What I did last summer (part 5): starting the footbridge

As I will describe in my future posts on the design of the outdoor layout, it was a given that there had to be a footbridge over the stream. Any garden railway needs paths to access the various parts of the railway, and as this is the front yard we also need a path to get from the driveway to the front door. The previously existing concrete path, as shown in previous posts, is now blocked by the pond. Furthermore, the new stream and pond completely bisect that lower part of the yard, so without a bridge people would need to detour a long way around to get from one side to the other.

When I drew plans for the front yard layout I put in various paths, but these were just general locations. One preference was to make a step-free path with a gentle slope from driveway to the house, over the stream, and have other paths branching off from that to the rest of the garden.

We plan a mix of different types of paths throughout the area, from simple stepping stones to formal boardwalks and possibly brick pavers. The intent is to give the garden a sense of variety and artistic interest. The main path to the house has to be walkable in all weather, which means it needs to accomodate a snow thrower and shovel, and thus it will be of the more rigid variety.

The order of construction for all these items was also pretty much a given. The pond/stream needed to be first, as building a stream under an existing bridge is harder. The bridge/path comes next so that people can now access the front of the house again easily. Then comes any major structures, such as a planned gazebo. After that we'll fill in with the track patterns, other paths, planters, town areas, etc.

So, with the waterfall running I started work on the bridge, knowing that I'd be tuning the water feature (last post) in parallel. The water began running on Friday, August 21. That weekend I put in the concrete footings. By the next Saturday, August 29, I had the basic substructure in place:



In between August 21 and 29 there was a mixture of planning and building. First, I had to locate the exact site of the bridge. I'll cover this in the future design post. Then I had to think about the bridge span (length between the supporting posts on either side of the stream) and the height. My early thoughts had assumed a relatively low, short bridge, but as I looked at the scene I felt that this would be way too much of a view block of the stream. Perhaps this was because the stream was modeled to be an F scale river, so every foot was carefully planned. Perhaps if I'd just built a standard stream with river rocks I wouldn't have cared if you couldn't see a few feet of any one section as they all looked the same. In addition a low-level bridge would have ruined the visual illusion of any people/building/animal scenes we model on or next to the stream.

So I decided to set the bridge posts back a few feet, and this meant a span of about 9 feet. That in turn, per all my books on deck building, mandated deep concrete footings that extend below the frost line.

I had never done anything related to a frost line before, so searched around on our county's local building code site and eventually determined that 30" is generally assumed to be the maximum frost line for most of the county, except the high elevations in the mountains. I asked around at the building stores and they seemed to agree with this. So I bought 4 10" diameter concrete tube forms which are 4' long and cut them to 3.5' to give me an extra foot as buffer from the frost line. Then bought 16 4' long rebar bars and a pile of concrete bags and went to work.

Ideally you'd rent a trailer auger to dig each hole quickly. I didn't, mostly because I'm often a cheapskate (it's usually not considered worth it for only 4 holes) but also because it would have been very hard to get such a heavy piece of equipment into place on the south side of the stream. I have all the normal fencepost digging tools, but it still took 90 minutes for each hole to site it, dig it out, then install the tube and tap in the rebar. Actually, about 80 minutes of that was just the digging. As the whole gets deeper you can remove only a little dirt at a time. In the process I learned that the frost line, which can be seen by looking at the soil profile, is only about 22-24" in our front yard. We're on the high side for this county (7400 ft elevation) so I have to guess that the 30" frost line guideline already has a safety measure factored in.

Then came the concrete. All this had happened in one day and I was tired and the light was falling fast. I didn't know what to expect -- I'd never mixed concrete before but I had mixed 36 60-pound bags of mortar for the water feature and I knew that took a lot of time. I was thrilled to discover that concrete mixing was 10x faster, or more, than mortar -- that it took longer to measure the water required than to actually mix the bag. So that went fast. I added and leveled a metal 6x6 post holder into the top of each concrete footing, and that part of the project was done.

For posts I picked 6x6 brown landscape timbers. They have the advantages of size and strength, and are designed for ground contact so they are extra-resistant to decay. Traditionally these are available only in a sick chemical green color, but recently a brown version has been produced, usually costing only $17 or so instead of $16 for a green 6x6x8. The only problem is that these are at least 6" wide, whereas the metal post fittings are designed for a "nominal" 6x6, which is really only 5.5" wide. I addressed this by sawing a slot in the post on one side so it can slide onto the metal post fitting. Then used lag screws and deck screws to hold the thing into place. I was a bit disappointed that the posts wobbled a bit -- if I do this again I'll use the 18" deep "retrofit" metal post holders instead -- but that is apparently to be expected. Once the whole bridge was anchored and braced the wobble was gone.

For the bridge path substructure I relied on a bunch of 2x8 lumber. For beams crossing a 9' span they typically recommend 2x10 or evey 2x12, however the books all say 4x8 can work too. I chose the 2x8 because I wanted to keep the height of the substructure as thin as possible -- I wanted the top of the bridge to be as low as possible, to minimize the distance a person had to climb to get there, but keep the underside of the bridge as high as possible to minimize the view blocking. I couldn't find 4x8, but for each side I put together 2 2x8 planks of pressure treated (p.t.) fir then added a third redwood 2x8 to the outside. The redwood isn't as strong as p.t. fir, but it looks tons better. The redwood almost certainly isn't needed for strength, but it doesn't hurt. Each 3x2x8 beam was attached to the top of posts on each side of the stream. Then a pair of 2x8s were used as a cross-beam between each pair of posts.

Finally, a single 2x8 was extended down the center of the bridge, between the cross-beams, to serve as a joist. The bridge is about 38" wide, meaning the gap between beams is about 29" -- much too far a distance to cover with 2x6 redwood planks without expecting serious bending or even breakage. From my 1995 deck building experience I learned that an 18" joist separation is ideal for 2x6 redwood planking. Much wider leads to plank bending, but less than 18" doesn't provide any benefit.

On Saturday, August 29 my last task of the day was staining the redwood. I've found that I like to pre-stain it before putting it on the deck because that way I get one coat of stain on the sides before installation. The next day I cut the planks to size and my daughter Emma helped attach them:



I added a second coat of stain, and except for railing the central part of the bridge was finished. The kids loved this additional novelty to the front yard ... a few days later my wife took this *non-posed* picture of them sitting together watching the stream:



With that done I began immediately working on the ramp from the north side of the bridge to the driveway. I'd done deck work before, but never with a sloped surface. As usual, I had my doubts mid-way through the project but the results turned out well. I'll cover that in the next post.