Enhancing Shear Strength of Post-tensioned Precast Concrete Bridge Girders Using External Prestressing: Experimental and Analytical Investigation
Creators: name: Ragab, Nora
Publisher: name: Graduate Studies
Year: 2015
Subjects: subject: Engineering--Civil
subject: pretensioning
subject: post-tensioning
subject: external
subject: ducts
subject: internal
subject: Bridge
subject: Shear strength
subject: Finite element modeling
subject: Push-off tests
subject: compression tests

description: The web thickness in precast post-tensioned bridge girders is an important bridge design parameter that dictates the shear strength of the girder. Intensive research work has been done to assess the effect of the presence of ducts on the compressive strength of the diagonal struts between inclined shear cracks. Codes reduce the design capacity in the presence of ducts by using a smaller effective web thickness. However, not enough research has studied the direct effect of the presence of ducts on the shear resisted by concrete in the diagonally cracked or un-cracked zones of the web. To achieve a better understanding of the effects of post-tensioning ducts in the web on compressive and shear strength, experimental and analytical investigations are conducted. A finite element model is developed and validated using experimental results published in the literature to study the effect of duct on the crushing strength of the web. The model is used to conduct a parametric study to investigate the effect of the ratio of the duct diameter to the web thickness, the inclination angle between the axis of the duct and the direction of the load, and the height of duct in the web. Moreover, a series of 27 push-off concrete specimens (600 x 400 mm) with and without ducts have been tested to examine the effects of different duct diameter-to-web thickness ratios on the direct concrete shear strength. A 3D finite element model for the push-off specimens, based on the theory of plasticity is successfully developed and verified against the experimental results. The model is able to predict the modes of failure of the test specimens. The study suggests two equations to be used to calculate the effective web thickness for a web with un-grouted duct. A published equation (Muttoni et al. 2006), validated in the present work, is recommended for the effective web thickness in calculating the limit of shear resistance of the web to avoid crushing of the concrete. The second equation, derived using push-off test results, is to be used for calculating the portion of the shear force resisted by concrete. The effectiveness of external post-tensioning as an alternative to conventional internal post-tensioning is also investigated. An external post-tensioning design was proposed for an actual internally post-tensioned bridge girder built in Calgary. The effective web thickness of the internally post-tensioned girder was calculated by the proposed equations and used as the web thickness for the externally post-tensioned alternative. A finite element model was developed and validated for each girder. The externally post-tensioned girder proved to have a satisfactory behaviour with considerable material and labour cost savings compared to the internally post-tensioned girder.