Issue 50

C. C. Silva et alii, Frattura ed Integrità Strutturale, 50 (2019) 264-275; DOI: 10.3221/IGF-ESIS.50.22 265 Figure 1 : Lateral distortional buckling (Oliveira [1]). Continuous and semi-continuous composite beams can be used with castellated sections, which are structural elements with multiple hexagonal openings of the same shape, regularly spaced in the web. The main reasons for the use of castellated beams in steel structures are increase of resistant capacity and flexural stiffness around the axis of greater inertia, provided by the increase of the total height of the cross section; elements lighter than a profile without openings of the same height, thus reducing the total weight; larger free spans, reducing the number of columns and foundations, leading to faster and more economical assembly; possibility of passing ducts or pipes through the openings and aesthetic gain, since the openings incorporate to the environment a modern appearance. Adding the potential of castellated steel beams for use in composite floors, the effect on material economy is even more promising once the steel-concrete composite beams are a very effective floor system due to the considerable increase in floor stiffness, steel weight reduction and the lower height of the beam-slab section (Queiroz et al. [2]). The Brazilian standard ABNT NBR 8800:2008 [3] provides a procedure for the determination of the lateral distortional buckling-moment resistance capacity of the continuous and semi-continuous steel-concrete composite beams, similar to the EN 1994-1-1:2004 [4] European standard. This procedure depends on the determination of the elastic critical moment, calculated considering the behavior of the inverted “U-frame” mechanism. A fundamental parameter for this determination is the rotational stiffness of the composite beam which, in turn, depends on the web stiffness. It should be noted that these code procedures apply only to composite beams without web openings. In the literature there are several studies on lateral distortional buckling (LDB) of continuous composite beams without web openings, among them Roik et al. [5], Chen [6], Dekker et al. [7], Hanswille et al. [8], Calenzani [9], Calenzani [10], Chen and Wang [11], Ye and Chen [12], Wang [13], Guo et al. [14], Zhou et al. [15], Zhou et al. [16], Oliveira [17], Amaral [18] Dietrich [19], Dias [20] and Oliveira [1]. The number of researches of castellated composite beams in regions of hogging moments is still limited. Salah and Gizejowski [21, 22, 23] performed several studies with alveolar beams (cellular and castellated openings) to analyze the influence of beam slenderness (length to height ratio), distortional buckling modes, profile height and steel type. Piassi et al. [24] developed an analytical model in order to obtain the expression of the rotational stiffness of composite cellular beams and the results were compared with numerical models developed in the ANSYS 15.0 [25] program. However, there are no studies on the rotational stiffness of the web castellated profile. Therefore, this paper presents a numerical study, validated with results of the literature, with the aim of proposing an equation for the calculation of the bending stiffness of the web of steel-concrete composite beams, which can be used to determine the rotational stiffness of the inverted “U-frame” mechanism and obtain the elastic critical moment of lateral distortional buckling. L ITERATURE R EVIEW Inverted “U-frame” mechanism he European standard EN 1994-1-1:2004 [4] and the Brazilian standard ABNT NBR 8800: 2008 [3] suggest the use of the inverted “U-frame” mechanism to determine the elastic critical moment of LDB, in which the concrete slab is considered to be over two or more parallel steel beams, as shown in Fig. 2. T