Damage Threshold of Near-Cross-Ply and Angle-Ply Tubes Used in Concrete Filled FRP Tubes Loaded in Flexure
Concrete-filled fibre-reinforced polymer (FRP) tubes (CFFTs) have been studied extensively, but a lingering question remains regarding the vulnerability of the exposed tube to accidental damage or vandalism. In this thesis, the effect of damage induced in the tubes on the flexural strength of CFFTs is studied. Two commonly available types of tubes are explored, one with near-cross-ply having nearly orthogonal fibre arrangements, and one with large (±55°) angle-ply fibre arrangements. Eighteen CFFTs using the former tube and twelve CFFTs using the latter tube were tested in four-point bending, including undamaged control specimens to establish their full nominal strength (Mno). Full wall thickness cuts were introduced in the glass-FRP tubes in the circumferential and longitudinal directions, on both the tension and compression sides. The cuts also varied in length, as a ratio of the outer perimeter πD, and their proximity to neutral axis was varied for one type of tubes. It is shown that the most critical cut for both tubes is the circumferential one on extreme tension side. The reduced-to-control ultimate moment ratio (Mn/Mno) of the angle-ply CFFTs reduces almost linearly from 1.0 to 0.36 as the cut length increases from zero to 20% (πD). This is much less critical than in near-cross-ply CFFTs, where (Mn/Mno) drops sharply from 1.0 to 0.55 at a 2% (πD) cut, then gradually to 0.25 at a 20% (πD) cut. The maximum difference between angle-ply and near-cross-ply CFFTs seems to occur around the 10% (πD) cut, where (Mn/Mno) ratio of the near-cross-ply CFFT is only half that of angle-ply CFFT. As the location of a 3% (πD) circumferential cut shifted from extreme tension to an 80° angular location from the bottom of a near-cross-ply CFFT, (Mn/Mno) increased from 0.52 to 1.0. Longitudinal tension cuts caused (Mn/Mno) to drop linearly from 1.0 to 0.47 at a 22% (πD) cut length, which was similar for both tubes. Compression side longitudinal and circumferential cuts were less critical with (Mn/Mno) being 0.8 to 0.9. For practical design purposes, a case study based on ultimate static strength showed that CFFTs similar to the ones tested in this study can tolerate a circumferential tension cut up to 30% (πD), for both tube types, before they have to be taken out of service or repaired.