View Full Version : Intro and question

Greg Jones
08-06-2005, 6:55 PM
Hello all,

I was told about this site by a fellow from another forum that is related to police matters.

A bit about myself, I live in Manitoba, which is located above ND.
I have been in the construction industry for the past 10 yrs mainly new home building but also some renovations and other things here and there.

Now the fellow that told me about this site said that someone would have the answer to my question, so please dont let him down. :D

What I want to do is build a miniature stone bridge in my backyard. It would be approx. 3' wide and 6'-8' long.

Now my thoughts were to create a base out of some heavy duty lumber and then just build up the sides with fieldrock which is plentious out here.

So any ideas? Thanks in advance

Rob Russell
08-06-2005, 9:41 PM
Hey Greg,

Welcome to SMC!

I believe that you've got some engineering challenges ahead of you to build this bridge. You could certainly build a substructure out of pressure treated lumber, but you'd need to size the depth of the structural members to handle the load of any foot traffic and the dead load of all the stone.

I say "foot traffic" - is this a bridge you want to walk across, perhaps take a daughter's prom pictures on, or is this purely decorative? If you build a wooden structure, would it be flat or arched? If arched, how you would you tie the ends of the framing members together? If flat, how would you support the weight in the middle of the span? Are you intending to hide any framing so you can't see it or is it acceptable to see the framing below.

Another alternate would be some sections of steel I-beam. A few of those would give you the strength and stiffness to build a flat bridge and put the stone on top of it. A good coat of rust-preventative paint as an undercoat and than a topcoat to blend with the stone and you won't notice the steel that much.

Just some comments and questions.


Greg Jones
08-06-2005, 11:10 PM
Yes this is supposed to be a working bridge. I just finished up a man-made river through the yard along with a waterfall that joins into it. This bridge is to go over that river.

Now the river is at most 2' wide and so this bridge is more decorative than anything else.

What I was thinking was building a frame out of railroad ties and trying to arch it. Then use the rocks and mortar for building up the sides. My main concern was that the bridge would be stiff enough to where I would not have to worry about the mortar cracking and then giving way. I realise that over time and especially up north here the water and ice working in there will break it down but I would like it to last as long as possible.

Am I crazy? :confused:

Vaughn McMillan
08-07-2005, 12:31 AM
Hi Greg -

Welcome to the Creek. We're full of ideas here, and many of them are right. Bonus!

I have a bit of a civil engineering background, including bridge inspection, and a little rock bridge like you're thinking of should be pretty easy to do. One approach that might work for yours would be to use a half section of corrugated metal pipe (culvert material), then build up the rock on either side (the abutments) to the point where you can create the stone pathway. Carefully done, you could probably hide the CMP pretty well. You might not even need to mortar the rocks, depending on the shape of the rocks and the look you're wanting. I got the idea from this picture I found on Google:


Another fun (but probably costlier) approach would be to build a curved plywood form, pour a 3" or 4" reinforced concrete deck, then remove the form and sheath the exposed concrete with stone. You'd also need some type of abutment on both sides of your stream -- concrete would be a likely choices for these. As long as the abutments are firmly anchored (and not going anywhere) the arch in the concrete deck should be plenty sufficient to support the dead weight of the stone and any reasonable live loads you put on it. you'd need to build the abutments first, then anchor the arched deck to only one of the abutments and letting it "float" on the other (to allow for the little bit of expansion and contraction the deck will have with temperature extremes). I'd think #4 rebar at 8" to 12" centers in a 3" arched slab would provide ample overkill to support your expected loads, although closer rebar spacing would likely reduce the extent of cracking. (*See Concrete Rule #3 below.) Offhand, I'd think your rebar should be about 1" off the bottom of your deck form. For a flat slab, you'd want it in the lower 1/3 of the slab, but if the arch is pronounced enough, the rebar won't need to do much work at all, since the deck would mostly be under compression from the loads. Keep in mind these are suggestions off the top of my head based on field experience, not textbook calculations. Someone here with a bit more of a structural engineering background could probably give a more precise spec.

The formwork would be a fun carpentry challenge. too.

I hope this helps give you some workable ideas -

- Vaughn

* Being in the construction industry, you probably already know the Three Concrete Rules:

1. It's almost always gray
2. It gets hard
3. It cracks