pile cap 2

[shabirali922]

hi i am new to bridge design and i am having trouble transferring live load on the pile cap can some one help i have a 70 ft simply supported span and my loading is hs20-44

 

[BridgeEI]

FHWA has posted some design examples for a complete bridge design. There are numerous other examples on the internet as well as other programs that can do an autogeneration of LL for you, such as RC-Pier. The FHWA site is linked here

http://www.fhwa.dot.gov/BRIDGE/lrfd/examples.htm

Take a look through it and it may resolve some questions about the process. However, determining which lane combinations/placements to determine the greatest effect takes lots of exercise doing so and experience.

The manuals on the FHWA site are LRFD which means the loading is HL-93. The difference is that HL-93 is an HS20 truck AND a lane load with the truck. You can always refer to AASHTO for further clarification about the loading and about the "lever rule". Off the top of my head I can't remember which sections they are in.

 

[shabirali922]

but i am using aashto standard specifications

 

[Qshake]

If you're using HS20, you're not using the most recent edition. And that may be fine if you're not working for a DOT but rather a private client who has requested a design based on an older AASHTO.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[bridgebuster]

Could you elaborate on the question? You state "pile cap" and "simply supported span"; my first instinct is to assume you mean an abutment foundation resting on piles. However, some people use the term "pile cap" to refer to a "cap beam" in a pile bent pier.

__________________ <--Cap Beam
1 1 1
1 1 1
1 1 1 <--- columns
---------------------------------- Ground Surface
1 1 1
1 1 1 <-- continuation of columns

If you're trying to figure out the load on a "cap beam" in a pile bent pier, follow the link that BridgeEI posted, download the steel bridge example, go to Chapter 8. It shows how to distribute the loads into the beam. It's the same procedure for Standard Specs or LRFD.

 

[shabirali922]

but i still dont't know how to transfer live load on a pier cap that is at the top of pier

 

[Qshake]

Do you have an integral bridge abutment or an open seat abutment?

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[bridgebuster]

If I'm make the correct assumption regarding your question:

Look at the attached sketch. I added some notes. Once you calculate the stringer reactions on the pier cap, then you can determine your pile/column loads.

 

[shabirali922]

i have determined live load on prestressed concrete I girder according aashto lane loading can i use reaction from the girder (dead load +live load) and place it as a point load on the cap beam will that be a correct procedure

 

[bridgebuster]

As long as you are sure that you arranged the lane load on your cross section in such a way that you achieved the maximum stringer reactions. That's why I attached the sketch. The reaction that you use to design the stringer is not necessarily the same one that you apply to the pier.

 

[Qshake]

Bridgebuster, for an open seat abutment that is a way to calculate the beam and vertical pile loads for live load. But for an integral abutment, it's really much simpler. Which is why I asked which type is being used.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[bridgebuster]

I've never designed an IA bridge - they're not popular here - I've seen some design examples but no consistent or for that matter detail method of applying live load.

I would like to learn more.

 

[BridgeEI]

You can place the dead load reaction on the cap. However for the live load, you need to look at various combinations of loaded lanes. For instance, one loaded lane may produce the more extreme effect (reaction) on a girder than if two lanes were loaded. Keeping this is mind, the HS20 truck also "floats" in its lane, therefore you may need to look at it at the side of the lane, middle of the lane, etc. If the live load reaction you have obtained is via hand, then you will need to perform the above procedure. If the reaction is from a computer program that uses influence surfaces, such as a grillage analysis, then you may be able to use that reaction depending whether the program "floats" the design trucks. One has to know the limitations/assumptions of their software.

Shorter said, is the live load reaction you have obtained from hand calculations or from a software program?

 

[shabirali922]

i have obtained live load on the girder by hand calculation and i placed a udl of .648 k/ft on the girder and for shear condition placed a 26 k point load on the girder end i think that will give me maximum stringer reaction

 

[shabirali922]

bridgebuster i will have to place point load or udl on the deck what will be the magnitude of point will it the half of the axle load of hs20-44 loading or if i place udl its total load will be equal to axle load of hs20-44

 

[bridgebuster]

The lane load and the truck load are not numerically equal.

If you use point (wheel loads) loads - each wheel is one-half the axle.

If you use a lane load you still need to include the 26k concentrated load for shear.

 

[BridgeEI]

Since you obtained the reaction from hand, you will have move the trucks across the lane to determine the maximum reaction on each girder. If you look at the link that was posted above and open either PDF to the pier design chapter it will explain the process much better. Even the though the document is for LRFD, the analysis is still the same in ASD. The analysis is the same between the two, what changes are the loads.

 

[yakpol]

Ignore the girders for live load distribution on capping beam. Apply wheel loads (each wheel load equals to the half of HS20 reaction at the pier) directly on the capping beam. Move them across within designated lanes to maximize the forces. You may want to calculate instantenious axial stiffness of the piles (EA/0.75L). The spring supports will produce better force distribution than rigid supports for continuous beam analysis.

This is a very common approach. Only three-dimensional grillage (or FE) analysis will give more precise results.