In the last few decades, hybrid simulation has become widely used for understanding the response of structural components and systems under extreme loading conditions. Large-scale, three-dimensional (3D), multi-axial testing facilities with six degrees of freedom (6DOF) are versatile testing systems that can be used for testing a variety of structural systems and components. The University of Minnesota Multi-Axial Subassemblage Testing (MAST) facility, which was originally developed in the 1990s, has had recent upgrades to its 6DOF multi-axial pseudo-dynamic hybrid simulation capabilities. One of the well-recognized challenges in using such 6DOF multi-axial setups of this size is the friction within the system, especially in the swivels of actuators, which can lead to numerical instabilities in hybrid simulation if not mitigated or compensated for properly. As such, when using such setups, friction effects on the stability of hybrid simulation must be understood. Characterizing the friction within the setup is not only crucial for understanding whether friction effects must be compensated for during the test, but it is also important for adopting the appropriate friction compensation scheme. Given the inherent complexities of such over-constrained large-scale multi-axial setups, due to their size, capacity, and the intricate interaction within the actuators, characterizing the friction within the system is not trivial. This paper provides an overview of the MAST system and its features, a brief review of selected past projects, and the architecture of the newly upgraded hybrid simulation capabilities. The effects of friction on the stability of hybrid simulation are discussed, and commonly used methods for managing friction or compensating for it are presented. The experiments used to characterize the performance of the MAST testing facility, including the internal friction within the system, are presented. These experiments can serve as a framework for internal friction characterization in similar test setups and can be used by laboratories that are new to using 3D, 6DOF multi-axial test setups. In the end, a suite of validation multi-axial pseudo-dynamic hybrid simulation tests was performed, where all 6DOFs of the MAST system were used in the hybrid simulation control loop. The validation hybrid simulations were performed on a three-story moment resisting frame structure, under the 1994 Northridge earthquake. One of the columns within the building was physically tested at MAST, while the rest of the structure was modeled numerically first in OpenSees, and afterward in Ansys in a repeat test. Results from the pseudo-dynamic hybrid simulation are presented and compared with purely numerical predictions, which validated the system performance. The hydrostatic friction bearings incorporated within the MAST system ensured negligible friction compared to the capacity of the system, with the friction not exceeding 0.23% for the worst-case scenario.

Unbonded post-tensioned rocking walls have demonstrated superior seismic performance with greatly reduced damage and exceland representative research on rocking walls are summarized in this paper. Some inconsistencies and voids in the major design parameters for rocking walls are identified. A brief description is provided for two rocking-wall specimens tested under quasi-static cyclic loading. Force flow and failure mechanisms of rocking walls observed from the tests were studied, and it is discovered that they are very different from those of special structural walls. The test data showed that the concentration of compressive strain in concrete at the corners of rocking walls was a local behavior such that the need for confinement reinforcement higher above the toe region was diminished. Fiber grout weaker than concrete in rocking walls used as ductile bearing materials at the wall-foundation interface is a reasonable alternative to ACI 550.7. Design recommendations for height and volumetric ratio of confinement reinforcement are provided. A requirement for the aspect ratio of rocking walls stricter than that in ACI 550.7 is proposed to prevent shear sliding of the walls.

Hybrid simulation is an innovative method that combines an analysis model of a structural system with physical tests of one or more substructures. The analysis model is typically a finite element analysis (FEA) model that outputs displacements applied to the physical substructure using a control system operated in displacement control. For stiff specimens, the displacement commands can be so small that the control system has difficulty imposing the command displacements accurately. To do hybrid simulation with a stiff specimen, force control is desirable. Cascade control, which features two layers of closed loop control, is proposed to address this issue. The inner control loop has force control mode that provides accurate control for hybrid tests with stiff specimens. The outer control loop is in displacement control mode for accepting displacement commands from an FEA model. The effectiveness of the cascade control method in conducting hybrid simulation of stiff test specimens was evaluated with three sets of tests. For each set of tests, the results of both cascade control and displacement control methods were compared. The three test cases covered a wide range of variation from specimen size, test equipment, model type (2-D vs. 3-D), experimental element type (beam-column vs. truss), and test speed (slowdown 10 times in Test Case 1 and 2 versus 100 times in Test Case 3). In all cases, cascade control proved to be an effective method for conducting hybrid simulation with a stiff specimen.

