The Experts below are selected from a list of 303 Experts worldwide ranked by ideXlab platform
Don B. Chaffin - One of the best experts on this subject based on the ideXlab platform.
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The effect of strict muscle Stress Limits on abdominal muscle force predictions for combined torsion and extension loadings.
Journal of Biomechanics, 1995Co-Authors: Richard E. Hughes, Don B. ChaffinAbstract:Abstract The objective of this study was to determine to what extent the central nervous system activates torso muscles so as to equalize the largest muscle Stresses. Two optimization models that treat large muscle Stresses differently were formulated. One model minimized spinal compression force subject to the lowest possible muscle Stress Limit, and the other model minimized the sum of cubed muscle Stresses. Experimental conditions were determined for which the two models made different muscle force predictions. Specifically, the models predicted different rectus abdominis activity levels for tasks involving torsion and extension moment loadings. Surface electromyography was used to evaluate the model predictions. Applied loads were chosen to assure that the rectus abdominis EMG exceeded 30% MVC. Analysis of variance indicated that rectus abdominis activity was not affected by torsion loading at the p
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The effect of strict muscle Stress Limits on abdominal muscle force predictions for combined torsion and extension loadings.
Journal of biomechanics, 1995Co-Authors: Richard E. Hughes, Don B. ChaffinAbstract:The objective of this study was to determine to what extent the central nervous system activates torso muscles so as to equalize the largest muscle Stresses. Two optimization models that treat large muscle Stresses differently were formulated. One model minimized spinal compression force subject to the lowest possible muscle Stress Limit, and the other model minimized the sum of cubed muscle Stresses. Experimental conditions were determined for which the two models made different muscle force predictions. Specifically, the models predicted different rectus abdominis activity levels for tasks involving torsion and extension moment loadings. Surface electromyography was used to evaluate the model predictions. Applied loads were chosen to assure that the rectus abdominis EMG exceeded 30% MVC. Analysis of variance indicated that rectus abdominis activity was not affected by torsion loading at the p < 0.05 level of significance in a statistical design having 90% power, which was consistent with the predictions of the model that minimized the sum of cubed muscle Stresses. Thus, it was concluded that equalization of the largest muscles Stress was not the paramount objective of the central nervous system in the tasks studied.
Richard E. Hughes - One of the best experts on this subject based on the ideXlab platform.
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The effect of strict muscle Stress Limits on abdominal muscle force predictions for combined torsion and extension loadings.
Journal of Biomechanics, 1995Co-Authors: Richard E. Hughes, Don B. ChaffinAbstract:Abstract The objective of this study was to determine to what extent the central nervous system activates torso muscles so as to equalize the largest muscle Stresses. Two optimization models that treat large muscle Stresses differently were formulated. One model minimized spinal compression force subject to the lowest possible muscle Stress Limit, and the other model minimized the sum of cubed muscle Stresses. Experimental conditions were determined for which the two models made different muscle force predictions. Specifically, the models predicted different rectus abdominis activity levels for tasks involving torsion and extension moment loadings. Surface electromyography was used to evaluate the model predictions. Applied loads were chosen to assure that the rectus abdominis EMG exceeded 30% MVC. Analysis of variance indicated that rectus abdominis activity was not affected by torsion loading at the p
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The effect of strict muscle Stress Limits on abdominal muscle force predictions for combined torsion and extension loadings.
Journal of biomechanics, 1995Co-Authors: Richard E. Hughes, Don B. ChaffinAbstract:The objective of this study was to determine to what extent the central nervous system activates torso muscles so as to equalize the largest muscle Stresses. Two optimization models that treat large muscle Stresses differently were formulated. One model minimized spinal compression force subject to the lowest possible muscle Stress Limit, and the other model minimized the sum of cubed muscle Stresses. Experimental conditions were determined for which the two models made different muscle force predictions. Specifically, the models predicted different rectus abdominis activity levels for tasks involving torsion and extension moment loadings. Surface electromyography was used to evaluate the model predictions. Applied loads were chosen to assure that the rectus abdominis EMG exceeded 30% MVC. Analysis of variance indicated that rectus abdominis activity was not affected by torsion loading at the p < 0.05 level of significance in a statistical design having 90% power, which was consistent with the predictions of the model that minimized the sum of cubed muscle Stresses. Thus, it was concluded that equalization of the largest muscles Stress was not the paramount objective of the central nervous system in the tasks studied.
Yilan Kang - One of the best experts on this subject based on the ideXlab platform.
