20th Analysis and Computation Specialty Conference

The current state-of-practice in the U.S., and elsewhere, for designing elastomeric seismic isolation bearings utilizes closely spaced intermediate steel shim plates with thin rubber layers that result in shape factors ranging from 15 to 30. Such high shape factors (HSF) produce a vertical stiffness several thousand times larger than the horizontal stiffness thereby providing isolation only in the horizontal plane (2D). While the large vertical stiffness has been thought to be desirable to minimize rocking in slender structures with an elevated center of mass it also results in a lower period of vibration in the vertical direction that can align with the dominant frequency content of the vertical component of earthquake ground shaking. A low shape factor (LSF) bearing concept to achieve three-dimensional (3D) isolation was explored in the past for the nuclear industry. Though this research demonstrated, through analysis and a prototype design, that the LSF concept could effectively provide isolation in both the horizontal and vertical directions, system level testing and implementation were never realized. This paper presents the results of an analytical, parametric, study aimed to further explore the low shape factor concept to achieve three-dimensional isolation. The results of this study suggest that 3D isolation might be achieved for low and mid-rise structures using the LSF concept if the bearing shape factors are less than four and supplemental vertical damping is included at the plane of base isolation.

This paper presents the process for establishing and validating the design properties of Triple Pendulum^TM seismic isolation bearings which were utilized to achieve the superior seismic performance of the New San Bernardino Court facility. The analysis parameters of the nonlinear elements used in the analysis model representing the seismic isolation bearing properties were confirmed through a prototype bearing test program and specification completed during the construction document design phase. The prototype test program and specification was developed considering ASCE 7-05 Chapter 17 requirements and preliminary structural analysis results including horizontal and vertical force-displacement characteristics, as well as axial load demands on the bearings. The final prototype test results verified the characteristic properties of the bearings at various vertical loads and displacements at real-time earthquake motions and the results validated the assumed properties used in the structural analysis and design. The real-time dynamic test program extends current ASCE 7 prescriptive requirements.

A 5-story steel moment frame building was tested at E-Defense in August of 2011 with three different support configurations: supported by a triple friction pendulum isolation system, supported by lead rubber bearings in combination with cross linear bearings, and in the fixed-base condition. Nonstructural components and contents were installed on the 4th and 5th floors. The isolated buildings were subjected to strong excitations with the goal to approach the displacement limit of the base-isolation devices. The triple friction pendulum system was subjected to a variety of large ground motions, but did not reach its displacement limit as the friction was observed to be larger than during initial bearing characterization. The lead-rubber isolators were subjected simultaneously to large displacements and some tension. Nonstructural component damage and content disruption due to strong vertical excitation was observed in both isolation systems and in the fixed-base configuration.

Many simple structures yield and develop inelastic deformations, while the accelerations are limited by the yield strength. These structures develop permanent deformations (and possible damage). Some structures, which employ special materials or innovative structural systems, may yield and recover without damage. These are known as nonlinear-elastic structures. Often the post yielding excursions are very large and may exceed the deformability capacity of the structure. Introducing damping in form of supplemental devices, it is possible to control such deformations and keep them within acceptable limits. The modified systems can be designed using control theory and implemented approximately while minimizing the difference between the required control forces to the supply provided by protective devices and damping. While the above design method may require special skills in control of structures, a spectral analysis method can be applied instead, with good approximations. The paper presents the approach, the model for the design of the control system and the design spectra needed for approximate computations. The procedure is exemplified using a three stories moment resisting frame model that shows the efficiency of negative stiffness devices added to "emulate" nonlinear-elastic behavior and contribution of dampers to control deformations during "yield excursions".

This study investigates the effects of anchored constrained VE layers on the flexural response of simply-supported Euler beams under base excitations. Two surface damping treatments were considered: one VE layer anchored at one end, and two VE layers anchored at their different ends. Each anchorage is realized with a thin stiff layer in tension, such as a fiber reinforced polymer sheet, bonded to the surface of a VE layer and anchored to one end of the beam for maximum shear deformation in the constrained VE layer. Non-uniform shear deformation in VE layers is taken into account in the new solution formulation. Sensitivity analyses are performed to understand and quantify the effects of various parameters on flexural responses of the structures. The performances of various configurations are compared and the two-end anchored configuration is found most effective in vibration suppression.

Previous research has shown the effectiveness of integrated redesign of civil engineering structures with control systems. A procedure for design of controlled structural systems was developed using a two-stage approach: (1) a design of an optimal control system using a Linear Quadratic Regulator (LQR) algorithm and (2) a redesign using an optimization procedure to match the performance of the controlled system obtained in (1). However past research was applied to fixed based structures considering relatively stiff soil conditions. It has been suspected, though, that using such techniques in soft soil sites while considering soil structure interaction, may diminish some of the advantages observed in past research. This paper examines the effect of soil-structure interaction on the seismic performance of integrated redesign. A linear SDOF steel portal frame, a 6 story building and a linear 9-DOF shear-type structure with different soil types and with a rigid surface foundation have been considered analyzing soil-structure interaction effects.

