There is a large stream that runs beside my house. It is generally pretty low, but it floods heavily at least once a year. For years I have built temporary bridges out of logs with the expectation that it will be washed away or covered up. These temporary bridges were always short and therefore relatively low and vulnerable to floods. When I came across some 24 ft long scaffold trusses I knew exactly what to do with them.
The plan was to create a bridge that can safely support an ATV (we call them four wheelers) and be used for foot traffic. It had to be tall enough to survive flooding and weather resistant enough to last for years. The entire cost for Lumber, concrete, and fasteners was around $300. The scaffolding pieces I obtained for free along with the scrap steel that the railings were constructed from. If you aren't as lucky enough to snag these freebies it may cost a little more.
This project was completed in the course of two weekends. If you focused and didn't get distracted by fishing and other projects it could be completed in one weekend.
Step 1: Site Preparation
Once you have chosen the location for bridge clear the area. Cut down any trees that may fall and damage your bridge. Luckily there was only one tree I had to remove. It was leaning pretty severely and while it was still healthy I didn't want to take any chances of it falling in the future.
My uncle runs a logging company and was between jobs. I was able to borrow his extremely large and powerful chipper to make some mulch and clean up around the house. This portion is by no means necessary, but was pretty cool so I thought I'd include some photos. For those interested this is a 150HP diesel chipper that can disintegrate an 18" log.
Step 2: Foundation and Truss
These scaffolding trusses are 24' long and can support a 2100 lbs point load each. The deck materials weigh around 900 lbs, a four wheeler around 400 and a rider around 200. With a total expected load of around 1500 lbs and a total capacity of 4200 lbs, that leaves 2,700 lbs of overhead. Even if the estimated weights are exceeded there are a couple things working with us. The first is that 2,700 lbs is a lot of room for error, the second is that when manufacturers post a safe working limit or SWL it already has a safety factor built in. This means that even tho the SWL is 2100 the truss could actually support much more weight before it fails. This should never be something relied upon when designing a bridge, just something to ease your mind about the strength.
The trusses were gently drug from a site over the mountain to the work site using a four wheeler and some care. This wasn't the best way to transport them, but getting the 20' trailer up our driveway is not an easy feat. Once delivered the trusses were put into place. Place the truss 36" apart from each other. This spacing was chosen based on the wheel base of an average four wheeler. Using rocks support and level the trusses. Note that these are not level along the length of the bridge, just from side to side. Be sure to complete this step for both sides and double check that nothing changed after all adjustments are made.
Find some pipe that is 1-1/2" OD and cut it into 4 equal pieces. Using a sledge hammer drive the pipes into the ground. The truss has 2 holes on each end for pinning them into place. Drill holes through the pipe and bolt with rated 3/8" bolts. This will prevent the truss from moving on the pipes.
Once the pipes are in place dig footings for each side. The footings should be about 5' long, 18" wide, and at least 4" deep. Drive 3/8" x 4' rebar into the ground on each side. This part may not be necessary, but I was concerned with the footings sliding since they are built on such a steep bank. Be sure to reinforce the footing with rebar and tie the all the rebar together with bailing wire.
I only built a concrete form on one side and that was simply because it was on the side of a very steep bank. On the lower side I dug a footing and poured the quickcrete right into the hole. Each side required five 80lb bags of quckcrete. Be sure to keep the quickcrete below the bottom of the truss. Covering the truss could result in it rusting and ultimately failing.
Step 3: Stringers
Stringers were placed 20" apart on center. To attach the stringers to the truss I used 1-1/4" conduit clamps. Be sure to use the type with attachment points on both sides. These happened to be the perfect size and worked very well. Keep in mind that these conduit clamps should not have any real weight on them. They are used in such a way to prevent the deck from sliding off the truss. All the weight from the deck is transferred directly from the 2x4 to the truss.
