If I have a curved surface, like the support timber in a small bridge, how can I put floor boards on the bridge that will follow the curved surface?

Is the curve a radius arc?

I think I would try:
Find the center point.
Rotate the first board to sit on the surface.
Rotate with the control key (copy rotate) about the center point to the opposite end.

Before ending the command press the / key.
Enter the number of planks it will take to fill in the curve.
This might take some time and trial and error.

Calculator for radius from an arc's width and height:
http://www.handymath.com/cgi-bin/rad2.cgi?submit=Entry

You will also find it calculates the arc length, so you can do some math on your board width to calc the number of planks you will need.

yes the curve is a radius arc... 5 inches over 10 feet.
how do I work out the size of circle here?
does this mean I need a "handle" on the board that reaches to the center of the circle?
OR, the other side of the circle...
I'm thinking of something with the "follow me" tool, some way to select the path, then copy along it.

Ok,
I found the center of the circle as follows:
-copy the support timber, move 10 feet away and explode.
-explode the curve
-using the line tool, find the midpoint of a section of the curve, and draw the line toward the center of the circle, then repeat this from another section of the circle.

the point where these two lines cross is the center, and your calculator agrees with the figure(30ft 2 5/16in)

unless the board is perfectly lined up, this process is not accurate...
I know I've seen this done on Youtube somewhere.

try setting the board / plank on the very center of the arc in a horizontal orientation, at the midpoint of the bridge, then rotate / copy to the one side X number of times... should give you better accuracy that trying to manually align the end board and rotate / copy around.

copy these and inverse scale them and move them back to the other side of the bridge.

i'm sure there is an automated script out there for this.

gotcha, that works with accuracy

nice.

yes, the trick with sketchup is often figuring how to use geometry and repetition to get what you want.

Sorry I'm late to the party. Here's a discussion of a plugin to find the centerpoint of an arc:

http://forums.sketchucation.com/viewtopic.php?f=180&t=18963

Watch the video to see how to use it.

nice!
i think you need to post a sketchup image at similar perspective...