In reply to Trangia:
Bottom rope forces are double abseil forces and I'd add the follow important differences:
- In bottom rope scenarios that the position of the anchor is less likely to be in the best place when compared with abseiling (because the line of the climb might not be in line with the anchors).
- You definitely want the anchor to permit the belayer to move around (to put this another way the anchor should not fail if the belayer moves)
- The rigger is usually in a different position of responsibility. By and large most people rig abseils for their own descent by contrast top-rope riggers rig top ropes for other people's safety.
- On an abseil, at the point of commitment, the anchor is in sight and has usually be checked immediately before every use. Top-rope anchors are usually out-of-sight and usually used repeatedly without rechecking.
Overall I'd take equal or more care rigging a TR anchor than a lead belay anchor and if I was unhappy about the anchor holding a lead fall there is no way I'd use it for a top rope scenario.