AASHTO Lane Load 1

[jreit]

Hello. I had a question about the impact factors applied to AASHTO loads.
They are applied for the truck loads but are they also applied for the lane load, (18k and 0.64 klf for moment and 24k with 0.64klf for shear)?
The AASHTO code specifies the impact loads because of the effect of moving vehicles, but does the single concentrated load also have the same effect?
It does say the lane load is based on a series of slowly moving trucks but I'm not sure whether to apply the impact factor to the lane load also.
Thank you.

 

[TehMightyEngineer]

AASHTO LRFD 3.6.2.1 is what you need:

"...The dynamic load allowance shall not be applied to pedestrian loads or to the design lane load."

Maine EIT, Civil/Structural.

 

[mike20793]

The impact load factor is used in lieu of a dynamic analysis to account for the bouncing of the vehicle as it travels across the uneven bridge deck. Since the lane load is used to represent standstill traffic on the bridge, there would be no need for an impact factor. Hope that helps.

 

[mike20793]

Also, I'm not sure what loads your referring to for moment and shear in your post, but make sure you apply all the axle loads with the lane load on the influence line so your getting the worst case loading.

 

[yakpol]

It seems you are referring to the old AASHTO 17th Edition, which is no longer used. Current AASHTO LRFD (2002, 6th Ed) uses different loads and applies impact factor to the truck only.

 

[jreit]

Appreciate the input everyone.
I was referring to the 17th edition because we were analyzing design loads for an older bridge and calculating the difference between the old loads and current HL-93 loading scenarios.
Current AASHTO LRFD code does spell out the procedure more clearly.
@mike - Thanks. I thought as much but wasn't sure of the reasoning behind it. The 18k and 24 k were concentrated loads in addition to the uniform lane loads. They are, as yakpol pointed out, from the 2002 edition. They were analyzed in the manner you outlined.

 

[mike20793]

If you are load rating an existing bridge, it is preferred to use the LRFD code even if the bridge was designed with the LFD code. There are many papers available that compare the results and show that LRFD is adequate and produces good results. Note that the FHWA only prefers it and, in the end, the rating procedure and load posting/rehabilitation is up to the Owner. For new design, you will want to use the LRFD code. It is based off heavy research and is based on probabilities of failure for the given load. I can point you towards some good papers if your interested in learning more.

 

[jreit]

@mike - I would appreciate it if you could point me in that direction. Thank you.

 

[mike20793]

Here is one from the Transportation Research Board that overviews the rating procedures and compares them. I'll try to find a few more this evening.

http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_w28.pdf