Two large-scale structural assemblages, which had equivalent PreWEC (Precast Wall with End Columns) rocking-wall systems but different surrounding structures, were tested under quasi-static cyclic loading to study the impact of wall-floor interaction on the performance of rocking-wall buildings. The first specimen (PFS1) included cast-in-place (CIP) edge columns and a CIP unbonded post-tensioned slab with rigid wall-floor connections, which maximized wall-floor interaction. The second specimen (PFS2) included precast edge columns and a precast slab with special vertical-isolation wall-floor connectors, which minimized wall-floor interaction. Test results showed that both structural assemblages demonstrated great seismic performance with limited damage and self-centering behavior. The strength of PFS1 was more than twice that of PFS2; PFS2 encountered less damage and had better self-centering performance than PFS1, but it possessed less energy-dissipation capacity. The difference in the performance of the two assemblages was mainly attributed to the different gravity load transfer paths and constraint effects of the surrounding structures. Contributions to the strength and the self-centering behavior of the two assemblages from different structural elements were quantified and compared.

A large-scale structural assemblage was tested under quasi-static loading to investigate the seismic performance of a rocking-wall structure assembled by precast members. An innovative PreWEC (precast wall with end columns) system, which consisted of a precast non-bearing rocking wall, energy-dissipating O-connectors, and adjacent load-bearing end columns was used as the rocking-wall system. The structure surrounding the PreWEC included precast edge beam and columns and a floor system formed by untopped precast planks. Wall-floor connections were used to isolate the floor from vertical movements of the wall. Test results showed that the floor was successfully isolated from the uplift of the wall throughout the test. The surrounding structure barely increased the strength of the PreWEC system, as anticipated. The entire specimen was almost damage-free at 2% design drift and demonstrated excellent self-centering performance even after experiencing 5% lateral drift. Design recommendations are provided for the key components of this type of precast rocking-wall structure (wall-floor connections, end columns, and floor-beam connections).

In this article, we illustrate the potential of aerodynamic modifications of road signs to reduce wind-induced vibrations. Using a real-world sign structure operated by the Minnesota Department of Transportation, we focus on two modification variants, one based on the simple removal of secondary panels and one based on the addition of drag reducing rear extensions to the main panel. Our main analysis tool is a computational fluid dynamics framework based on the finite element method, which is validated against experiments with a scaled sign model that were conducted in the towing tank and the wind tunnel of the St. Anthony Falls Laboratory. We demonstrate computationally that aerodynamic modifications constitute an effective way of reducing the vibration amplitude in the example structure for head wind at the average operating wind speed. The present study can be seen as a first step towards establishing the use of aerodynamic devices for road sign structures.

University of Minnesota M.S. thesis.May 2018. Major: Civil Engineering. Advisors: Lauren Linderman, Catherine French. 1 computer file (PDF); xv, 152 pages.

Fieldand model-scale experiments were conducted to quantitatively assess the effects of wind loading on Rural Intersection Conflict Warning System (RICWS) highway sign structures. A field-scale RICWS was instrumented with acceleration and linear displacement sensors to monitor unsteady loads, dynamics, and displacement of the sign under various wind events classified by cup and vane wind velocity measurements. To complement the field-scale results, tests on a 1 : 18-scale model were conducted under controlled laboratory conditions in the St. Anthony Falls Laboratory towing tank and wind tunnel facilities. Aerodynamic effects on the sign structure were identified through analysis of the mean and oscillating drag and lift forces. Vortices periodically shed by the structure induced forces at a frequency governed by the Strouhal number. The shedding frequency overlapped with the estimated natural frequency during strong wind events, leading to possible resonance. Amplified oscillations were additionally observed when the wind direction was parallel to the structure, possibly due to an aeroelastic instability. The findings highlight the relevance of aerodynamic effects on roadside signs or similar complex planar geometries under unsteady wind loading. (c) 2020 American Society of Civil Engineers.

A numerical parametric study was conducted with two primary objectives: (1) to investigate the accuracy of existing AASHTO shear distribution factors for prestressed concrete girder bridges in the United States; and (2) to provide recommendations to more accurately rate bridges for shear and permitting. Results from the parametric study indicated that the longitudinal stiffness (girder composite longitudinal moment of inertia divided by cube of span length) to transverse stiffness (transverse deck strip moment of inertia divided by cube of beam spacing) provided an indication of the elastic shear distribution in nonskewed bridges. Shear distributed more broadly to adjacent girders when the ratio of longitudinal to transverse bending stiffness was lower. The stiffness ratio can be used as a screening tool to delineate cases for which AASHTO shear distribution factors are conservative or unconservative relative to results of detailed finite-element or grillage analyses. Because grillage analyses can account for the distribution of shear associated with truck axle position, they can be used to better determine permit truck demand. (C) 2019 American Society of Civil Engineers.

First Name is required invalid characters Last Name is required invalid characters Email Address is required Invalid Email Address Invalid Email Address

A large-scale structural assemblage was tested to investigate the interaction between "Precast Wall with End Columns" (PreWEC) and surrounding structure. The PreWEC consisted of a precast rocking wall, steel end columns, and energy-dissipating elements termed "O-connectors". The cast-in-place surrounding structure included gravity columns and an unbonded post-tensioned floor with rigid wall-floor connections. Test results showed that damage to the wall and the grout beneath the wall was minor through 5% drift. Although local damage occurred to the floor at 4% drift, it did not affect the structural integrity of the floor. The resistance provided by the surrounding structure greatly increased the strength of the PreWEC, resulting in a high base shear demand on the wall. Although the inelastic action developed in the surrounding structure, which in turn increased the energy dissipating capacity of the test assemblage, the PreWEC system is expected to self-center when subjected to seismic loads.

