AASHTO loading interpretation for Pier design 3

[dgidwani]

For a highway bridge pier supporting simple spans on either side, should the pier be designed for one HS20 truck per lane (positioned right above the pier for maximum total reaction), or for the sum of the maximum reactions from the spans on two sides?

The latter implies two trucks being considered simultaneously, and AASHTO seems to require only one HS20 truck per lane on highway bridges. What is the generally accepted procedure, and what is the correct code interpretation?

Thanks for your help,
DG

 

[dgidwani]

Rowe,
Your para 2 statement - ("as many truck loads (aka axle loads) shall be placed ALONG a 12 foot lane (in the longitudinal direction) to produce the maximum stress for whatever component you're designing.&quot - doesn't gel with AASHTO max. moment and shear tables (App.A).
In the tables AASHTO have used only one truck no matter how long the span, even 300ft.

 

[Qshake]

I agree with ROWE 100% and believe that ROWE has summarized the discussion to date well.

You can't go back to the simple span tables when rebutting simply because they are looking at one simple span not two or more. So that arguement doesn't hold water.

A rear axle placed on the bearing of the second span (of a two span bridge) at a common pier and the front axle of a truck placed as close to the first truck in the first span (of a two span bridge) will give the maximum reaction to the pier. That is it, plain and simple.

R M F R M F
_______|__|__| |__|__|_____________
^ ^ ^ ^
common
pier

The sketch above shows the placement of two trucks for maximum loading. R = rear axle, M = middle axle and F= front axle. Each truck is in the same lane, just bumper to bumper.

 

[rowe]

dgidwani

It's probable that, for those simple spans in the AASHTO table that can fit two truck loads (i.e. more than three axle loads) within the span, the lane load governs - thereby eliminating the need to put more than 1 truck load on one simple span.

One other suggestion: Why not make the deck spans continuous? It would lower the max. positive moments and eliminate a deck joint (which requires maintenance and affects rideability).

 

[dgidwani]

Rowe,
Consider a 200ft span. More than one truck could theoretically fit onto this span. Here are the max. moments:
One HS20 truck: 3322 k.ft
Lane loading : 4100 k.ft
Two HS20 trucks : 5855 k.ft
By AASHTO table : 4100 k.ft
Hence it is clear that the AASHTO table has considered only one truck per lane.
We could make the spans continuous, but say we had one mile of bridge deck. We would have to place expansion joints at some piers, and then have to decide how to figure the max reactions at those piers.

 

[yjung]

Hi all,
Whatever Qshake says, he is right.(I think he knows everything.)

I came to this forum about 3 years ago to ask some question about the bridge design. At that time I was an entry-level bridge designer. He answered my question in a good way. I believe ROWE did good job here too. I now have CA PE with 3 years pure bridge design experience.

I think dgidwani's question is very easy. Let me answer his question if Qshake and ROWE do not mind.

Problem: Simple spans each side on top of pier. I think the question is about the design of substructure, not superstructure. (I have never seen two simple span each side on the pier for any live load.(except for the constuction live load) This is not economical design.) BTW, to design this problem, First of all, a truck(whatever truck, manual says) has to be loaded for the max. Reaction at one side. The total reaction should be double to obtain the max pier reaction and times number of lane. Second, lane load should be loaded on a span and make it double to obtain the pier reaction. Third, use whatever the max is. If the span is long, lane load normally govern.

However, you must also calculate lateral load due to the bearing pad deflection from R+S+T(this is huge force if the bearing pad has a wide surface area and thickness.) and any other lateral load, wind or seismic etc. Actually, obtaining axial load is not that critical for pier design from my experience. U can even guess the axial load from LL(because its load is smaller than DL) and use the lateral analysis to design the substructure.

The code can't explain all the design situation. If I were the bridge designer for this problem, even though there is no reference for LL, I would do what I wrote above.

Anyway, happy NEW year Qshake and ROWE. I hope I see you here next year too. I still have many questions for you guys.

 

[dgidwani]

Yjung,
Is this procedure outlined in the manual you're referring to? Could you please share the reference information for the manual.
I don't have a problem with conservative approaches. But that wasn't my question. I'm just trying to see if anyone can tell me exactly what the code wants us to do in such a situation, and why; and if theres a reference describing it.
Thanks.

 

[Qshake]

dgidwani,

If there was a code prescription for this problem we would have written it by now.

The best we can do it point to article 3.11.2 and give you our interpretation. Please note that article calls for the engineer to provide a loading that "produces the maximum stress".

We've told you what we would do and yjung has also provided a viable alternate as well.

yjung, thank you for the kind words. I don't know as much as I would like and probably much less than I should. But after all the years it nice to be able to help others. Much happiness in the new year.

Qshake

 

[bridgedesigner]

dgidwani:
hi. i am bridgedesigner.
for vertical loads case, aashto some years ago managed, to accomodate trucks in the lanes, if you use hs20, you should know it dimension axis ti axis, the ones that cause the highest reactions, moments, or what you are analyzing for. there was another consideration, using equivalent load, those were an uniform load plus one vertical load for moment or for shear or reaction. you should compare the effects of accomodate the trucks by lane, taking care of number of lanes for %reduction, and compare with equivalent loads, that you should use by lane and that consider the lane totally loaded by the model truck you are considering. i hope this is useful to you. qshake, know a lot.

 

[rowe]

dgidwani,

you are correct concerning AASHTO's placing ONE truck on a simple span to determine the "Truck" load reaction. More than one truck on the span will be idealized by the "Lane" (aka "truck train") load.

Admitting my error however, does not preclude the application of one "truck" load on each adjacent SIMPLE span to determine the max reaction at the common pier due to "truck" loading.

 

[tincan]

Where is Simon & Garfunkles "Bridge over Troubled Waters"when you need them? The piers are usually designed for the various load combinations envelope. The extreme force effect of the loadings include:
...design tandem + lane load
...design truck w/variable axle spacing + lane load
...at the interior piers, 2 design trucks + lane load at the multiple lane factor of 0.90

We normall use two trucks (both headed in the same direction) w/14' axle spacing and vary the spacing between the trucks to obtain the maximum loadings.

A good discussion. But with Halloween fast approaching, we all should be looking forward to the GREAT PUMPKIN.

Best, Tincan.