Méthodologie de Recherche

      Flexural behaviour of cold-form steel sections subjected to static load

Author: Oulfa Harrat

Abstract. Currently, cold-formed steel sections (CFS), such as "C" and "Z" sections, are commonly used for structural design. However, such open sections are subject to certain instability modes, such as flexural buckling. It is therefore important to deal with this kind of problem and to find a solution to increase the strength of these elements. For this purpose, a numerical finite element analysis is performed by ABAQUS 6.14 software of cold-formed steel beams of C and Ω (hat) shape sections. The models are 1m long and 4mm thick, and were examined for performance against instabilities. These beams are subjected to a static load in the middle varying in intensity (the 3-point bending test) in addition an analytical study was made based on Eurocode 3. The results of both sections are compared and a very good agreement with the analytical results was found.

Keywords: Cold-formed steel, Flexural buckling, Instabilities, ABAQUS.

Introduction [Modifier]

Structural members in steel construction are classified into two categories: hot rolled sections and cold formed sections. Cold-formed steel sections (CFS) are preferable to hot-rolled sections because of their greater versatility and because they are well suited to economical construction [1]. Cold-formed steel is also known as lightweight steel. In civil engineering, cold-formed steel elements are typically used in industrial buildings as purlins, columns, trusses or structural members, storage racks, vehicle bodies and various types of equipment [2]. In recent years, the use of CFS structures has increased rapidly due to significant improvements in manufacturing technology. CFS elements are manufactured from steel sheets and are formed into different shapes, either by press-bending of sheared sheets or coils or, more commonly, by rolling at room temperature [3, 4]. CFS structural elements maintain their position in the lightweight construction sector. This is due to the significant advantages of CFS: high strength-to-weight ratio, ease of transport, possibility of using all conventional joining methods, cost effectiveness, very good quality control, dimensional stability and flexibility in the manufacture of profiles compared to hot rolled sections [5, 6]. 

The problems of cold-formed sections are associated with the phenomenon of instability. The verification of this instability is in many cases a difficult problem to deal with especially in open sections.       

Our objective is to study the behaviour of open section beams made of cold formed sections of shapes "C" and "Ω" by the finite element method using the software "ABAQUS" based on instabilities plus verification with the Eurocode3 part 1.3 approach.

 A comparative study to bring out the advantages and disadvantages of each of the two calculation procedures.

Presentation of the models
[Modifier]

For the purpose of this paper, the section to be studied was chosen to be a "C" shaped section with dropped edges and a "Ω" shaped cold formed section (Fig. 1). The beams are of 1m span and are subjected to a static load in the middle that varies in intensity (the 3-point bending test). The beams are made of cold rolled structural steel S220 according to ISO4997.

                            Fig. 1. Cross-sections of form (C) and (Ω) beams.

Table 1. Mechanical properties of cold-formed steel

Element	Density γ (kg/m3)	Young's modulus E (MPa)	Coeff of poison  ν	Plastic Module (MPa)
Steel	7850	210000	0.3	220

Mesh [Modifier]

For the meshing of the beams by finite element, refer to the ABAQUS library and according to a case study proposed by the latter. In our study, we use elements of the S4R type: A thin or thick double curved shell with 4 nodes, reduced integration with finite deformation of the membranes. 

Results and discussion [Modifier]

For a thickness of 4mm and a concentrated load of 20kN and for the models without rounding, a concentration of stresses in the load application area was observed, where the profile Ω showed a lower condition compared to model C. The edges on the outside increased the profile strength Ω with a difference of 46% between model C and Ω.

For a load of 20 KN and for the models without rounding, model Ω showed a lower displacement condition compared to model C, a reduction of 41%( Fig. 3).

                   Fig. 3. Comparison of stress and displacement states in the C and Ω sections.

• Figure 4 shows the distribution of the stress state with the failure modes of the beams. 

                                               Fig. 4. Failure modes of treated beams.

Conclusions

• It is clear that the hat-shaped section (Ω) is more stable than the shaped section (C).

• A significant deformation state in the central zone (load application zone), a 50% decrease in deformation of the C model compared to the Ω model.

• The manufacturing cost of the hat section (Ω) is higher than that of the section in (C). Apart from this cost consideration, the hat section (Ω) is quite strong than the (C) section.

References [Modifier]

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   [2]    Reza, W., Selvan,  S.S.: Experimental study on flexural behaviour of cold formed steel channel and i sections providing angle stiffener on the web, Indian Journal of Science and Technology, 9(35), (2016)..

   [3]    Mansuri, S., Parmar, P.K.A.: Numerical investigation of flexural strength of cold form built-up beams, International gournal of advance research in science and engineering, 6(01), pp.109-116, (2017).

   [4]    Parvati, S., Joanna P.S., Samuel J., Eapen Sakaria P.: Flexural behavior of cold formed steel beams With end stiffeners and encased web, International Journal of Engineering Research & Technology, 3 (11), pp.1276-1297, (2014).

   [5]    Khadavi1, A., and Tahir M.M.:  Prediction on Flexural Strength of Encased Composite Beam with Cold-Formed Steel Section. Proceedings of the 3rd International Conference on Construction and Building Engineering (ICONBUILD), pp.1-7, (2017).

   [6]    Sureshbabu,S., SenthilSelvan S.: Experimental Investigation on the Flexural Behaviour of Cold Formed Corrugated Steel Channel Sections. International Journal of Innovative Technology and Exploring Engineering (IJITEE), 8(6S3) , pp. 124-132, (2019).