Behaviour of stainless steel and cold C sections with large web openings in bending and shear.

This research addresses the use of thin steel C sections with large circular, hexagonal and diamond web openings in applications where service integration in beams is required or where thermal bridging of members crossing the building envelope has to be minimised. The effect of these openings in terms of combined bending and shear effects on stainless steel and thin steel C sections is evaluated. The behaviour of thin steel C sections with large web openings in shear is a new subject and the knowledge gap is mainly concerned with the local buckling around and between openings which affect the ability of the perforated section to resist shear. Simply supported beams were considered in this research. The main part of the research was divided into various parts as follows: Simple theories were developed for the design of thin steel with circular web openings (Tangential Stress Method) and with hexagonal web openings. The Tangential Stress Method is a method in which the tangential stresses around the web openings are determined as a function of the applied shear force. A total of 16 tests on pairs of C sections with web openings was carried out. Three groups of beams were tested as follows: 1- Stainless steel C sections of 210 mm depth and 70 mm width with 150 mm diameter openings at 50, 100 and 250 mm edge distances were tested. Two groups of stainless steel were tested; Austenitic and Lean Duplex (LDX) grades of 2 and 3 mm thickness. For beams with isolated and widely spaced web openings, Vierendeel bending associated with local buckling was the mode of failure. Beams with closely spaced web openings failed by web-post buckling. 2- Galvanized steel sections of 250 mm depth and 63mm flange width with 150 and 180 mm diameter web openings at 60 and 90 mm edge distances were tested in 1.5 and 1.8 mm steel thicknesses. The failure of the C sections with isolated web openings was due to Vierendeel bending associated with local buckling. For closely spaced web openings, the failure was due to web-post buckling and twisting of the top flange. 3- Galvanized C sections with diamond and hexagonal web openings were tested to investigate the shape of the web openings effect on the behaviour of the beams subject to shear. The diamond-shaped openings were 180 mm deep and hexagonal openings were 167 mm deep. The failure of beams with isolated diamond-shaped web openings was due to buckling of the un-supported web next to the openings. For the C sections with pairs of openings, it was due to web-post buckling and twisting of the top flange. The bending resistances of the two C sections were then predicted from the parametric study and were compared with the design resistance calculated using section properties to BS EN 1993-1-3 and BS EN 1993-1-4. The tangential stresses using the method presented in Chapter 4 were calculated at the failure loads in all tests for beams with circular web openings and compared to the measured steel strengths fy at 0.2% strain. The ratio of the direct tangential stress to steel proof strength σ/fy varied between 0.70 and 1.21 for the stainless steel beams and between 0.5 and 0.8 for the galvanized steel beams. This shows that the Tangential Stress Method is reasonably accurate. Linear and non-linear finite element (FE) models were defined to investigate the behaviour of the thin C sections. ABAQUS software was used for the finite element analysis. An extensive parametric study was conducted to study the effects of opening diameter, opening spacing, and span to depth ratio of the beams. The failure load for each beam was determined using the Riks Analysis Method (explained in Chapter 8) ignoring the effect of web imperfection which was found to have a little effect of on the failure load. The comparison between buckling analysis with different imperfection values and the Riks analysis for the two beams sections is presented in Chapters 10 and 11 for the various openings configurations. The results from the FEA were in good agreement with the test results and showed the effect of the opening depth to the beam depth ratio (h0 /h), opening spacing (so) and the thickness of the web (tw) on the section resistance. The section resistance obtained from the finite element analysis for all models was in good agreement with the test results and the proposed theory. In the final part of this research, the additional deflection due to the loss of the shear and bending stiffness at the position of web openings was investigated and simple formulas were developed in Chapter 6 to predict the additional deflection of perforated beams. Linear finite element analysis was considered for comparison and the results were in good agreement with the proposed theory