| Fiber-reinforced polymer-concrete-steel double skin tubular columns（DSTC）is a new kind of hybrid column that consists of an outer tube is composed of FRP,an inner tube is composed of steel and concrete could be filled in the space between both tubes.In this paper,Hybrid Ribbed fibre-reinforced polymer-concrete-steel double-skin tubular columns（R-DSTC）is a new form of DSTC by welding ribs on inner steel tube.16 R-DSTC and 8DSTC were tested under monotonic axial compression in this experiment.Specimens are composed of inner steel tube made of Q690 high strength steel plate（HSS）,outer GFRP tube and sandwich high strength concrete（HSC）.The parameters including the cross-section void ratio,the presence/absence of ribs,the quantity of ribs,the length of ribs,the thickness of FRP tube and the strength of concrete on the axial compression performence were investigated.The main research work and results of this article are as follows:（1）It is found that the failure mode of R-DSTC is similar to DSTC,which shows inward buckling of inner steel tube in many places and the fiber breaking of FRP outer tube.The buckling deformation of the inner steel tube of DSTC presents a ring shape and concentrates in the middle of the steel tube,while the buckling deformation of the inner steel tube of R-DSTC distributes between the adjacent stiffeners and concentrates in the middle and upper part of the steel tube.The findings illustrate he bending stiffness of the stiffener is higher than the inner steel tube,so the setting rib is beneficial to restrain steel tube inward buckling and ameliorate the performance of HSS.（2）The axial load-axial strain curves of specimens with HSS and HSC exhibit an“ascending-descending-ascending-failure”behavior.Setting ribs can improve the initial stiffness of the first stage of the axial load axial strain curve,effectively reduce the deteriorating range of the descending section,and improve the ultimate bearing capacity of the specimens.The above effects enhance with the increase of stiffener setting and size.Compared with the DSTC,the average ultimate bearing capacity of R-DSTC can be increased up to 29.8% if other parameters are determined.The ultimate axial strain of R-DSTC is obviously larger than DSTC.This results indicate that the composite effect of three materials in R-DSTC is better than DSTC,that could bring into full play to the potential of the three materials.（3）The cross-section void ratio reflects the influence of the concrete cross-sectional area on the specimen performance.The load deterioration ranges of smaller cross-section void ratio spceimens are 32.0% and 30.5%.While the load deterioration ranges of bigger cross-section void ratio spceimens are 24.7% and 27.7%.The average fracture strain are higher than the latter by 12.1%.While the average ultimate axial strain of the former is22.3% smaller than the latter.In other words,the specimens with smaller hollow ratio has higher integral stiffness,higher bearing capacity,but its ductility is inferior.（4）The ultimate bearing capacity of specimens as the thickness of GFRP tube increases.Only if the number of ribs is variable,the average ultimate bearing capacity of GFRP outer tube with 4.5mm and 2.2mm is increased by 11.6%,4.4% and 8.9% respectively.In the middle and later loading stage of axial load-strain curves,the restraint whose interlayer concrete as well as inner steel tube and the slope of the secondary rising section magnify with the increase of the outer tube thickness of GFRP.（5）Based on the original FRP confined concrete formula at ultimate state,a new formula was put forward,which considers HSC＆UHSC strength.Furthermore,a new model is proposed,which considers the cross-section void ratio and stiffener.The axial compression behavior is predicted precisely by the new model proposed in this paper.At the same time,compared with the original model,the prediction whose model proposed is more accurater. |
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