Deck Options for Trail Bridges
Bill Tindall, E.Tn.
Adding a deck to a trail bridge is going to involve putting some sort of supportive covering(deck) over supports called stringers that run parallel to the trail and bridge across some support structure-a pier, trestle, etc. Picture 1 shows an example of our stringers.
New pedestrian bridges in the national forests often use laminated beams for stringers as shown in Picture 2.
Rail posts can be fastened to the sides of the stringers. Deck boards, usually 3 x 6, are screwed directly to the stringers. Ideally the interior width of a bicycle bridge is at least 8’ wide. The lumber for this kind of bridge becomes expensive to build in this width compared to converting a RR trestle style bridge that has sound ties.
Rails to Trails bridges begin with RR bridges that are either a wooden trestle structure or steel I-beams. Many trail projects inherit a RR shortly after it is abandoned. The bridge supports and ties are in sound shape and all that is required is adding a deck.
In our case the RR was abandoned in 1983. The ties are all unusable. The trestles need repair but thankfully the stringers were clad in galvanized steel and for the most part are sound or at worst have minor decay at the ends that touched dirt as shown in Picture 1.
Previously we tried two approaches to adding a deck. On our first restored bridge 3 x 6 deck boards were screwed to the stringers as shown in Picture 3. This size board is a custom order from the wood treatment companies and this fact nearly doubles the price in addition to the fact that these boards need to be 10’ long not 8, and 50% thicker. The next bridge used new replacement ties as shown in a previous post. The $100 cost for each tie made this option expensive.
Bridge ties are longer and larger than the ties that set on rock RR bed. Used bridge ties are not common. However, a source was located and now we had a source of nearly new 10’ long ties for $20 each.
We had to decide which was the cheapest bridge deck design to build. Installing ties, curbs and runners to fasten the deck boards to is obviously more labor than screwing deck boards to stringers. But at $20 a tie on the recently approved 24” centers, instead of 16”, the tie option is substantially less lumber cost compared to adding 3 x 6” x 10’ deck boards to the stringers.
The former RR engineer designing these bridges would not easily let go of the “tie look” for the bridges so we were forced to build his first 4 with ties. The hard part of the tie approach is getting 400 pound ties onto the bridge and into position. We tried everything. It took a month to set the ties on the long river bridge.
(note the big guy and what is on his feet. A second after I took this picture the tie skidded off the cart and crushed his toe. )
The remaining bridges are remote, but not over 15’ off the water. We found that a tracked excavator (they can go anywhere) of much larger size than shown on the bridge, coupled with a skilled operator could snatch a tie from a storage pile at the end of a bridge and place it where it needed to go. A crew of 3 positioned the ties on an 83’ long bridge in 3 hours. By the end of the day they were screwed to the stringers and the runners and curbs were added. And so the labor of using the tie approach became almost insignificant.
Recent calculations show that adding used ties back to the stringers, adding runners and curbs, then decking with 2 x 6 x 8’ deck is the cheapest route to adding a deck. (and it made some happy that the bridge will look like a RR bridge) Compared to the first bridge shown in the earlier post we are saving by using used ties, fastening runners with Spax screws and using 2 x 6 runners rather than 2 x 8. Going to 2 x 6 runners yielded another saving. We now use “cull” deck and rail boards for the runners. We were culling 5+% of this lumber when using a #1 grade and having no use for it.
