Oklahoma Suspension Bridge

Step 2 Posts, Beams and Anchors

Next came the towers and anchors. Because the only way to keep the tension as low as possible, reducing the strength requirements of all connectors, is to have as much sag as possible, not exceeding approximately a 6 to 1 ratio of span to max sag.  More typical designs use a range of 12 to 1  to 8 to 1 in order to minimize sideways sway. The 16 ft posts plus the 8x8 beam on top, 2 ft of bury and about 8 in above grade to the top of the joists gave me about 13.5 ft total distance for sag plus a minimal suspender length at mid span. More bury would have been better, but would have reduced the max sag and increased tension in the catenary.

I laminated 6 posts, cut 1 post in half for the top beams and 1 post in quarters for the anchors. The anchors are designed for 3.5 ft of bury with spikes on all sides, and embedded in as much concreted as is feasible - I aimed for 1500 lbs of concrete approximately 3 ft in diameter with at least 18 in of compacted dirt on top. I don't know what the total uplift resistance is, but I estimate it is in excess of 3000 lbs. This is lower than I wanted, but as much as I could do. The spikes are 10 in (I think) long and similar to a really, really big nail. They are pounded in about 40%. They are what the concrete bears upon, whether in up lift (anchors) or in down force (tower legs). There are 6, as I recall, in each anchor, along with two bolt ends.

The anchors had 1/4 in steel plates drilled for an eye bolt for the anchor cable and additional holes for lag bolts. The towers had similar steel plates on both sides tying the beams to the posts and allowing for and eye bolt and eye nut for catenary and anchor cable connection. 

I chose a fixed connection for the cables rather that a saddle as is customary in suspension bridge because I was concerned that during installation of the bridge the unbalanced loads on the towers would be too great for the minimal bury I used. With the anchor cables installed, I had no worries the towers would fall into the pond during installation!

The towers were erected with the assistance of my compact tractor as well as two human assistants. The west side tower has a tree interfering with the approach and was very difficult to raise. The east tower, with the knowledge gained on the west tower and no interference from trees went up almost easily - by comparison, at any rate.

The towers are designed for a 5 ft width between the inner faces of the posts. The eyebolt assembly is designed so that the catenary cable and suspenders will fall just outside the handrail.

Comments

[rprough]

Do you think creosoting the anchors before inserting into the ground would be a good idea?

[Jakebutnottheone]

Any treatment of the anchors is a good idea, but I personally am not comfortable with the idea of creosote. There are other ground line treatments available, and I may use one of those. In the long run, I will probably replace the anchors with steel screw anchors. Greater, and more certain, uplift resistance as well as a far longer life.

[rprough]

Forgot about the steel screw type. What comes to mind is the tiedown anchors used for a mobile to secure it. Good amount of uplift resistance.

[Jakebutnottheone]

The manufactured home tie downs are also often screw anchors. Using those anchors I could double up each anchor and provide the uplift I desire. I haven't yet decided how I shall do it.