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Newsletter  Summer 2010
Welcome to the latest edition of the LimitState newsletter, where you will find topical information on LimitState software and its practical application. This edition includes an article on performing slope stability analysis using LimitState:GEO, an article on modelling missing masonry in LimitState:RING, plus a tip on how quickly select and modify multiple objects in both LimitState:GEO and LimitState:RING.
News  LimitState at Summer Conferences and ExhibitionsIn the last few months you may have seen LimitState personnel at one of a number of industry exhibitions or conferences taking place in the UK or mainland Europe. You may have seen us presenting a technical paper or manning the LimitState stand, and in some cases giving live software demonstrations. We hope to attend further events during the coming year, so if you see us please feel free to come across and say hello! Recent events:


News  LimitState Secure Major Network Rail ContractLimitState recently secured a contract to supply Network Rail with their groundbreaking geotechnical software, LimitState:GEO, which uses new technology to rapidly evaluate the safety of geotechnical constructions such as slopes, retaining walls and foundations. Network Rail has purchased a multiuser network license which means the LimitState software can potentially be used by engineers in company offices nationwide. LimitState will also provide Network Rail with training in using the software in conjunction with new European design standards, the Eurocodes, which became mandatory on 1 April. The introduction of the Eurocode for geotechnical design marks a paradigm shift in the way geotechnical constructions will be designed. More... 

Quick Tip  Selecting and Modifying Multiple Objects in LimitState ApplicationsDid you know that there are several ways of quickly selecting multiple objects of a similar type in both LimitState:RING and LimitState:GEO? Basic  Use CTRL and SelectBy holding down CTRL whilst clicking, you can select multiple objects  if these are of the same type then their properties will be displayed in the Property Editor for you to edit. This is useful if you have a small number of similar objects to select and modify. Advanced  Use Rectangle Select and the Property EditorIn both LimitState:RING and LimitState:GEO, you can select a large block of objects by choosing the 'Rectangle Select' tool and drawing a rectangle around the objects of interest. This method is useful if you have a large number of objects to modify and / or want to ensure that you have not excluded one by mistake.
Once the objects are selected, you can filter out specific types using the dropdown menu at the top of the Property Editor and then modify the common properties of these en masse (see Figure).
Alternatively, both LimitState:RING and LimitState:GEO also allow you to filter the objects before you select them:
In both cases, to avoid later confusion, you should remember to set the selection filter back to normal before continuing to work with your model. 

LimitState:GEO Application Note  Principles of Slope Stability Limit AnalysisIntroductionThis Application Note outlines the principles applied when performing a slope stability analysis using LimitState:GEO. In a conventional slope stability analysis (e.g. using the method of slices) a predetermined slip surface is assumed and the stability of the failing soil mass is evaluated by comparing resisting and disturbing forces/moments. Usually many trial slip surfaces are investigated and the most critical one identified. This typically requires specification of a search zone and entry and exit points, and can be very sensitive to the shape of slip surfaces used (e.g. circular or noncircular).In contrast the general purpose limit analysis procedure used by LimitState:GEO does not require the form of the collapse mechanism to be prespecified. However, the use of a general purpose procedure does mean that an particular approach must be adopted to identify the critical failure mechanism. Slope Stability Limit AnalysisIn LimitState:GEO, it is necessary to drive the problem to collapse by applying a multiplier (or Adequacy factor) to an unfavourable load. In a foundation problem this is conceptually straightforward: the load on the foundation is increased until the underlying soil fails, indicating that the ultimate limit state has been reached. Similarly, in a slope stability problem the self weight of the soil forming the slope could be increased until collapse occurs. However, a simple example will help show that this is not necessarily the best approach, particularly for problems involving purely frictional soils. Thus see Figure 1, which shows a brick placed on a plank of wood. For a given angle of inclination of the plank, the brick will either be stable or unstable. However, factoring up the weight of the brick will not affect its stability, i.e. applying the Adequacy factor to the self weight of the brick will not drive the problem to failure (and will typically return a solution of *unstable* or *locked* in LimitState:GEO). Alternatively, consider a laboratory model of a slope stability problem with the model contained within a tank, such that the tank may be pivoted about one end as depicted in Figure 2. In order to establish how close to the point of stability the slope is, the tank could be slowly tilted from the horizontal to a steeper and steeper angle α_{crit} until failure occurs. The larger this angle the more stable the slope. Typically a factor of safety is required in terms of a factor (F) on soil strength such that the slope collapses with soil properties c'/F and tan φ'/F. The problem then becomes one of finding F such that α_{crit} = 0^{1}. Alternatively, from a Eurocode 7 perspective (Design Approach 1, Combination 2, and Design Approach 3), a specified factor is applied to the soil strength and stability evaluated. In this context a value of α_{crit} ≥ 0 indicates stability. As will be seen, either of these principles can be applied when using LimitState:GEO. Application in LimitState:GEOTilting the tank is analogous to modifying the body forces exerted by any mass of soil or structure. Within the frame of reference of the tank, the vertical body force becomes mg cosα and a horizontal body force is introduced mg sinα, as shown in Figure 2b, where m is the mass of the body and g the acceleration due to gravity. In LimitState:GEO, horizontal body forces can be introduced using the Seismic Actions facility in the Property Editor. By setting the Horizontal Accel. k_{h} (g) to 1.0, Adequacy (on k_{h}) to True (and ensuring Adequacy is not set on any other parameter), this requires the software to find the horizontal acceleration required to cause collapse. This will be returned as the Adequacy Factor (AF). This can be converted to an equivalent value of α_{crit} using the expression α_{crit} = tan^{−1}(AF). Thus the problem is stable if AF ≥ 0.0. It is important to note that:
A worked example of a slope stability problem using LimitState:GEO may be found in Application Note LSAN2.


LimitState:RING Application Note  Modelling Missing Masonry BlocksA common question that we are asked is:
The answer is yes  by following the method outlined below:
NoteFollowing this course of action, the thrust lines will not display correctly after solving. You should therefore hide them by clicking the 'Show / Hide Thrust' button in the View toolbar (normally 3rd button from the bottom on the left of the screen). Example FileThis example file includes a model where the 4 crown blocks have been removed from the intrados ring using the method described above: 