Question
I would like to analyse a bridge that is skewed in plan. Can I do that using LimitState:RING?
Answer
Since LimitState:RING is 2D analysis software, it is most suitable for the analysis of bridges
which span squarely between abutments. Skew bridges tested in the laboratory (Melbourne &
Hodgson 1995) have exhibited distinct 3D failure modes which cannot be replicated using a 2D
analysis tool. This must be borne in mind here and engineering judgement applied with caution when deciding on the applicability of the advice below.
Given the comparative computational expense and lack of availability of mainstream
3D analysis tools, some codes of practice pragmatically permit the use of 2D analysis methods
for skew bridges, and LimitState:RING can be used in such cases. For example, guidance on the assessment bridges with skew up to 30 degrees is available in codes of practice such as Annex G of BA16/97 and cl 7.7.7 of CS454. Here it is assumed that the skew span acts in the same manner as a square span. This may not be the case in real life - direction of brickwork and span:skew ratio play a large part in determining the behaviour of the structure (the larger the span, the lower the angle of skew up to which the "square behaviour" assumption holds true).
In LimitState:RING, users could potentially make allowance for the skew of a bridge by:
- Modifying the span - i.e. using the skew value rather than square length, and
- Modifying the load conditions if the skew is such that each end of an axle is deemed to be acting on different parts of the bridge when viewed in elevation
A previous version of the software (RING 2.0) was used to model the TRL Barlae test bridge, which has a skew of 29 degrees. The results were conservative and close to the measured values (RING2.0 reported capacity = 92% of experimental capacity).
As mentioned, this information is intended as guidance only and the ultimate responsibility for any assumptions made lies with the engineer, who must be confident that the model and results are a realistic representation of the real-life situation.
References
Melbourne, C. & Hodgson, J. 1995 The behaviour of skewed brickwork arch bridges, 1st Int
Conf. on Arch Bridges, Bolton, pp. 309–320.