Plastic buckling and axial crushing of concrete-filled steel tubes: usage of multiple wood blocks

This paper aims to lighten the weight of concrete-filled steel tubes using wood blocks. Different proportions of wood to concrete were used in a way that wood blocks were substituted with a fraction of concrete in small-scale columns. The behaviour of the composite members was compared with the reference specimens, i.e. fully concrete-filled tubes. Carbon Fibre Reinforced Polymer (CFRP) was also employed as a confining material for some specimens, the effect of which was investigated. The weight reduction that timber provided was evaluated against the axial capacity of each specimen. A new index is introduced to quantify the structural efficiency with a view to evaluating capacity change against the weight. A new parameter is defined in this paper to determine and quantify the transitional response of columns under compression between different peaks loads on load-displacement graphs. This parameter can further help reduce potential risks of failure/collapse at post-peak stages. A key finding was that the number of timber blocks in specimens with the same volume of timber did not affect the axial capacity, which paves a path for designing timber blocks in multiple numbers. Despite a significant difference between the compression strength of timber and concrete, the capacity reduction provided by timber was not proportionate (was smaller) to the strength difference of the two materials, nor was proportionate to the weight reduction. This disproportion, which can be due to the interaction of all three materials, is structurally desirable especially for the seismic design where the total weight plays a key role. This can be attributed to the usage of stronger and thicker steel acting as a confining material, which is advised for similar structures based on the results of this study. •Plastic Transition Triangle (PTT) is defined for the first time to quantify the transition area of load-displacement.•A new index (Ei) is introduced to quantify the structural efficiency to evaluate capacity change against weight.•The number of timber blocks in a certain volume of timber did not affect the capacity.