The cast steel Yielding Brace System (YBS) is a new device for enhanced seismic performance of concentrically braced frames that is characterized by its full, symmetric stable and repeatable hysteretic response and its large ductility capacity. The YBS system is comprised of two specially designed cast steel connectors that are welded to one end of each diagonal brace in a building's lateral force resisting system. The axial force in the brace is transferred to the lower beam-column joint through triangular-shaped flexural yielding fingers. In order to develop the YBS concept and successfully design and test a full-scale prototype several small scale tests were conducted on specimens similar to the yielding fingers of the YBS. The result of this test series was an experimentally validated low-cycle fatigue prediction model using the Coffin-Manson relationship and Minor's rule. This prediction model was used to design a full-scale YBS prototype with a nominal brace yield load of 250 kips (1110kN). The prototype was full-scale tested in a frame to simulate actual in-situ boundary conditions with a pseudo-dynamic test protocol derived from Appendix T of AISC 341. In addition, a non-linear cyclic model was developed to predicted the cyclic response of the YBS and enable the modeling of an entire building equipped with this device. This model was validated through comparison with the full-scale prototype tests.

Weakening and damping of structures has proven to be an effective method for mitigating the structure's response. This approach has drawn further attention after the invention of negative stiffness device (NSD), developed by the authors. Preliminary analytical and experimental studies reported on the NSD have revealed that by adding the NSD to a single story structure the base shear demands and peak acceleration of the main structure are reduced significantly and the inter-story deformations are contained by adding a passive damper. In this paper an analytical study is carried on an inelastic multistoried shear building to demonstrate the effectiveness of placing NSDs and dampers at multiple locations along the height of the building. It has been shown that by placing a NSD in a particular story the superstructure above that story can be isolated. It has also been shown through simulation studies that the NSD will limit the amount of energy transmitted to the superstructure from the ground excitation. Essentially, NSD acts as a vibration isolator. Large base deformations is one major limitation in base-isolating the structural systems but using NSDs this can be overcome as the isolation is achieved over the height of the building and not confined to the base. It has been shown through the simulation studies that by placing NSDs in all the lower story's the acceleration of the superstructure and base shear can be reduced significantly without affecting the drifts. Simulation results of a nine-storied 1:3 scale inelastic shear building subjected to periodic ground motion and Kobe fault normal ground motion demonstrate the response of the multi-storied frame can be attenuated by incorporating NSDs.

Supplemental energy dissipation in the form of nonlinear viscous dampers is often used to improve the performance of new structures or to retrofit the existing ones. The optimal amount of nonlinear damping is needed to achieve the desired performance of inelastic structures. Using dimensional analysis, this paper proposes a dimensionless nonlinear damping ratio that decisively quantifies its effects on responses (e.g. drift and total acceleration) of inelastic structures subject to pulse-type near-fault ground motions. It is demonstrated that the added nonlinear damping is not always beneficial for inelastic structures, resulting in the increase of their total accelerations under certain ground motions. A critical structure-to-pulse frequency ratio exists, under which an optimal nonlinear damping needs to balance between the increases of total acceleration and the reductions of the structural drift.

Situations that justify structural measurements continue to increase in number and in importance. Measurement-data interpretation has become the principle scientific challenge for such activities. This paper summarizes a study of quantifying modeling uncertainties using the Grand-Mere Bridge located in Canada. This case study demonstrates that modeling errors involve systematic biases and that these biases can affects data interpretation and ultimately, the quality of infrastructure-management support. The impact of systematic errors on traditional data-interpretation approach is discussed.

This paper presents the strain sensing capability of a wireless and batteryless smart-skin sensor array. The sensor design is based on a folded patch antenna. When the patch antenna is under strain/deformation, its resonance frequency varies accordingly. The frequency variation can be easily interrogated and recorded by a wireless reader based on a backscattering mechanism. The patch antenna utilizes an inexpensive off-the-shelf radiofrequency identification (RFID) chip for signal modulation and anti-collision, in order to avoid interference among multiple sensors. Since the RFID chip harvests electromagnetic energy from the interrogation signal emitted by the reader, the patch antenna itself does not require other (internal) power source and, thus, serves as a batteryless and wireless strain sensor. In this preliminary investigation, several prototype folded patch antennas have been designed and manufactured to form a wireless strain sensor array. Tensile testing results of the wireless strain sensor show strong correlation between interrogated resonance frequency and strain experienced by the sensor. Minimum interference is observed among multiple sensors in the same neighborhood, when interrogated individually by a reader.

While integrated health monitoring systems are often presented as a solution to the national infrastructure crisis, this project adopts a unique variation on this theme by actively seeking to remove barriers that have precluded non-expert citizen's direct use of these systems. Often the expense and need for expert involvement in most structural health monitoring programs means that the vast majority of bridges, including those that are structurally deficient and in greatest need of assessment, cannot benefit from these efforts. By recognizing that our target end users have limited budgets and large inventories of bridges, this project delivers to these end users a rapidly re-deployable, self-organizing wireless sensor network. However, by providing end users full freedom to deploy these units at will, this project must do something all competing efforts have not: operate without full knowledge of where sensors are placed not only in detecting damage but also in relaying that information through a wireless network. This paper focuses specifically on how this wireless network will be activated in a restricted fashion, the hardware necessary to do so, and the sensing elements that will be activated to detect damage packaged for rapid, repeated, re-deployment by end users through autonomous localization.

