- #1

Guy L'Estrange

- 6

- 0

I'm planning to build a suspension bridge in my backyard and i'd like some help with sag and force calculations.

The bridge will span 18m across a stream (anchor to anchor) in my backyard. I'll have 2 x strands of wire rope (1 at either side of the walkway) to span the gap. I've calculated that the weight of the wire rope, timber planks, and fastners will be approximately 650kg total for this span. I'd like up to 4 adults to be able to cross the bridge at any time so that would equate to an extra 320kg (4 x 80kg). I'd like to minimise the sag as it's only a shallow creek however I realize that the smaller the sag, the more force I'm generating at the anchors. I'm estimating the sag would be between 5-10% at the anchors although this would be difficult for me to measure acurately in the field. The easiest way for me to measure the sag would be to stringline the actual span then measure the vertical distance (with a tape measure) to the bottom of the curve.

Is there a version of a catenary equation (or other) whereby measuring the sag in the middle of the bridge (with my tape measure), I could therefore calculate the sag in degrees at the anchors and thereby calculate the estimated force at the anchors?

Any help would be greatly appreciated, also if anyone could point me towards any reference material for "suspension bridges calculations for dummies".

Thanks,

Guy.