Which code to use? A general question.

[TehMightyEngineer]

Recently I've been working on a single span, single lane bridge for an industrial facility. This is to replace a ridiculously deficient bridge that looked like it was built by someone who had never opened an AASHTO code and was clearly built using steel S-sections stolen from a floor system (obvious splices and bolt holes for where the connections use to be). The bridge spans 30 feet into the turbine room of a small hydro-electric facility.

Thankfully the people with the checkbooks decided that before they put a heavy crane across the bridge they should probably let an engineer take a look at it. We recommended a full replacement as repair involved a new bridge deck, a new abutment, plating all 4 beams, and cutting out 2/3 of the beam ends and replacing with CJP welds. They agreed.

So, that leads me to today. I've been working on this for a while and am quite comfortable with the design but I keep getting this annoying problem of which code do I design to? It's industrial, private use, slow speed, single lane, and predictable usage so I really don't feel that AASHTO is fully warranted. However, this seems out of the full realm of ACI as my impression is ACI 318 is more for buildings and 350 is more for environmental and chemical structures. For the steel should I use AISC or AASHTO? What about the abutments; AASHTO or can I just design it using the CRSI retaining wall tables and design?

So, my question is this. When you're faced with this problem what do you typically do? How does an engineer determine what code is appropriate? Or, more simply, what is the usual "standard of care" for an engineer in this situation? Should it be designed to all codes (most conservative), only one code, or should I pick and choose based on how I feel each code applies to the situation (what I did)?

P.S. I ended up using AISC for the steel beams, AASHTO for the loads and general design guidance (but not AASHTO LRFD factors), ACI for the bridge deck, CRSI for the abutments but checked to comply with AASHTO's soil loads, and local DOT details and design guides for the small stuff (expansions joints, etc.).


Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[BAretired]

I wish you luck on your test.


BA

 

[Quanski]

For the load, I suggest you consult your client to determine what live load is expected and then determine what AASHTO loading to use. All components of the bridge should be designed in accordance with AASHTO requirements.

 

[TehMightyEngineer]

@BAretired: Thanks!

@Quanski: Well, what is your reasoning for having every component be designed per AASHTO? For example, AASHTO requires that bridge joints be designed for a 75% dynamic load allowance. This just isn't feasible on this bridge as it's a single lane bridge 13 feet wide with a narrow 10 foot wide doorway on one end. If someone is taking a HS-20 vehicle over this at more then 10 mph they're going to rocket off the side of the bridge before they ever destroy the expansion joint.

However, I'm not saying your wrong, simply wondering why you say everything has to be designed per AASHTO?

Oh, and yes, we consulted with the client so that they understand that we're going to rate the bridge for the HL-93 load.

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[woodman88]

But have you talked to the AHJ and the Fire Department about the bridge?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[TehMightyEngineer]

@woodman88: I was wondering that last night if there is anything I need to do with the local town. I wouldn't think so since we're replacing the bridge in kind and as it's a special use, industrial facility my understanding is they usually leave that to state authorities and let them police themselves (they would never get anything done if they had to get a permit signed off each time they wanted to do a structural upgrade).

As for the fire department I hadn't considered that. I don't think they'll need anything since I imagine all they care about is if they can get a firetruck over it. I'll take a look through NFPA and see what applies.

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!