View Full Version : Any structural/concrete engineers out there?

Paul Downes
10-27-2008, 10:11 PM
I'm in the process of building a bridge over a friends creek. The span will be around 28' and that's not counting extra length on the ends for safety. 28' is about what just gets me from bank- to- bank. Because I don't have poles long enough, ( I figured I would need 36 to 40 ft. to be safe) and not liking the possibility of flex in the middle, I have decided to put in a couple of piers in the middle for support. The idea is to sink in some 3 ft. dia. drain tiles upright in the middle of the creek with 12" dia. galvanized culvert in the middle of those all filed with cement and re-rod. The 3' culvert would be about 3' high with the smaller culvert rising up to around 7 or 8 ft. I have been thinking that the 12" culvert/cement posts will be a bit undersized to take the compression from the loads. We planned to just make a 4x4 quad bridge, but it seems to be not much more work to build something capable of supporting a truck or tractor. Any learned opinions out there?

Matt Meiser
10-27-2008, 10:32 PM
No idea, but the DEQ (http://www.michigan.gov/deq/0,1607,7-135-3313_3681_28734---,00.html) is probably going to be interested in your project, especially if you want to place a structure in the creek but probably regardless. Since the state is looking for revenue, I'm not sure I'd do anything to raise their ire. Remember, we're talking about an agency who tried to collect $10,000/day (http://www.snopes.com/humor/letters/dammed.asp) from beavers for an unauthorized structure. And times were good back then!

Tom Wilson66
10-27-2008, 11:02 PM
Check with your local county engineer, he can tell you what you need to do to obtain the permits needed to build the bridge. He can also tell you of a registered structural engineer who will be needed to design the bridge.

Paul Downes
10-27-2008, 11:03 PM
Shhhhh Matt, We figure that if they find out we can just remove the pilings. It is a natural creek but actually falls under the county drain rules. I have worked with both agencies removing log jams from the local rivers and am going to be careful with the construction. The creek is a rather short one and only drains a few square miles. When I worked on some of their projects we were also building wing dams and stream deflectors. Their main concern is to control flooding. we worked to improve the fish habitat and increase the stream velocity to increase silt removal. I can't fathom that there would be any problems with the design I'm thinking about. I just don't want it washing out in a 50 or 100 year deluge. We did get 7-8 inches of rain a few weeks back and the creek didn't overflow it's banks.

Jim Becker
10-28-2008, 8:15 AM
Paul, I think Matt is correct. Be sure all the eyes are crossed and tees are dotted 'cause it could get very expensive otherwise...more than it already is. There may be specific requirements for how the mid-stream piers are constructed, too, if they are not just pilings driven into the stream bed.

Mike Hess
10-28-2008, 8:58 AM
I'm a Civil Engineer in Pennsylvania, and - while I don't know what Michigan's requirements are, I recommend that you contact your municipal or county engineer. Here in PA, there would be a significant design/permitting process.

Beside the permitting process, here are some technical things to consider:

Frost depth - Your center pier(s) and the ends of the span would all need footings that extend below the frost depth so that it doesn't heave when the ground freezes. This could cause the structure to crack.

Footings - the size of the footings depends on the mechanical properties of the soil, which varies from place to place. You need to spread the weight borne by each point of contact with the ground over enough area that the soil can support it.

Erosion - the center pier and possibly the ends of the span would need some kind of protection from erosion. You'd be suprised how much soil one strong storm event can wash away.

Hydraulics - A 50 or 100 year storm is a LOT more water than you see from base flow. What might look like a ridiculously large opening to convey the base flow can be woefully inadequate when you get a heavy rain. Water's heavy, and can easily wash a way a structure that's improperly designed.

I hate to be the bearer of bad news, but bridge design - even for a small bridge, is a lot more complex than most people think. And for good reason - there's a lot to it. It's not too hard to design a structure that can stand up to base flow conditions, but heavy storms and high flows will subject the structure to some pretty strong and complex forces.

Why not just find a shallow place to drive your quads across?

