Effects of cyclic loading on the long-term deflection of prestressed concrete beams

Lihai Zhang,Priyan Mendis,Wong Chon Hon,Sam Fragomeni,Nelson Lam,Yilun Song 사단법인 한국계산역학회 2013 Computers and Concrete, An International Journal Vol.12 No.6

Creep and shrinkage have pronounced effects on the long-term deflection of prestressed concrete members. Under repeated loading, the rate of creep in prestressed concrete members is often accelerated. In this paper, an iterative computational procedure based on the well known Model B3 for creep and shrinkage was developed to predict the time-dependent deflection of partially prestressed concrete members. The developed model was validated using the experimental observed deflection behavior of a simply supported partially prestressed concrete beam under repeated loading. The validated model was then employed to make predictions of the long-term deflection of the prestressed beams under a variety of conditions (e.g., water cement ratio, relatively humidity and time at drying). The simulation results demonstrate that ignoring creep and shrinkage could lead to significant underestimation of the long-term deflection of a prestressed concrete member. The model will prove useful in reducing the long-term deflection of the prestressed concrete members via the optimal selection of a concrete mix and prestressing forces.

Contact forces generated by fallen debris

Jing Sun,Nelson Lam,Lihai Zhang,Emad Gad,Dong Ruan 국제구조공학회 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.50 No.5

Expressions for determining the value of the impact force as reported in the literature and incorporated into code provisions are essentially quasi-static forces for emulating deflection. Quasi-static forces are not to be confused with contact force which is generated in the vicinity of the point of contact between the impactor and target, and contact force is responsible for damage featuring perforation and denting. The distinction between the two types of forces in the context of impact actions is not widely understood and few guidelines have been developed for their estimation. The value of the contact force can be many times higher than that of the quasi-static force and lasts for a matter of a few milli-seconds whereas the deflection of the target can evolve over a much longer time span. The stiffer the impactor the shorter the period of time to deliver the impulsive action onto the target and consequently the higher the peak value of the contact force. This phenomenon is not taken into account by any contemporary codified method of modelling impact actions which are mostly based on the considerations of momentum and energy principles. Computer software such as LS-DYNA has the capability of predicting contact force but the dynamic stiffness parameters of the impactor material which is required for input into the program has not been documented for debris materials. The alternative, direct, approach for an accurate evaluation of the damagepotential of an impact scenario is by physical experimentation. However, it can be difficult to extrapolate observations from laboratory testings to behaviour in real scenarios when the underlying principles have not been established. Contact force is also difficult to measure. Thus, the amount of useful information that can be retrieved from isolated impact experiments to guide design and to quantify risk is very limited. In this paper, practical methods for estimating the amount of contact force that can be generated by the impact of a fallen debris object are introduced along with the governing principles. An experimental-calibration procedure forming part of the assessment procedure has also been verified.

Contact forces generated by fallen debris

Sun, Jing,Lam, Nelson,Zhang, Lihai,Gad, Emad,Ruan, Dong Techno-Press 2014 Structural Engineering and Mechanics, An Int'l Jou Vol.50 No.5

Expressions for determining the value of the impact force as reported in the literature and incorporated into code provisions are essentially quasi-static forces for emulating deflection. Quasi-static forces are not to be confused with contact force which is generated in the vicinity of the point of contact between the impactor and target, and contact force is responsible for damage featuring perforation and denting. The distinction between the two types of forces in the context of impact actions is not widely understood and few guidelines have been developed for their estimation. The value of the contact force can be many times higher than that of the quasi-static force and lasts for a matter of a few milli-seconds whereas the deflection of the target can evolve over a much longer time span. The stiffer the impactor the shorter the period of time to deliver the impulsive action onto the target and consequently the higher the peak value of the contact force. This phenomenon is not taken into account by any contemporary codified method of modelling impact actions which are mostly based on the considerations of momentum and energy principles. Computer software such as LS-DYNA has the capability of predicting contact force but the dynamic stiffness parameters of the impactor material which is required for input into the program has not been documented for debris materials. The alternative, direct, approach for an accurate evaluation of the damage potential of an impact scenario is by physical experimentation. However, it can be difficult to extrapolate observations from laboratory testings to behaviour in real scenarios when the underlying principles have not been established. Contact force is also difficult to measure. Thus, the amount of useful information that can be retrieved from isolated impact experiments to guide design and to quantify risk is very limited. In this paper, practical methods for estimating the amount of contact force that can be generated by the impact of a fallen debris object are introduced along with the governing principles. An experimental-calibration procedure forming part of the assessment procedure has also been verified.

Aerodynamic performance evaluation of different cable-stayed bridges with composite decks

Rui Zhou,Yaojun Ge,Yongxin Yang,Yanliang Du,Lihai Zhang 국제구조공학회 2020 Steel and Composite Structures, An International J Vol.34 No.5

The aerodynamic performance of long-span cable-stayed bridges is much dependent on its geometrical configuration and countermeasure strategies. In present study, the aerodynamic performance of three composite cable-stayed bridges with different tower configurations and passive aerodynamic countermeasure strategies is systematically investigated by conducting a series of wind tunnel tests in conjunction with theoretical analysis. The structural characteristics of three composite bridges were firstly introduced, and then their stationary aerodynamic performance and wind-vibration performance (i.e., flutter performance, VIV performance and buffeting responses) were analyzed, respectively. The results show that the bridge with three symmetric towers (i.e., Bridge I) has the lowest natural frequencies among the three bridges, while the bridge with two symmetric towers (i.e., Bridge II) has the highest natural frequencies. Furthermore, the Bridge II has better stationary aerodynamic performance compared to two other bridges due to its relatively large drag force and lift moment coefficients, and the improvement in stationary aerodynamic performance resulting from the application of different countermeasures is limited. In contrast, it demonstrates that the application of both downward vertical central stabilizers (UDVCS) and horizontal guide plates (HGP) could potentially significantly improve the flutter and vortex-induced vibration (VIV) performance of the bridge with two asymmetric towers (i.e., Bridge III), while the combination of vertical interquartile stabilizers (VIS) and airflowdepressing boards (ADB) has the capacity of improving the VIV performance of Bridge II.

Improving Soil-Water Characteristics and Pore Structure of Silty Soil Using Nano-aqueous Polymer Stabilisers

Wei Huang,Cuiying Zhou,Zhen Liu,Hao Sun,Jiaxin Du,Lihai Zhang 대한토목학회 2021 KSCE JOURNAL OF CIVIL ENGINEERING Vol.25 No.9

Polymer soil stabilisers have been shown to improve soil strength and erosion resistance, for example on slopes where topsoil can be easily eroded. However, fundamental mechanisms of soil stabilisers in soil enhancement through modifying the soil-water characteristic and microstructure of soil (e.g., distribution of pore size) have not been fully understood. This gap in understanding is not conducive to the subsequent development and engineering applications of soil stabilisers. This paper reports a series of experimental studies on the influence of nano-aqueous adhesive (NAA) in modifying the pore size distribution and water content of the soil. The results show that NAA can change the soil-water characteristic curve and has the potential to increase water retention capacity, which is essential for vegetation growth on a slope. In addition, they indicate that NAA could improve soil microstructure and pore uniformity by blocked small pores. The results also show that the NAA could reduce wilting coefficient, improve air entry value, and thereby protect vegetation, which helps stabilise the slope.