PERFORMANCE DETERIORATION OF SLOTTED-BOLTED CONNECTION DUE TO BOLT IMPACT AND REMEDY BY RESTRAINERS
The behaviour of slotted-bolted connections when the maximum slip travel exceed the available slot length is presented. The drastic degradation behaviour of slotted-bolted connections under bolt impact is demonstrated. Remedy by means of the concept of restrainers is proposed, which is also advantageous in suppressing the build-up of resonance motion by virtue of the change in system stiffness at different stages. Nonlinear dynamic analyses were performed on a prototype 6-storey steel building equipped with friction dampers with and without restrainers. Three historical earthquake records from California, i.e. the 1989 Loma Prieta, the 1966 Parkfield, and the 1940 El Centro records, were considered in the analyses. In the worst loading case, dampers with restrainers are superior to that without restrainers, resulting in reduction of the peak inter-storey drift by 16% and the peak slip travel by 15%. The shorter slip travel obviously provides extra safety to the system. Increasing the maximum restraining force is generally beneficial in reducing the peak inter-storey drift. However for steel bracings, the restraining force limit should be kept sufficiently below the buckling capacity to avoid buckling of the bracings. Furthermore, a higher restraining force would induce higher storey base shear. Hence, the adequacy of the foundation has to be properly addressed as well in retrofitting of buildings.
Α NEW SEISMIC ENERGY ABSORPTION DEVICE THROUGH SIMULTANEOUSLY YIELD AND FRICTION USED FOR THE PROTECTION OF STRUCTURES
Passive energy dissipation systems do not require external power to generate system control forces and hence, are easy and cheap to implement in a structure (existing or new). The new seismic energy absorption device, which is presented in this paper, is an improvement of the device CAR, proposed by Papadopoulos et al (2008) and it is incorporated at the ends of the steel diagonal bars. The improved device, through replaced the traverse elastoplastic regulation bolts with superimposed steel blades, aims at avoiding material fatigues of the bolt because of the repeated load cycles. Also it has many advantages. First of all, it limits the maximum axial forces developed in the steel diagonal bars of the structure, with the appropriate choice of dimensions and characteristics of the superimposed steel blades. Also, allows additional horizontal relative floor displacements of the structure without increasing the axial load of the steel diagonal bars, which means increase of the absorbed seismic energy. The main advantage of the proposed new device is that it can function reliably under large cycles of dynamic load, where large horizontal relative floor displacements develop. At the same time, when the horizontal relative floor displacement reaches a characteristic value, then the new device locks, so that the buckling of the compressed diagonal bars is decisively removed, so introduces second degree of protection of the structure in the case of very strong earthquakes. So this improvement is capable of producing higher axial forces and deformations, before the collapse. In order to investigate the suitability of the new proposed device CAR1 described in the present paper, nonlinear analyses have been carried out using, on one hand, a static pushover procedure with lateral floor monotonic incremental static forces with a triangular distribution in elevation and, on the other hand, nonlinear response history analysis with suitable seismic artificial accelerograms. Two different structures of reinforced concrete are used, of which the first is a one storey planar structure with five frames and the second is a three storey planar structure. Also each structure is being analyzed using the pushover analysis and time history analysis, as regards three different cases, the initial pure r/c frame without strengthening, the simple strengthening frame and the strengthening frame with the new proposed device CAR
Engineering bio-inks for 3D bioprinting cell mechanical microenvironment
Three-dimensional (3D) bioprinting has become a promising approach to construct­ing functional biomimetic tissues for tissue engineering and regenerative medicine. In 3D bioprinting, bio-inks are essential for the construction of cell microenviron­ment, thus affecting the biomimetic design and regenerative efficiency. Mechanical properties are one of the essential aspects of microenvironment, which can be char­acterized by matrix stiffness, viscoelasticity, topography, and dynamic mechanical stimulation. With the recent advances in functional biomaterials, various engineered bio-inks have realized the possibility of engineering cell mechanical microenviron­ment in vivo. In this review, we summarize the critical mechanical cues of cell micro­environments, review the engineered bio-inks while focusing on the selection princi­ples for constructing cell mechanical microenvironments, and discuss the challenges facing this field and the possible solutions for them.