Structural performance of long span composite beams with high performance materials and practical constructional features

Abstract

Objectives: In order to improve structural accuracy and efficiency of long span composite beam design, an advanced numerical investigation into the structural performance of composite beams with high performance materials and practical constructional features was conducted. After extensive calibration against test data of single and double span composite beams, advanced two and three dimensional finite element models with the following features have been established successfully: - Geometrical and material non-linearity with initial geometrical and mechanical imperfections. - Interfacial non-linearity with deformable shear connectors of limited ductility. - Elasto-plastic material models for high performance steel with various degrees of strain hardening. - Advanced reinforced concrete material models with normal to high strength concrete. - Full deformation ranges against different failure criteria: steel stresses, concrete strains, overall deflections, slippages of shear connectors - Effect of propping (or shoring) during construction. Figure 1 illustrates typical deformed shapes of the finite element models of both single and double span composite beams at large deformations. [Figure 1 : see article file for the details of the abstract] Extensive parametric studies using the numerical models have been conducted to provide detailed 'stress and strain' information of composite beams. After careful data analysis and interpretation, structural understandings as well as important design data and quantities are obtained which are readily applicable to assess and predict the structural performance of long span composite beams against various failure criteria. Key findings of the numerical investigation are presented as follows: a) For composite beams with full shear connection, the force acting onto each of the shear connectors is relatively small, and hence, it is operating along the ascending part of the load-slippage curve with very small slippage. Hence, composite action between the concrete slabs and the steel sections are readily developed whilst the maximum slippages in the shear connectors are typically smaller than 1 mm. b) For composite beams with partial shear connection, the force acting onto each of the shear connectors is significantly increased, and hence, it is operating along the non-linear part of the load-slippage curve with significant slippage. Hence, considerable slippages in the shear connectors in the range of 2 to 5 mm will occur in order to mobilize sufficient forces in both the concrete slabs and the steel sections for the development of composite action. Unpropped construction is often more favourable as the slippages of the shear connectors are smaller when compared with those in propped construction. c) However, for long span (≥ 12 m) composite beams with large steel sections (≥ 550 mm) operating at relatively low degrees of shear connection (< 0.5), the assumption of highly ductile shear connectors in many codified methods is not always warranted in reality. In general, it is conceived that there will be significant strength reduction in a typical shear connection owing to local crushing and cracking of concrete under high localized forces. Nevertheless, it is possible to adopt that only 95 % of the moment capacities of the composite beams are readily mobilized, and this reduces the required slippage of the shear connectors down to the range of 5 to 10 mm. This is generally considered to be readily achieved in many shear connectors with rationally designed and detailed reinforced concrete. d) For composite beams with different material grades in steel and concrete, it is found that the moment capacities of the composite beams are readily increased by about 20 to 70 % if the yield strengths of the steel sections are increased from 355 N/mm² to 460 and 690 N/mm² with C30/37 and C80/95 concrete respectively. The moment capacities of the composite beams are also readily increased by about 20 % if C80/95 concrete is used instead of C30/37 concrete with steel sections of any grade, i.e. S355, S460 or S690. e) Furthermore, it is also demonstrated that the use of an elasto-plastic stress distribution as shown in Figure 2 allows accurate prediction on the moment capacities of composite beams with a wide range of steel and concrete materials. A comprehensive set of analytical formulations is proposed in predicting the moment capacities of the composite beams with high performance materials and deformable shear connectors. [Figure 2 : see article file for the details of the abstract] Significance of the research work: This research work provides detailed understanding on the structural behaviour of long span simply supported and continuous composite beams with practical configurations. These practical constructional features include i) shear connectors with different load-slippage curves, ii) different construction methods, iii) different levels of strain hardening, and iv) high performance materials with various combinations. Moreover, detailed understanding on the structural behaviour of composite beams with normal to high strength materials is also provided, together with a design equation utilizing an elasto-plastic stress distribution across the depths of the composite cross-sections. The proposed design method is readily incorporated into modern codes of practice, and it deals with composite beam design with various practical constructional features, which are significantly different from the conventional arrangements. Advanced finite element models have been established successfully for composite beams with practical configurations and constructional features. Due to the high efficiency of the proposed finite element models, the computational resources for the execution of those models are found to be readily available in modern research offices. Moreover, two dimensional finite element models have also been successfully calibrated, and structural engineers are strongly encouraged to employ these models in their practical work to exploit the full advantages offered by composite construction.