This paper presents the numerical simulation of novel Micro-Electro-Mechanical- System (MEMS) based piezoresistive strain sensor in order to monitor the localized strain in structures such as crack tips. The MEMS strain sensor developed in this study is made of single crystal silicon, and has 100 μm width and 400 μm length which prevents the strain gradient effect at the vicinity of stress concentration regions. The device has a novel U shape configuration where the open side of the U shape faces the crack tip and provides additional strain amplification. The silicon has piezoresistive property, which can have a gage factor reaching up to 165. Rigid substrate effect of silicon package on the strain sensor has been minimized based on the geometrical feature which has been used to amplify the stress in the sensor. The MEMS strain sensor has been optimized using COMSOL Multiphysics software, which enables the combination of multiple physics solutions in a single model, which are structural and electrical in this study.

The Rock Island Arsenal Government Bridge, built over the Mississippi River in 1896 between Rock Island, IL and Davenport, IA, is just one of over two hundred bridges owned by the United States Army. The swing span of the Rock Island Arsenal Government Bridge has the ability to rotate 360° in either direction and can lock each end of the span on either abutment. The bridge carries highway and rail traffic; the swing span allows for the passage of barge traffic on the river. The Army regularly inspects and maintains their bridges to ensure their functionality. To supplement the regular inspections of the swing span of the Government Bridge, the US Army Engineering Research and Development Center (ERDC) has installed a structural health monitoring system composed of both a fiber optic sensor network and a wireless smart sensor network. This multi-framework system measures strain, acceleration, and orientation and uses these metrics to perform structural health monitoring of the bridge. The monitoring is designed to automatically record the measured changes in strain caused by swing events and record the accelerations measured during train events and other scheduled times of day. The integrated monitoring system has been successful in recording the necessary multimetric sensor data and using it to monitor the swing span of the Government Bridge.

Time history analysis of structures for seismic loads requires a set of simulated design ground motions for this application. Spectral matching is the process of adjusting an actual earthquake accelerogram to match a target design spectrum. This procedure is very favorable for the development of design ground motions due to the evolution of worldwide network of seismic monitoring stations and hence the availability of many actual earthquake ground motion databases that reflect variability of important seismological parameters such as magnitude, distance, and rupture directivity. A time-domain spectral matching procedure is presented in this paper, wherein a recorded accelerogram is decomposed into its wavelet components at different frequencies through a continuous wavelet transform algorithm using a mother wavelet, and scaled iteratively such that the resultant time history is compatible with the target spectrum. It is shown that the Morlet wavelet can be used successfully to represent the mother wavelet for spectral matching. The procedure is illustrated by scaling a number of earthquake records to match the ASCE-7 Standard design spectrum at different seismic zones.

The nonlinear modal analysis (NMA) methodology for analyzing moment-resisting framed structures is here extended to include frame elements with rigid-end offsets. Four key components of the NMA methodology for conducting dynamic analysis of nonlinear framed structures include: (1) the force analogy method for material nonlinearity; (2) stability functions for geometric nonlinearity; (3) the state space method for dynamic analysis; and (4) modal superposition for computational efficiency. Under this NMA methodology, the nonlinear stiffness matrices capturing full panel zone rigidities and centroidal locations of plastic hinges in moment-resisting frames are completely derived in conjunction with satisfying global equilibrium, joint compatibility, and element force-displacement relationship. Nonlinear dynamic analysis of a 10-story steel frame with rigid-end offsets is then performed and results are compared with those obtained using Perform-3D (P3D) commercial software. Good agreement between NMA and P3D results indicate accuracy of the proposed methodology even though it is based on different assumptions.

The objective of this paper is to present new attenuation relationships for free-field peak ground accelerations (PGA) using a fuzzy methodology approach. The used database was compiled from recent earthquakes in Taiwan and USA with magnitudes 5 or greater; and recorded on three different site conditions classified as rock, soil and soft soil. Uncertainty is a condition associated with essentially all aspects of earthquake related science and engineering. The main sources of uncertainty in the estimation of ground motion lie in the characterization of site geology, calculation of closest distances, determination of seismic shaking properties, and in the geotechnical properties of earthquake motion monitoring sites. However, fuzzy logic and statistical analysis can be used to quantify the uncertainties associated with these fundamental features. Fuzzy sets are developed as fuzzy model inputs by dividing the local geologic conditions into three groups as rock, soil and soft soil; epicentral distances into three groups as near, intermediate and far, whereas the output is the horizontal components of peak ground accelerations (PGA). The prediction model is tested with an independent data set for its successful prediction capability. The combination of fuzzy sets to describe natural language and statistics to quantify uncertainties inherent in earthquake ground motion lead to a rational analytical tool. This approach can be used to assess the potential variability in parameters that influence the earthquake intensity.

The characterization of seismic hazard through stochastic ground motion models, providing a description of the entire earthquake-acceleration time-history, is discussed in this paper. A probabilistic framework is presented for assessment of seismic risk and an innovative sensitivity analysis is discussed for identification of the importance of the various risk factors within this context. An illustrative example is presented that considers risk quantification in terms of the life-cycle repair cost, implementing an assembly-based vulnerability approach to calculate the latter.

