I regularly walk under along Pacific Avenue under the Granville Street Bridge. If you’re on the north side of the street, and you look up (which is advisable, given the pile of crap that’s usually at street level) you’ll see this odd engineering graphic on the underside of the bridge. I’ve always wondered what it’s meant to indicate.
Here’s a wider shot that gives a little more context. Are there any engineers (or bridge-savvy folk) out there who can offer an explanation? It obviously has something to with the angle of the bridge, or the cable. Incidentally, it’s not a drawbridge–the bridge isn’t supposed to move.
I went on a long walk yesterday and took the camera.
DarrenBarefoot.com 
There are one of these on the other side too, on 4th as it goes under the bridge. I’ve wondered about it for years.
What’s the metal thing beside it? Could they be related?
Darren, you’re on the right track.
That sign indicates the various angles of slope that the bridge goes through during the average day. There is expansion and contraction in the concrete due to temperature changes, there are also load variations on the bridge deck, depending on traffic. The metal contraption next to it controls the amount of tension in the bridge. Too much tension and they ‘relieve’ some of it. Not enough and they can tighten the steel tension rods. Also, as the bridge ages, it loses some of its integrity (sags, much like our body parts). The rods are tightened each year (or whenever) to keep the bridge looking young (place both hands over your ears, and pull back and up).
This is closely monitored due to the strict seismic codes that our city inforces since we are sitting along the ‘ring of fire’.
I knew that metal cable thing had to be involved. I’m guessing the circles show the times of day, right?
Thanks for the explanation, anonymous person.
This is an easy one. That sign designates the ‘angle of the dangle’ and has been in use since Roman times.