BEHAVIOR OF DOUBLE SKIN (CFRP/STEEL) TUBULAR CIRCULAR HIGH STRENGTH MORTAR COLUMNS UNDER AXIAL COMPRESSION LOAD

Abstract

ABSTRACT A hybrid double skin tubular members (DSTM) consist of three composite materials in one section, include fiber reinforced polymer (FRP) tube out ward, steel tube in ward and concrete in between. These three materials give the member unique properties like protect column from harsh environment, reduce amount of concrete by presence of inner steel tube that allowed to the passage of services. Therefore, consider sustainable structure element, as well as rising strengthening of members and excellent corrosion resistance. This study presents experimental program include 24 columns with two types of circular columns, concrete filled FRP tube (CFFT) and double skin tubular column (DSTC), with dimensions 100 mm*310 mm that divided into five groups which constructed and testing under axial compressive load till failure. With variables, numbers of layers of outer FRP tube, compressive strength of mortar, ts/d ratio and ds/d ratio of inner steel tube. The experimental results showed that the mortar was effected confinement in the hybrid DSTC led to very ductile behavior of stress strain relationship, the number of outer FRP layers was the most effective parameters on DSTCs behavior. The nominal confinement ratio was affected by thickness of outer FRP tube and compressive strength of concrete inside DSTCs. By increase number of layers of outer FRP tube from 1 to 2 and 3) layers, the strength enhancement ratio increasing by II (48%, 72% and 155%), the enhancement of axial strain increasing by (114%,123% and 274%) and the ductility rising by (149%,183% and 266%). When increasing mortar compressive strength from 45 to (50, 55 and 60) MPa, the strength enhancement ratio was about (50%, 40%, 41% and 48%), enhancement of axial strain decreasing by (215%, 164%, 157% and 114%) and the ductility decreasing by (174%, 160%, 167 and 149%). By using ts/d ratio of inner steel tube from 0.015 to (0.02 and 0.03), the strength enhancement ratio decreases by (48%, 38% and 41%), the enhancement axial strain increase by (114%, 126% and 134%) and the ductility was about (149%, 137% and 150%). By increase ds/d ratio of inner steel tube from 0.25 to (0.32, 0.38 and 0.48), the strength enhancement ratio decreased by (61%, 53%, 50% and 48%), the enhancement of axial strain was about (191%, 167%, 130% and 114%) and the ductility decreasing by (223%, 176%, 146% and 137%). When the ds/d ratio decrease the behavior of specimens similar to the behavior of CFFT. By compared between CFFT and DSTC the enhancement ratio is approximately more in CFFT than DSTC, and presence of inner steel tube in DSTC is supported to concrete in order to less press concrete on outer FRP tube.