Cooper 80 Uniform Loading

[structbear]

Does anyone know if there is a code accepted Design Uniform Loading behind a retaining wall Structure. I seem to remember seeing 800 psf. I just don't remember where

 

[PEinc]

If you are talking about a Cooper E-80 railroad surcharge, then yes, there is a standard load for this.

The Cooper E-80 surcharge is the vertical load resulting from one axle of a special engine. In the heaviest loaded portion of this engine, the axles are spaced at 5 feet on center. The axle load is 80,000 pounds. The surcharge is 80,000 lbs / (5'cc x 8.5' tie length) = 1882 psf vertical strip load pressure. This 1882 psf vertical pressure is then used with Boussinesq's equation for a strip load behind a rigid wall. Most railroads and AREMA specs require the calculation of horizontal surcharge pressure be made for a rigid wall. This gives higher pressures than with a flexible or semi-flexible wall. If you go to a railroad's engineering web site, it may show you the equation and define the variable.

What I don't like about the Boussinesq equation is that it gives the same horizontal surcharge pressure for all soil types. The equation gives the same lateral pressure whether the retained soil is dense, well graded gravel, a loose silt , or a soft or hard clay. It just doesn't make sense. But the railroads do not care because the equation gives conservatively high design loads.

 

[DougP100]

Hi:

A standard surcharge load would depend on where the wall is, who it is being constructed for, and what it is used for and/or retaining. Many times the owner of the wall may set these values through their design manual if it is a public entity.

In absence of such information, from experience I generally use 250 PSF for a wall surcharge. I have seen this number in a few design manuals by Pile Buck, etc. I use it as a standard for loadings adjacent cofferdams, and have never had an engineer tell me to increase it.

AASHTO uses 2' of fill as a surcharge number to simulate passing traffic loads behind retaining structures (AASHTO 3.20.3), which is also more or less 250 PSF depending on your section of the country.

Personally, I have only used 800 PSF in situations that call for regular heavy loads, such as a marine terminal bulkhead at a port. The Standard Handbook for Civil Engineers (Merritt) gives a range for these loads from 600 to 800 psf, with up to 1000 PSF if justified (For instance, if the pier handles heavy metal ingots).

PEInc above has also covered the other instance where heavy surcharges would be required, which is for retaining structures near railroads. In these instances, the owner of the railroad will ordinarly dictate what surcharge load is to be used. I had a cofferdam several years ago that used the Cooper loading described above. Although I did not actually do that design, I was PM in charge of its constuction and got to witness first hand the effects of such heavy surcharges on construction materials and methods.

Hope this helps.

 

[PEinc]

Structbear specifically inquired about "Cooper 80" (I assume he means Cooper E-80 railroad loading). Most railroads require this loading. However, some, such as Amtrak, require even higher surcharge loading be used in wall design. Amtrak requires Cooper E-80 PLUS 50% for impact. This is an extraordinary amount of surcharge (2824 PSF vertical load, possibly per each adjacent track). Depending on how close the wall is to the end of the rail ties, you could be designing for well over 1000 PSF lateral pressure in addition to the earth and water pressures. It's very important to know your railroad's exact requirements before wasting time using an "incorrect" design parameter. Railroad engineers are busy and certainly in no hurry to review a design submitted by another engineer.

 

[DougP100]

PEInc.:

I beg your pardon, Sir.

I have re-read Structbear's Posting:

"structbear (Structural) May 5, 2003
Does anyone know if there is a code accepted Design Uniform Loading behind a retaining wall Structure. I seem to remember seeing 800 psf. I just don't remember where"

I see NO reference to the word "Cooper", specific or otherwise. He doesn't even mention the word "railroad". He has merely requested input regarding a uniform surcharge behind a retaining wall. The factors I proposed were merely suggestions based upon my experience, along with a few references.

And you will note that TWICE within my response I noted that the owner will ordinarily dictate what surcharge they want to use, first in the initial paragraph, and again in the paragraph where I referred to your response - favorably if I might add.

I see no reason for the tone of your response. This is a forum of suggestions, and I offered mine.

DougP100

 

[PEinc]

DougP100,

My, aren't we testy this morning! Re-read the entire posting. Look at the title of the thread at the top of the page - Cooper 80 Uniform Loading. That title did not get there by itself. I assume structbear wrote it and wanted information on that type of loading.

Your original response was a good one. It just did not have any information on Cooper E-80 railroad loading.

 

[structbear]

I was asking about Cooper E 80, not AASHTO. A 250 psf is definitely not enough for a railroad surcharge. The loading described in the responses above appear to be a strip loading accross the width of the track placed a cetain distance away from the wall. This is more af an exact analysis. What I was looking for was something like a simplified analysis where a uniform surcharge is applied entirely behind the wall, such like AASHTO gives with the 250 psf surcharge (This includes impact). Your imput is appreciated.

