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

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Hi Hassan

This is well presented and I have forwarded it to our bridge group and our 
association here in BC.


Victoria, BC
Email: vicpeng(--nospam--at)
Please consider the environment before printing out this e-mail

-----Original Message-----
From: seaint-seaosc(--nospam--at) [mailto:seaint-seaosc(--nospam--at)] On 
Behalf Of Abolhassan Astaneh Sent: May-24-13 6:18 PM
To: seaint-seaosc(--nospam--at)
Subject: Re: [SEAINT-SEAOSC] AP: Wash. State bridge collapses into Skagit 
River; sends vehicles, people into water


In this case of course the initiating cause of collapse was the impact of 
truck hitting the sway braces. But not all bridges being hit by a force 
that they are not designed for should totally collapse.  Especially due to 
inherit ductility of steel as a material, steel structures can take quite a 
 lot of beating before you can actually collapse the entire span. But, 
there is very important exception to this and that is all bets on ductility 
and redundancy are off if the bridge is "fracture critical" which this 
bridge was.    In other words, it would have made any difference if this 
bridge was opened to traffic the day before yesterday and its rating of 
course would be 9/9 at the time of opening. Still because it was 'fracture 
critical" if it was hit by the same truck , it would collapse the same way. 
 As you know, "Fracture Critical" bridge is a bridge that if a single 
member or connection fails  (fractures, buckles or yields) the entire 
bridge can collapse.  essentially fracture critical trusses are trusses 
that are determinate.  As we know from our Static course, in a determinate 
truss you can find member forces  by using only equilibrium, which is nice 
and you really do not need computers, but, if you remove a member or a 
connection from a determinate structure , it becomes unstable and 
collapses.  That is

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