The Bridge of Size
I needed a bridge. A serious, hardcore bridge.
When I lived at Shoal Creek I was faced with a moving puddle about 30 or so feet across at the wet parts. From bank to bank, though, it's closer to 50 feet, which makes for one helluva clearspan bridge. Since most of my then 120 acres was across the creek, I needed a Bridge. A serious, hardcore brdige. A bridge that makes you think, "Yeah, that'll do." Contractors were quoting north of 20 grand for anything of usable size, so that was out. Now bridges are Hardcore Bare Knuckles Engineer Stuff from the word go, right? So there gotta be a book somewhere on how to build 'em.
No Way That’ll Work.
Each beam weighs about six and half tons and is reinforced with surplus prestress cable I found in six and eight foot chunks. There are twelve cables arranged six over six, three inches and six inches from the bottom respectively. The beams were simply formed up with wood and poured with concrete. Concrete has great compressive characteristics, but is fundamentally useless in tension. The bridge is designed so that the top of the concrete fails in compression before the reinforcing steel hits yield. This means that the bridge starts cracking if you about to take swim, as opposed to just dropping you in the river. I always apperciate a little notice before something fails catastrophically, even if I am doing something silly like driving a concrete truck across a little bridge in the woods.
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The concrete design was straight from the Civil Engineer's handbook although the crane
operator was worried about them breaking "like a potato chip"!
I can tell you from experience that with the exception of a live band, nothing draws a crowd like a crane.
This picture has my neighbor Frank Hedrick and Jeff The Mason helping me and my Dad land the second beam. The huge block in the background is the concrete abuttment set back from the water far enough to not disturb the bank. With the exception of the new DOT Special bridge, there's been really no environmental impact at all.
You can always hire a professional to design, build and install just about anything. With a little knowledge, you can usually do it yourself and get pretty good results. That said, anything structural is worth running past a professional engineer before driving across it. If you happen to be an engineer, by all means get a crane, book a band, and have a concrete raising barbeque. It’s a helluva good time.