Effectiveness of steel concrete composite beam over RC beam for long span structures

Abstract

In this study effectiveness of steel concrete composite beams for long span structures has been investigated. The aim was to provide a choice to the design community for selecting proper framing system while designing long span structure. Performance of composite and RC beams has been studied based on strength and serviceability parameters. Numerical analysis has been done to understand the behavior of composite and RC beam. Three different spans have been considered in the study. Six (6) different Finite Element (FE) models were prepared in ETABS to analyze, design and select optimized beam sections for 60ft, 80ft and 100ft spans based on strength requirement. A three storied structure was selected for analysis and design. Similar framing system and loadings were considered for both RC and composite beams. 50% composite action was considered while designing the composite beams. For designing the composite beams, pinned connection was considered at beam ends. However, deflection of composite beams was calculated for fully restrained, partially restrained and pinned connections at the ends. Selected beams were then analyzed to check the serviceability performance. Short and long term deflections were calculated for both Composite and RC beams. Numerical simulations were conducted on RC and Composite beams using ABAQUS. Beams selected in the first step were loaded at one third points of the span. Load was increased up to failure of concrete or steel to obtain the moment vs. displacement curves. Moment vs. deflection curves were prepared from the results obtained from analysis in ABAQUS. The ultimate moment capacity and corresponding vertical displacements for composite and RC beams were compared for the selected span lengths. The results of current study showed that considering strength, serviceability and ductility criteria for long span floor systems, steel concrete composite beams performed better as compared to RC beams. Composite beams showed higher flexural capacity and improved ductility for the selected span lengths. Similar depth of composite and RC beams was selected for a particular span length. Flexural capacity of composite beam was found to be 30% to 60% higher than that of RC beam. This increase in capacity is affected by the end connections and partial composite action. Short term deflection of RC beam was observed to be slightly lower than that of composite beam under service load condition. However, long term deflection of RC beam was found to be about 4 to 5 times higher as compared to composite beam. The serviceability criteria in design can be easily satisfied with a shallower Composite beam as compared to RC beam. The finite element analysis showed that the deflection of the composite beam at the ultimate capacity point is 150% to 250% higher as compared to the RC beam ensuring improved ductility of composite beams.