Over the past two decades, there has been significant interest in research relating to concrete-filled tubes, and a corresponding penetration of this technology into practice. This paper aims to expound upon the effect of timber cores on the structural response of concrete-filled circular tubes under compression. A timber infill with different shapes and geometries surrounded by concrete and encased in a steel tube was employed. The effects of the combination of infill elements on the failure, axial capacity, ductility, and structural efficiency (weight versus capacity) are exhaustively set forth. For the specimens with the highest timber to concrete ratio, the capacity was enhanced by about two times the capacity of the hollow steel specimens. For these specimens a significant reduction in the total weight of the composite element was obtained relative to the fully concrete-filled specimens. These specimens showed the highest ductility among the other specimens. In addition, greater ratios of energy absorption to the mass were obtained for the specimens with different timber cores in comparison to the equivalent values for fully concrete-filled tubes, which is quite desirable in many practical scenarios. It is found that the use of timber as an inner core element in this new composite yields promising results in decreasing the weight and yet enhancing the capacity, ductility, and energy absorption, and can be a good alternative to double-skin concrete-filled steel tubes.

Composite timber beams strengthened by $U$ -shape steel sections and carbon fiber–reinforced plastic (CFRP) are studied experimentally in this paper. Specimens with various strengthening, i.e., steel only, CFRP only, and a combination of the two, were considered under three-point flexural tests. Failure, displacement and strain response, ductility, bending capacity, and structural efficiency were evaluated for the present tested models. Dramatic enhancement of the capacity in addition to improved deflection and ductility were gained for the strengthened beams relative to the plain specimens, indicating the effectiveness of the reinforcement on the flexural strength of such composite beams.