the practical range for the design of these systems to attenuate earthquake loads.

Decision-making on economic maintenance of earthquake-resistant reinforced concrete structures

Abstract: It is necessary to repair or reinforce earthquake-resistant reinforced concrete structures during their service time since their safety performance may gradually deteriorate due to environmental factors such as concrete carbonation, steel erosion, natural aging, or earthquake damage. This paper adopts a benefit-cost analysis and puts forward an economic decision-making method on preventive maintenance, pre-earthquake reinforcement, and post-earthquake repair and dismantling of structures, with consideration to earthquake damage accumulation, natural aging, and durability damage. It is of crucial theoretical significance and application value to the decision-making on the maintenance of structures.

Seismic Analysis and Rehabilitation of a 15 Story Pre-Northridge Steel Moment Resisting Frame Building

Abstract: This paper summarizes the seismic analysis and rehabilitation design of a fifteen story steel moment resisting frame building in Oakland, California, less than five miles from the Hayward Fault The building was constructed with connection details that were found to be vulnerable to fracture in the 1994 Northridge earthquake Previous testing of the existing connections indicated that the welded connections were highly vulnerable to fracture, and consequently the building presented a risk to life safety in the event of a major earthquake Seismic strengthening to meet the State of California Guidelines for the Seismic Retrofit of State Buildings Risk Level III resulted in a requirement to reduce the story drifts to 15% in a 475-year return period event After considering several retrofitting schemes including connection strengthening, BRBF's and seismic isolation, a scheme that included strengthening sixty percent of the existing pre-Northridge moment frame connections and adding viscous dampers was selected Since the existing moment connections had performed so poorly in previous testing, and because the size of the members in this building exceeded those that had been previously tested, a series of four full-scale tests were performed to confirm the validity of the connection upgrade design Two phases of advanced analysis techniques were performed in the rehabilitation design; a study phase with multi-mode two-dimensional nonlinear pushover analyses along with single-degree-of-freedom nonlinear dynamic time-history analyses; and a working drawings phase with nonlinear response history analyses of building frames to refine the scheme and perform member checks The extra steps taken beyond typical engineering practices were intended to provide better assurance that the project's performance goals would be met during the design basis seismic event