Study on Pure Flexural Buckling Behavior of Web of Castellated Composite Beams with Regular Hexagonal Hole

Castellated components have been widely used in high-rise buildings and long-span structures in recent years because of their advantages such as high bearing capacity, great bending stiffness and convenient crossing pipelines. For the castellated component, the local buckling of the web is one of its main failure modes, and the buckling will cause a rapid increase of the local deformation of the web until the work is terminated, which may result in the overall instability of the components and the structural chain reaction. In the traditional steel structure design, the local buckling of the web is avoided by limiting the height to thickness ratio of the web. In the castellated component, the inter-hole pier and its bridge are likely to have local buckling problems, which is more complicated than the solid member. At present, most of the researches on the local stability of castellated beam members are carried out around pure steel castellated beams, and there are few studies on castellated beams considering concrete slabs. Therefore, it is necessary to study the pure flexural buckling of castellated composite beams.
On the basis of the two pure steel castellated beams, two simply supported castellated composite beams were designed and fabricated. The static test of these two castellated steel beam-concrete composite beams was carried out at the four points. By observing the location of buckling on the steel beam and shape of the cracks in the concrete slab, the web buckling performance, bearing capacity and hole angle strain were analyzed, and compared with the pure steel castellated beams, in order to study the effect of the stiffening rib between the floor and web on the failure pattern, stress distribution and bearing capacity of castellated steel beam specimens. The finite element software ABAQUS was used to establish the model to carry on the numerical simulation, by comparision between the test result and the finite element simulation result, the model result is consistent with the test result.Based on the experimental model, the influence of height to thickness ratio and the opening ratio on the pure flexural buckling and the ultimate bearing capacity of castellated composite beams was analyzed.
It shows that pure castellated beams buckle at the upper flange and the corresponding bridge plate, and the composite beams buckle at the pier plate and bridge slab. Concrete slab can effectively improve the yield load and ultimate load of composite beams, improve the ductility of specimens, and avoid the occurrence of local buckling. Setting stiffeners between the webs of composite beams can reduce the stress concentration at the corner of the holes and avoid buckling of the pier plates between the holes. There are two buckling modes of the composite beams, both of which are the semi-wavy out-of-plane instability of the piers between holes. The difference lies in the out-of-plane displacement distribution.The displacement of the first buckling mode centrosymmetric distributes with respect to the middle of the beam span and the displacement of the second buckling mode centrosymmetric distributes with respect to the middle of the beam span.The height to thickness ratio is a decisive factor affecting the failure mode and pure flexural buckling of composite beams. When the height to thickness ratio is less than 80, strength failure occurs. When the height to thickness ratio is greater than 80, buckling failure occurs. Reducing height to thickness ratio of web can effectively improve the buckling resistance of specimens and increase the buckling resistance of specimens. Opening ratio is an important factor affecting the failure mode and pure flexural buckling of composite beams. Increasing the opening ratio within a certain range can increase the buckling load of specimens.