Practical application of CFD for wind loading on tall buildings

This paper is concerned with assessing the scope of appicabiity for computational fluid dynamics(CFD) in the field of structural engineering, with a particular reference to tall buildings. Modern design trends and advances in engineering materials have encouraged the demand for taller and more slender structures. This pattern induces inherent structural flexibility; these cases exceed the limitations of the quasi-static method offered by current codes of practice. Wind tunnel testing is the traditional solution for such dynamically sensitive structures. However, even this scaled modelling approach is clouded by some uncertainties, including scaling the Reynolds number and assuming damping values for the aeroelastic model. While CFD cannot be used as a replacement for wind tunnel testing, there are results within the literature to suggest it has the potential to act as a complimentary tool - provided it is used within its capabilities. The paper outlines the various turbulence models that are available and summarises the extent of their application in a practical structural engineering sense. It also details the user-defined criteria that must be satisfied and discusses the potential for simplified models in tall building CFD analyses, with a view to promoting more efficient and practical solutions

Web crippling design of cold-formed duplex stainless steel lipped channel-sections with web openings under end-one-flange loading condition

Cold-formed stainless steel sections are becoming more widely used in the residential and commercial sectors due to their high corrosion resistance and high strength-to-weight ratio. However, their susceptibility to web crippling at points of concentrated loading is well-known to be an important design issue. In addition, web openings are also become popular, as they improve ease of installation of services. This paper presents the results of an investigation into the effect of web crippling on cold-formed duplex stainless steel lipped channel-sections, having such openings, under the end-one-flange (EOF) loading condition. 728 non-linear elasto-plastic finite element analyses are undertaken, with web openings located either centred above the bearing plate or offset to bearing plate. The effect of the size of the web opening, length of bearing plate and location of the web opening is considered. Strength reduction factor equations are proposed, that can be used to take into account such openings in design

Coupled element and structural level optimisation framework for cold-formed steel frames

Optimisation of cold-formed steel (CFS) structures can be challenging due to the complex behaviour of thin-walled CFS sections affected by different buckling modes. In this paper, a coupled framework is presented for element and structural level optimisation of CFS portal frames, under serviceability limit state (SLS) and ultimate limit state (ULS) conditions, using Genetic Algorithm. First, CFS lipped-channel beam sections are optimised with respect to their flexural capacity determined in accordance with the effective width method specified in Eurocode 3 (EC3). The relative dimensions of the cross-section are considered as the main design variables, while the EC3 plate dimensions and slenderness limits and a number of manufacturing and end-use constraints are taken into account in the optimisation process. The results show that the optimum CFS sections exhibit significantly higher (up to 84%) ultimate capacity compared to the standard lipped channel sections with the same plate width and thickness. The structural level optimisation is then carried out to obtain the optimal design solution for a long-span CFS portal frame with knee braces under SLS and ULS conditions. Compared to conventional optimisation using standard cross-sections, it is shown that the proposed coupled framework leads to more cost-effective solutions (up to 20% less structural material) by using the more efficient CFS cross-sectional shapes optimised for generic applications. The results also indicate that optimising the frame geometry and knee brace configuration can noticeably improve the structural performance and reduce the required structural weight, especially when both ULS and SLS conditions are considered

Design and optimization of cold-formed steel sections in bolted moment connections considering bimoment

The load transfer mechanism in cold-formed steel (CFS) bolted moment connections is mainly through the bolt group in the web of beam elements, which may lead to relatively large bimoment and warping deformations. While the bimoment effects can be considered in the Direct Strength Method (DSM), ignoring the fact that the bolt-group length in the conventional design process can lead to nonconservative solutions. This paper presents an alternative analytical design approach using Eurocode 3 (EC3) effective width method to determine the ultimate flexural strength of CFS bolted moment connections by considering bimoment effects. The results compare very well with previously published experimental test data as well as detailed finite-element models developed in this study. It is shown that a short bolt-group length may lead to up to 25% reduction in the flexural strength of the CFS bolted connections. However, a longer bolt-group length generally results in a moment capacity almost equal to the flexural strength of the CFS channel section. Shape optimization is then conducted using a genetic algorithm (GA) to improve the flexural capacity of the connections by taking into account the bimoment effects. The main design variables are considered to be the relative CFS beam cross-sectional dimensions, while the plate slenderness and dimension limits suggested by EC3 as well as a number of manufacturing and practical end-use constraints are incorporated as design constraints. It is found that, compared with standard cross-sectional dimensions, the optimized sections can improve the flexural strength by as much as 36% for a bolt-group length equal to the depth of beam element

