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Bridge Collapse in MN 29

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Risk Assessment is a "subjective" process; and it's also a group process.

It's fairly simple to rank the importance of bridge components and understand the consequence of their failure.

Identifying the hazards (especially the non-structral) can be fuzzy, although there are consultants who specialize in this area.

How much damage or potential loss of life is acceptable? It's a political decision. And the decision is somewhere between Dirty Harry ("Are you feeling lucky today?") and Josef Stalin ("If one man dies it's a tragedy; if a million die, it's a statistic.")
 
NYSDOT instituted a Bridge Safety Assurance program nearly 20 years to identify and address various vulnerabilities such as seismic, scour, steel details, concrete details, collision and overload.

Each vulnerability is identified through a qualitative process. It's a very useful tool. The process helps to identify potential problems and prioritize where funds are best spent.

The following is a link to their website:


It's not the same as the risk assessment concept, which is being used by some agencies to address terrorist threats.
 
It seems there is some misunderstanding of the risk assessment process. Risk assessment does not set an acceptable level of fatalities. And it really has little to do with terrorist threats. Although, I am sure there are some risk assessments being done specifically for terrorist threats. It merely calculates the risk of a structure. Comparison of the risks for various bridges is a more logical way of making the decisions on priorities for bridge repair or replacement. The methodology I am recommending is well summarized in the following links.


 
From one of the papers referenced in cvg's post. He may not set the numbers but he certainly considers lives loss in an economic return on investment sense.

Quoting from Risk-Based Decision Making in Water Resources
Proceedings of 4th Conference/EF/WR Div./ASCE
Santa Barbara, CA/October 15-20, 1989

From a paper by David S. Bowles.

"The assessment of human safety can be presented as a histogram of probabilities of life loss events, or the cost-to-save-a-life. The latter should not be confused with placing a value on human life, which is entirely different. The cost-to-save-a-life is calculated as the cost by which a rehabilitation alternative exceeds its economic benefits expressed per life that would be expected to be saved by investing in the alternative. Thus, it can be considered as the cost of providing safety on a per-life-saved basis. Comparisons can be made with the cost-to-save-a-life that society is willing to pay in other areas involving involuntary risks, such as environmental risks, in order to judge acceptable levels of investment in public safety. It is also possible to calculate the expected number of lives that would be lost on an annual basis (similar to risk cost); however, this generally results in relatively small numbers, which are difficult to interpret. Other approaches to quantifying human safety are therefore considered to be preferable."

"The decision is especially sensitive and difficult where lives are at risk, or where large investments will be required to improve safety with little or no effect on project benefits, except of course to their expected longevity considering the reduced likelihood of dam failure."

I'll give him credit for one statement, "Risk Assessment procedures cannot be substituted for good engineering".









 
"the cost-to-save-a-life that society is willing to pay "

Well, he is trying to simply look at the inverse of lives lost...i.e. cost of lives saved. That is a much more pleasant way of looking at it I'll admit.

But the other key phrase here is "comparison....with other areas of risk."

That is, or should be the main focus of any economic/risk evaluation. That you have multiple ways to spend your money to mitigate risk in society or add to the general welfare.

Raising millions of dollars in taxes to simply flood the bridge construction market will definitely save lives. But perhaps more lives could be saved in many other ways for the same dollar.

 
Even though they are of different designs and constructions there is commonality between the two failures.

The bearings froze.
The supports moved.
Problems with expansion joints.

Chronology of 35W

Posted by prost
&u=
Look at the Forensic Inspection Report for the Dunn Memorial Bridge Ramp Structural Failure on the following site.

Posted by bridgebusters
 
The Dunn Memorial Bridge Ramp is interesting .Going back to my earlier post I'll ask again - can anyone give details of a new type of expansion system we have here on our new bridge ? A stainless steel plate on the girder resting on a teflon covered plate .
 
Robertmet - which new bridge are you referring to?


The Roebling canal crossing you mentioned, is that the one in the vicinity of Lackawaxen PA?
 
Purely conjecture by someone not versed in the art of bridge design and building.

Considering thermal expansion on a long span of this design (35W) what would happen if both ends were locked either at the bearings or the first expansion joints and one end was suddenly released, earthquake style?

If the ends were locked would thermal expansion cause the center of the arches to rise or would the expansion be taken up by elastic deformation of different members?
 
If the expansion bearings were locked up, the thermal deformation of the bridge as a whole will be taken by elastic deformation in the piers.

Regards,
Qshake
[pipe]
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
 
Bridgebuster, the Roebling bridge is one of three aquaducts he built on the Delaware and Hudson Canal.It crosses the Delaware at Lackawaxen PA. Four miles downstream is our new bridge at Barryville NY.I Googled 'bridge bearings ' and found which has links to various makers of bridge bearings and expansion joints.Very interesting ,apparently these type of bridge bearings [polymer] are common now.
 
I would surmise that there is not a design procedure for looking at a structure with frozen bearings as a standard load case. I would think, however, that this certainly would be looked at if frozen bearings were discovered in the field.

It is possible that the piers may deform to absorb some of the thermal movement, however, there may other undesirable affects that could lead to failure before all thermal forces afre absorbed
 
To check the bridge members with seized bearings, you would just consider the ones that are seized as pinned supports rather than rollers. This bridge had one pinned support, 3 rollers when built. The rollers that are now seized would then have to be checked for the horizontal thrust, which might be difficult considering the advanced corroded state of the bearings. Qshake may be right, that the thrust would be taken by bending of the piers, but only if the bearing is not the weak link.
 
I wonder if someone has an opinion as to how URS determined that there were 13x4=52 fracture critical members on the bridge, and they were all chord members. The University report had previously identified some web members as critical. Those tension diagonals in the first bay adjacent to the main piers look critical to me.
 
Robertmet - elastomeric bearings are quite common now; long life; no maintenance; they don't lock-up.

hokie66 - to find the FCM's you remove a member, analyze the structure, see what happens; repeat the process for all tension members.
 
bridgebuster - You'd be surprised how often and quickly a PTFE bearing can lock up.

I've seen several examples of PTFE bearings locked up at the stainless steel/PTFE interface only to deform in shear the elastomer below. In some cases the elastomer tore away from the sliding surfaces.

All it takes is for some debris to mar the surface of the PTFE. Or anything introduced that can raise the coefficient of friction from 0.02 (which is nothing) to 0.2 which could mean a lock up for the normal force on the bearing.

Regards,
Qshake
[pipe]
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
 
Qshake - I was referring to a strictly elastomeric bearing not the pot type ones with PTFE. I've seen the problems you mentioned, including a bridge where the sliding surfaces were painted.

As they say, "if it moves, salute it; if not, paint it."
 
Bridgebuster - ok. I do recall that saying from back in the old barrack days!



Regards,
Qshake
[pipe]
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
 
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