Momentum Distribution of Near-Zero-Energy Photoelectrons in the Strong-Field Tunneling Ionization in the Long Wavelength Limit

We investigate the ionization dynamics of Argon atoms irradiated by an ultrashort intense laser of a wavelength up to 3100 nm, addressing the momentum distribution of the photoelectrons with near-zero-energy. We find a surprising accumulation in the momentum distribution corresponding to meV energy and a \textquotedblleft V"-like structure at the slightly larger transverse momenta. Semiclassical simulations indicate the crucial role of the Coulomb attraction between the escaping electron and the remaining ion at extremely large distance. Tracing back classical trajectories, we find the tunneling electrons born in a certain window of the field phase and transverse velocity are responsible for the striking accumulation. Our theoretical results are consistent with recent meV-resolved high-precision measurements.Comment: 5 pages, 4 figure

Shear Resistance Prediction of post-fire reinforced concrete beams using artificial neural network

In this paper, a prediction method based on artificial neural network was developed to rapidly determine the residual shear resistance of reinforced concrete (RC) beams after fire. Firstly, the temperature distribution along the beam section was determined through finite element analysis using software ABAQUS. A residual shear strength calculation model was developed and validated using the test data. Using this model, 384 data entries were derived for training and testing. The input layer of neural network involved parameters of beam height, beam width, fire exposure time, cross-sectional area of stirrup, stirrup spacing, concrete strength, and concrete cover thickness. The output was the shear resistance of RC beams. It was found that use of BP neural network could precisely predict the post-fire shear resistance of RC beams. The predicted data were highly consistent with the target data. Thus, this is a novel method for computing post-fire shear resistance of RC beams. Using this new method, further investigation was also made on the effects of different parameters on the shear resistance of the beams

Shear Strength of Fiber Reinforced Reactive Powder Concrete I-Shaped Beam without Stirrup

Due to the high compressive strength of steel fiber reinforced (SFR) and Reactive Powder concrete (RPC), higher degree of prestressing is available for the beams using fiber reinforced reactive powder concrete. Hence, a slenderer structural component can be designed, which could save self-weight, work spacing, and potential reducing the costs. However, comparing to conventional reinforced concrete (RC) beams, studies on post-tensioning SFR-RPC beams subjected to shear failure are fewer. In this study, a series of shear tests for four large-scale beams (one SFR-RPC beam and three post-tensioning SFR-RPC beams) are made to quantify the effects of levels of prestressing on shear load capacity of SFR-RPC beams. Finally, the test results together with data published from existing literatures are compared to the design strength calculated in accordance to different standards. It is found that the current code provisions will underestimate the shear load capacity significantly. In addition, an analytical model is proposed to predict the shear load capacity of the test specimens. A good correlation is observed with a mean analytical model to experimental strength ratio of 1.02 and coefficient of variation of 0.12

Effects of High Strength Concrete on Progressive Collapse Resistance of Reinforced Concrete Frame

Increasing terrorist activities in the past decade brought requirements in design buildings, especially government or commercial buildings, under extreme loading conditions. One of the devastating consequences due to extreme loading is the possibility of progressive collapse. Although extensive studies had been carried out in the past decade on load resistance mechanism of reinforced concrete (RC) frames in preventing progressive collapse, the effects of high-strength-concrete (HSC) on progressive collapse resistance capacity is still unclear. Therefore, in this paper six tests of reinforced concrete frames with different span-to-depth ratio and concrete strength were conducted in the present study. Among them, three are HSC frames and the remaining are normal strength concrete frames. It was found that the use of HSC could further enhance the compressive arch action (CAA) capacity, especially for those with low span-to-depth ratio. On the other hand, HSC can reduce the tensile catenary action (TCA) capacity at large deformation stage, primarily because of higher bond stress between concrete and rebar,leading to earlier fracture of the rebar.The analytical results from the model were compared with the test results. It is found that the existing CAA model could accurately predict the CAA capacity of HSC frames as well as NSC frames.However, existing model is hard to predict the CAA capacity of the frames with relatively small span-to-depth ratio (less than 7) accurately

