The implementation of Advanced Assessment Methods for Concrete Bridges
The flexural behaviour of reinforced concrete slabs at collapse is usually significantly different from the behaviour predicted by an elastic analysis. Hence the quality of a load assessment rating for such structures is highly dependant on the method of analysis employed. Confirming this, a recent audit of the United Kingdom’s (UK) ten year trunk road bridge assessment programme has revealed the most prevalent causes of bridge assessment failure are overly conservative and inappropriate analysis techniques (Parsons Brinkerhoff, 2003). Almost half of the sub-standard structures included in this audit were reinforced concrete slabs with 64% of these failing in a flexural mode.
A major assessment and strengthening programme is now in its infancy in the Republic of Ireland (ROI) and this research project stems from this current climate. By drawing on the experience gained from the UK’s assessment programme an advancement in the understanding of the appropriate techniques to use when modelling the structural behaviour of bridges would alleviate the number of condemned structures and potentially lead to significant savings with the assessment scheme in the ROI. This thesis examines the use of plastic and non-linear finite element (NLFE) analyses for bridge load rating and has shown that the use of elastic analysis methods for assessing concrete bridges results in a significant under-estimate of strength in certain cases.
In order to identify appropriate methods to adopt when undertaking bridge capacity assessments, different aspects of structural theory as applied to slab load rating were investigated. Validation of the COBRAS yield-line program was undertaken by analysing the classic square clamped plate under a uniformly distributed load (UDL) problem. A novel failure mechanism for this particular problem has been developed for inclusion in the COBRAS yield-line program. Fox (Fox E.N., 1974) provided an exact theoretical solution to the problem. A yield-line analysis with COBRAS using this new mechanism successfully predicted the collapse load of the square plate to within 2.2% of Fox’s exact solution.
This yield-line method is applied to six reinforced concrete slab bridges previously assessed using linear elastic methods. A marked increase in load rating is observed in many cases, with the largest benefit coming from a structure previously deemed to have a load capacity of 3 tonnes (t) as a result of inadequate capacity in transverse hogging reassessed to have the full 40 t capacity when analysed using the yield-line method. In other cases where only very low percentages of steel reinforcement are present only relatively smaller benefits are derived from employing the yield-line method. The yield-line method is also applied to a structure with longitudinal reinforcement only (i.e. no transverse steel) and the implications of such are discussed.
A sensitivity analysis on the compressive membrane enhancement factor required to achieve a 40 t rating on three sub-standard bridges has been conducted. As little as 2% membrane enhancement is required in one case to achieve a live load increase from 18 to 26 t. A qualitative study on the amount of membrane enhancement available through fixed boundary action is also conducted. It is shown for small span-to-depth ratios up to 20% membrane enhancement may be available through friction at the supports alone.
The results of an elastic grillage analysis were improved upon by assuming cracked section properties for the transverse members thereby reducing the transverse hogging moments and resulting in a live load increase from 7.5 to 18 t. The possibility of a lower-bound optimisation within a grillage analysis are discussed.
NLFE analyses were undertaken using the DIANA program. The use and limitations of NLFE analysis are investigated using a two-dimensional plane stress model of an under-reinforced beam and comparing the predictions obtained with experimental test data. In particular the choice of the various parameters used for NLFE analysis are investigated. The sensitivity of these analyses to boundary conditions is also highlighted.
COBRAS, the computerised yield-line method used in this thesis, is recommended over DIANA NLFE analysis program for the assessment of reinforced concrete slab bridges where elastic grillage methods are likely to yield overly conservative estimates of flexural capacity. An elastic analysis is recommended where a structure has inadequate ductility to enable the formation of a full collapse mechanism predicted by a yield-line analysis.
[Cambridge University | CUED | Structures Group | Geotechnical Group]