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mechanical characterization of the load distribution on the cutterhead ground interface of shield tunneling machines
Tunnelling and Underground Space Technology, 2015Co-Authors: Qian Zhang, Gan-yun Huang, Zhende Hou, Zongxi Cai, Yilan KangAbstract:Abstract The paper proposes a mechanical characterization and approximate calculation method to determine the load acting on the cutterhead–ground interface of an Earth Pressure Balance (EPB) shield tunneling machine. Through a mechanical decoupling analysis of the coupling interaction between the cutterhead and the ground, the basic expressions for the normal and tangential Stresses acting on the tunneling interface between the cutterhead and the ground are developed. For ease of use in engineering applications, the Stress expressions are modified and simplified with the consideration of several engineering factors, such as the Stress Limit that the ground can bear, the overburden, and the topological structure of the cutterhead. A “bilinear” description model for the load distribution is then established; the effectiveness of this model is discussed and verified by a comparison with the laboratory experimental results and the numerical simulations in existing research. The equations for the approximate calculation of the thrust and torque on the cutterhead are also presented. This work may improve the simplified hypothesis that the cutterhead load is uniformly or linearly distributed in the current equipment design and load calculation, thereby providing a theoretical reference for cutterhead design and layout and for the analysis and calculation of loads in the tunneling process.
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Mechanical characterization of the load distribution on the cutterhead–ground interface of shield tunneling machines
Tunnelling and Underground Space Technology, 2015Co-Authors: Qian Zhang, Gan-yun Huang, Zhende Hou, Zongxi Cai, Yilan KangAbstract:Abstract The paper proposes a mechanical characterization and approximate calculation method to determine the load acting on the cutterhead–ground interface of an Earth Pressure Balance (EPB) shield tunneling machine. Through a mechanical decoupling analysis of the coupling interaction between the cutterhead and the ground, the basic expressions for the normal and tangential Stresses acting on the tunneling interface between the cutterhead and the ground are developed. For ease of use in engineering applications, the Stress expressions are modified and simplified with the consideration of several engineering factors, such as the Stress Limit that the ground can bear, the overburden, and the topological structure of the cutterhead. A “bilinear” description model for the load distribution is then established; the effectiveness of this model is discussed and verified by a comparison with the laboratory experimental results and the numerical simulations in existing research. The equations for the approximate calculation of the thrust and torque on the cutterhead are also presented. This work may improve the simplified hypothesis that the cutterhead load is uniformly or linearly distributed in the current equipment design and load calculation, thereby providing a theoretical reference for cutterhead design and layout and for the analysis and calculation of loads in the tunneling process.
Ehsan Masoumi Khalil Abad - One of the best experts on this subject based on the ideXlab platform.
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implementation of the forming Limit Stress diagram to obtain suitable load path in tube hydroforming considering m k model
Materials & Design, 2009Co-Authors: Ramin Hashemi, Ahmad Assempour, Ehsan Masoumi Khalil AbadAbstract:Abstract The purpose of this work is to obtain a theoretical forming Limit Stress diagram (FLSD) and a suitable load path in tube hydroforming. The methodology for prediction of hydroforming Stress Limit diagram is based on Marciniak and Kuczynski (M–K) model. The numerical approach is based on the modified Newton–Raphson method. The comparison between the experimental and theoretical results for hydroforming Stress Limit diagrams as predicted by different methods indicates that the present approach is suitable for prediction of necking in tube hydroforming. Then the forming Limit Stress diagram determined by the proposed methodology is implemented to obtain the suitable load path in tube hydroforming. As a case study, the finite element (FE) model for bulge forming of straight tube has been constructed and verified with published experimental data. Ultimately, the suitable load path has been obtained by applying Adaptive Simulation Method in ANSYS Parametric Design Language (APDL).
Oguzhan Bayrak - One of the best experts on this subject based on the ideXlab platform.
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Effects of Increasing Allowable Compressive Stress at PreStress Transfer
ACI Structural Journal, 2010Co-Authors: David B. Birrcher, Oguzhan Bayrak, Michael E. KregerAbstract:This paper evaluates the benefits and weaknesses of increasing the allowable compressive Stress at preStress transfer in preStressed concrete members. A historical background of the allowable release Stress in compression is provided. A simple design example is used to quantify potential production and design benefits of increasing the allowable Stress and the applicability of these benefits is discussed. Test results of 36 pretensioned beams that were subjected to a range of maximum compressive Stresses at release are reported and analyzed. The results suggest that the compressive Stress Limit at preStress transfer must govern the design of the pretensioned member to achieve significant production or design benefits from increasing the allowable Stress, but that production and design benefits cannot be maximized simultaneously. The findings indicate that although increasing the allowable release Stress in compression at midspan of a member to 0.65fci' or 0.70fci' is possible, members subjected to midspan release Stresses in excess of 0.70fci' displayed premature flexural cracking. Before design codes are changed, however, additional testing on full-scale specimens is recommended.
