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LimitState:RING  Frequently Asked Questions
Q1. What is the 'adequacy factor' ?
It is generally useful to compute the factor which would, when applied to some specified pattern of live loads, lead to collapse. This factor (or ‘multiplier’) is commonly termed the ‘adequacy factor’ and its determination for a given bridge is the principal goal of a normal LimitState:RING analysis. When appropriate partial factors are included in the model, this adequacy factor must be greater than 1.0 for a safe structure. For example, if a 1kN single axle load is specified and LimitState:RING indicates a computed adequacy factor of for example 154, this means that the load which would cause collapse is 154kN. Alternatively if a 100kN single axle load was specified, the adequacy factor computed would be 1.54 (1.54 x 100 = 154kN).
When the applied load comprises a series of axle loads, the adequacy factor is the multiplier which, when applied to all axle loads simultaneously leads to collapse. For example, if a 1400kN rail vehicle comprises four 200kN axles and four 150kN axles and LimitState:RING indicates a computed adequacy factor of 3, this means that the loading at failure comprises four 600kN axles and four 450kN axles (3200 = 600kN; 3150 = 450kN).
Q2. When I specify end abutment blocks, I seem to get a very low predicted adequacy factor. Why?
If an end abutment is specified then often LimitState:RING will predict a sliding failure just below the skewback at the top of the abutment.
In reality when soil is present behind an abutment such a failure mode would be resisted by horizontal soil pressures. Currently in LimitState:RING the abutment option is really meant for situations when a beam and arch span are adjacent to one another, when the possibility of the above failure mode is real.
Q3. Is LimitState:RING being constantly updated?
Yes, LimitState regularly update their software. Click here to read the LimitState:RING release history.
Q4. Has LimitState:RING been validated against fullscale test results?
Yes. LimitState:RING is a continuation of software originally developed as a means of interpreting the results from fullscale laboratory tests, where good correlation was found.
Interested users are referred to the experimental validation and industrial validation pages, as well as the reference list at the back of the LimitState:RING User Manual.
Q5. How does a multispan analysis work using LimitState:RING? Does it involve manually balancing thrusts from adjacent arches?
LimitState:RING uses rigorous optimisation techniques to find the critical collapse load factor, which can be found when the yield, mechanism and equilibrium conditions of plastic analysis are all simultaneously satisfied. No balancing of thrusts is required.
In other software, by balancing thrusts from adjacent spans, the user is basically attempting to carry out this process manually (the yield condition being deemed to be violated when the line of thrust lies outside the thickness of the pier).
Q6. How does LimitState:RING deal with transverse distribution of the load through the fill?
LimitState:RING allows the user to specify an angle of transverse load distribution. This is used to compute the effective bridge width (when the autowidth calculation is enabled).
Q7. I am modelling a multiring brick arch bridge. I get very different answers depending on whether I define each ring separately or simply specify an overall barrel thickness. Why?
Defining each ring separately sets up a series of separate rings which can interact with each other via interfaces which the user may specify to be frictional. Such a configuration is usually significantly weaker than a single barrel of thickness equal to the sum of all the rings  hence a lower predicted capacity will result.
Q8. I am modelling a multiring brick arch bridge. I am not sure if ring separation (delamination) is present. What assumption should I make when using LimitState:RING?
This is a difficult question. It depends on your judgement of the strength of the bond at the joint between rings.
If the strength is low, or nonexistent (as it certainly is in many cases) then you should treat the barrel as a series of separate rings, interacting via frictional interfaces.
It should also be remembered that:
 The square scaling law for stresses in gravity structures means that a large bridge needs to have a greater interring bond strength to avoid ring separation than a geometrically similar small bridge.
 The effect of your assumption on carrying capacity can be dramatic and it will normally be prudent to experiment with differing levels of ring separation (e.g. separating bottom ring only, or partial separation in crown region only for example).
Q9. The more bricks I specify per ring, the lower the failure load factor. Why?
Relative rotations (hinges) and sliding movements between blocks are a necessary feature of failure mechanisms. These can only occur at the interfaces between bricks/block units. Thus the fewer units that are specified, the less options the optimiser has open to it when trying to find the minimum load factor.
In practice when the number of units exceeds say 40 or 50, changes in the computed load factor as a result of adding further units will generally be small.
Q10. Can I model 'slender' piers using LimitState:RING?
Yes, LimitState:RING can model piers of any height/width.
Some codes of practice distinguish between 'stocky' and 'slender' piers in multispan masonry arch bridges, with bridges containing piers that are deemed 'slender' requiring a full multispan analysis. However, the distinction is somewhat artificial as in some cases the critical failure mode in a bridges comprising 'stocky' piers will actually involve more than one span (indicating that a single span analysis will be nonconservative). The key thing is that LimitState:RING will identify the critical failure mode, whether this involves one, two or even more spans.
Q11. Do I need to apply global factors of the type used in the MEXE method to the computed LimitState:RING adequacy factor (e.g. profile factor, material factor, condition factor etc.)?
Generally speaking the answer is 'no' as LimitState:RING directly takes account of e.g:
 the actual profile of the arch;
 the actual masonry strength specified;
 the effects of defects such as ring separation and mortar loss.
However, as LimitState:RING is a 2D modelling program the global effective width should be reduced e.g. if longitudinal cracks are present.
Q12. Is LimitState:RING a finite element (FE) package?
LimitState:RING is a flexible and powerful tool, but it is not based on finite element analysis methods.
Finite element analysis is undoubtedly a convenient numerical procedure for the analysis of continua. However, masonry arches comprise blocks separated by joints and it is therefore more convenient to use a different procedure which takes account of their inherent 'discontinuous' nature. Hence LimitState:RING uses 'rigid block analysis', a computational limit analysis procedure which is very different from conventional finite element analysis.
For example, when analysing a masonry arch bridge using conventional finite element analysis, a nonlinear analysis generally has to be performed in order to analyse the ultimate 'collapse' limit state. Numerous input parameters are involved and, because of the incremental nature of the analysis, numerical convergence problems are commonplace. This means that the time and cost necessary to perform an analysis are generally high.
LimitState:RING requires very few input parameters and the ultimate limit state is analysed directly, using a simple and robust linear optimization algorithm.