In reply to stewieatb:
> The ADT effect clearly doesn't apply, because the bottom angle is effectively zero. Look at the diagram I just posted.
When abbing the scenario quite clearly is a death triangle however as I'll explain there is also little difference when lowering quite often too.
Ok for ease of explanation let's imagine climbers B and C weigh the same and Climber A weighs B + C. Also lets call the lowers off LO1 (the left one) and LO2 the right one, and also assume climber A B and C are free hanging on the rope
Do you think it makes much difference if climbers B and C hang on the rope as per your diagram (call this scenario X) or only climber A does (at the same hanging height) making a death triangle (Call this scenario Y?
In fact it makes very little difference as long as the bolts are placed fairly closely together and the lengths of rope to the climbers are long. Neither scenario is going to magnify the loads on the bolts and in fact both bolts will take less load then either one of them being loaded in isolation.
This is because of the way the death triangle works to act as if the anchors were independently attached to the Centroid of the (death) triangle and pulled down from there. So much the same as with belay anchors the load will get magnified once the angle between two anchors exceeds 120 degreesand the greater this angle the greater the exponential magnification. Or in the case of a death triangle once the strands leaving climber A to LO1 and LO2 exceed an angle of 60 degrees between them / equilateral triangle. When the angle is smaller than this the anchors share the load when its greater the load is magnified (but there is some redundancy at least).
With the case of lowering off the angles at the centroid of the triangle will be 90 degrees so will be sharing the load, however there is nothing forcing B and C being in vertical alignment with each other (other than them free hanging in this example but you get what I mean) or indeed preventing ropes crossing over which could create exactly the same (or worse) scenario as a death triangle.
In the real world because belay bolts are placed fairly close together with fairly low loads and because the angles to the anchors are generally not multiplying forces the death triangle is not something we need to consider in terms of safety. However for setting up a belay it would be quite easy to equalise the bolts to create a scenario where the forces would be multiplied and this coupled with the potentially higher forces involved its worth making sure the anchors are equalised to minimise the force applied to them.