Fracture Critical Members- Floorbeams

[greendrake]

In the BIRM. Ref. BIRM pg 10.2.7

Floor Beams: A floorbeam may be fracture critical if it satisfies one or more of the following. Some states consider any floor beam to be fracture critical even if none of the following criteria are met. So, most FB are considered FC unless FE model proves otherwise.
• Flexible or hinged connection to support at the girder/floorbeam connection.
• Floorbeam spacing greater than 14ft
• No stringer supporting the deck
• Stringers are configured as simple beams

With no stringers supporting the deck at what spacing does it become (by inspection) that the floorbeams can be considered redundant and can no longer need to be inspected as FC members?
Can anyone give background on how the 14ft criteria is based on?

Thank you

 

[bridgebuster]

The 14' spacing could be "arbitrary" i.e. based upon the 14' truck axle spacing.

I think a girder floorbeam system with 14' floorbeam spacing and no stringers would always be FC.

 

[wiktor]

The 14 feet criteria is likely based on the size of average passenger car and possibility of falling through in case of floor beam failure. I will not trust completely FE analysis - as all computer models could not predict the movement and behavior of the structure during the collapse - the Minnesota bridge being the best example, as some "studies" claimed extra redundancy via bracing system.
I agree with Bridgebuster that 14' spaced floorbeams w/o stringers are always FC. Some redundancy could be provided by a deck system, but in most cases the deck will be jointed over the floorbeam, or every second floorbeam.