Modeling of through-thickness stress state in adhesive joints connecting pultruded FRP bridge decks and steel girders

The through-thickness performance of adhesive joints connecting pultruded FRP bridge decks and steel girders subjected to loading in the transverse bridge direction was investigated. Deck-to-girder adhesive joints can be assumed as being loaded by uplift forces only (and not by additional bending moments) if the adhesive layer has a minimal thickness of approximately 10 mm. If possible, however, steel web stiffeners should not be connected to the upper flange in order to allow free flange rotation. The tensile stress distribution in the adhesive joint is non-uniform with high stress concentrations below the FRP webs of the cellular deck and above the steel girder web. Alternately inclined deck webs cause significantly higher stresses below the vertical webs. An analytical model allowing the calculation of a representative approximation of the complex uplift stress state in the joint is proposed and validated by numerical and experimental results. Based on this model, a design method for adhesive joints connecting FRP decks and steel main girders can be established. © 2009 Elsevier Ltd. All rights reserved

Through-thickness performance of adhesive connections between FRP bridge decks and steel main girders

Martin Schollmayer

FRP bridge decks offer several advantages compared with conventional concrete bridge decks, particularly their much lower weight, but also their resistance against corrosion as well as easier installation and maintenance. The poorly conceived methods available for connecting FRP bridge decks with their supporting structures – normally steel girders – nonetheless constitute a disadvantage. Connections involving studs or bolts are not appropriate in this case, since FRP is a very brittle material that offers no ductile properties. Bolted connections usually result in much higher stress concentrations, while adhesive bonding is a more material-adapted connection method since larger surfaces can be linked together, thus ensuring reduced stresses. The bridge system investigated in this thesis consists of a pultruded FRP bridge deck bonded to steel main girders, the bridge's main structural components, which have to transmit the dead and traffic loads to the supports, whereas the bridge deck is spanned in the transverse direction perpendicular to the steel girders. Uplift forces caused by the load-bearing behavior of the bridge deck transverse to the bridge axis lead to through-thickness tensile stresses in the adhesive joint. The main objective of this thesis is the description of the structural behavior in the transverse direction. This includes analysis of the stresses in the adhesive connection as well as determination of the strength of the joints. Analytical and experimental investigations were carried out. It was shown, that the tensile stress distribution in the adhesive joint is non-uniform with high stress concentrations below the FRP deck webs of the cellular deck and above the steel girder web. Alternately inclined deck webs thereby induce significantly higher stresses below the vertical webs. A method for the calculation of the stress state in the adhesive layer is proposed which is validated by numerical and experimental results. The material strength of the connection in terms of a combination of tensile through-thickness and shear stresses is established. The total safety factor of the joint was higher than the safety factor of the FRP deck for bending between the main girders. A possible failure process would not start in the adhesive connection between bridge deck and steel girder which eventually could lead to additional failure of other structural members. The system is redundant. Fatigue loading up to 10 million cycles showed no stiffness degradation. The results of this thesis prove the existence of a good load-bearing behavior under static and fatigue loads of adhesively-bonded joints between pultruded FRP bridge decks and structural steel girders, where the adhesive connection is loaded with uplift forces and moments acting in the bridge deck, in addition to the shear in the connection layer due to composite action. The basis for a design method for adhesively-bonded connections between pultruded FRP bridge decks and steel girders is provided.

EPFL2009

Résistance au déversement des poutres métalliques de pont

Raphaël Thiébaud

Steel and steel-concrete composite bridges made of plate girders represent an economical structural form and are largely used for short and medium spans. Two parallel plate girders connected by cross-beams define a twin-girder bridge. This type of bridge transfers the loads from the deck to the supports essentially through bending and shear of I-girders. Associated with the two resistance modes are the following instability modes: lateral torsional buckling (LTB) and local buckling. Design experience of steel bridges made of plate girders shows that LTB determines the dimension of the upper flange of an I-girder at mid-span during construction phase, and the dimension of the bottom flange next to the intermediate supports during construction by launching or in service. In practice, LTB verification is carried out using reduction curves (i.e. buckling curves) which decrease the resistance of a beam with respect to the sensitivity to LTB; thus, the more sensitive the beam, more severe the reduction. A comparison of different standards shows reduction curves available for steel bridge girders have non-negligible differences which can reach 30 % of the resistance. A more refined study of the buckling curves shows that it is semi-empirical in nature, where the empirical part is mainly based on numerical and experimental research for building sections. These findings justify the goal of this thesis which is to propose a verification method for LTB that better takes into account the special features of bridge girders. To reach this goal, study of parameters that influence LTB, like re-sidual stresses, geometric imperfections, cross-beams, loading and beam geometry, is required. In this research, the experimental studies carried out on the residual stresses propose a new model which takes into account flame-cutting and welding of thick plates used in the fabrication of steel bridge girders. Experimental measurements of geometric imperfections performed on real bridge girders show the difference between fabrication tolerances and imperfections measured. These results are then used as input data for numerical simulations of LTB by two approaches: beam and bridge. The beam approach, which considers a simple span I-girder with fork support and a constant bending moment, reveals that the influence on the resistance to LTB can reach 10 % for the residual stresses and 12 % for the geometric imperfections. The bridge approach, which considers a twin-girder bridge with cross-beams and a constant load on the upper flange, shows that the influence reaches 17 % for cross beam spacing and 27 % for the type of cross-beams (frame, truss, diaphragm). Based on the experimental and numerical results, a design method for LTB is proposed. It recommends the use of reduction curve c for SIA263:2013 or reduction curve c of Eurocode EN1993-2:2006.

EPFL2014

Through-thickness performance of adhesive connections between FRP bridge decks and steel main girders

Martin Schollmayer

EPFL2009

Résistance au déversement des poutres métalliques de pont

Raphaël Thiébaud

EPFL2014