Structural analysis and design is often conducted under the assumption of rigid base boundary conditions, particularly if the foundation system extends to bedrock, though the extent to which the actual flexibility of the soil-foundation system affects the predicted periods of vibration depends on the application. While soil-structure interaction has mostly received attention in seismic applications, lateral flexibility below the ground surface may in some cases influence the dynamic properties of tall, flexible structures, generally greater than 50 stories and dominated by wind loads. This study will explore this issue and develop a hybrid framework within which these effects can be captured and eventually be applied to existing finite element models of two tall buildings in the Chicago Full-Scale Monitoring Program. It is hypothesized that the extent to which the rigid base condition assumption applies in these buildings depends on the relative role of cantilever and frame actions in their structural systems. In this hybrid approach, the lateral and axial flexibility of the foundation systems are first determined in isolation and then introduced to the existing finite element models of the buildings as springs, replacing the rigid boundary conditions assumed by designers in the original finite element model development. The evaluation of the periods predicted by this hybrid framework, validated against companion studies and full-scale data, are used to quantify the sensitivity of foundation modeling to the super-structural system primary deformation mechanisms and soil type. Not only will this study demonstrate the viability of this hybrid approach, but also illustrate situations under which foundation flexibility in various degrees of freedom should be considered in the modeling process.

Inspired by the seminal paper by Michell (1904), engineers have investigated several tools for the optimization of structural shapes and systems to be employed during the design process. Structural optimization attracts increasing interest in the building industry, especially in the design of high-rise buildings. By selectively distributing the structural members in the building, the efficiency of the resulting design can be optimized; often aesthetically pleasant form can be achieved. Although Michell trusses represent a valuable starting point in defining optimal structural systems, analytical solutions have been derived only for relatively simple load conditions and geometries. Recently, along with the increased computing power, efficient numerical methods have been developed to generate optimal structural solutions for various problems. This paper presents recent developments in optimization methods which have been used for the design of novel structures. Concepts of growth, genetics, and emergence are introduced as context into the fundamental concepts presented. Growth patterns in nature can service as guidelines of how an optimal structure might be conceived. Genetics provide an understanding of how the blueprint of life, DNA, can be used to develop optimal solutions using genetic algorithms. Emergence theory suggests that interactions among parts serve to create something that is not realized until individual parts work in concurrence to comprise the whole. This concept is displayed through topology optimization. Several high-rise structures are described as examples of how these concepts can be applied to the development of structural systems.

In recent years, the building industry has seen vast technological developments. All projects are handled primarily in digital CAD and BIM environments, and three-dimensional modeling is becoming an industry standard. Moreover, the concept of integrated project delivery (IPD) is spreading, as organizations such as the AIA have endorsed the IPD system. In short, IPD relies on new technologies, and leverages early contributions of knowledge and expertise, to save time and reduce requests for information and change orders during the construction phase. If the project is planned thoroughly early on, and utilizes emergent technologies, the project owner can save time and cost in the construction process.

Topology optimization is a systematic, free-form approach to the design of structures. It simultaneously optimizes material quantities and system connectivity, enabling the discovery of new, high-performance structural concepts. While powerful, this design freedom has a tendency to produce solutions that are unrealizable or impractical from a structural engineering perspective. Examples include overly complex topologies that are expensive to construct and ultra-slender subsystems that may be overly susceptible to imperfections. This paper summarizes recent tools developed by the authors capable of mitigating these shortcomings through consideration of (1) constructability, (2) nonlinear mechanics, and (3) uncertainties.

Several practical tools for the characterization of the optimal layout of a truss system have been developed in recent years based on fundamental theorems like Maxwell's and Michell's. The paper will revisit such theorems and demonstrate their relevance in optimal truss system design. In addition, the latest findings on the geometric aspects of optimum trusses will be presented in relation to the three-point problem for a cantilever truss and more general three-force problem.

Though topology optimization has been applied to many fields, ranging from mechanical to aerospace engineering, more work must be done to tailor it to the needs of the structural engineer, especially in regards to the design of high-rise buildings. Thus, this work aims to improve its application to structural engineering by describing an integrated topology optimization approach involving continuum and discrete finite elements to design the lateral systems in structural braced frames for high-rise buildings. The approach is implemented using concurrent continuum finite elements and discrete beam/truss elements to simplify and improve the overall design process by creating optimal geometries for a given volume of material. For example, after an engineer develops a structural frame consisting of beams and columns sized for gravity loads, topology optimization on the continuum (e.g. quadrilateral) elements is used to create a conceptual design for the braces of the lateral system resulting in highly efficient structures. Several practical examples are demonstrated to show the importance and relevance of this work to the structural design industry.

Modern formulation of structural optimization was initiated in 1960. During the evolution and maturation of this design methodology some researchers promoted the idea of generating a set of problems to test the performance of numerical algorithms for different classes of optimization problems. Recently the Technical Committee of optimum Structural Design of ASCE agreed upon defining a new set of benchmark cases of structural optimization showing, as much as possible, resemblance with real engineering problems in order to show the applicability of optimization methodologies. As a part of that set the volume minimization of the cables material of a cable stayed bridge is presented. The bridge has a main span of 658 m and is composed of 160 cables. Geometrical and mechanical data of each bridge element are very similar to many structures of this type commonly present in real engineering design.

