Effect of construction method on shear lag in prestressed concrete box girders
Most of the previous researches conducted on shear lag of box girders were only concerned about simple types of structures, such as simply supported and cantilever beams. The structural systems concerned in these previous researches were considered as determined and unchangeable. In this paper, a finite element method considering shear lag and creep of concrete was presented to analyze the effect of dynamic construction process on shear lag in different types of concrete box-girder bridges. The shear lag effect of the three types of a two-span continuous concrete beam classified by construction methods was analyzed in detail according to construction process. Also, a three-span prestressed concrete box-girder bridge was analyzed according to the actual construction process. The shear lag coefficients and stresses on cross sections along the beam including shear lag were obtained. The different construction methods, the changes of structural system with the construction process, the changes of loading and boundary conditions with the construction process and time, the prestressing, and creep were all imitated in the calculations. From comparisons between the results for beams using different construction methods, useful conclusions were made.
PRESTRESS INFLUENCE ON SHEAR-LAG EFFECT IN CONTINUOUS BOX-GIRDER BRIDGE
The shear-lag effect in a thin-walled box girder has been recognized and introduced in the technical literature. In contrast however, a discussion of shear-lag effect caused by the combination of prestress force and self-weight of a box girder is limited. In this paper, the shear-lag effect is treated by the principle of superposition by considering that the configuration of a prestressed tendon takes the form of a broken straight line. Two equal spans of a continuous box girder of constant depth with a shear-lag effect are used as an illustrative example. If λ\N denotes actual stress due to bending under symmetrical loading over the stress computed by elementary beam theory, a compound parameter \IN\N\I\dy\Nθ\N/ω\N\IL\N is a measure of the shear-lag effect in this particular example, where \IN\N\I\dy\N is effective prestress force, θ\N is the inclined angle of tendon at support, ω\N is the intensity of uniformly distributed load, and \IL\N is the span length of bridge. On the \IN\N\I\dy\Nθ\N/ω\N\IL\N and λ\N diagram, an asymptote exists. Usually in engineering practice \IN\N\I\dy\Nθ\N/ω\N\IL\N is equal to or greater than unity. Based on this approach, some preliminary conclusions are presented for this study.
A DESIGN AID FOR SHEAR LAG CALCULATIONS.
A semi-empirical method, termed the "3-bar method", for calculation of shear lag effects in box girders is described. Explicit solutions for different support and loading conditions are presented which enable the amplified edge stresses due to shear lag to be calculated directly. Results given by the 3-bar method are compared with those obtained from the interim design rules (idr) for box girders and there is good agreement. The ease of application of the proposed method, exemplified by the calculations in the appendix, make it suitable as an aid during initial design stages. Situations in which the effects of shear lag are important are identified. High stress amplification in heavily stiffened steel girders and at intermediate supports of continuous girders is noted.(a) (TRRL)