PS. Save the piss'n for the contractors, we need to stick together.

 

[structbear]

Actually if I remember right, impact typically is not required for buried structures. So AMTRAK's requirement is conservative

 

[PEinc]

structbear,

I have designed many retaining walls along railroad tracks. Lately, I have not seen any railroad accept any surcharge load other than a Boussinesq Cooper E-80 (minimum) strip load. What we haven't discussed is how many adjacent railroad tracks and surcharges must be included in the design. It is highly unlikely that more than one Cooper E-80 train would pass by at the same time. However, with multiple adjacent tracks, you could have more than one surcharge applied to the wall. You may have to ask your railroad if Cooper E-80 needs to be used for each adjacent track or is there some lower surcharge load (perhaps E-65) that can be applied to a second track and beyond.

To help minimize the wall loads, check AREMA's recommended soil properties and try using Coulomb earth pressure coefficients rather than Rankine. Sometimes AREMA's soil values are better than what some people normally would use. However, you have to satisfy yourself that, whatever soil values you use, they are appropriate for actual conditions.

I hope this information has been useful to you.

 

[DRC1]

Typically Railroads carying freight on a main line will specify a Cooper E-80 loading with or without impact. Light rail, side tracks and yards may allow less. This should be discussed with whoever is reviewing it for the railroad.
A Cooper E-80 train is a steam locomotive with 4 driving axles at 80 kips ea and 1 idle axle at 40 kips. The locomotive is followed by a coal tender with 4 axles at 52 kips. The locomative and tender are followed by an identical set of locomotive and tender, which in turn is followed by an ulimited length of cars at 8 kips per foot of track. A Cooper 60 loading would be 60/80 of the Cooper E-80 loading.
Although I have never seen specific instructions to do this anywhere, I usually take the total load of the locomotives which is 1136 kips and divide it by the length of the locomotives and tenders, which is 119 ft times the length of the ties which is typically 8 ft. this gives me a contact pressure at the top of the stone of approx.1.2 kips per sq. ft. A 50% impact would take that to 1.8 ksf.
It has been my experience that if the wall is at least 7 ft of the ties, the loads are managable. Once you get closer than 7 ft or so, the loads become much tougher to deal with.
Good Luck!

 

[PEinc]

I have not seen a railroad allow me to distribute the engine load over the full length of the engine due to the uneven distribution of the engine weight. Railroads I deal with are very specific in what they want for surcharge loads. For example:

Conrail, Norfolk Southern, and Metro North Commuter each require Cooper E-80 loading and specify an 8.5' wide Boussinesq strip load of 1882 psf vertical pressure. E-80 is defined as 80 kips spread over a 5 feet wide axle spacing and 8.5 feet tie width = 1882 psf.

CSX requires an 8.5' wide Boussinesq strip load with 1800 psf vertical pressure.

Amtrak requires Cooper E-80 plus 50% impact which is 120 kips distributed over an area 5' x 8.5' = 2823 psf.

As DRC1 said, check with the railroad's reviewer for the required load.

 

[DRC1]

I have used the full length of the locomotives and tenders for years without a problem. First of all there is a large mat of ballast under the ties which spreads the wheel loads very effectively. Second it is my understanding only two Cooper E-80 trains were made. They were used to haul 16" guns to the Rocky Mountain Arsenal. The Cooper E-80 load is a somewhat conservative loading to begin with. Third there will be some bridging of te soil and load transfer of the retaining structure to support a localized transitory load. I would talk to your reviewers to see if they would support a reduction of load on that basis, because a 2.8 ksf load has got yield a rather substantial wall section.

 

[VoyageofDiscovery]

PEinc,

I don't understand your 1882psf, I use a Boussinesq line load along the ties at 5ft spacings, not 8.5ft strip loads across 5ft. The AREMA states that the load is uniformly distributed on the surface of the ballast over the length of the tie.

So for example, a 15ft wall and a Cooper E85, for the sake of convenience with a 8.5ft tie length, my lateral pressure is approx. 1700psf.

Regards

VOD

 

[PEinc]

DRC1,
You are correct. But lately, I've yet to see a railroad reviewer back off from what the specs require unless there is a very unusual circumstance. What railroads have you been dealing with? In the past, I have applied railroad surcharge loads in several different ways, all less conservative than using the 1882 psf Boussinesq strip load. I have had each of theses approaches approved and rejected many times. I have never had a problem with any of the designs or surcharges used. As you said, it all depends on the reviewer. But in the last 10 years or so, it seems that the railroads want what the spec says and all of the specs I receive call for the heavy strip load.

 

[DRC1]

PE Inc. -
Mostly Amtrak & Metro-North. I find it very interesting that we have all been doing this for a while, and we all do it differently with rather significant differences. The loads are very well defined, and the wall or sheeting design critea is well defined. But the is no uniform direction on how to take the load and establish a wall pressure. Perhaps that issue should be raised.