Fracture Critical Bridge

[SKJ25POL]

Hi
If it sounds basic I apologize for my question, however I am trying to understand what is a Fracture Critical bridge?
And where the word Fracture coming from and applies?
Where the Fracture occurs in the bridge?

Thank you

 

[Lomarandil]

I don't have AASHTO with me to give a precise definition, but the general concept is a non-redundant structure.

If any bridge is non-redundant to the degree where a brittle failure (e.g. fracture) would result in significant loss of capacity and or collapse, that bridge is considered fracture critical. (And subject to additional inspection or design requirements).

Common examples are two-girder or truss bridges.

----
just call me Lo.

 

[MIKE_311]

The wording varies but basically it's defined as "a steel tension member or steel tension components of members whose failure would be expected to result in a partial or full collapse of the bridge."

Think a two girder bridge. If the girders are fracture critical, if one girder fails the bridge is probably coming down. However the diaphragms on a two girder bridge are not fracture critical because the bridge likely wouldn't collapse if a diaphragm was compromised.

Here is pretty good summary -

https://www.fhwa.dot.gov/bridge/120620.cfm

These components require extra material testing in construction and require a special inspection to looks for cracks.

 

[SKJ25POL]

Thank you very much Lomarandil (Structural) and MIKE_311 (Structural).

So is this redundancy issue not true about bridge piers with only one or just two piers on a bridge girders support?
If the one pier mid-span support collapses then no matter how many girders are sitting on it, the bridge will come down?
I don't know why the focus is just the numbers of girders, there are tons of other stuff makes the bridge weak and high risk for collapse?

 

[r13]

I believe probability and size effect in play, as piers are usually well protected from extreme forces, and much stiffer than girders.

 

[Lomarandil]

Mike correctly added that the concern is focused on steel (or timber?) elements in tension, because of the sudden and difficult to detect nature of failures possible in those cases.

For elements in compression, more ductile failure modes are common.

For elements of other materials, failure is often preceded by warning signs that are more easily caught during inspection.

----
just call me Lo.

 

[MIKE_311]

So is this redundancy issue not true about bridge piers with only one or just two piers on a bridge girders support?
If the one pier mid-span support collapses then no matter how many girders are sitting on it, the bridge will come down?
I don't know why the focus is just the numbers of girders, there are tons of other stuff makes the bridge weak and high risk for collapse?

Well design codes require non redundant structure to be design for an additional capacity. As was mentioned, the issue is sudden collapse. Extreme conditions can an do occur, but they are designed for or to a level the relates to the probably of occurrence. It's the ones that no one sees coming that are troubling. Steel in tension, with an unnoticed crack, can crack without warning and if the structure in non-redundant, it will come down or be severely compromised.

The FCM designation requires additional material testing and more strict routine inspection practice to look for cracks in these members.

 

[SKJ25POL]

Gentlemen,

So am I right that fracture control bridge is not necessarily only for the "steel girder" bridges? This applies to "concrete post-tensioned beam and deck bridges" as well as "wood beam" or truss bridges?

Also with steel girder bridges that are redundant do we have fracture control case? if yes, then where is the origin or source of fracture risk?

Thank you for education. Just the term of Fracture Control is a bit ambiguous to me.

 

[TrussBridgeboy]

There have been several continuing education seminars on the topic from AISC/NSBA. I haven't checked but you might find them on the AISC website. It has been a hot topic at the last few NASCC (North Am. Steel Const. Conferences) too. "Fracture Critical" is often confused with "non-redundant" and they are close in intent but not exactly the same. "Fracture Critical" designation applies only to non-redundant steel members in tension (I'm not sure about post tensioned concrete) and simply invokes enhanced QC measures (and slightly more conservative design) so that there are no "initial imperfections". Therefore, the "flawless" steel can't grow a crack without the initial imperfection. However, it can still be hit by a truck or boat and come down, so it is "non-redundant". The compression top chord of a truss (on a two truss bridge), or a leg of your two leg pier, while not fracture critical, is non-redundant and the bridge can be wiped out be a falling tree or vehicle.

 

[CANPRO]

SKJ25POL, where are you practicing? Someone might be able to give you a more local definition of fracture critical.

I believe you have misunderstanding with how this term is applied. There is no such thing as a fracture critical "bridge", you can have a fracture critical member (FCM) within a bridge structure. Definition a fracture critical element where I live is "members or portions of members, including attachments, in a single load path structure that are subject to tensile stress and the failure of which can lead to collapse of the structure".

I can't speak to FCM in concrete or wood, I'm a steel guy. But to elaborate on your steel girder example - if you have a single or two girder structure, your tension flange (and a portion of the web) might be a FCM (definitely is for a single girder). If you have multiple lines of girders then a tension flange failure might not necessarily lead to collapse, but will have a major impact on the load carrying capacity of the bridge - it is now defined as a primary tension member (PTM). If the entire girder is subject to tensile stress (tie-girder for example), then the entire girder could be a FCM.

 

[OSUCivlEng]

Fracture Critical Members and non-redundant members are not necessarily the same thing. There are many piers that are not redundant, but are not fracture critical because they are constructed with concrete, not steel.

What I find really scary is a fracture critical pier beam supported by a 2 column pier in the middle of a busy highway.

 

[r13]

Fracture Critical Bridges
Fracture Critical is a classification for members (e.g., girders, beams, piers) of a structure that are steel, have a higher risk of cracking, and are non-redundant (failure of a Fracture Critical Member may lead to a progressive failure of the entire structure). Fracture Critical Members (FCM) require an "arm-length" inspection everytwo years at minimum.