Design Methods for Flexural Capacity of Variable Cross-Section Inverted Trapezoidal Steel Beams

As a new type of structural member， variable cross-section inverted trapezoidal steel beams have been widely used in large space structures due to their favorable mechanical properties and cost-effectiveness. These beams possess a more complex geometric configuration compared to conventional structural members such as H-shaped beams， box-section beams， and circular steel tubes. There are many factors affecting the geometrical form of such members， resulting in a great difference in their bearing capacity. However， the Standard for Design of Steel Structures（GB 50017—2017） does not provide a calculation method for their flexural capacity， resulting in a lack of theoretical calculation methods for engineering applications. This study investigates the secondary variable cross-section inverted trapezoidal steel beams in the Shenzhen Dayun Integrated Transportation Hub project. Formulae for calculating sectional properties were derived， and finite element models were developed using ABAQUS for parametric analysis. The mechanical mechanism and variation patterns were examined， leading to the proposal of a flexural capacity coefficient β for such beams. Based on the parametric analysis results， a theoretical formula for calculating flexural capacity was established， and a design procedure was outlined. A standard model for two-segment variable cross-section inverted trapezoidal steel beams was established with appropriately defined end-section parameters to study the variation law of flexural capacity. The research focused on the effects of the flange width-to-thickness ratio， web height-to-thickness ratio， ratio of top-to-bottom flange widths in the inverted trapezoidal section， and taper rate on the flexural capacity. The mechanisms of sectional plastic development under these parameters were revealed. Results demonstrated that the flexural capacity increased with the ratio of top-to-bottom flange widths and the taper rate， while it decreased with an increase in the web height-to-thickness ratio and flange width-to-thickness ratio. The bearing capacity coefficient β was proposed based on the elastic limit moment of the end sections， comprehensively considering the member's elastoplastic flexural capacity. Through multiple linear regression analysis， a theoretical formula for calculating the flexural capacity under both concentrated and distributed loads was derived. Comparison with numerical simulation results showed good agreement. For engineering applications， a design procedure was summarized， incorporating overall stability analysis and providing formulae for checking cross-sectional strength and local stability， thereby offering a design basis for practical projects.