Discrete Element Analysis of the Load Transfer Mechanism of Geogrid-Ballast Interface under Pull-Out Load

Geogrids have been extensively used in subgrade construction for stabilization purposes of unconfined ballast. Based on well-calibrated microparameters, a series of geogrid-reinforced ballast models with different geogrid sizes and particular structures were developed to reproduce the mechanical behavior of the geogrid under pull-out load in this paper. And the rationality of the DEM model is verified by comparing the evolution law pull-out force measured by laboratory tests and numerical simulations under comparable conditions. Moreover, the macro pull-out force and the internal force distribution of the geogrid were analyzed, and the contact force statistical zones of the particle system were divided accurately according to the results. Meanwhile, both the force transfer mechanism in the geogrid-ballast interface and the sectionalized strain of the geogrid were discussed. And results unveil that the pull-out load is transmitted along the longitudinal ribs to the transverse ribs, and nearly 90% of the load is transmitted to the contact network (in statistical zone 1) in front of the first transverse rib, resulting in strong interlocking between the particles occurs in statistical zone 1. And the second transverse rib is the strength dividing line between strong and weak contact forces. Then, additional pull-out tests on the control groups were conducted, and the sectionalized strain of the geogrid and the peak pull-out force, as well as the energy dissipation were systematically analyzed. In addition, the proposed method used in simulation holds much promise for better understanding of the reinforcement mechanism and further optimizing the performance of geogrid-reinforced structures.

Internal Force Transfer Mechanism and Bearing Capacity of Vertical Stiffener Joints in CFDST Structures

This paper firstly studied the internal force transfer mechanism of vertical stiffener joints in concrete-filled double steel tubular (CFDST) frame structures on the basis of finite element modeling (FEM). Analytical models of shear force and bending moment were established through the appropriate material constitutive equations and equilibrium theory. Then, the proposed models were used to predict and evaluate the shear and bending resistance of the vertical stiffener joint. Six joint specimens were tested to verify the rationality of the theoretical models, and the design suggestions for construction were subsequently discussed. The analysis indicated that the vertical stiffener together with the anchorage web played a dominated role in the internal force transfer mechanism. The computed bending resistance obtained by the tension model agreed well with the measured experimental data, and the shear resistance in the panel zone was sufficient to guarantee the ductile failure in the test. The vertical stiffener determined the plastic hinge so as to ensure the strong connection between the CFDST column and the steel beam. The ribbed anchorage web was an effective way of increasing the shear and bending resistance.

Experimental study of the force transfer mechanism in transition zone between composite column and reinforced concrete column

The current EN 1992 provides structured information related to the design of reinforced concrete columns or reinforced concrete column beam connections. On the other hand, EN 1994 gives enough information on the design of composite columns but none of the current codes provide details about a possible transfer zone in the case of usage of RC and composite column solution. The current study tends to fill the gap between these two norms. In the current experimental campaign, carried out in the frame of the European research program SmartCoCo, it is presented as a calibration method for a tentative design method which has been elaborated by one of the authors based on theoretical strut and tie reasoning. The objective of the current paper is to present the results of the experiments and aims to validate the theoretical approach for calculating the force transfer mechanism in the transfer zone. The experimental campaign comprises of 4 columns and 4 column-beam connections, all of them being composed by a RC part and a composite. The tests are performed on vertical column, simply supported with a width of 350mm, length of 380 mm and a height of 3850 mm with a regular concrete quality (C25/30). This contribution describes the test specimens, summarizes their design, presents a selection of the most relevant results from analog and digital measurements and a short interpretation of the obtain results. We concluded from this set of tests that the new design method is able to explain the force transfer mechanism with a good accuracy and can therefore be considered as a suitable solution for designing practical cases.

Design of SC Walls and Connections in Nuclear Facilities

Steel-plate composite (SC) walls and associated connections can be designed based on the provisions of Appendix N9 to AISC N690s1. AISC N690s1 is Supplement No. 1 to AISC N690-12 specification for safety related steel structures in nuclear facilities and was published in October 2015. AISC is currently in the process of developing a design guide to further enable the use of this specification. This design guide will explore the provisions of this specification in detail and discuss different possible design methodologies. SC wall details at the beginning of the design process are based on typical plant layouts, shielding requirements and prevalent practices. The spacing of tie bars and steel anchors in SC wall needs to ensure the faceplate does not undergo buckling before steel yielding. The steel anchor additionally need to be spaced to ensure that (i) interfacial shear failure does not occur before out-of-plane shear failure, and (ii) the yield strength of the faceplates is developed over the development length. The tie bars need to have sufficient tensile strength to prevent splitting failure of SC walls. The elastic analysis of the SC walls is performed using a finite element analysis. The analysis needs to consider cracked transformed stiffnesses and equivalent material properties. The analysis will be conducted for operating thermal and accident thermal load combinations. The individual demands and the combination of demands need to be compared with the available strengths. The SC walls need to be adequately detailed for openings, meet construction and fabrication tolerances, and satisfy the Quality Assurance and Quality Control requirements. The designed SC walls needs to be safe for impactive and impulsive loads. SC wall panel may need to be (i) anchored to basemat, (ii) connected to another SC wall panel, or (iii) connected to RC slab. The SC connections can be designed as full strength connection or over strength connections. The connection needs to have a well-defined force transfer mechanism. The connection required strength is determined from the design demands of SC walls and the connection design philosophy. The available strength is determined from the individual strengths of connectors participating in the force transfer mechanism.