Yu, Tao, and Wu 2008
This paper studies the possibility of using thin-walled hollow structural steel columns filled with very high strength self-consolidating concrete (SCC). The main parameters varied were section types, circular and square; slenderness ratio, from 12 to 30; and load eccentricity ratio, e/r (e is load eccentricity, r=D/2, D is overall section dimension), from 0 to 0.6. Comparisons were made with predicted column strengths using the existing codes such as AISC, EC4, and DBJ13-51-2003.
Experimental Study, Results, and Discussions
Twenty-eight specimens, including eight stub columns and 20 beam-columns were tested. All steel tubes were manufactured from mild steel sheet. The average yield strength of the steel was found to be 58.6 ksi with a modulus of elasticity of 30,000 ksi and an ultimate strain of about 0.3. The fresh properties of the SCC mixture were as follows: slump flow: 10.6 in; unit weight: 4030 lb/yd^3; flow speed: 0.063 ft/s; and flow distance 23.6 in.
One end of each tube was tack welded by a steel bottom plate with a thickness of 0.47 in. The SC was then filled in layers of about 20 in. without vibration at intervals of about 3 min. to ensure compaction. A high-strength epoxy was used to fill the longitudinal gap so that the concrete surface was flush with the steel tube at the top. Another 0.47 in. thick steel plate was welded to the top of each specimen. The average cube strength was 17.6 ksi and the modulus of elasticity was 6,180 ksi.
The eight stub columns consisted of four identical circular section specimens and four identical square section specimens. All specimens had a width or diameter of 3.937 in., tube thickness of 0.075 in, and length of 11.8 in. The length was chosen to avoid the effects of overall buckling and end conditions. A load interval of less than one tenth the estimated load capacity was maintained for about 2 minutes. Failure results from these tests indicated that the failure mode of square columns was local outward folding failure mechanism while that for circular columns was a shear failure model. Tests indicated that circular columns were more ductile than square columns. Authors provide a table of calculated strength indexes for the column stubs.
Twenty tests on concentrically and eccentrically loaded beam-columns were carried out. The axial load was applied on the specimen through a stiff top platen with an offset triangle hinge, simulating pin-end supports. Specimens were incrementally loaded with each interval being maintained for about 2 minutes. Measured deflection curves were described as being close to a half sine curve in shape. The typical failure mode was overall buckling failure; buckling generally occurred near mid-height of the beam-columns. Ductility for very high strength SCC filled steel tubes is generally smaller than for normal strength concrete filled steel tubes.
Comparisons made with predicted section capacity using existing codes such as AISC, EC4, and DBJ13-51-2003 found that these codes are generally acceptable for prediction of member capacities of high strength SCC filled high strength steel columns. However, it seems some codes do give slightly higher predictions on the member capacities of columns with square sections.
Reference
- Yu, Q., Tao, Z., and Wu, Y.-X. (2008). “Experimental Behaviour of High Performance Concrete-Filled Steel Tubular Columns.” Thin-Walled Structures, 46(4), 362–370. doi: 10.1016/j.tws.2007.10.001