FAQs

Crossing thrusts

The usual form of this question is "The thrust is in the pier but is coming out of the next span of the arch, surely this is a fail?"

The picture referred to is as below

Answer

Zones of thrust cannot actually cross. Once they meet, they become one, but the way Archie-M handles this up to version 2.2 is simply to allow them to cross, assuming that users will understand the implications.

A better solution is to truncate the thrusts at a vertical line through the extrados at the springing. This is what Arcie, Multi, and Archie-win did but it seemed to be much more difficult to do in C++ and we never managed to make it work.

A better solution yet is to consider the domain within which forces operate. This means that if the thrust rises above the arch extrados, its position should be calculated on the vertical line on which the fill is divided, not on the extension of the radial line between segments. To his shame, Bill only worked this out in Nov09 and the change will be incorporated in version 2.4.

This picture will need updating when we sort out the minor calculation error!!