What is structural optimization? The answer depends on who you ask. One practicing engineer will likely have a different answer than an engineer in academia or a representative in the structural engineering software community. That said, two different practicing engineers will likely have somewhat different views, as would two different professors or software vendors. The authors of this paper represent private practice and the software industry. Several benchmark problems and design tools in structural optimization (some existing in the literature and a couple of new ones) will be reviewed and discussed. The paper will discuss how a practicing engineer would formulate a benchmark problem in optimization, from choosing the appropriate design variables (discrete, continuous, etc.), determining the constraints (such as limits on strength and/or deflection), to defining the desired objectives (least weight, least cost, etc.). There are numerous optimization methods well known for solving benchmark problems, i.e., gradient based methods (such as Newton's Method), population based methods (such as genetic algorithms) as well as other search methods. A practicing engineer, however, may choose to use more conventional software to solve the problem. The paper will investigate the solutions found using conventional structural software and discuss some of the capabilities of existing software to truly perform optimal design.

Structural systems for tall buildings have evolved to produce higher lateral stiffness more efficiently, and the efficiency of a structural system is significantly influenced by its geometric configuration. Therefore, once a particular structural system is selected for a tall building, it should be configured very carefully to maximize its structural efficiency and, at the same time, satisfy other non-structural design requirements integratively. This paper investigates optimal configurations of today's prevalent structural systems for tall buildings. Among various structural systems developed for tall buildings, the systems with diagonals are generally more efficient because they carry lateral loads by their primary structural members' axial actions. When the primary lateral load resisting system is located over the building perimeter, the system's efficiency can be maximized. Tall building structural systems with perimeter diagonals include braced tubes and more recently developed diagrids. Braced tubes of various column spacing and diagonal configurations are comparatively studied. Diagrid structures of various uniform and varying angle diagonals are studied to determine more efficient configurations. Another structural system widely used for today's tall buildings is outriggers structures. Optimal stiffness distribution between the building core and perimeter mega-columns, connected to the core through outrigger trusses, is investigated for outrigger structures.

In this paper, consideration is given to candidate benchmark problems that utilize continuum topology optimization to find long-span structural forms that maximize stability. It is posited that by beginning the structural design process with forms of optimal stability as opposed to forms that are inherently unstable, one avoids getting locked into inferior designs that will be problematic at all stages. This issue is examined here by introducing some problems in which the goal is to obtain sparse lightweight structural forms that maximize critical buckling loads computed from linearized buckling analysis.

This paper presents an overview of a proposed tall building design platform that has the aim of determining the optimal external shape and structural system for tall buildings subject to aerodynamic loads. The platform is intended to bridge the gap between the traditionally manual conceptual design stage and the more automated detailed design stage in an attempt to define a new generation of innovative tall buildings. Shape optimization will be used to sculpt the optimal external profile of the building, topology optimization will be utilized to determine the optimal configuration of the structural system, and reliability-based optimization algorithms will be used in the detailed design stages. This paper presents the results of several preliminary studies concerning the topology optimization stage of the platform, performed on simplified building structures, to highlight the concept and benefits of this approach.

Real-time hybrid simulation provides an economical and efficient experimental technique for performance evaluation of structures under earthquakes. A successful real-time hybrid simulation requires accurate actuator control so as to achieve reliable experimental results. The time delay due to servo-hydraulic dynamics, if not compensated properly, would lead to inaccurate or even unstable simulation results. The nonlinearities in both servo-hydraulic systems and experimental substructures however make the actuator delay difficult to be accurately estimated in practice. Actuator control therefore presents a great challenge for structural application of real-time hybrid simulation technique. This paper presents a recently developed adaptive compensation technique for real-time hybrid simulation. Adaptive control laws are established to minimize both phase and amplitude errors in the servo-hydraulic actuator response. A large-scale magneto-rheological damper subjected to band-limited white noise generated random displacement and variable current input is utilized to experimentally demonstrate that the proposed adaptive scheme enables accurate actuator control to be achieved by reducing not only the tracking error but also the associated energy error.

Real-time hybrid testing (RTHT) has become a promising alternative to reduce the cost and operational demands when evaluating the performance of damping systems for seismic response attenuation in building structures. While damper devices can be physically tested, buildings can be represented with computational models to avoid the expense of testing a large, integrated specimen. Therefore, reliable simulation tools are needed to accurately recreate the behavior of the computational component under real-time execution demands; also, appropriate control schemes for testing the experimental component using hydraulic actuators are required for high fidelity RTHT implementation. In this paper, a novel RTHT platform appropriate for the study and evaluation of damped buildings is presented. The main components of the platform are described, including the computational tool for performing non-linear dynamic analysis of steel frames and the advanced robust control strategy for the hydraulic actuators. Additionally, initial results of a validation experiment using a re-configurable steel moment-resisting frame are presented to demonstrate the accuracy and efficiency of the proposed RTHT framework.

Real-time hybrid simulation combines physical testing (experimental substructuring) and numerical simulation (analytical substructuring) such that the dynamic performance of the entire structural system can be considered during the simulation. A grid-based real-time hybrid simulation technique is introduced as a means to perform real-time hybrid simulations of complex structural systems where the analytical substructure poses a large computational demand. Real-time hybrid simulations of the 9-story ASCE benchmark structure with large-scale magnetorheological (MR) dampers are performed at the Lehigh NEES Equipment Site to demonstrate the multi-grid real-time hybrid simulation procedure and illustrate the ability to significantly reduce the time to perform the state determination of the analytical substructure. The 9-story building structure is modeled as the analytical substructure, with the experimental substructure consisting of large-scale MR dampers that are located in the structure. The analytical substructure is divided into two parts and implemented onto a computational grid consisting of two parallel xPCs that run MathWorks real-time Target PC software package. The restoring force data from these two xPCs are synchronized together along with the measured damper forces from the experimental substructure, and processed in a real-time manner for each time step of the hybrid simulation. The results of real-time hybrid simulation are compared to those of numerical simulations to validate the new test methodology.

