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RE: AASHTO Load Combination

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Rich,

 

Your application of the load factors is correct. Groups IA and IB are for checking overload (AASHTO Articles 3.5 and 3.22.5). You ought to speak with your client to determine if the bridge owner requires a check for special permit vehicles, in which case you would use Group IB. You would need to ascertain the special permit vehicle axle spacing and axle/wheel loads from your client (assuming your client is the bridge owner). Group IA is used for overload checks when the design live load is less than HS20.

 

You indicated you’re working on a footing for a pier. For the footing, you’re correct in not applying impact. Please refer to Article 3.8 for application parameters for impact if you are doing any other portion if the pier or the bridge.

 

Last, please verify (if you haven’t already) with your client (bridge Owner) that the AASHTO Standard Specifications (ASD and LFD) are in fact the design code to be followed. Most state DOT’s prescribe the current AASHTO LRFD Bridge Design Code, which uses a different live load model (HL-93), and as an LRFD methodology, uses different factors than for LFD. However, for rehabilitation/retrofit of existing bridges, the AASHTO Standard Specifications are sometimes prescribed.

 

Hope this helps and hasn’t been confusing.

 

Best of luck,

Tom

 

Thomas J. Cummings, PE

Manager - Bridge Design Practice

Gannett Fleming Engineers and Architects, P.C.

One Penn Plaza, Suite 2222

250 West 34th Street

New York, NY 10119

P:  (212) 967-9833 Ext. 5244

F:  (212) 268-6684

E:  tcummings(--nospam--at)gfnet.com

 


From: Rich Lewis [mailto:seaint04(--nospam--at)lewisengineering.com]
Sent: Friday, October 27, 2006 12:24 PM
To: seaint(--nospam--at)seaint.org
Subject: AASHTO Load Combination

 

I rarely do any DOT work so I am not familiar with the AASHTO standards. 

 

I am working on a simple spread footing design for a bridge pier.  I’m looking at the load combinations on Table 3.22.1.A.  As I understand this I use these load factors with Equation 3-10.  I have a question concerning LFD combinations I & IA.  I see them as follows:

 

Group I – (1.3*1.0)Dead + (1.3*1.67)Live = 1.3Dead + 2.17Live

Group IA – (1.3*1.0)Dead + (1.3*2.20)Live = 1.3Dead + 2.86Live

 

Am I interpreting this correctly?  Do I have a live load factor of 2.86?

 

Thanks.

 

Rich