Question for civil engineers

[ToryGattis]

I'm not a civil engineer, but have a question for one if any of you are on this board. Please forgive me if I'm using incorrect terminology.

I was just driving and saw a flatbed 18-wheel truck with two giant square concrete culvert sections in the back. You know, essentially giant square concrete pipes - maybe 6-8' on a side? It seemed tremendously wasteful to me that a truck can only carry 2-3 of these when it must take dozens or hundreds to build a drainage line of any length. So here's my question to civil engineers: do you think it might be possible to design a similar system where each box was cut into two L-shaped sections along two corner edges? The L-shape would stack/nest much more space-efficiently on an 18-wheeler truck bed. The the L's could be put together into a box on-site.

Now I realize that creates a weakness along those cut edges vs. a solid box, but here's my solution: when nesting them in the ground, alternate the cut corner edges from section to section. I wish there was an easier way to draw or describe this.

Imagine it this way. If you were standing at one end of the pipe as they were assembling it, the first section would be an L with another (L rotated 180 degrees) on top of it. That creates a weakness on the top-left and bottom-right corners. The next section moving forward would put an (L rotated 90 degrees) with another (L rotated 270 degrees). That one has a weakness in the top-right and bottom-left corners, but it's strong in the top-left and bottom-right corners, and the nesting with the first section helps strengthen that section and hold it together. Each nested section holds the section before it and after it together.

Any thoughts? Is this feasible? What would be required?

[jjt]

(I'm not a civil engineer, although I do hold an ancient B.S.C.E.)

I would think that pre-fabricating things like concrete sections is *much* cheaper than assembling them on-site. It's probably pretty hard to line up two massive L-shapes along their length out in the field. While it's true that the pre-fabrication means more waste in transportation, that's a much smaller cost.

Speculating further, every joint/seam/step in a conduit (like any baffle) is going to reduce the amount of water it can carry and present an opportunity for an obstruction to form. The more uniform the section, the smaller it can be to carry a given flow. This is bound to have a big impact on the cost, mainly through making the hole you have to make to put it in smaller, or allowing you to carry more flow given your already existing space constraints.

I remember as a civil engineering student being frustrated by the "waste" inherent in, say, having to pick from a standardized set of I-beams (or whatever) instead of designing something more optimal (and more importantly, fun). Of course with a little more thought, obviously having standards is vastly cheaper system-wide - mills roll standard shapes, their properties go into handbooks, and codes and best practices keep people from getting killed by clever optimizers doing stuff like this:

http://en.wikipedia....alkway_collapse

Much better account than the Wikipedia article, but partially excised, on pp. 221-230 of this excellent book:

http://books.google....q=hyatt&f=false

Now that I'm spending a lot of time looking at how to model the cost and benefits of various system- and subsystem-level design choices in a manufacturing system, I think *that's* the interesting part, not the actual detailed engineering...

jt

[ToryGattis]

Thanks. All good points. I also hadn't considered the leaking that might occur, although that might be fixable with some heavy duty caulking. I was assuming the L-shapes would have grooves+tabs that would make them very easy to line up and assemble. I was also assuming they would be manufactured and transported to keep those cost savings.

[jjt]

Thanks. All good points. I also hadn't considered the leaking that might occur, although that might be fixable with some heavy duty caulking. I was assuming the L-shapes would have grooves+tabs that would make them very easy to line up and assemble. I was also assuming they would be manufactured and transported to keep those cost savings.

I know you weren't questioning the idea of standards or anything; that's just where my mind wandered to.

So, just considering a different standard - your L-shape vs. a box, the thing is that transport is just really cheap comapred to everything else. Sure you save some truck trips, but now you need to lift twice as many big heavy things and put them into place (and then join them, etc.)

Of course, this is just all speculation on my part. For all I know, built-up boxes from L-sections *are* used somewhere.

jt

[editor]

I was just driving and saw a flatbed 18-wheel truck with two giant square concrete culvert sections in the back. You know, essentially giant square concrete pipes - maybe 6-8' on a side? It seemed tremendously wasteful to me that a truck can only carry 2-3 of these when it must take dozens or hundreds to build a drainage line of any length.

There is also anothher possibility to consider -- weight restrictions on the truck.

Most places I've lived, trucks have looked pretty much like they do in Houston, except that Houston was the first place I lived that allowed double trailers.

Now that I've moved to the Northwest, they're MUCH more sensitive about the roads here. Trucks are MUCH longer, with MANY more tires to spread the weight of the load over a greater surface area so the road isn't as badly worn. And it's not just tractor-trailers. Pretty much any truck larger than a bread truck is longer here than I'm used to, with many more wheels, and often articulated for no apparent reason. Dump trucks have this bar between trailers that's so long (again, spreading the weight) that it requires flashing lights in the middle. On the plus side, the roads are remarkably smooth compared to the crap I'm used to from the east coast.

Also, remember that the flatbed you saw was likely going to a construction site, so it was probably going to have to drive off-road, so there's the possibility of a heavy load getting it stuck in soft ground. Watch a couple of episodes of Swamp Loggers on the Discovery Channel and you'll get an idea of the kind of off-road driving low-boy trailers do.

[RedScare]

Considering that the load is only traveling 20 to 30 miles (there are several concrete manufacturing companies in Houston), I cannot imagine why you even think that a 30 minute trip by truck to deliver a couple of several thousand dollar culverts is considered inefficient. But, even if it is considered inefficient, the various government agencies specing out these jobs would never allow the equivalent of a pre-broken culvert to be placed underneath its roads and highways, especially in the movable clay that Houston sits on.

Here's a link to a site listing all the ways pre-cast concrete is superior...and more efficient.

http://www.americanconcrete.com/commercial/box_culverts/box-culverts.htm

[Highway6]

I cannot imagine why you even think that.....

Really? Another civil (pun intended) conversation starting its downward spiral.

Your points are solid.... but -1 for the typical Red mocking delivery.

[editor]

Really? Another civil (pun intended) conversation starting its downward spiral.

Your points are solid.... but -1 for the typical Red mocking delivery.

Civility is a lost art on the internet. But good for you for calling him out on it.

[Highway6]

But good for you for calling him out on it.

[sarcasm] I'm guessing your photo wont escalate the situation at all... [/sarcasm]

-1 for you too editor.

Edited September 26, 2010 by Highway6

[ToryGattis]

So, summarizing what I'm hearing, the trucking cost is insignificant in the bigger scheme of things (and weight limits might be an issue anyway), and even if it were, the L-shape creates complexities at the work site and unacceptable weakness compared to the fully-formed box shape (although I haven't heard any details on that or if that could be compensated for by good design). I didn't know the cost of the concrete vs. trucking. I kinda thought the concrete was a lot cheaper and the truck ride more expensive. It doesn't sound like that's the case in urban areas, although in more remote areas with longer trucking distances it could be more of a problem.

[editor]

So, summarizing what I'm hearing, the trucking cost is insignificant in the bigger scheme of things (and weight limits might be an issue anyway), and even if it were, the L-shape creates complexities at the work site and unacceptable weakness compared to the fully-formed box shape (although I haven't heard any details on that or if that could be compensated for by good design). I didn't know the cost of the concrete vs. trucking. I kinda thought the concrete was a lot cheaper and the truck ride more expensive. It doesn't sound like that's the case in urban areas, although in more remote areas with longer trucking distances it could be more of a problem.

I'm inclined to agree with the notion that trucking is cheaper than concrete. When I was between jobs once I worked data entry at a trucking company. I was really surprised how cheap you can ship something huge across the country.