Bridge design and engineering, how?

Toiyabe · Nov 28, 2005

For a simple steel beam design, calculating the capacity of the bridge itself is fairly simple. Wood and concrete are somewhat more difficult. I say fairly simple because there are lots of little things that make the difference between a sophmore engineering student and an experianced professional engineer. Still, the basic principals can be learned from books especially if you already understand the concepts of stress, strain, bending moment and moment of inertia.

Designing the foundations is more of an art because it's so site-specific, and takes experiance beyond what can be learned in books. That's something you'll just need to be very conservative on.

If you are interested in doing some calculations yourself, I'd recommend spending a day at the nearest college or university with an engineering library. They should have a basic statics text from which you can learn how to calculate shear and bending moment from point loads and distributed loads. The AASHTO "Standard Specifications for Highway Bridges" has information on the sort of loads you should be considering. The AISC "Manual of Steel Construction" has tables of information on the characteristics of standard steel shapes - most likely you'll want a "W Shape" (aka wide-flange I beam). Buying all of those books will run you about $500.

KentT · Nov 28, 2005

( Larry,

Yeah, I tried somthing like this on another part of the Army site....turns out to be blocked by a log-in. I'm finding this all over the the .mil and .gov sites (Army Corp Eng. as well).

Apparently we can thank 9/11 on this change. I've also heard the Bush administration is labeling everything they can "secret" regardless of its actually use....thousands upon thousands of documents are now classified. Oh well. Maybe somebody has a copy and can email it or has it posted on the web. )

They are NOT making these materials Secret, but they are restricting access to them -- for good reason. Much of the information that used to be visible on .MIL websites by anyone is now restricted to only those viewers coming from another .MIL web address...

But, that doesn't mean the documents are classified. Actually the trend has been to make fewer documents classified, instead of more, because of the additional "handling and storage" costs associated with classified info...

Classified information must be stored in a safe, and it must be signed for and controlled, similar to the "chain of evidence" related to evidence used in trials... It's a PITA for everyone involved.

drzmatt · Nov 28, 2005

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."
Benjamin Franklin

MikeD74T · Nov 28, 2005

I needed a 24' span capable of supporting 8000 lbs live load. went to a structural steel yard to price it out. they calaulated the beam size and had used beams availible. Paid $400 for 4 -26' beams and spacers cut to length delivered 25 miles. The beams are 18" tall but I don't know the weight per foot. The original hemlock deck only lasted 5 years so I made it into a covered bridge when i replanked it last year. Since you have a short span check with a precast company. You can usually get concrete plank cheaper than you would expect. harder to erect by yourself, usually need a crane. But crane rentals aren't that expensive either if you have good access.

JimMorrissey · Nov 28, 2005

Nice....I'm pretty sure that's the one. It's the general engineering document.

Thanks.

JimMorrissey · Nov 28, 2005

"restricting access" .....that's correct. Restricting access seems to be the standard pattern We should all be frightened that somebody might get ahold of the bridge building or road building documents. God have mercy on us all.....

MMM · Nov 29, 2005

I've been wondering about mobile home frame spanning a narrow 10' section of a creek that blocks access. I know where there are a few that burned and all that's left is frame.I'm just not bright enough to find info whether it'll support 8500lbs of tractor and cutter. Don't want to find out the hard way.
Anyone heard of using these frames?

v8dave · Nov 30, 2005

( For a simple steel beam design, calculating the capacity of the bridge itself is fairly simple. Wood and concrete are somewhat more difficult. I say fairly simple because there are lots of little things that make the difference between a sophmore engineering student and an experianced professional engineer. Still, the basic principals can be learned from books especially if you already understand the concepts of stress, strain, bending moment and moment of inertia.

Designing the foundations is more of an art because it's so site-specific, and takes experiance beyond what can be learned in books. That's something you'll just need to be very conservative on.

If you are interested in doing some calculations yourself, I'd recommend spending a day at the nearest college or university with an engineering library. They should have a basic statics text from which you can learn how to calculate shear and bending moment from point loads and distributed loads. The AASHTO "Standard Specifications for Highway Bridges" has information on the sort of loads you should be considering. The AISC "Manual of Steel Construction" has tables of information on the characteristics of standard steel shapes - most likely you'll want a "W Shape" (aka wide-flange I beam). Buying all of those books will run you about $500. )

Since I asked the original question, I've worked with a Mechanical Engineer who loaned me the books to do the calculations. Turns out it's all in just one book. But, it's been so long since I was in school, I wasn't familiar with the terms. Being basically lazy, I've asked for help from the ME and we're going to figure it out with the standard I-beam shapes/materials listed in the book. Hey! I'm no longer in school and it's now ok to ask for help from your buddies. In fact in the real world plagarism (sp?) is a good thing. No company wants to pay to develop an approach over again if the old one is still good and quicker to apply.

v8dave · Nov 30, 2005

( I needed a 24' span capable of supporting 8000 lbs live load. went to a structural steel yard to price it out. they calaulated the beam size and had used beams availible. Paid $400 for 4 -26' beams and spacers cut to length delivered 25 miles. The beams are 18" tall but I don't know the weight per foot. The original hemlock deck only lasted 5 years so I made it into a covered bridge when i replanked it last year. Since you have a short span check with a precast company. You can usually get concrete plank cheaper than you would expect. harder to erect by yourself, usually need a crane. But crane rentals aren't that expensive either if you have good access. )

Mike, great idea, I hadn't thought of this. This is even easier than doing the calculations myself. Steel yards are not very close, but it's do-able.

Louisiana_John · Nov 30, 2005

A cheep way to build a easy bridge is buy a old big truck flat bed trailer. Buy from a salvage yard cut the running gear off sell back to them. If you want to cut a pipe like you said in your first post go get a section of a rail tank car that has been cut. they are 10' in DIA and around .75" for most thicker for others. That is if there is a scrapping place close. Being in CA I don't think you will find one. Old flat car and box car bottems make good bridges. They are on the heavy side.
The big truck trailer you could leave long and dig out on each side or cut and pour footings.
I need to span a 30" bayou soon. I am waiting to see were a gas well goes, it might get done for me with a road.