I have been exercised for some time about the issue of limitations on the powerful technique of thrust lines (or, in our case, zones of thrust) as tools for visualising force flow. One major issue is the question of what bit of force gets included at which stage of the thrust line. If the structure is relatively narrow (like an arch rib) and the thrust is contained within it, there is no particular problem. In a large blocky structure or where the thrust wanders into the backing on an arch, things get more interesting as this picture (and some explanation) shows.
First a picture of the traditional thrust line for Bargower with an HB load. This has been editted down to the bare essentials.
Now look what happens if we work out where the thrust leaves the fill part associated with the segment.
The upturned end looks very strange but is actually the same thrust shown above.
The picture below shows a bit more detail of what is happening at the end there.
The grey portion of the arch shows the last segment and the fill it is assumed to support. The yellow bit of thrust line is the related section of thrust calculated on the assumption that the action of the forces can be assumed to take place between the boundaries of the arch segment projected outwards. It seems that this is unsatisfactory because the effect of the forces on the thrust line is entirely separated from the area within which the forces are generated.
An alternative approach would be to work out where the thrust leaves the domain of the loads concerned. ie where it leaves the grey area. This occurs at the point where the last section of thrust projected backwards cuts the vertical division in the fill. That is an easy calculation but produces an unexpected result which can be seen in the red piece of thrust zone. The thrust moves rapidly upwards as it approaches the end of the arch.
This effect is created by the hockey stick shape of the grey zone. the blob at the bottom creates an anticlockwise moment that throws the thrust upwards. The calculation is correct but the division of the structure turns out to be unreasonable.
Actually, the point at which the thrust leaves the structure is sensible. It is the route it takes to get there that doesn't make sense.
We are working on this. Those few of you who have version 2.4 will note these strange upturns on arches which approach the vertical at the end. In previous versions the thrust has been allowed to continue out of the arch but this also has draw backs for the overall image produced
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