Steel Design

Wind loads are a major threat for old corroded steel structures constructed near the sea where wind speeds can be very high. This paper presents a case study of a wind-induced failure analysis of an existing steel structure and the proposed retrofitting methods. The examined steel structure was constructed in the 1970s in Syros, Greece and is currently operating as an athletic centre. The first part of this study presents the wind-induced failure analysis, from which a domino effect is documented. A corroded bracing that was buckled due to wind load governs the reduction of vertical load carrying of the steel structure and creates an asymmetry under horizontal loading before a number of other steel members failed due to buckling. To understand the structure’s performance, failure analysis, as well as time history and incremental dynamic analysis, were performed. The second part of this paper presents the proposed retrofitting methods for improving the vertical load carrying capacity under wind loads. The goal was to improve the load-carrying capacity of the structure so as to comply with current design European codes. In addition, enhancement of the dynamic properties of the strengthened structure was demonstrated using modal analyses. The structural behaviour was determined in a more precise manner via non-linear wind time-history and incremental static analyses. The analytical results explain the development of failures in the existing structure.

A corrugated web beam is a built-up beam with thin walled corrugated web. T RI WP, designed as a web profiling to avoid the failure of the beam due to loss of stability before the plastic limit loading of the web is reached. The web profile is a modified section of a trapezoid web profile steel section which is the eccentric stiffeners of the trapezoidal section; that is eliminated and changed to the slanting stiffeners. This paper develops a three-dimensional finite element model using LUSAS 14.3 to study on the effect of the triangular steel beam web profile (T RI WP), box beam in deflection and stress compared to that of the normal flat beam (FW). All specimens are cantilever beam which are fixed at one ends. The flange is constant with variable web profiled. Nonlinear analysis was used in analysing the deflection and stress of the flat plate model, box beam and triangular web profile (T RI WP). The FEA throw LUSAS 14.3 program can be used as a very useful tool in predicting the deflection of cantilever beams and can provide very detailed information for the distribution of stress in beams with different profile web.

In South Africa engineers are starting to use the Eurocode guidelines for steelwork design, and it is important to understand the implications and differences in results that are obtained when applying the different codes. This paper presents a comparison between the Eurocode 1993-1-1:2005 and SANS 10162-1:2005 hot-rolled steelwork design codes. Numerical comparisons of predicted member design strengths for the important modes of failure
and the complexity of calculations are presented, along with considerations regarding the
parameters used in design. The following are explicitly shown for both codes: (a) differences in the classification of commonly used H, I, PFC and equal L sections, (b) differences in tension resistance calculations, (c) comparisons of all axial buckling curves, (d) calculations for a selection of members in flexural buckling which have different classifications, and (e) a summary of the shear resistances of commonly used H and I sections. It is shown that, on average, Eurocode 3 predicts higher member design strengths than the SANS 10162 code for most failure modes, primarily because of material partial safety factors closer to unity, less conservative buckling curves and the consideration of plastic resistance of sections. These EC3 design capacities can be higher by up to 11% for tension, 35% in compression, 31% in bending and 51% in shear, although there are cases where strengths of up to 33% lower were calculated, such as for an IPEAA-200 in shear. Results are influenced by design geometric tolerances, which are based on section classifications. The Eurocode’s equations and design methodologies are more complex and computationally demanding. Since South Africa has started moving in the direction of adapting or adopting Eurocodes with the SANS 10160 Loading Code (from EN 1) and SANS 10100 Structural Concrete Code (from EN 2), it should be considered whether or not the steelwork code should be adopted or adapted in a similar fashion in the future.

Liquid storage steel tanks are vertical above-ground cylindrical shells and as typical thin-walled structures, they are very sensitive to buckling under wind loads, especially when they are empty or at low liquid level. Previous studies revealed discrepancies in buckling resistance of empty tanks between the design method proposed by the American Standard API 650 and the analytical formulas recommended by the European Standard EN1993-1-6 and EN1993-4-2. This study presents a comparison between the provisions of current design codes by performing all types of numerical buckling analyses recommended by Eurocodes (i.e. LBA-linear elastic bifurcation analysis, GNA-geometrically nonlinear elastic analysis of the perfect tank and GNIA-geometrically nonlinear elastic analysis of the imperfect tank). Such analyses are performed in order to evaluate the buckling resistance of two existing thin-walled steel tanks, with large diameters and variable wall thickness. In addition, a discussion is unfolded about the differences between computational and analytical methods and the conservatism that the latter method imposes. An influence study on the geometric imperfections and the boundary conditions is also conducted. Investigation on the boundary conditions at the foot of the tank highlights the sensitivity to the fixation of the vertical translational degree of freedom. Further, it is indicated that the imperfection magnitude recommended by the EN1993-1-6 is extremely unfavorable when applied to large diameter tanks. Comments and conclusions achieved could be helpful in order to evaluate the safety of the current design codes and shed more light towards the most accurate one.