Edit: Oh - to answer your initial question, with a 2000 psi concrete mix (pretty common, run of the mill concrete), a 12" diameter column could support over 100 tons of pure compressive load. Concrete is very strong in compression. That, however, is the least of the issues that you'll be facing.

Chris Damm
10-28-2008, 9:42 AM
Listen to Matt. Not only will the DEQ catch up to you, the DNR, EPA, and god knows how many other agencys will be on your butt. Not only will you have to remove the structure, there will be hefty fines to pay. If you go through proper channels you might get an answer in abot 10 years. I had property with a small creek (10' wide) running through the middle. I built a small bridge so I had access to the back of my property. Wrong move. I had at least 3 different agencys on me about it. It cost me thousands of dollars to get out of this mess.

David G Baker
10-28-2008, 10:41 AM
In my area of Michigan, if you are connected you can do as you please without fear of consequences.

Matt Ocel
10-28-2008, 3:33 PM
Oh Man!
You are crazy to build a bridge for so many reasons.
But the main reason would be enviromental impact.
Can you say "Sattelites"?

Had a buddy excavate a small pond, one day a parade of D.N.R., City, County, Watershed district people came pulling into his driveway and asked him what he did out there in that wetland, he said it always looked like that.
He changed his story real quick when they pulled out recent sattelite photos of the wetland that was there the week before.

Luckily he only received a slap on the wrist.

P.S. I've always wanted to build a bridge of that same nature.

Paul Downes
10-29-2008, 1:17 AM
Well, The area around the pilings will be rocked in, as well as the banks and I'm planning on placing a boulder in front of the upstream piling to deflect the water from it. As far as the stream bed goes it is hard sand and gravel. The area around there has a LOT of stones in it. I'm always joking with the farmer that he 'grows' more rocks than crops. I wasn't sure how much frost is a problem It would seem that if the stream doesn't freeze over that the ground underneath it wouldn't either. But I don't know if that's true. I certainly could sink the pilings in 4' if need be. I lost a half dozen traps in there a few years ago because I couldn't drive the stakes in that deep and a rain washed them all away. It is amazing how water can move rocks. When we built stream deflectors we had to winch boulders in place on the upstream side or the whole structure would be gone in the spring runoff.

As far as wetlands are concerned; the DNR has rules that limit the size of ponds dug in wetlands. I think it was something like if you put in a pond over 7 acres you had to create replacement wetlands for every acre over that amount.

I will have to make some descrete inquires concerning the rules and go from there.

We had been talking about building a rainbow bridge, and that maybe what is done if the current design isn't feasable.

mike holden
10-29-2008, 7:57 AM
Here in SE Michigan we had two plants divided by a small creek/drain. This was small enough that a shipping pallet would cross it, so thats what we used to keep our feet dry. Soon we had the Army Corps of Engineers knocking on the door wanting to cite us for "blocking a navigable waterway" - I kid you not! I mean, it wasnt large enough to float my kayak, but it was on the maps as a "navigable waterway" so....

Be Careful! Get a lawyer's advice. Dont do someting thinking you can remove it and it will be alright. Think of it this way: if you steal, replacing the theft does NOT mitigate the crime. So, just because you can reverse the bridge, does not mean you are free of obligations.
Be Careful Out There!


Ben Davis
10-29-2008, 10:43 AM

Erosion - the center pier and possibly the ends of the span would need some kind of protection from erosion. You'd be suprised how much soil one strong storm event can wash away.


Mike brings up an excellent point regarding erosion. Also note that the erostion and sedimentation around a pier is not what you think, especially in higher velocity shallow settings. Albert Einstein's son, Hans, was a pioneer in this field and there are several brainy quotes by his old man regarding how mathematically stupid it was to try and predict sedimentation.

Basically the leading edge of the structure erodes and the trailing edge has sediment deposit. The structure will typically first fail in the upstream direction which is quite a bizzare thing to watch. This sedimentation problem is most pronounced with circular shaped structures.

You'll likely never have these problems though.