The behaviour of curved hybrid girders

Master Thesis


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University of Cape Town

Curved girders are used in bridges to fit predefined alignment. Hybrid girders are an innovative use of high strength steel enabling optimising moment capacity. Previous studies of curvature and hybrid girder effects have been disjointed, focusing on curved homogeneous girders and straight hybrid girders. There are no generally accepted curved girder equations and this has implications in the study of curved hybrid girders since the hybrid effects become apparent in the inelastic range. Furthermore, the range of radius to span ratio where available analytical procedures can be applied is not known. A total of 48 girders are investigated, 12 of which are straight. The girders are all simply supported, un-braced and loaded at midspan. The load-deflection behaviour of curved hybrid girders is investigated. Stress plots of the girders are obtained at ultimate load. The radius to span ratio is varied from 5 to 50 for 5m span girders and from 5 to 30 for 8m span girders. Three steel grades are used to obtain hybrid girder configurations, with higher yield steel always used in the flanges. The web-flange yield steel combinations used are 350MPa/460MPa, 350MPa/690MPa and 460MPa/ 690MPa. A finite element model using ADINA version 8.4 is used to investigate curved girder behaviour. The collapse analysis option is used to trace behaviour as the load is incremented automatically to a prescribed displacement. Available experimental data is used to check the validity of the modeling assumptions. The presence of curvature radically modifies a girder's load pattern by causing additional lateral bending moments. Lateral bending moments reduce the vertical load carrying capacity of a girder and cause the flanges to be unequally stressed. For the girder and spans investigated, there is a reduction of 57% in ultimate load for radius to span ratio (R/L) of 5 compared to a straight girder of similar proportions and span. The effects of curvature reduce as R/L increases and this is observed in the 5m homogeneous girder with R/L of 50 which attained more than 91% of the straight girder load capacity. The 8m girder with R/1 of 30 attained more than 83% of the equivalent straight load girder capacity. The hybrid girders investigated had load-deflection curves close to corresponding homogeneous girders with flange steel grade, reaching more than 97% of the ultimate load capacity of reference homogeneous girders. The hybrid factors as proposed in the simplified design procedure are adequate and can be applied to analytical equations that predict curved homogeneous girder loads. The available analytical equations give conservative loads for both hybrid and homogeneous girders compared to the finite element method when R/1 is 5 and are unconservative for higher rations.

Includes bibliographical references.