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Integral Bridge AButment on 1 raw of piles
3

Integral Bridge AButment on 1 raw of piles

Integral Bridge AButment on 1 raw of piles

(OP)
I am amending a geotechnical report and making geotechnical design for a an integral bridge (one span of 30m) in a motorway to cross railway line. The abutment is 10 m high above the ground surface and founded on bored piles (mostly CFA piles but may be the 27 m piles will be constructed by rotary auger), as it is 6 m away from the rail lines. The ground is 4-5 m of soft and weak recent alluvium soils (clay, peat, silty sand), underlain by about 4 m of sand and gravel, underlain by firm to stiff clay then stiff silt. The stiff silt and clay layers are underlain by about 3-4 m weathered sandstone. Below the weathered sandstone, come the intact weak to moderately weak sandstone (qu=12MPa and RQD=0-38%, with inclined discontinuities). The rock head depth below the ground level ranges from 8.5 m in one side of the 200m-long abutment (with no stiff silt and clay layers between the sand and gravel layer and the bedrock)to 26 m on the other side (with the stiff silt and clay layer existing for 15 m depth). The 10 m high embankement (behind the abutment)will be carried by embankment piles+tension membranes (not ideal solution but will be used). The fill behind the abutment will be light weight fill (Maxit with unit weight=5kN/m3).
The structure engineer want to found the integral bridge on one row of end bearing piles (0.9m diameter), that are socketed in the intact rock. I would like to ask about:

1- In the structural engineer solution, The piles and abutment will work like a vertical beam, encastre at the bottom and not fully fixed at its top (connection with the bridge deck). In this case, the pile row will be loaded with vertical loads (100kN/m), lateral loads, and bending moment. Am I correct in seeing that this is not acceptable solution, as the piles should either be axially or laterally loaded, and that at least a second pile row is needed to avoid a bending moment at the pile cap?

2- If the answer is yes, what should we do (we need, if possible, to stick to the piles solution for non-engineering reasons)?

3- If the answer is No, how to consider the combined loading effects in estimating the allowable pile load?

4-I used the Goodman (1980) relations, mentioned in Prof. Das Book, to estimate the allowable bearing capacity for the end bearing piles. I used a safety factor 6 to get the allowable compressive strength for the bedrock samples, estimated the ultimate bearing stress at the pile tip using Goodman relation, and a safety factor 3 to get the allowable bearing stress for the pile using the uniaxial strength of the bedrock.For a Pile diameter of 0.9, the allowable point load is 1055kN but Tomlinson book says that New York Foundation Code restirict the carried load by any single pile to 600kN for weak rock, and 1500 for medium strong to strong rock, and 1000kN for hardpan over laying rock. Is my choice of allowable load acceptable or unsafe?

5-Are there regulations/guidelines to choose the number of piles to be tested in the field, and the number of piles for which pile integrity tests should be conducted? If the answer is yes, would you tell me where to get such information?
Thank you

RE: Integral Bridge AButment on 1 raw of piles

Deep foundation design of bridges is very elaborate and am wondering if you could run your numbers with a senior engineer in your locale.  Your computed geotechnical capacity seems very low given the diameter of the shaft.  

You haven't mentioned where the ground water table depth is and if you're in seismically active region.  Often, scour studies will govern the depth.  If you are in seismic region, the foundation/abutment will act more like a beam column and in non-seismic areas-the structural is right-like a vertical beam.

Realize that the NY building code is best suited for geology and past experience of structures in New York at the time the code is printed.  

As for number of rows, I agree that double row are better.

Because the shaft is in hard strata at the bottom and has a pile cap at the top, I would lean on having slenderness ratio less than 50.

RE: Integral Bridge AButment on 1 raw of piles

For integral abutments, single row of pile is the best solution for the SE.  THe reason is flexibility for thermal movement in the bridge.  This is especially true for single spans.

The pile which are normally very flexible (steel H-piles) offer the benefit of moving with the superstructure and yet maintain stability.  They also allow for the SE to maintain a design based on no moment transfer to the pile - or at least very little of it.  

Using bored piles or large diameter piles of concrete removes the flexibility and also increases the stiffness of the pile relative to the deck so that moment transfer is likely between the superstructure and substructure.  THis is not the behavior that we desire for integral abutments.

Regards,
Qshake

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

RE: Integral Bridge AButment on 1 raw of piles

(OP)
Thanks for the replies. I like to mention the following:
1- Allowable Pile Point Load is 2375 kN (I put Pall for another pile diameter in my previous mail).

2- Ground water level is about 1m below ground level.

3- The site  is in northwest of England, so no earthquake.

4- Can you advise me about design manuals that cover the piles when loaded with vertical & Lateral Loads, as well as Bending moments? In shallow foundations, for example, the load inclination reduce the bearing capacity (using the factors ic, iq, and igamma). Is there a similar reduction in piles design?

Many thanks for your comments

RE: Integral Bridge AButment on 1 raw of piles

The American AASHTO Design manual covers the US reqiurements for piles and load tess. Although not directly applicable, it may provide guidence.
 As Qsake pointed out, you want flexibility for an intrigal abutment. H piles, as he sugessted or concrete filled pipe are better choices. The cfa piles will not have the performance you are looking for in a pile.

RE: Integral Bridge AButment on 1 raw of piles

Another good example for this type of bridge is the USS Highway Design Examples which goes through the process of a vertically and laterally loaded integral bent.  The only thing to be careful of is the dated material on determining pile fixity - though some information is better than none.

Another reference is a UK book on Integral Abutments but I can't recall the actual title right now.  Perhaps you can look at teh website for Taylor and Francis publishers.

Regards,
Qshake

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

RE: Integral Bridge AButment on 1 raw of piles

The May 2001 issue of the NCSEA/CASE/SEI magazine "Structure" has a good article by Edward P. Wasserman (Director of the Tennessee DOT) on intrgral abutments. It provides a design method for single row H-piles. The publisher may be contacted at publisher@structuremag.org.  

RE: Integral Bridge AButment on 1 raw of piles

(OP)
Thanks for very much your contributions. Would you kindly tell me more information about what the USS Highway design examples are (where are they published)? I am not living in the USA and searching in google brings a lot of documents (Thousands).

RE: Integral Bridge AButment on 1 raw of piles

AlaaElnahas -

You can see the details of the USS Highway Design Handbook at the following link:

http://www.aisc.org/Template.cfm?Section=Browse_by_Category&Template=/Ecommerce/ProductDisplay.cfm&ProductID=143

This is at the American Insitute of Steel Construction website.  You will also find a chapter on integral abutments for steel bridges.  This chapter is authored by Ed Wasserman which Jacktrades mentions above.  It doesn't have any examples of the design though.

Regards,
Qshake

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

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