Four competing bridge deck designs are compared on the basis of life cycle costs (LCC). Two of the designs are reinforced with steel. The remaining two are reinforced with glass fiber reinforced polymers (GFRP). Typical of Canadian prairie environments, the decks will face periodic exposure to deicing salts '" a factor influencing both the designs and projected maintenance needs. Combining the resulting stream of costs over a 100-year service life and required rate of discount permits the computation of Present Worth LCC (PWLCC) for comparative purposes. Uncertainty in performance and other variables is captured through a systematic integration of sensitivity and risk analyses to produce expected value results and corresponding risk profiles. The results will aid the Manitoba Department of Infrastructure and Transportation select deck designs for two upcoming bridge projects. Publication Year 2008 Title Durability in a Salt Solution of Pultruded Composite Materials Used in Structural Sections for Bridge Deck Applications. Author Fam Amir; Boles Raouf; Robert Mathieu Source Journal of Bridge Engineering. 2015/5. Content ID 04015032 (Refs.) URL http://dx.doi.org/10.1061/(ASCE)BE.1943-5592.0000768 Abstract This study addresses the durability of glass fiber-reinforced polymer (GFRP) pultruded structural sections used in bridge-deck applications, namely, a flat plate with T-shaped ribs (R-GFRP) and a corrugated plate (C-GFRP). Standard coupons were aged for up to 224 days at 23, 40, and 55 degrees C in separate baths of 3% salt solutions simulating deicing conditions. The tensile-strength retentions and Young's moduli were measured periodically. Data were assessed using ANOVA. Microstructure assessments using differential scanning calorimetry, Fourier transform infrared spectroscopy, and scanning electronThis study addresses the durability of glass fiber-reinforced polymer (GFRP) pultruded structural sections used in bridge-deck applications, namely, a flat plate with T-shaped ribs (R-GFRP) and a corrugated plate (C-GFRP). Standard coupons were aged for up to 224 days at 23, 40, and 55 degrees C in separate baths of 3% salt solutions simulating deicing conditions. The tensile-strength retentions and Young's moduli were measured periodically. Data were assessed using ANOVA. Microstructure assessments using differential scanning calorimetry, Fourier transform infrared spectroscopy, and scanning electron

For the successful implementation of long-term monitoring strategies of prestressed concrete structures, the expected behavior of the structure must be accurately quantified before anomalous or damage-related readings can be properly identified. In situ structures may be subject to large variations in temperature, which can have a significant impact on measured deformations, and continued creep and shrinkage of the concrete further complicate long-term predictions. The goals of this and a companion paper are to present the methodology for extracting the time-dependent behavior of a posttensioned concrete box girder bridge from structural monitoring data in the presence of changing temperatures (this paper) and to compare predictions of long-term time-dependent deformations computed using finite-element analysis with the extracted time-dependent monitoring data (the companion paper). To investigate the interactions between temperature and time-dependent behavior for in situ monitoring data, strains and expansion joint deflections from the St. Anthony Falls Bridge, a posttensioned concrete box girder bridge on I-35W in Minneapolis, Minnesota, were collected over a period of 5 years. A methodology based on linear regression was used to separate the time-dependent deformations from the temperature-related deformations given a variable coefficient of thermal expansion (CTE). The total temperature-related deformations were captured by functions based on the average bridge temperature, the thermal gradient through the depth of the superstructure, and the average squared temperature of the bridge, which was proposed because the CTE was observed to vary with temperature. On examination of the extracted time-dependent readings, the deformation rates were found to decelerate during the winter and accelerate during the summer. To enable direct comparison between the measured results and the creep and shrinkage predictions from finite-element models assuming constant temperature, an Arrhenius-adjusted time formulation was used, which normalized the measured deformations under varying temperatures to those expected from a constant reference temperature. This procedure for processing the time-dependent measured data enables a comparison with time-dependent finite-element model results conducted at constant temperature. (C) 2017 American Society of Civil Engineers.

