That is exactly the information I was looking for. Thank you so much. I will try to do some research on the hows and whys of bridge failures, but I expect you are right as well. Overloaded vehicles, drunk drivers, too big farm equipment all play a role.
My feeling is that a one lane bridge in the country is not a big deal, The farmer I'm dealing with though is happy that River Road is closed. He doesn't get that much traffic now.
The term "fracture critical" is two scary words put together. When the first example people name is the I-35W bridge, the term becomes less a technical term and more emotional. I'm not a civil so I talked to a friend who is. He thinks the term as used is vague. It certainly means that something bad will happen if a single unit of the structure fails, but what this bad thing actually is--the structure fails at dead load, max live load, or just unacceptable loads being transferred to other members--varies with who is using the term.
One person might describe a structure as fracture critical if the loss of one member transfers loads to other members in excess of their design loading, which is usually very conservative and thus might result in no immediate effect on the structure. Another might reserve the term for a structure that will immediately collapse upon the failure of a single member.
In short, when someone speaks ill of an old truss claiming it's fracture critical, we need to ask whether they mean total failure, damage to a portion, unacceptable deflections, or simply loads outside of the margin of safety for the remaining structure. The term sounds scary to the average listener and we should make certain that anyone using it is prompted to explain exactly what they mean in terms of what can happen to the structure being discussed.
It might be interesting to look at some of the trusses that are documented on this site as having failed to see how they failed. It seems the cause is usually a grossly overloaded truck and the damage ranges from a tire sized hole in the deck to total collapse. Being able to show actual data might give some perspective to discussions on the safety of older trusses.
Clark. What actually are the percentages behind fracture critical. Can we devise some tests. I understand section loss and functionally obsolete and redundancy. Also as far as loading it is a number. Safe has no number. Risk seems to have a number but it changes as to what is acceptable. Really sincere about trying to shut the spin down and testing seems to be one way. Julie
I'm not sure why Dr. Kim chose this bridge or site to post his information.
He's correct in stating that truss bridges lack some of the redundancy built into other designs (they are "fracture critical"), and metal will crack and fail due to fatigue over time.
He seems to be suggesting that one way around this shortcoming, as an alternative to complete replacement, is to add the steel arch we've seen used to strengthen some older trusses. It's debatable whether adding this additional structure truly "preserves" the original structure, but it does prevent its complete removal. In that sense, he seems to share our belief that sometimes replacement is not the best option.
Unfortunately, most older trusses have other shortcomings such as width, vulnerability to crash damage, deteriorating abutments, etc. that make their preservation less than the ideal engineering solution. My metaphor would be maintaining an old car. I love my '63 Mercedes but, even though it was ahead of its time in terms of engineering, there is no way I can justify using it as a daily driver. The improvements in safety and efficiency in the last 50 years cannot be duplicated in the '63 without altering it unacceptably.
A highway engineer would be irresponsible to suggest keeping old technology for bridges just as an automotive engineer would be for suggesting building a car without seat belts, a collapsible steering column, disc brakes, or crumple zones.
In order to be practical to preserve, older bridges must have some valuable quality beyond their use for allowing safe, easy movement across an obstacle. Getting a large enough group of people to value these qualities will always be the key to preservation.
(I have letters behind my name--mostly scarlet....)
Thanks for the reformat. Engineers always point to the I35 Bridge collapse. That bridge was built by engineers in 1967, and it had some problems from that engineering. Today engineers continue to take out fractions of a bit of steel to save money, they don't trust pins, they think iron is tired and old because its old, but unless it's gone through the serious damage, and even then, it can be fixed, welded, riveted.
This letter is an example of one that went to every SHPO and DOT in the country. It is tragic about the I35 bridge collapse, but it sparked a one-way conversation that needs to end now.
Using the words fracture critcal on an iron bridge that has withstood the test of time seems wrong. We are able to beef these bridges up and the redundancies are built into the over engineering of the time.
Our engineers run the numbers, tell us what we need to replace and how to fix the rest, and we go on. But this type of engineering sets us back.
Please sir, let's continue the conversation. Although I have no letters after my name, I do understand the concepts and I've seen the work in progress.
The .docx format is the new MS Word format. I've converted it to the older .doc format so those without the latest Windoze software can read it. The short document speaks of the advantage of reinforcing existing trusses with arches, a structurally sound idea but not in line with preserving the original structure.
View attachment #1 (Microsoft Office Document, 15360 bytes)
I couldn't open your document. I am always interested in what engineers are saying.
The engineers that I work with understand that iron bridges are still strong. They were engineered with more material that engineers today with their computer models. Redundancy was built in with two eyebars and the pin connections. The rust on iron is not significant and these bridges can be fixed.
Why don't you post what you want to say or put it in a.doc document. I would like to read it. Thanks. Julie
See attachement --Concerns for most truss bridges for the safety.
View attachment #1 (Zip archive data, at least v2.0 to extract, 13668 bytes)