Keywords: composite beam, open-web, composite construction, steel and concrete.

Composite construction makes possible the exploration of the advantages of the two main materials used in civil engineering: concrete and steel. As a result of the requirement to pass a duct through a beam as a consequence of weight minimization, or architectural motivation; the steel I-beam in composite construction may need holes in the web. A hexagonal hole pattern is an option and may be produced from two "T" rolled sections welded by their webs after the hole pattern is cut-off. This paper deals with a numerical study of the stresses acting in the weld line of I-beams with hexagonal holes [1] in the web. The study is performed using finite element (FE) models [2] discretizing the entire steel I-beam section considering a complete interaction between the steel and concrete slab.

A composite beam system with holes in the web has the advantage of reducing the beam weight and, moreover, the holes allow ducts to pass through the beam web. Different hole patterns offer diverse design options to architects. The composite I-beam analysed in this paper has a hexagonal hole pattern produced from zigzag cuts of period () along the span of a rolled I-beam with a full-web. After the cuts, two rolled "T" beams are formed. One of the "T" section may be rotated or displaced to produce the desired hexagonal hole pattern of the new open-web I-beam. For simplification, this new Hexagonal open-web I-Beam is named in this paper as "HoIB". The new HoIB section is taller than the original IB. The increase in the height (and as a result an increase in the flexural resistance) is achieved without adding any steel material; thus, the HoIB has the same weight of the original I-Beam.

After the assemblage of the new HoIB, the connecting weld between the two "T" sections will be at the centre line. When this new HoIB is used in a composite construction (due to the concrete slab) the NA moves towards the concrete slab. This fact produces higher stresses in the welding region. Considering a complete interaction between concrete and steel section [3], the composite beam using the new HoIB may have the NA located on: (a) the concrete slab, or (b) the HoIB flange, or (c) the I-beam web.

In this research paper, the stress [4] on the weld are determined numerically with 3D FE models. This study is limited to composite beams simply supported at the ends, and under uniform loading along the beam spans [5]. The cross section for the original I-Beam in this paper is the W200x15.

Stress analysis from the FE results and analytical formulations proposed are presented in the paper. The stress results demonstrate that the weld string under higher stress is always near the ends of the beam next to the supports, where the shear stress is at maximum and bending moment is at a minimum. The values of the weld stresses decrease in the direction of the middle of the beam span, where the bending moment is at maximum and the shear stress is at minimum. It is observed that the weld stresses decrease as the NA position in the concrete slab goes towards the HoIB upper flange. This fact also produces a reduction in the bending moment strength causing also a decline in the values of the weld stress. Moreover, for short spans, high shearing stress takes place at the support region causing stress increments at the weld string. This is explained because for constant bending moment value at the middle of the beams, the beam with shorter span is under higher uniform loading. For this reason, higher shearing stresses at the support regions are observed.

1

Cimadevila, J.C., Gutiérrez, E.M., Rodríguez, J.A.V., "Vigas Alveoladas", Bellisco Ediciones Tecnicas y Científicas, Espain, 2000.

2

"ANSYS user manual (Revision 5.2), reference manual", United States of America, 1995.

3

N.B.R-8800, "Projeto e Execução de Estrutura de Aço e de Estruturas Mistas Aço-Concreto de Edifícios", Brasil, 2003

4

Saeed, M., "Finite Element Analysis", Prentice Hall, United States of America, 1995.

5

Veríssimo, G.S., Fakury, R.H., "Design of Steel and Composite Beams with Web Openings", Journal of Constructional Steel Research, 46:1-3, Paper No. 108, 1998. doi:10.1016/S0143-974X(98)00136-9

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