Engineered cementitious composites (ECC) are a fiber-reinforced high-performance material developed as a replacement for conventional concrete. ECC is characterized by high damage tolerance, shear resistance and energy absorption. This paper provides detailed information regarding the hybrid tests performed as a part of a broader experimental program, which was conducted to comparatively investigate the behavior of reinforced concrete (RC) and reinforce ECC structures. The capabilities of the testing facility and the details of the experimental setup and software used to perform hybrid tests are provided. The results of this comparative investigation performed at the component and system levels favor ECC over conventional concrete in terms of the considered seismic response metrics. The advanced small-scale testing equipment alongside the hybrid simulation methodology allows for evaluation of the system performance at a fairly low cost compared to other experimental methods.

In the field of earthquake engineering, and more generally in structural dynamics and control, experimental verification is critical. For large structural systems, full-scale experimental tests may not be economically or practically feasible. However, hybrid simulation (where the simulation is partitioned into numerical and physical components), provides the capability to isolate and physically test critical components of a structure in an efficient manner, while still fully capturing the dynamic behavior of an interaction with the entire structural system. Real-time hybrid simulation (RTHS) conducts these tests in hard, real-time to ensure that any rate-dependant characteristics of the physical component are accurately represented. Furthermore, testing at multiple geographically distributed laboratories can optimize the use of distributed resources found in the Network for Earthquake Engineering Simulation (NEES) equipment facilities. Leveraging multiple equipment sites for RTHS poses great challenges due to the hard real-time nature of RTHS and the inherent and unpredictable network delay associated with geographically distributed testing. This paper describes the framework, sensitivity analysis, and resulting tests of a series of geographically distributed RTHS successfully conducted between the University of Connecticut (UConn) and the University of Illinois (Illinois).

This paper investigates the calibration and validation of a new ultra highperformance concrete (UHPC) named Cortuf using LDPM-F, the Lattice Discrete Particle Model for fiber reinforced concrete. The LDPM-F is a discrete meso-scale model that can accurately describe the macroscopic behavior of concrete in elastic, fracturing, softening, and hardening regimes. LDPM-F has been verified extensively through the analysis of a variety of experimental tests and can reproduce with great accuracy the response of concrete under uniaxial and multiaxial stress states in compression and tension, and under both quasi-static and dynamic loading conditions. The model is calibrated herein by simulating: (1) unconfined and confined compression tests as well as 3-point bending tests on plain Cortuf and (2) single fiber pull-out tests. Afterward, quasi-static compression and tensile validation and prediction experiments were performed. The numerical results are compared to the experimental results both graphically and through failure modes.

In this study, the analysis of reinforced concrete structures is performed by adopting a coarse-grained approximation of a recently developed meso-scale model for concrete, the Lattice Discrete Particle Model (LDPM). The coarse-grained system is obtained by scaling the particle size with a "coarsening factor" .. > 1. The coarse system cannot reproduce all aspects of the mechanics of the fine system, consequently, the coarse-scale model parameters need to be calibrated from the behavior of the fine-scale system by matching some aspects, e.g. the macroscopic stress-strain/load-displacement curves under different loading conditions, of the fine scale response. The coarse-scale parameters are optimized by the least-square minimization procedure. The effectiveness of the procedure is verified by comparing full fine-scale solutions and coarse-grained solutions for cases of interest in which the full meso-scale problem can be actually solved.

As recently discovered, disregard of the size effect is a major source of uncertainty in the work-of-fracture (or Hillerborg's) test of the fracture energy and in the calibration of the softening law underlying the cohesive (or fictitious) crack models or the nonlocal softening damage models for concrete and other quasibrittle materials. The classical work-of-fracture test of a notched beam of one size, pioneered for concrete by Hillerborg et al., can deliver acceptable results only for the total fracture energy G_F. It is shown that the same complete load-deflection curve of a notched specimen can be closely approximated with stress-separation curves in which the values of tensile strength f'_t differ by 77% and the values of initial fracture energy G_f by 68%. This ambiguity means that that the one-size work-of-fracture test alone cannot provide a sufficient basis for quasibrittle fracture analysis, especially not for predicting the load capacity of quasibrittle structures. It is found, however, that if this test is supplemented by size effect tests with a sufficient size range, the cohesive crack model or nonlocal softening damage model can be identified unambiguously and render the fracture analysis of quasibrittle structures unambiguously realistic. Vice versa, the size effect fracture tests alone do not suffice for determining the total fracture energy G_F. They provide a sufficient basis only for maximum load predictions of normal structures, but not for computing the post-peak softening with the energy absorption capability, nor the peak loads of extremely large quasibrittle structures. However, if the size effect tests of sufficient range are supplemented by one complete softening load deflection curve of notched specimens, an unambiguous identification of the peak loads and post-peak responses of structures becomes possible. To this end, the notched specimen tests must be conducted within a certain optimum size range in relation to the material inhomogeneity size. This range is established by extending the previous analysis of Cusatis and Schauffert. A further important general implication of the demonstrated non-uniqueness is that numerous recent studies in which multiscale or other softening damage models are verified by a few tests of one size only are virtually worthless.

