Building the bridges-post installation
Bill Tindall, E.Tn.
Installing posts
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We begin with a bridge that has been decked with curbs.
We found the decks are not straight as a result of variance in the height of the ties beneath and variance in the trestles that support the stringers. A top rail that undulates looks bad. To get the top rail straight each post must be custom cut to length so the tops line up, with the length determined as follows.
Posts are permanently secured at each end of the bridge at the target height. A line is then tightly stretched between end posts. Measurements from the tie tops to the line reveal the height of the post at this position that will result in a straight top rail.
In our case we simply cut a 4 x 6 x 8’ in half to yield approximately two 4’ posts with the top angle of the post 12 degrees. The “approximately in half” is easier said than done. These end posts must be of a height that the sum of the posts that fill in between the ends work out to not have waste from cutting 8’ post timbers approximately in half. For example, if the bridge sags in the middle the end posts must be cut some less than 4’. The first post might be 47” yielding the second post from this 8 footer 49” to be placed where the bridge sags. With a bit of measuring and calculation it can be worked out that cutting the posts shorter than 4’ yield all the posts required that are longer than 4’…..for the most part. We had one bundle of 4 x 6 x 8’ that was in fact 8' ¾'’ and this pack saved the day on the long bridge that undulated 1 ½” in height relative to true horizontal.
So, we have our end posts set and of optimum height. We measure from the top of the tie to our string and write down the length. A table of posts is generated, for example, post #2 47 ¼, post #3 47 3/8, etc. A chop saw is set up, 4 x6’s hoisted up to the saw and the posts cut and numbered. Numbered posts are loaded onto a cart for transport across the deck to the location they are to be installed.
As they are loaded onto the cart the pilot hole for the 6” Power lag which fastens the post to the curb from the back side is drilled. A template locates this hole and they take but seconds to drill.
Next fastening hardware is distributed down the bridge at each post location. The 6” Power lag is started into the back side of the posts.
Now comes the key part. A tape measure is stretched down the curb to provide a running distance from the start post. Using this tape measurement as a guide the angle bracket is screwed to the curb with 2- 8” Powerlags. The edge of the angle bracket is located such that when the post is secured to the angle bracket the centers of the posts will be exactly! on 4’ centers.
Posts are trivial to install with two people. One person sets the post on the tie and the second drives in the 6” Powerlag from the back side of the post with a battery powered driver. It is critical that the post edge align with the edge of the angle bracket to yield posts whose centers are exactly on 12’ centers. The through bolt is not installed at this time, nor is it important to plumb the post at this time. It takes on the average only three minutes to install a post.
With the center of every post spaced exactly at 4’ the rails can be cut exactly at 12’. All the rails for the bridge except the ending rails are cut, loaded on the cart and dragged across the bridge for installation.