Bridge Design Help 3

Gatorboy · Aug 13, 2003

Perhaps someone can help or at least guide me in the proper direction with my bridge-building decision/design. This is a large post, so I hope I don’t lose a lot of you from reading it because of its length.

OVERVIEW:
I have a piece of property in Fallston, Maryland that has a stream running the entire width of it, and where my house (someday) will be built is on the far side of the stream. I need to build a bridge that will be strong enough to allow fire trucks to cross or I think getting insurance will be a foregone conclusion. I would also like it to be strong enough to handle the occasional cement truck.

I have contacted one structural engineer who came out to my property, but I he never came up with any ideas on his own – I was coming up with several scenarios, and all he said was, “Yes, you could do that”, and in the end he said he would go back to his office, make some calls and get back to me with a quote. Well, he quoted me $2000 to come up with some plans, but still had not determined the best plan of action or design to be taken. I turned down his offer, and have been having a hard time finding a decent Structural/Civil Engineer in my area with experience in bridge designing. There are a couple large firms who said that my project was too small for them to work on. Where can I find a listing of these guys (or gals) to give them a call? I’m not trying to be stingy, I just want someone who KNOWS what can/can’t be done.

PRE-REQUISITES:
I do not want to touch the water in the stream, for if I do, then the Army Corps of Engineers will need to get involved, and I don’t want to have to deal with them. So the abutments need to be made on the outside of the banks, and no use of box culverts in the stream will be considered.

SPECIFICS:
- The bridge would span approximately 30’-40’ depending on how far away from the stream banks I put the abutments.
- There is a 4’ delta in height from the low and high side of the stream banks.
- The low side of the stream bank lies in a 100-year flood plain
- Pictures of location:

http://www.pbase.com/gatorboy/gallery/bridge

MY (remedial) DESIGN THOUGHTS:
- I want to build 95% of this bridge by myself – so simplicity is key.
- 4 steel I-beam stringers (size I have no clue), but more than 4 is probably needed.
- One lane bridge, so I am thinking the bridge should be 10-12’ wide
- Decking would be 8x8’s or railroad ties
- I want to build a timber footer and timber-bent abutment on the low side for it is very rocky and should keep costs down from needing big excavating equipment for digging. A picture of my thought is found at:

http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/3-34.343/chap7.htm#par1

(Scroll down to Figure 7-6)
- The abutment on the high side would be made of a timber-pile abutment (same web page Figure 7-7)
- I am totally in the dark about the size of the I-beams needed, and even if my ideas can produce a bridge to fit the rating requirements.

All help will be greatly appreciated.

:: D A V E H O F F M A N N
:: g a t o r b o y

Qshake · Aug 13, 2003

You should be able to find some one in Maryland who can do this for you. Perhaps you need to look into a civil engineering firm and not just a structural engineer. A lot of structural engineers never touch work that has earthwork involved with it. However, most civil engineering firms have civil/site engineers as well as structural engineers.

What you are asking for is not difficult, however, it is more complicated than what you asking for.

As a few examples....

The stream you are describing seems to hold flowing water year around. However, what you really need to do is assess what the maximum flow elevation is for a specific flood event (i.e. 2 year, 25 yr, 50 yr, 100 yr or 500 yr). Otherwise while it rains several miles upstream and your house is on fire, the fire truck will have to wait to traverse the stream. Your bridge should be 2' above the expected high water elevation for a 50 or 100 yr. event. but if you want to gamble you can go lower. Note that if you do go lower some thought should be given to designing for stream forces otherwise your bridge will be carried away.

What kind of bearing capacity do you have with the soil on site? You're talking about a shallow foundation and one made of wood. You need to make sure that the foundation is large enough to distribute the load to the soil without bearing failure or sliding failure and so on. Also, if you design for a more frequent flood event, you'll need to consider what will happen to the abutments during the flood. Will they be scour resistant? If so, how?

The actual span and steel I-beams is the easy part of the analysis. At 30 or 40' you should have no trouble spanning the creek at all with four beams. That also considers a 10-12' deck width.

And don't forget about a barrier of some type at the edges of the bridge. If not for you perhaps someone else who might not take it easy on the span.

Again, a competent civil or bridge engineer can do the structural part with relative ease.

Gatorboy · Aug 13, 2003

Qshake,

Thank you for the Civil Engineering Firm idea.

As far as the flood levels -- I do have that information, and on the 100-year level, it never should overflow the bank on the high side. So, bridge height isn't a concern. However, determining the bearing on the soil is something I will have to investigate.

I know that for someone who designs bridges for a living, this is probably a piece of cake. I just need to find someone locally to put their seal of approval on their or my design with their inputs.

Thanks again.

:: D A V E H O F F M A N N
:: g a t o r b o y

Qshake · Aug 13, 2003

Yes, I agree with you.

Also on the idea of the flood data, I guess I should have pointed out that the extreme highwater is often determined by observing the existing features for watermarks etc. Hence you need not have really have this investigated since its on your property. However, its wise to make sure the bridge is out of the floodway.

Good luck.

Gatorboy · Aug 14, 2003

I was just thinking about Rock Gabions -- are they a good choice for abutments in an area that can get occasional flooding? It seems to me that the water could flow through the gabion creating less eroding action than a concrete or timber abutment.

:: D A V E H O F F M A N N
:: g a t o r b o y

Qshake · Aug 14, 2003

I think in your situation it is a good choice for retaining the fill at the abutments with gabion walls. I see a lot of gabion walls used as channel liners for large storm water channels in my area in the midwest. What is absent in those walls are serious degradation so something is working right!

JohnFarley · Mar 11, 2004

I know I'm a little late on this response. If you are still having difficulty finding an engineer, try The David R. Schmidt Co.. He's a solo practicing structural engineer in Abingdon.

Also, I know you want to do this work yourself, but you may want to consider a pre-engineered bridge by Steadfast or Continental. You would still need to provide the abutments, but you'd get the bridge dropped into place in less than a day.

Gatorboy · Mar 11, 2004

John,

Thanks alot for the heads up on a local structural engineer.

:: D A V E H O F F M A N N
:: g a t o r b o y

maek · Mar 11, 2004

Gatorboy,

Other things to consider:

Make you sure you have the jurisdictional limits of that wash that you have in order to avoid a 404 Permit (getting Corps involved). You don't want to encroach that if you can help it. It might be on your property, but I'm not sure how much weight that carries if the wash extends beyond your property limits and is impacted by your project.

Also, given the size of the job, $2000 is not too unreasonable for a fee (I'm surprised he came in that low), so you may want to keep your options open along those lines.

As far as loading is concerned, an HS-20 load will definitely cover all bases and allow any highway legal truck to access your property, including fire engines. Typically for concrete bridges, live load (trucks, pedestrians, etc.) only account for about 20-30% of the design since dead load typically governs -- the ratio is different for steel, but my experience is primarily with concrete. That doesn't exactly help, but it gives you an idea as to how much vehicular loading enters into the equation.

Just a little bit of rambling for you. Hope it helps.

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