AASHTO Girder Moment and Shear Distribution Factors

[TS59401]

We typically design temporary detour bridges to the old ASD AASHTO specs, however this time the state is asking us to use the 2014 version. The old ASD method gave distribution factors based on wheel line, thus we divided the simple span live load by 2 prior to applying the appropriate distribution factor.
(Lane live load moment/2)*DF = Girder Live Load Design Moment

It appears that the newer version of AASHTO now applies the distribution factor to the Land load and no longer bothers with a wheel line load.
(Lane live load moment)*DF = Girder Live Load Design Moment

Do I understand this correctly??? Does it explicitly tell us how to use the DF anywhere in AASHTO.

Thanks,
Tom

 

[BridgeSmith]

The distribution factor (DF) in the LRFD is the fraction of the axle load that is applied to a girder, thus it is essentially half of the wheel fraction (WF) from the Standard Specifications (ASD & LFD). A WF of 1.0 would mean the load of one line of wheels is applied to the girder, while a DF of 0.5 means that half of a line of axle loads is applied to the girder, which is the same magnitude.

Unfortunately for you, there's far more differences than just the change from WF to DF in designing by LRFD. The truck and lane loads are similar, but the load combinations are different (truck+lane, 90% of truck train + lane, etc.) Calculation of the DF is considerably more detailed, as well. Sorry, nothing as simple as WF = S/5.5, etc. or one simple equation for live load moment in LFRD. You've got a lot of catching up to do, unless you can sub it out to someone who's up to speed on LRFD and has the software to make quick work of the design. It just depends on how often you anticipate doing similar work in the future, whether it's worth the considerable time it will take to get up to speed on the new AASHTO LRFD.

 

[ernestog84]

The HL-93 live load is a combination of the lane load AND the design truck/tandem load at the position of greatest load. You must determine which has a greater load for that bridge, a design truck or a design tandem. For a simply supported span, see the following site for a tabulation of all these values (

http://www.dot.ca.gov/des/techpubs/manuals/bridge-design-aids/page/bda_9_1.pdf)

The live load distribution factors are determined based on the equations in AASHTO. The equations are determined by the type of bridge, the position of the girder (interior vs exterior), single vehicle vs multiple vehicle, and a different factor for moment and for shear. Don't forget to include the Multiple Presence Factor and the Impact factor. See the following example (

https://www.fhwa.dot.gov/bridge/lrfd/pscus051.cfm)

 

[TS59401]

Thank you for the confirmation. I have noticed that the distribution factors are much more complicated, and mercifully we are only dealing with simple spans (tabulated LL forces). I have been through this bridge by hand, and it is pretty involved under LRFD. Do either of you have any software suggestions for simple bridge structures (future work). Simple meaning steel girders, no appreciable skew or curve. Thanks again.

 

[TS59401]

I should add that we also use a timber deck and even the new code gives us a little break for timber plank decks. Little victories.

 

[BridgeSmith]

Our (WYDOT's) BRASS-Girder program would certainly work for you. Could be more than what you want to spend if all you're doing is a few simple span bridges, since it also analyzes girder systems for multiple continuous spans. The other options I know of are more advanced and also more expensive. There's likely spreadsheet solutions for calculating distribution factors. Max moments on a simple span are fairly straightforward from there.

 

[ernestog84]

I don't have an software suggestions, but it shouldn't take more than an afternoon to set up your own spreadsheet to calculate these factors, similar to the time it takes to familiarize yourself to new software.