We present a reduced order multiscale explicit nonlocal (ROMEN) model, which consists of the following features: a). Systematic upscaling: derivation of coupled coarse-scale (macro) equations from fine-scale (micro) equations using mathematical homogenization theory. The upscaling procedure incorporates dynamic effects into a unit cell model. b). Model reduction with explicit nonlocality of micro-phases: reducing the complexity of solving a fine-scale problem to a manageable size. Size effect has been incorporated into the fine-scale model by defining nonlocal micro-phase fields by averaging eigenstrains over characteristic length in the physical problem domain. c). Model characterization: solving an inverse problem for constitutive parameters (fiber-reinforcement, cement) by minimizing the error between experimental data at coupon level and multiscale simulation results. d). Material optimization: optimizing microstructure (geometry, volume fraction of fibers, interface properties) based on user-defined design criteria, such as energy absorption. This module allows designing optimal Very High Strength Concrete (VHSC), Fiber Reinforced Concrete (FRC), and other types of special concrete materials. The ROMEN model has been verified to be mesh size independent and validated against experimental data for the three-point bending problem.

A model for early age concrete mechanical behavior and deformations, which makes use of the microplane model M4 and the solidification-microprestress theory, is presented. The viscoelastic behavior is described according to (1) the solidification theory, in which short-term aging and partially recoverable creep deformations are modelled through a solidification process assumed to be a function of the degrees of hydration and silica-fume reaction and (2) the microprestress theory that models long-term concrete aging and irrecoverable creep deformations. Cracking and nonlinear behavior are modeled by using a modified version of microplane model M4 which is extended to include a realistic aging model. Numerical examples, which deal with the analysis of concrete specimens, both in adiabatic and isothermal conditions, and loaded at different ages, are presented in order to show (1) the calibration procedure of model parameters and, after that, (2) the validation of the model for early age behavior of concrete without any further change of the parameters.

Disproportionate collapse is an extreme loading event requiring consideration of all available load paths within a structure. Current criteria quantify acceptable performance limits through the use of seismic documentation. For steel moment frame structures, specifically, ASCE Standard 41-06 is used to quantify a loss of moment resisting capacity in the beam-column connections. This currently serves as the performance limit for the disproportionate collapse analysis of steel moment frame structures. A case study is presented featuring the disproportionate collapse analysis of an existing 33-story steel moment frame building using the nonlinear dynamic finite element program, LS-DYNA. The limitations encountered using ASCE 41-06 performance limits for disproportionate collapse analysis are discussed and finite element modeling considerations are enumerated. Furthermore, recent research on the collapse performance of seismic steel moment frame connections supporting this case study's conclusions regarding the limitations of ASCE 41-06 for disproportionate collapse analysis is discussed and recommendations for further research are proposed.

This study utilizes virtual work method to calculate individual member contributions to the vertical displacement at the point on which collapse is initiated. An arbitrary column at the base of the structure is removed to initiate the collapse. The study follows a two-step procedure. The first step addresses internal strain energy stored in the system under gravity loads only. The second step continues from the previous step and it involves an application of a virtual load at the location of the column removed. The analysis at this step is an incremental-iterative procedure so that strain energy increments stored in the system are calculated at each iteration while the load is continuously increased. Each member's contribution to the deflection at the point of load application is continuously monitored. A detailed breakdown of member's different response components (i.e., displacement participation factors such as axial, shear, flexural, torsional, connection and panel zone responses) are reported.

A novel approach for the implementation of the progressive collapse guidelines of UFC 4-023-03 in an existing building is presented. The existing building is a 5 stories concrete flat plate structure with perimeter beams. It was determined that the beams and columns would be overstressed as a moment frame due to the light reinforcement of the beams and lack of continuous top reinforcement. The proposed solution is to hang the columns from a new steel transfer girder, located on top of the roof, which is designed to span over the lost column. Since concrete has limited tensile strength, new high strength Dywidag steel rods are used to carry the load from the floors up to the transfer girder. The progressive collapse event was modeled as a time history in SAP2000. It was found that a strain compatibility analysis was necessary in order to obtain an adequate estimate of the force distribution between the concrete columns, concrete column reinforcement and the new steel rods. For this particular building, it was found that most of the load was carried by the column and its reinforcement up to the roof level. As a result a very robust connection was required at the upper levels to transfer the load from the damaged column to the rods and up to the steel transfer girder. The analyses presented herein take into account a lower yielding stress in the rebar due to insufficient development length and considers various concrete tensile capacities. Finally, the design of the steel transfer girder has a considerable impact in the calculation of the peak load experienced by the system. If the girder is allowed to develop plastic deformation during the collapse event - but remains elastic in the steady state condition - the peak response forces at the column, steel rods and connections are considerably smaller from the ones obtained when the girder remains elastic.