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Allowable tensile Stress Limit at preStress transfer
Structures Congress 2009, 2009Co-Authors: Robin G. Tuchscherer, Oguzhan BayrakAbstract:This research program was funded by the Texas Department of Transportation (TxDOT) to determine the cause of flexural cracking of preStressed beams at release. Girders that exhibited cracking were 54-inches deep, relatively short in length (60-feet and less), and contained highly eccentric strand configurations.
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Tensile Stress Limit for PreStressed Concrete at Release: ACI 318-08
ACI Structural Journal, 2009Co-Authors: Robin G. Tuchscherer, Oguzhan BayrakAbstract:Crack control is important for new preStressed beams because cracked beams will either not pass inspection or will need expensive repairs. This paper describes a study to investigate current tensile Stress Limits and crack control provisions. The tensile Stress Limit for preStressed concrete beams at release was evaluated experimentally for seven 54 in. (1370 mm) deep bridge beams. The purpose of the experimental program was to characterize the mechanical properties of concrete, including the relationship between the inplace strength of a beam and its representative material strength. Beams fabricated as part of this study cracked with an applied Stress less than half of the apparent strength determined from material tests. The ACI 318 tensile Stress Limit was not sufficient to prevent cracking. It was concluded that Limiting the extreme fiber tensile Stress will adequately prevent cracking at release. The amount of crack control reinforcement specified in ACI 318-08, Section 18.4.1 was sufficient to Limit cracks at release to a width ≤ 0.004 inches. Conventional linear-elastic design assumptions provided accurate predictions of Stresses at the location of preStress transfer.
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Allowable Compressive Stress at PreStress Transfer
2008Co-Authors: Brian Schnittker, Oguzhan BayrakAbstract:In 2004, the Texas Department of Transportation (TxDOT) initiated Project 5197 to investigate the feasibility of increasing the allowable compressive Stress Limit at preStress transfer. Initially, the live load performance of 36 specimens was evaluated by Birrcher and Bayrak (TxDOT Report 5197-1, 2007). Report 5197-4 presents the subsequent research conducted based on recommendations of Birrcher and Bayrak (2007). In this portion of TxDOT Project 5197, 45 Type-C beams and 10 4B28 box beams were tested to experimentally determine their cracking load. The Type-C beams were produced in four different fabrication plants using conventionally consolidated concrete. The 10 4B28 box beams were produced in two fabrication plants using concrete mixture designs of both self consolidating concrete as well as conventional concrete. For all specimens, measured cracking loads were compared to predicted cracking loads. The data from the 45 Type-C beams and 10 box beams were added to the 36 beams investigated by Birrcher and Bayrak (2007) to compile a comprehensive set of data from 91 specimens. An appropriate maximum compressive Stress Limit was determined from the ability to accurately predict the load at which cracking occurred. As the maximum compressive Stress at preStress transfer was increased, a decline in cracking load prediction accuracy was observed. For the specimens subjected to high compressive Stresses at release (greater than 0.65f’ci), the concrete in the pre-compressed tensile zone was subjected to the non-linear inelastic range causing microcracking to occur. This non-linear behavior (due to microcracking) was unaccounted for in preStress losses or standard design equations (P/A±Mc/I). Based on the analysis of the results, an increase of the allowable compressive Stress Limit at preStress transfer to 0.65f’ci is justified. Additionally, the use of self consolidating concrete with a maximum compressive Stress of 0.65f’ci is not recommended.
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Effects of Increasing the Allowable Compressive Stress at Release of PreStressed Concrete Girders
2007Co-Authors: David B. Birrcher, Oguzhan BayrakAbstract:In this report, the feasibility of increasing the allowable compressive Stress Limit at preStress transfer of 0.60f'ci is evaluated. For this purpose, the live-load performance and initial camber of pretensioned beams was investigated. Static-load tests were performed on 24 scaled and 12 full-scale specimens that were subjected to a maximum compressive Stress at release ranging from 0.46f'ci to 0.91f'ci. From the comparison of the measured and predicted cracking loads of these test specimens, an increase of the allowable compressive Stress to 0.65f'ci was justified. For the specimens subjected to release Stresses exceeding 0.65f'ci, premature cracking in flexure was detected due to nonlinear deformation at release and associated microcracking. In addition, an initial camber database of information from 223 pretensioned girders was compiled. The data suggested that increasing 0.60f'ci did not affect the ability to accurately estimate initial camber. However, for a given section, higher cambers were detected as the compressive Stress at release increased. Lastly, a simple technique for improving the initial camber estimates of conventional girders was presented as a feasibility study for future work.
