HL-93 Loading to Piers for Simple Spans 2

[cap4000]

Can someone explain the difference between continuous and simple span reactions to piers per LRFD 3.6.1.3.1. Why 2 trucks on the continuous span and only 1 on the simple span. The lane load is for both conditions. Any tips will be appreciated. Thanks.

 

[cap4000]

See pdf page 21 of 24 for the diagram.

http://www.scdot.org/doing/bridge/pdfs/BD_manual/Files/Chapter_13.pdf

 

[tumbleleaves]

I read most of it (no LRFD tables), but it was still good. Thanks cap4000.

I have always wondered how the lane load was arrived at since per the 1935 AASHTO it was meant to model a truck train of mainly H15 trucks 30 ft apart (for the H20 and HS lane loading). I am at a loss as to what span and conditions were used for correlation between the truck train and the lane-load?

Sounds like they did something similar this time as well, and left it to the imagination, once again?

 

[bridgebuster]

cap4000 - The way I read the passage on page 21, for simple spans you position the truck for the maximum reaction at each bearing, which is what we normally do.

 

[cap4000]

I believe the continuous span condition is the correct one. One truck should be placed over the support and the other could be placed less than 50 feet away.

The rationale for the lane load coud be derived something like this. H15 is 30,000#/12'w/40'truck = 62.5psf over a 10 foot lane width.

You would think that AASHTO would want to explain this furter as you could have a hammmerhead pier with 6 lanes. Here in the New York area the traffic is extremely heavy.

 

[tumbleleaves]

cap4000, I wasn't paying close attention (skimming) and mistakenly thought the 2nd post was someone responding to your question--and that the 2nd post answered the question.

Now that I see my mistake...
A continuous superstructure of a central pier will "draw" more of the load to the support, particularly when loaded in both spans: each span will behave like a fixed-pinned arrangement. Take a look at your continuous beam tables, whichever ones you are using.

The AASHTO folks decided to specifically exclude this loading condition for the pinned-pinned condition, when it is less likely to control, whether it controlled or not, seems inconsistent on its face, particularly when this loading is used for substructure design as well, but without knowing the reasoning (per the questions of my previous post)...just go by the code and "plug an chug".

Does appear the new code has a lot of undue and unnecessary complexity (especially LRFD for footings and retaining walls eeehgaad). Did notice some changes that made sense too.

I understand why the ADA, environmental, building codes, and funding processes are increasingly complicated because of a higher level of competing interests, politics and legalities. The bridge code should be the example and the saving grace, but alas.

 

[cap4000]

Just for a comparison with a 100 foot simple span without impact.
HL-93 M= 2,320k-ft, R= 97.3k
Standard Edition M=1,520k-ft, R=65k
The previous AASHTO editions you added 26k at the reaction for shear plus a lane load or truck load.
I think each span in LRFD should have a truck placed right at the support regardless whether its fied or simple plus the lane load.