A monitoring strategy for investigating the long-termlongitudinal deflections at the expansion joints of the I-35WSt. Anthony Falls Bridge was developed as a means of inferring the integrity of the bearings. During the duration of this study, the deflections were primarily caused by time-dependent behavior, which tends to be highly uncertain, and changes in temperature, which are comparatively predictable. The proposed monitoring strategy uses separate anomaly-detection routines to monitor the expansion joints for quickly developing problems, such as bearing lockup, and slowly developing problems, such as unexpected changes in the rate of time-dependent bearing movement that might be indicative of slow degradation. The quickly developing anomaly-detection routine presented used Bayesian regression to combine the uncertainty in the time-dependent deflection predictions with the scatter of the data to realize coherent bounds for discerning problems that occur instantaneously or over the course of several weeks. The slowly developing anomaly-detection routine used the rates of the measured timedependent behaviorwith respect to theArrhenius-adjusted age to discern deterioration developing over time frames fromseveralmonths up to several years. These rates were expected to decrease with time according to a power function for a structure with no damage. The system was tested on the collected linear potentiometer data from the I-35W St. Anthony Falls Bridge to investigate the incidence of false positives. Use of model code time-dependent provisions for the two anomaly-detection routines minimized false positives. To test the efficacy of the developed system in identifying true positives, artificial perturbations were introduced to the collected data. These perturbations were intended to mimic probable quickly developing and slowly developing problems that the bridge might encounter. The system successfully identified the introduced quickly developing and slowly developing perturbations unless sensor failure followed by a delayed sensor replacement occurred at approximately the same time as introduction of the perturbation. (C) 2017 American Society of Civil Engineers.

Two T-shaped reinforced concrete wall specimens were subjected to reversed cyclic loading quasi-statically to failure. Both represented half-scale wall assemblages of a 6-story prototype building. Modifications to the wall detailing were incorporated to study the effects of longitudinal reinforcement distribution and splicing, shear lag, increased amounts of shear reinforcement, and increased dimensions of the boundary elements beyond original code-based requirements. In addition, the minimum number of stories required to capture important aspects of multi-story wall behavior through physical experiments was investigated. Distributing the longitudinal reinforcement across the flange, rather than concentrating it within the boundary elements, was found to reduce crack widths, damage to the wall, and shear sliding across the wall panel. Concentrating large amounts of reinforcement in the flange tips tended to increase shear lag effects in the web-direction loading, but led to moderate increases in the in-plane strength and deformation capacity in the flange direction. Locating the lap splices at the second-story level avoided problems with localized damage observed in cases where lap splices are located at the wall-flange interface. Increasing the amount of shear reinforcement and dimensions of the boundary elements did not have a significant impact on behavior. A minimum of two stories was found to be necessary to characterize the behavior of this 6-story prototype structure; it was sufficient to capture the height over which plasticity occurred. (C) 2017 American Society of Civil Engineers.

Typically, measurement and analysis of concrete time-dependent behaviors such as creep and shrinkage are performed under the assumption of constant temperature conditions. However, many structures in the field are subject to variable seasonal and daily temperatures. This paper explores how changes in temperature affect the time-dependent behavior of concrete structures, with a particular focus on posttensioned concrete bridges. Temperature-dependent creep, shrinkage, aging, and relaxation models were incorporated into structural finite-element analyses examining a posttensioned concrete beam under variable thermal loading. The impacts of uniform temperature changes on the time-dependent deflections, strains, and stresses were accounted for using an Arrhenius-adjusted age based on the structure temperature. However, nonuniform temperature changes, such as thermal gradients through the depth of the cross-section, caused time-dependent behavior that could not be accounted for using the Arrhenius-adjusted age based on the average structure temperature. Time-dependent vertical deflections and especially longitudinal stresses throughout the structure were altered by the repeated application of thermal gradients. The different creep and shrinkage strain rates through the depth of the section, driven by the temperature and stress differences from the applied thermal gradients, induced residual stresses. The presence of these residual stresses implies that time-dependent stress changes from a creep and shrinkage analysis at a constant temperature cannot be linearly superimposed with the stresses caused by thermal gradients computed using an independent elastic analysis.

First Name is required invalid characters Last Name is required invalid characters Email Address is required Invalid Email Address Invalid Email Address

This paper presents a technique for computing the time-dependent behavior of reinforced and prestressed concrete as a composite material. This technique, when implemented in a finite-element model and assuming that the concrete and steel reinforcement act as a composite material, is computationally advantageous compared with explicitly modeling the concrete and steel materials separately. The method is developed assuming linear viscoelasticity and uncracked sections. The approach starts by first approximating the creep compliance functions for the viscoelastic concrete as a Kelvin chain model. This approximation allows the viscoelastic behavior to be framed as a rate-type creep law, which converts the analysis to an equivalent elastic problem, simplifying the computations. This approach, originally developed for plain concrete, is extended in this paper to account for the effects of linear elastic reinforcement. Several implementation examples are provided documenting the viability of the method for problems of uniaxial, multiaxial, and bending behaviors. For the presented cases, the composite method is shown to provide similar results compared with models containing explicitly modeled reinforcement. The paper concludes with a discussion regarding how to extend the methodology to the general case with linear viscoelastic reinforcement and matrix materials.