The application of Fiber Reinforced Polymer (FRP) strips as Tie Elements in strengthening of existing concrete slabs to resist progressive collapse is investigated. In the Tie Force approach it is essential that the FRP strips, designed to behave as Tie Elements, possess the deformation capacity required for large deflections to develop catenary action. The Tie Force approach and the behavior of FRP strips acting as Tie Elements are discussed. The properties of Carbon Fiber Reinforced Polymers (CFRP) and Glass Fiber Reinforced Polymers (GFRP) are compared and it is concluded that the GFRP is more adequate as Tie Element to fulfill the requirement of slabs to undergo a rotation of 0.2 rad. In order to transfer the gravity loads away from the damaged area, an application of Fiber Anchors is introduced to properly connect the concrete slab to the strips. The FRP strips and the FRP anchors from a current retrofit project will be presented.

The stability of steel columns that have suffered localized damage is investigated through analytical and computational means. An analytical model based on the Rayleigh-Ritz technique is proposed and used to study the effect of damage extent and location on column load carrying capacity. The analytical model is run in parallel with detailed finite element models that are validated using available experimental results. Analytical and simulation results show that localized flange loss in a column could lead to a severe reduction in its axial resistance.

This paper discusses the analysis procedure adopted for the performance evaluation of Wuhan Center in Wuhan City, China. The 88-story, 438-meter tower will become the tallest building in central area China upon completion. The selected structural system consists of a composite concrete core, five exterior belt trusses, three sets of steel outriggers and 16 concrete filled steel pipe super columns. Given the scale and importance of this building, a rather sophisticated evaluation of seismic performance during a severe earthquake (2% probability in 50 years or 2475-year return period) was performed. Two synthetic and five recorded ground motions were selected for the nonlinear time-history analysis. These ground motions were scaled to meet China Seismic Code provisions BG50011-2010. The performance levels for immediate occupancy, life safety and collapse prevention were set based on ASCE 41-06, "Seismic Rehabilitation of Existing Buildings" (2006). Complex 3-D computer models with geometric and material nonlinearities were generated for nonlinear dynamic time-history analyses. The detailed performance assessments of main structural elements such as coupling beams, core walls, super columns, outriggers and belt-trusses are presented in this paper.

Avoidance of collapse is one of the most important objectives when designing structures in seismic regions. This study focuses on collapse risk quantification and, in particular, deaggregation of the mean annual frequency of collapse. Similar to the deaggregation used in probabilistic seismic hazard analysis, which provides information about the magnitudes, distances, and epsilon values that primarily contribute to the seismic hazard, deaggregation of the mean annual frequency of collapse is a powerful tool that identifies the ground motion intensities that primarily contribute to the collapse risk of a given structure. A detailed description of how to conduct this deaggregation is presented, and the concept is illustrated using a 4-story steel special moment frame building designed according to the 2003 International Building Code. The collapse risk of this structure is evaluated for sites in the western, central, and eastern United States. Despite significant differences in the seismic hazards at each site, it is shown that intensities corresponding to the lower half of the collapse fragility curve dominate the collapse risk at all sites. It is concluded that emphasis should be placed on the lower half of the collapse fragility curve as opposed to focusing on the median collapse intensity as is currently done.

In seismic design, moment resisting frame is designed to form the overall sway mechanism under severe earthquake. However, it is possible to form the weak column mechanism with the rise of strength of beams and panel zone due to strain hardening. The causes to collapse the weak column type frame are strong bi-axial seismic force, P-Ä effect and deterioration of restoring force. In this study, a series of response analyses of weak column type multi story steel frames under bi-axial ground motion are carried out. In the analysis, realistic hysteresis model of story including deteriorating range is used. From analytical results, collapse behavior and ultimate earthquake resistance of steel frames governed by deterioration of columns are evaluated.

Base isolated buildings are typically important facilities expected to remain functional after a major earthquake. However, their behavior under extreme ground shaking is not well understood. A series of earthquake simulator experiments were performed to assess performance limit states of seismically isolated buildings under strong ground motions, including pounding against a moat wall. The test setup consists of a quarter scale three-story frame isolated at the base with Friction Pendulum Bearings (FPS) and a simulated moat wall. The building model is scaled from a professionally designed isolated three-story steel Intermediate Moment Resisting Frame (IMRF). An effort was made to properly scale the strength and stiffness of the frame relative to the bearings properties so that realistic yielding mechanisms can be obtained. The moat wall was modeled as either a rigid triangle steel stopper or a concrete wall of various thicknesses with soil backfill. The moat wall gap was set to various displacement increments to examine the sensitivity of this parameter and also to assess the effects of impact on the superstructure at different velocities. The test results indicate that pounding can induce yielding in the superstructure and is largely dependent on the gap distance and wall flexibility. Numerical model is proposed that can capture the nonlinear structural response and pounding interface behavior. The pounding interface is modeled by two spring elements in series that separately account for the material response at the contact interface and the flexibility of the moat wall.

The Christchurch Women's Hospital, completed in March 2005, is the only base-isolated building in the South Island of New Zealand. The displacement capacity of the base-isolation system and the super-structure ductility capacity are designed to meet 2000-year return-period demands. Detailed structural evaluations after the 2010 Darfield Earthquake and the 2011 Christchurch Earthquake revealed damage only to sacrificial non-structural components at the seismic gaps. Because the structure was not instrumented at the time of these earthquakes, basic design information, ground motion records from nearby sites, and first-hand accounts of the motion in the structure are used to estimate the responses to the main shock and a large after- shock. Results from this modeling effort are used to corroborate reports of structural response from staff present at the time of the main shock and aftershocks. Subsequent instrumentation of the structure is described and will be used to collect aftershock responses for validation of numerical models.