Experimental Study on Seismic Behaviour of RC Frames with Different Infilled Masonry

Six 1/2 scaled, single-storey, one-bay frame specimens were tested in this study to investigate the seismic behavior of masonry infilled reinforced concrete (RC) frames subjected to lateral loading. The variables investigated include types of masonry and types of openings. The crack patterns, failure modes, load-displacement hysteretic loops, stiffness degradation, and energy dissipation capacity are presented and discussed. It is found that the infilled wall (with or without openings) could improve the behavior of RC frames significantly. Moreover, as expected, the infilled frame with higher strength masonry performed better than those with relatively low strength masonry. Furthermore, the openings may detriment the stability of the infilled walls. The concentric widow opening has worse effects than the eccentric door opening. The proposed analytical model could determine the load resisting capacity of bare frame and infilled frame with reasonable accuracy

Effects of tai chi on postural control during dual-task stair negotiation in knee osteoarthritis : a randomised controlled trial protocol

Stair ascent and descent require complex integration between sensory and motor systems; individuals with knee osteoarthritis (KOA) have an elevated risk for falls and fall injuries, which may be in part due to poor dynamic postural control during locomotion. Tai chi exercise has been shown to reduce fall risks in the ageing population and is recommended as one of the non-pharmocological therapies for people with KOA. However, neuromuscular mechanisms underlying the benefits of tai chi for persons with KOA are not clearly understood. Postural control deficits in performing a primary motor task may be more pronounced when required to simultaneously attend to a cognitive task. This single-blind, parallel design randomised controlled trial (RCT) aims to evaluate the effects of a 12-week tai chi programme versus balance and postural control training on neuromechanical characteristics during dual-task stair negotiation. Sixty-six participants with KOA will be randomised into either tai chi or balance and postural control training, each at 60 min per session, twice weekly for 12 weeks. Assessed at baseline and 12 weeks (ie, postintervention), the primary outcomes are attention cost and dynamic postural stability during dual-task stair negotiation. Secondary outcomes include balance and proprioception, foot clearances, self-reported symptoms and function. A telephone follow-up to assess symptoms and function will be conducted at 20 weeks. The findings will help determine whether tai chi is beneficial on dynamic stability and in reducing fall risks in older adults with KOA patients in community. Ethics approval was obtained from the Ethics Committee of the Affiliated Rehabilitation Hospital of Fujian University of Traditional Chinese Medicine (#2018KY-006-1). Study findings will be disseminated through presentations at scientific conferences or publications in peer-reviewed journals. ChiCTR1800018028. [Abstract copyright: © Author(s) (or their employer(s)) 2020. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.

Nondestructive Post-fire Damage Assessment of Structural Steel Members using Leeb Harness method

Assessment of steel damage is one of the key methods in retrofitting and reconstruction of the steel structure after fire. The traditional assessment method is to cut the samples from the steel members and check the levels of damage. This method will damage the structural member and the process is time consuming. In this paper, a quick, simple and efficient nondestructive detection method to measure the strength of steel after fire is developed using so called Leeb hardness method by means of establishment the relationship between the residual strength of steel members after fire and the Leeb hardness, the post-fire steel strength can be fast determined without damage to the structural members. In this paper, in total 120 Chinese H-shaped steel sections were selected for testing the Leeb hardness after fire. The influence of the parameters such as the duration of the fire exposure, cooling mode, steel grade, stress state and location of the Leeb hardness test on the test results was investigated. The relationship between the steel Leeb hardness and the parameters were developed. In addition, regression functions between the residual strength of steel members after fire and the Leeb hardness was established based on these test results which can accurate predict the residual strength of the steel members after fire, providing the engineers a new fast assessment method for the residual strength of the steel after fire