Cut pressure treated 2x4's to 5' lengths and make a mark 1' in from each side. The marks will indicate where to line up the 2x4 when you attach it to the truss. Attaching all the stringers proved a little difficult based on the height of the bridge. I ended up stacking all the 2x4's on the trusses and scooted across the bridge installing the stringers one by one. This was probably interesting to watch, but it worked well. Note that some of the stringers are spaced slightly closer together as a result of the vertical supports interfering with the conduit clamps.
To prevent the truss from splaying in or out I created a linkage out of 3/4" EMT conduit. Any metal pipe or bar could be used for this. The truss had 1/2" threaded studs welded on the bottom of each support which these were attached to. Cut three pieces of conduit to 3'4" and hammer 2" of each end flat. Be sure to keep both sides parallel. Drill a 1/2" hole about 1" in from one side. Measure 3' from the center of the hole and mark the other side. Then drill a matching 1/2" hole on that side. You should now have a linkage with holes exactly 3' apart.
Step 4: Decking
The deck was fabricated using 5/4 pressure treated deck boards. Everything was assembled using coated deck screws to prevent rust. As with most lumber purchased from a home improvement store many boards were warped and bowed. I didn't have a pipe clamp with me so I used a ratchet strap to draw the boards together where necessary. Deck boards should be installed with gaps to allow for swelling. I used the wrench out of a circ saw as my spacer. It took some elbow grease, but I was able to get everything installed with an impact driver, a ratchet strap, and a pry bar.
Be sure to alternate the seams in your deck. Since the deck is 24' long I used 12' boards with alternating seams. Since I made the mistake of not pre-planning I had to install a board to catch the seams. This was as easy as adding 2 additional stringers and splitting the difference of each deck board. I had one set of boards that seamed in the middle and another that seamed about 6' in from the upper side. I left all of these boards long and cut them off at the same time using a circ saw. This kept me in a rhythm while decking and didn't require breaking that rhythm to use a saw after each section. Feel free to do this either way you like.
Once the deck was installed I couldn't resist doing a weight test. Even though I did the math and was certain it was going to hold the weight of a four wheeler I was excited to see it in action. So far so good.
Step 5: Railing
The hand rails were fabricated using 1"x2" 16 gauge rectangular tube. This was all left over from a play and was covered in construction adhesive. To keep cost down I opted to use these scraps and scrape all of the construction adhesive off. This was extremely time consuming and I was never able to get the scrap completely clean. The railings were designed to connect to the truss using U bolts and allow the sides to hinge down when needed. I don't know that I will even need to fold the sides down, but I know that it is possible if the need ever arises.
The railing supports were fabricated in a metal shop with all holes pre-drilled. This made assembly a breeze. To prevent rust everything was painted and sealed. The pieces are about 5'4" wide and about 4'0" tall. Since they mount under the deck this still leaves us with a 42" railing height (which is OSHA standard if any one was curious).
Once all the Railings supports are attached, proceed adding the 2x4 railings. Position the 2x4 and then using a 3/8" bit drill through the hole already in the support and into the wood. Install using a carriage bolt and a nylock nut. Carriage bolts have round heads and are easy on hands. Nylock nuts have a nylon ring in the tip that prevent undesired loosening. Be sure to use a nylock nut instead of a regular nut and lock washer. When dealing with wood that is prone to swelling and contracting lock washers should be avoided. If the wood contracts the pressure against the nut is released and it could become loose over time.
Step 6: Finishing Touches
By this point you should be mostly finished. There are only a few details left to make this a functional long lasting bridge.
One detail left is building an approach (aka a ramp leading up to a bridge). Stack large rocks up to and cover with gravel. Avoid using dirt or soil as this will hold moisture and could rot the wood quicker. The gravel will allow all the water to drain. Chances are if you are building a bridge you will have access to plenty of rocks. If so desired the approach could be made from concrete or wood.
Be sure to paint all of the exposed metal surfaces. The conduit clamps used under the deck are coated, but just to be safe they were painted as well. Some of the paint on the scaffolding had worn off so this was touched up as well.
Overall I am very happy with the bridge and so is the rest of my family who use it on a regular basis. It is extremely rigid and doesn't look too shabby.