It is well acknowledged that muscles that are elongated while activated(i.e.,eccentric muscle action)are stronger and require less energy(per unit of force)than muscles that are shortening(i.e.,concentric contraction)...It is well acknowledged that muscles that are elongated while activated(i.e.,eccentric muscle action)are stronger and require less energy(per unit of force)than muscles that are shortening(i.e.,concentric contraction)or that remain at a constant length(i.e.,isometric contraction).Although the cross-bridge theory of muscle contraction provides a good explanation for the increase in force in active muscle lengthening,it does not explain the residual increase in force following active lengthening(residual force enhancement),or except with additional assumptions,the reduced metabolic requirement of muscle during and following active stretch.Aside from the cross-bridge theory,2 other primary explanations for the mechanical properties of actively stretched muscles have emerged:(1)the so-called sarcomere length nonuniformity theory and(2)the engagement of a passive structural element theory.In this article,these theories are discussed,and it is shown that the last of these—the engagement of a passive structural element in eccentric muscle action—offers a simple and complete explanation for many hitherto unexplained observations in actively lengthening muscle.Although by no means fully proven,the theory has great appeal for its simplicity and beauty,and even if over time it is shown to be wrong,it nevertheless forms a useful framework for direct hypothesis testing.展开更多
This paper reports a validation study involving sixth scale masonry model to replicate prototype tests carried out on five unit high masonry prisms. In order to test the applicability of small masonry models to real l...This paper reports a validation study involving sixth scale masonry model to replicate prototype tests carried out on five unit high masonry prisms. In order to test the applicability of small masonry models to real life problems, an investigation into masonry behaviour relevant to the serviceability requirement of masonry arch bridges was chosen as prototype test to validate the small scale masonry tests. Only representative masonry specimens were considered in the study;this corresponded to parts of an arch ring in a complete masonry arch. Two mortar designations;designation iv and designation v were used. These weak mortars tend to conform better to existing old structures. Loads were applied at four eccentricities of 0, 5, 9, and 14 mm from the centre of the specimens. This corresponds to e/d ratios of 0, 0.14, 0.25, and 0.39, where e is the eccentricity of the load and d the length of the transverse section of the specimens. The result shows that validation study corresponds with prototype study for low eccentricities;therefore, strength enhancement is seen over the concentric compressive strength. However, this does not apply at higher eccentricities as specimens were noticed to fail by elastic instability characterised by tension debonding of the top mortar joint.展开更多
The adequacy of using Fiber Reinforced Polymer (FRP) retrofit technique to restore the flexure-compression behavior of deteriorated bridge timber piles is examined experimentally in this paper. Sixteen specimens are t...The adequacy of using Fiber Reinforced Polymer (FRP) retrofit technique to restore the flexure-compression behavior of deteriorated bridge timber piles is examined experimentally in this paper. Sixteen specimens are tested monotonically under eccentric compressive loading. The specimens are first tested in their unretrofitted condition to determine their elastic properties. Each specimen is then cut and connected (posted) using the proposed FRP retrofit technique, and retested. The results show that the retrofitted specimens are capable of reaching same or higher strengths than that of the unretrofitted specimens with minimal reduction in their stiffness. Based on the experimental results, a design equation is presented to compute the volumetric ratio of FRP needed for retrofitting bridge timber piles under eccentric load.展开更多
文摘It is well acknowledged that muscles that are elongated while activated(i.e.,eccentric muscle action)are stronger and require less energy(per unit of force)than muscles that are shortening(i.e.,concentric contraction)or that remain at a constant length(i.e.,isometric contraction).Although the cross-bridge theory of muscle contraction provides a good explanation for the increase in force in active muscle lengthening,it does not explain the residual increase in force following active lengthening(residual force enhancement),or except with additional assumptions,the reduced metabolic requirement of muscle during and following active stretch.Aside from the cross-bridge theory,2 other primary explanations for the mechanical properties of actively stretched muscles have emerged:(1)the so-called sarcomere length nonuniformity theory and(2)the engagement of a passive structural element theory.In this article,these theories are discussed,and it is shown that the last of these—the engagement of a passive structural element in eccentric muscle action—offers a simple and complete explanation for many hitherto unexplained observations in actively lengthening muscle.Although by no means fully proven,the theory has great appeal for its simplicity and beauty,and even if over time it is shown to be wrong,it nevertheless forms a useful framework for direct hypothesis testing.
文摘This paper reports a validation study involving sixth scale masonry model to replicate prototype tests carried out on five unit high masonry prisms. In order to test the applicability of small masonry models to real life problems, an investigation into masonry behaviour relevant to the serviceability requirement of masonry arch bridges was chosen as prototype test to validate the small scale masonry tests. Only representative masonry specimens were considered in the study;this corresponded to parts of an arch ring in a complete masonry arch. Two mortar designations;designation iv and designation v were used. These weak mortars tend to conform better to existing old structures. Loads were applied at four eccentricities of 0, 5, 9, and 14 mm from the centre of the specimens. This corresponds to e/d ratios of 0, 0.14, 0.25, and 0.39, where e is the eccentricity of the load and d the length of the transverse section of the specimens. The result shows that validation study corresponds with prototype study for low eccentricities;therefore, strength enhancement is seen over the concentric compressive strength. However, this does not apply at higher eccentricities as specimens were noticed to fail by elastic instability characterised by tension debonding of the top mortar joint.
文摘The adequacy of using Fiber Reinforced Polymer (FRP) retrofit technique to restore the flexure-compression behavior of deteriorated bridge timber piles is examined experimentally in this paper. Sixteen specimens are tested monotonically under eccentric compressive loading. The specimens are first tested in their unretrofitted condition to determine their elastic properties. Each specimen is then cut and connected (posted) using the proposed FRP retrofit technique, and retested. The results show that the retrofitted specimens are capable of reaching same or higher strengths than that of the unretrofitted specimens with minimal reduction in their stiffness. Based on the experimental results, a design equation is presented to compute the volumetric ratio of FRP needed for retrofitting bridge timber piles under eccentric load.
