Pricing a bridge design

[marwin23]

Hi,

My first post, so short introduction.
I've been for 3 months in a well-established MEP engineering company in Midwest (approx. 30 ppl), which after 30 years decided that they want to open a structural department. I was acquired to start it, and struggle with many issues, including proper pricing. My experience consists of approx. 15 years of design: 8 in Europe (mostly steel silo, some industrial buildings), and 7 years in USA (mostly mid-rise residential up to 16-sty, some warehouses). In the new office I am the only one structural and responsible for all. And office tries to catch whatever is available in the radius of 300 miles.

Recently we were approached by a contractor who is building a 120' long single span steel bridge based on its own experience. I visited his shop and one 40' long section is almost ready already. They want it to be rated for 150kips (single piece of equipment at the time) and it would be used in a mining industry. Quick analysis showed that their main girders are capable of no more than 90kips concentrated.

And here I come to few problems. Contractor wants me to design (or reinforced his existing) structure to support 150 kips, and does not know (he & me) a lot of info. Roughly I believe I can design it as a concentrated load, distributed to two girders and ignore all distribution between axis of truck. When I first looked on that I said: $1000 for each type of beam, and $1000 for each connection. He has few types of beams which are identical (main girder, cross beams, longitudinal beams, cantilevers on both sides, diagonal braces), and slightly more connections (members to each other + splices). It will add up to ca. $15,000. The more I think of it, and the more contractor is interested in our service, the more I am nervous.

He excludes supports from our service, and I am fine with that. But still I am not sure whether I did not price it too low. First time since years I opened AASHTO today (and old version) and started to have some doubt. Can someone give me an idea, whether it is wise and responsible to design non-public bridge and whether it seems to be doable for the price I provided? I once in my live designed pedestrian passage at the height of approx. 30ft, and the length of approx. 50' using just steel design principles (connecting two school buildings in East Coast Area).

Any suggestion or a piece of information which would make me more enlightened would be greatly appreciated.

 

[Lomarandil]

Depending on the details of your bridge, distribution between girders may be more complex than simply dividing by two. However, for a 120' span, I agree that ignoring the different axles will be only mildly conservative.

A few other issues to consider:
What kind of lateral loads are you dealing with? Is this near seismic country (IL)? Or upper midwest where wind will control?
Delegating away the supports may be doable for a single span, but who will handle bearings? How about apron slabs?
What kind of barriers are required on the bridge? These can be non-trivial.
For a mining bridge, I'm assuming a straight (not skewed) configuration.
Don't forget fracture and fatigue considerations.

AASHTO has a decent set of documents, but you might also want to search out examples from the different DOTs and NCHRP for additional commentary.

----
just call me Lo.

 

[AS_PEng]

Furthermore to Lo's comments, depending on the girder configuration and deck width. you will need to consider off-center loading, with unequal distribution of live load. I've seen it as high as 60/40.

I wouldn't design it the span as a single concentrated load. That's may be way too conservative depending on the wheel configuration of the mine vehicle.

Also, don't forget about the dynamic load allowance.

 

[oldestguy]

:Looks like a good time to at least get someone of more experience to review your work. Miss something and it may hit you seriously. Price shpuld not get too mixed into this for that reason.