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[SEAOC] RE: [SEAOC] aircraft on bridges

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Martin

If I interpreted your question correctly, I think that you are concerned
about the term "impact".  There are different load cases, or impact
conditions, that I believe the ACI code is addressing.  The first case
is the "Impact" loading with regard to an aircraft "landing" on the
bridge.  Here it could be assumed that the reduction in applied load due
to the lift of the wings could be included (ie use the min. landing
speed of 170 mph).  The other instance of "impact" loading is due to a
loaded plane travelling (at a lower speed) across the bridge.  At these
lower speeds, less then 170 mph, the aircraft will still "impact" the
bridge as it rides across the structure.  Therefore, the 2.2 factor
should be used with the full weight of the loaded aircraft, ie don't
subtract the lift of the wings. 

This is just my opinion and I don't have a copy of the code in front of
me to review.

>----------
>From: 	Martin Johnson[SMTP:mwj(--nospam--at)EQE.COM]
>Sent: 	Friday, June 21, 1996 6:42 PM
>To: 	seaoc(--nospam--at)seaoc.org
>Subject: 	[SEAOC] aircraft on bridges
>
>I am using ACI 343R-95 to look at a bridge which is subject to 
>aircraft loads, and I have a question.  It defines loads for aircraft 
>landing as U = 1.3D + 2.2L + 2.2I, where I is 100% impact for 
>landing.  I presume that impact would be based on the full maximum 
>landing weight of the aircraft, but I question whether the L should 
>be the full landing weight, or the maximum landing weight, minus the 
>lift force on the wings at the minimum permitted landing speed (170 
>mph).  ACI is silent regarding this, it just says "live load".
>Note that the weight minus lift would reduce the live load to about 
>half.
>  If there are any bridge experts out there with an answer to this, 
>I'd appreciate your opinion.
>Martin Johnson
>EQE International, Inc.
>mwj(--nospam--at)eqe.com
>
>...
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