Are the side walls of the bridge specified to take directional load like that?
Right now this looks like a Darwin award candidate. They're very lucky that those walls contained enough rebar vertically to withstand this load, it's at the worst possible attachment point and at the worst possible angle (especially for the inner wall).
Edit: looking more closely at the picture it seems as though the cables under tension are actually running through the drainage ports of the back wall, the ropes at the top seem slack or loaded both top and bottom of the opposite so there is in fact mostly compressive load on the front wall and about 50% of sideways load on the opposing wall. Still, that structure was definitely not meant to be loaded like this.
So I "think" most of the load would be applied vertically, but that would be my biggest concern still.
Still seems a bit stupid to put that much additional load on a 70 year old pedestrian bridge though.
Based on that it was definitely built for higher load than just a few pedestrians.
Edit: The German language Wikipedia article actually confirms this [1]. It states that the bridge was built for a traffic load of 13 tons and that the new bridge was built in 1993 to increase capacity. The old bridge was thoroughly renovated after the new bridge had been constructed. The bridge actually is well documented and is under protection as an architectural landmark. It was exhibited in the MoMA for a long time.
Based on the amount of prep these guys put into this and the available documentation I don't see why they should not have checked if what they wanted to do is possible.
[1] http://de.wikipedia.org/wiki/Pont_de_Gueuroz
Edit 2: Some fascinating pictures from the time of construction: http://www.swiss-timber-bridges.ch/detail/193
That's weird. It definitely looks like it is the same bridge but that page says it has been removed.
The construction clearly shows a concrete bridge in the making (lots of concrete forms visible) whereas the heading of the page is 'Swiss timber bridges'.
No net sideways force. Any +X force on the left wall will have a -X force on the right wall. If there's a tension T in the ropes across the top they'll be pulling the walls inward with a force of T causing a torque of T*height.
Almost, you'd still have to subtract the friction component across the 90 degree angle where the cable hangs down from the edge near the porthole but that's probably relatively small compared to the total (depends on cable material and any kind of padding they put in between the cable and the concrete).
It would have been a lot better to sling those cables through the portholes on both sides and to make it loop the bridge, one person would have to rappel down to get that started but you'd have zero load on the walls.
The longer I think about this the more I think those guys and girls were extremely lucky and I hope that this will not inspire any copycats to try this with other bridges or even the same one. It's a long way down.
Apparently the bridge carried cars at one point (http://en.wikipedia.org/wiki/Gueuroz_Bridge) so the wall was rated for vehicle impacts, which is good.
Why they went over the top of the walls I don't know, I think they wanted to measure the ropes exactly instead of adjusting it the day of. Like, that was where they drew the line of "unacceptable risk"? I agree a loop through the drain holes would have made their lives easier in a bunch of ways, except the tripping hazard would have made it a very risky project...
Spherical cows look so much nicer too ;)
If that thing weighed 6 tons they may have made it out of the "designed to handle" range and into the "safety factor" range of inward forces on those walls. Surely those drainage ports weren't designed to handle upward loads?
I really can't believe some of them took their harnesses off while in the pool.
[0] - https://www.physicsforums.com/threads/wind-speed-100mph-to-p...
Some more evidence that they had considered these things would have been nice, but civil structures are quite a bit stronger than people here seem to be giving them credit for. For example, the vertical design load from people walking on a bridge like this is something like 100 lb/ft^2. Meaning the bridge is designed to hold one jacuzzi every 10 or 15 feet, depending on how wide it is.
It seems like it would have been a better idea to bring the cables/ropes in through the hole at the bottom of the wall, rather than over the top. The lateral compressive load would then have been taken up by the bridge itself, rather than the walls.