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This study explores the possibility of using low yield point steel (LYPS) as the material for seismic retrofitting braces without casing. While buckling restrained brace has commonly been considered for LYPS applications, it is demonstrated that a more effective usage of LYPS can be achieved without the casing, which not only provides economic advantages but also facilitate a more complete utilization of the excellent ductility of LYPS. In developing design procedures for noncasing LYPS braces, calculations for stability, such as those related to compactness requirements and buckling strength, become a major concern, as seemingly too optimistic values are obtained for them when using LYPS with a particularly low yield stress. Therefore, both monotonic and cyclic loading tests were first conducted to extend the current stability-related provisions to LYPS. Cyclic test results confirmed the outstanding hysteretic performance of noncasing LYPS braces compared to those fabricated from ordinary steels. Using the results from the cyclic tests and additional test-validated numerical analyses, a nonlinear static procedure was developed for the seismic evaluation of noncasing LYPS braces. This procedure includes guides for performing a pushover analysis for the retrofitted structure and setting the acceptance criteria. The developed seismic evaluation procedure is demonstrated through a case study, in which a steel moment-resisting frame is retrofitted with noncasing bracing. Noncasing braces made of LYPS are shown to be effective in improving seismic performance, whereas those made of ordinary steel are shown to be inferior for seismic retrofit. [ABSTRACT FROM AUTHOR]

Geopolymer ceramics undergo a series of thermal phase transitions, progressing from an amorphous geopolymer gel to a crystalline phase, and eventually to an amorphous glass phase as the temperature increases. However, there is a lack of mechanism understanding regarding to the crystallization process and the subsequent thermal degradation. Here, we fundamentally investigated the kinetics of nepheline formation in Na‐based geopolymer systems and its thermal stability up to 1400°C. Nepheline crystallization is controlled by bulk nucleation and three‐dimensional crystal growth based on the Avrami factor of 4.64, where the activation energy of nepheline formation is 350.59 kJ/mol. High thermal stability of geopolymer ceramics is achieved due to the appearance of nepheline up to 1400°C with the Si/Al ratio ranging from 1.40 to 1.94, while melting and amorphous structure are formed above a higher Si/Al ratio of 2.22. The nature of sintering for geopolymer ceramics consists of shrinkage, expansion and shrinkage corresponding to dehydroxylation, crystallization, and densification, leading to a thermal shrinkage of 21% at 1400°C. [ABSTRACT FROM AUTHOR]

Suspended ceiling systems are among the most frequently reported earthquake-vulnerable nonstructural elements. Despite their importance, a rational seismic design procedure for suspended ceiling systems has not been developed because of their complex details and behavior. In this study, the seismic performance of direct- and indirect-hung suspended ceilings was evaluated based on shake table tests. In particular, a large-size ceiling specimen, comparable to an actual constructed size, was tested using an array of two shake tables and a highly stiff test frame with overall dimensions of 13 (length) × 5 (width) × 3 (height) m. In addition, six smaller specimens were tested using one shake table. All the specimens were prepared as free-floating systems by permitting free movements at the four edges. Based on the test results, key engineering parameters related to acceleration amplification and natural frequency were identified and the overall seismic performance was evaluated. Compared with the extensive failure observed in the lay-in panel ceiling specimens, marginal damage occurred around the pounded edge in the continuous panel ceiling specimens because of the robust restraining effects of the screw-attached panels on the ceiling grid members. The median amplification factors observed in the tested suspended ceiling specimens were about 1.80 and 1.75 for the vertical and horizontal directions, respectively. The dynamic behavior of the tested indirect-hung suspended ceilings which used long hanger bolts was significantly different from the pendulum behavior observed in the direct-hung suspended ceilings. The unidirectional rotation-restraining characteristic of the hanger bolt connection imparted different behaviors in each orthogonal direction. The natural frequencies of the tested suspended ceilings were well predicted when the deformed shape of the hanger bolt was modeled as a single curvature bending in the rotation-free direction and a double curvature bending in the rotation-restrained direction. The damage evaluation of each ceiling system was also presented through incremental-intensity shake table testing. [ABSTRACT FROM AUTHOR]