Strength Capacity of Existing Bridge 1

[Shz713]

Dear Bridge experts,

I seek advice from experienced bridge forum users regarding calculating the capacity of bridge components like girders, abutments, slab and diaphragms. I am working on a bridge built in 90's and I want to get the nominal capacity of structural elements. How may I do this? Should I get max moment and shear from drawings? Any suggestions is appreciated.

Shoot for the Moon, even if U miss, U still land among Stars!

 

[MikeHalloran]

Hire someone who knows what they are doing.

Mike Halloran
Pembroke Pines, FL, USA

 

[Shz713]

Okay Sir

Shoot for the Moon, even if U miss, U still land among Stars!

 

[bridgebuster]

You can get a copy of the Bridge Evaluation Manual from AASHTO but it's expensive, close to $500. They have examples of bridge ratings, which I suspect You are looking for. You could also try googling Bridge Load Rating Calculations and will probably find some samples. If your bridge was built in the 90's it was probably designed using the Standard Specifications.

Ratings are for primary members - stringers, girders, floor beams, and cross frames in curved bridges. Diaphragms are typically considered secondary members. Abutments are designed the same way as a cantilever retaining wall.

 

[Lomarandil]

If you are performing a thorough analysis of the capacity, you'll want to reference AASHTO's Manual for Bridge Evaluation.

Before you do that though, you'll want to check in with a mentor to see what level of analysis is required (depending on your application). It's rare that drawings will publish useful member capacity information, but there may be some information that simplifies the process.

Or, as Mike said, find an expert.

 

[Shz713]

Thanks for your inputs,

Well I want to check the capacity of various components as-designed. Then, check against the current legal loads; it is mainly component assessment. I found the mentioned book in university library and have a look.

Shoot for the Moon, even if U miss, U still land among Stars!

 

[Shz713]

I gained valuable insights from 'manual bridge evaluation'. In the uploaded file, I have shown an example of rating from 'bridge engineering handbook'. As highlighted, the nominal moment and shear resistance were computed based on steel rebars ( and of course effective depth, neutral axis and etc.). What if there's limited info on this aspect in bridge design drawings and reinforcement details are not available?

Shoot for the Moon, even if U miss, U still land among Stars!

 

[bridgebuster]

If we're talking about a concrete beam and there isn't any design information available, you could/should field measure everything, take cores to determine the design strength of the concrete & use a pachometer or hand held GPR to approximate the reinforcement in the beam.

The AASHTO manual also gives values for concrete and rebar strengths based upon the year of construction. Another thing would be to look at plans for similar bridges if available - some old text books have details and spec requirements - and make an educated guess.

 

[Shz713]

For getting in-service properties, field test is planned for near future. I did look for similar bridge plans; the point is all of the available examples (even in AASHTO manual) only bridge with nice and clear drawings are illustrated to get the As, neutral axes (NA) and you name it; no one talked about hand-drawings of old bridges with respect to estimating nominal member strength. So even reading the hand-drawings detailing is quite cumbersome. I'm planning to ask some bridge engineers regarding this aspect, since it's out of academic scope.

Shoot for the Moon, even if U miss, U still land among Stars!

 

[Lomarandil]

Sure, there's no easy way to teach someone to read old as-builts. Sometimes, you can find ways to read and reasonably interpret the drawings. Sometimes, the drawings are too illegible or just not available, and you'll be forced to the field measurement bridgebuster mentioned. Or sometimes, you'll be able to read enough from the drawings and apply some engineering judgement, experience, and maybe a few field verifications to conclude that the illegible details match a detail shown elsewhere that you can read.

I'm in bridge demolition, so this is a hazard of every job we encounter. Oddly enough, it seems the drawings from the 30s are often preserved better than drawings of bridges built in the 50s.

 

[Shz713]

Indeed I will take all suggestions into account and will figure out how to effectively rate the bridge

Shoot for the Moon, even if U miss, U still land among Stars!