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Re: AP: Wash. State bridge collapses into Skagit River; sends vehicles, people into water

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Allen,
The primer being under the final surface paint maintains its elasticity, but the final paint being exposed to whether and especially to sun becomes brittle over the years. When this bridge collapsed , it hit the water surface , which has a very high stiffness and is like a rock at the instant of impact, the members bent and created large strains in the steel as well as the primer and the final paint. The final paint being brittle got fractured and flaked out and dropped. The primer being flexible stretched with the steel and remained on it.

This is like testing a specimen in the lab. We whitewash the specimen and when the base steel yields , the white wash does not yield and fractures and flakes out. That is the way we realize what parts have yielded.

I have used this notion of surface paint flaking out of areas of steel that yield so often during my investigation of post earthquake damage or failure investigation of collapsed structures. It is a great tool (by default). It is like all bridges are whitewashed and ready to be tested when an earthquake hits and then we go in and see which part of our specimen (i.e. the actual bridge!) has yielded.

As an example of how valuable such observation can be , when i was investigating the damage due to the 1994 Northridge earthquake to steel buildings and bridges, in case of steel bridges, almost in all cases I realized that the damage is not due to cyclic behavior. It was like someone has hit the structure with a hammer and caused a single line of yielding or fracture (in case of welds) , whereas in other earthquakes, I had seen yield lines and fracture lines that clearly indicated multiple cycles of back and forth action. Later, I learned from seismologists that Northridhe quake had a very strong pulse.

Best wishes.

Hassan

On 5/24/2013 7:33 AM, T.William (Bill) Allen, S.E. wrote:
Hassan -

Why would primer be exposed at some of the joints like that?

T. William (Bill) Allen, S.E. (CA #2607)
University of Southern California, M.S.C.E., 1983
University of Texas at El Paso, B.S.C.E., 1977

ALLEN DESIGNS
Consulting Structural Engineers
32302 Camino Capistrano, #206
San Juan Capistrano, CA 92675
V (949) 248-8588•F (949) 209-2509

-----Original Message-----
From: seaint-seaosc(--nospam--at)mail-list.com [mailto:seaint-seaosc(--nospam--at)mail-list.com] On
Behalf Of Abolhassan Astaneh Sent: Thursday, May 23, 2013 9:26 PM
To: seaint-seaosc(--nospam--at)mail-list.com
Subject: Re: [SEAINT-SEAOSC] AP: Wash. State bridge collapses into Skagit
River; sends vehicles, people into water

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