Finite element idealisation of a cold-formed steel portal frame

A simple linear beam idealization of a cold-formed steel portal frame is presented in which beam elements are used to idealize the column and rafter members, and rotational spring elements are used to represent the rotational flexibility of the joints. In addition, the beam idealization takes into account the finite connection length of the joints. Deflections predicted using the beam idealization are shown to be comparable to deflections obtained from both a linear finite element shell idealization and full-scale laboratory tests. Using the beam idealization, deflections under rafter load are divided into three components: Deflection due to flexure of the column and rafter members, deflection due to bolt-hole elongation, and deflection due to in-plane bracket deformation. Of these deflection components, the deflection due to bolt-hole elongation is the most significant and cannot, therefore, be ignored. Using the beam idealization, engineers can analyze and design cold-formed steel portal frames, including making appropriate allowances for connection effects, without the need to resort to expensive finite element shell analysis

Stiffness prediction for bolted moment-connections between cold-formed steel members

The authors have recently described a cold-formed steel portal framing system in which simple bolted moment-connections, formed through brackets, were used for the eaves and apex joints. Such connections, however, cannot be considered as rigid because of localised in-plane elongation of the bolt-holes caused by bearing against the bolt-shanks. To therefore predict the initial stiffness of such connections, it is necessary to know the initial bolt-hole elongation stiffness kb. In this paper, a finite element solid idealisation of a bolted lap-joint in shear will be described that can be used to determine kb; the results obtained are validated against experimental data. A beam idealisation of a cold-formed steel bolted moment-connection is then described, in which spring elements are used to idealise the rotational flexibility of the bolt-groups resulting from bolt-hole elongation. Using the value of kb in the beam idealisation, the deflections predicted are shown to be similar to those measured experimentally in laboratory tests conducted on the apex joint of a cold-formed steel portal frame

Ultimate strength of bolted moment-connections between cold-formed steel members

The behaviour and design of bolted moment-connections between cold-formed steel members, formed by using brackets bolted to the webs of the section, is considered. The particular problem of the moment-capacity of such joints being lower than that of the cold-formed steel sections being connected because of web buckling, caused by the concentration of load transfer from the bolts, is addressed. In this paper, a combination of laboratory tests and finite element analyses is used to investigate this mode of failure. It is demonstrated that there is good agreement between the measured ultimate moment-capacity and that predicted by using the finite element method. A parametric study conducted using the finite element model shows that the moment-capacity of a practical size joint can be up to 20% lower than that of the cold-formed steel sections being connected. Web buckling so-caused must therefore be considered in the design of such connections

Serviceability design of a cold-formed steel portal frame having semi-rigid joints

Details are given of a cold-formed steel portal framing system that uses simple bolted momentconnections for both the eaves and apex joints. However, such joints function as semi-rigid and, as a result, the design of the proposed system will be dominated by serviceability requirements. While serviceability is a mandatory design requirement, actual deflection limits for portal frames are not prescribed in many of the national standards. In this paper, a review of the design constraints that have an effect on deflection limits is discussed, and rational values appropriate for use with cold-formed steel portal frames are recommended. Adopting these deflection limits, it is shown through a design example how a cold-formed steel portal frame having semi-rigid eaves and apex joints can be a feasible alternative to rigid-jointed frames in appropriate circumstances

Design and development of a general cold-formed steel portal framing system

A general cold-formed steel portal framing system is proposed that uses simple bolted moment connections, formed through brackets, for the joints. Such brackets are easy to manufacture and the joints easy to assemble on site. The joints, however, are semi-rigid and the importance of this for the design of the frame is explored through a combination of numerical analysis and full-scale testing. A simple beam idealisation that accounts for the semi-rigidity of the joints in a frame analysis is described and verified using a combination of finite element shell analysis and laboratory tests. The efficiency of the proposed frame is compared to that of an equivalent rigid-jointed frame, including consideration of serviceability requirements and practical site constraints on the size of the brackets and number of bolts used for the joints. For certain combinations of frame geometry and imposed loading such frames are shown to provide very competitive alternatives to conventional rigid jointed hot-rolled steel construction

F. E.-assisted design of the eaves bracket of a cold-formed steel portal frame

Non-linear large-displacement elasto-plastic finite element analyses are used to propose design recommendations for the eaves bracket of a cold-formed steel portal frame. Owing to the thinness of the sheet steel used for the brackets, such a structural design problem is not trivial as the brackets need to be designed against failure through buckling; without availability of the finite element method, expensive laboratory testing would therefore be required. In this paper, the finite element method is firstly used to predict the plastic moment capacity of the eaves bracket. Parametric studies are then used to propose design recommendations for the eaves bracket against two potential buckling modes of failure: (1) buckling of the stiffened free-edge into one-half sine wave, (2) local plate buckling of the exposed triangular bracket area. The results of full-scale laboratory tests on selected geometries of eaves bracket demonstrate that the proposed design recommendations are conservative. The use of the finite element method in this way exploits modern computational techniques for an otherwise difficult structural design problem