AASHTO traffic surcharge on MSE Wall 2

[TehMightyEngineer]

Per AASHTO LRFD Bridge Design Spec. 5th ed. (2010) article 3.11.6.4 the live load surcharge can be ignored if it is applied on the backfill greater than 1/2 the wall height behind the back face of the wall. Makes sense.

Now, for a traditional wall this is easy and obvious. However, for a MSE wall what is the back face actually defined as? I think it should be the back of the reinforced earth area but my boss and P.E. thinks it should be the back face of the block wall in front of the reinforced earth. This makes sense to me but I'm not sure if it's the intent of the code. What is the opinion here?

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[dcarr82775]

Back face of the reinforced earth area in my opinion.

 

[TehMightyEngineer]

That would also seem to match most of the examples I've seen, but I can't tell if they're including it for examples sake or not.

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[RFreund]

For external analysis i.e. sliding and overturning of the soil mass, I would 'confidently' say from the back face.
For internal analysis i.e. facing, connection, tension in reinforcement, I would say back of wall face.

EIT

http://www.HowToEngineer.com

 

[PEinc]

For external analysis i.e. sliding and overturning of the soil mass, I would 'confidently' say from the back face of the reinforced soil mass.
For internal analysis i.e. facing, connection, tension in reinforcement, I would say back of wall face panels.

http://www.PeirceEngineering.com

 

[RFreund]

yikes left out the important part. Thanks PEInc!

EIT

http://www.HowToEngineer.com

 

[PEinc]

I thought that's what you meant but it needed a little clarification.

http://www.PeirceEngineering.com

 

[TehMightyEngineer]

Okay, that would make the most sense of all though all the design examples I find that include a surcharge load include it through-out which would lead me to believe that you either include the surcharge load in the calculations or do not include it at all.

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[BigH]

After this discussion - it would be interesting to see what the effect of the "AASTHO" surcharge on the back of the MSE wall would have on the various Factors of Safety - unless it is for a very small wall, I would doubt it has any influence. The main concern, in my view, would be on the front face panels - i.e., the strips in the area of the interface.

 

[TehMightyEngineer]

I'm in the process of making an engineering spreadsheet to design MSE walls for our company which prompted the question. I have a rough version done so I can answer that question in the morning.

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[PEinc]

For sliding and overturning anaylses, you don't want the surcharge above the reinforced soil mass because this would inflate the safety factors because the surcharge would increase the normal force for sliding and the resisting moment for overturning.

http://www.PeirceEngineering.com

 

[TehMightyEngineer]

@PEinc: Makes sense for the vertical live load. I'm referring to the lateral live load surcharge though so it would be un-conservative to ignore it if it was required for design.

@BigH: From my rough design spreadsheet for a 20 ft wall with a brokeback sloped backfill I get the following factors of safety with and without the surcharge:

FS Sliding w/surcharge: 1.119
FS Overturning w/surcharge: 1.184
Required soil bearing capacity w/surcharge: 10,089 psf

FS Sliding w/o surcharge: 1.278 (14.2% increase)
FS Overturning w/o surcharge: 1.522 (28.5% increase)
Required soil bearing capacity w/o surcharge: 9,016 psf (11.9% decrease)

So, it's about a 25% difference for a typical wall.

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[PEinc]

A vertical live load surcharge would be a downward load (normal force) for a sliding analysis and, after applying the active pressure coefficient to the vertical surcharge, would cause a lateral pressure for use in both the sliding and overturning analyses. A vertical surcharge above the reinforced soil helps prevent sliding and overturning but hurts internal stability, affecting the facing, the reinforcement, and total bearing pressure. A vertical live load surcharge beyond the back end of the reinforcement grids increases the sliding force, the overtuning force, and the maximum bearing pressure but does not help sliding resistance.

Those safety factor numbers do not mean much without us knowing where the surcharge is being applied. Also, I assume that these safety factors were calculated by an MSE wall computer program which is a slope stability program. If you consider the reinforced soil as a block mass and then hand calculate sliding and overturning with the live load surcharge in the two positions that I mentioned above, you should see a more meaningful example of how the surcharge location affects the safety factors for sliding and overturning (not internal or global slope stability).

http://www.PeirceEngineering.com

 

[Doctormo]

You confuse me, you reference AASTO LRFD then provide factors of safety in your example. I trust you mean CDR ratios.

Maximum live load over the structure is used in the bearing pressure calculation to go with the factored bearing capacity but not in the maximum eccentricity calculation (<B/4) which is the same as the sliding and overturning calculation. In all cases, live load is a driving force on the structure.

MSE wall design has its own design section so the some of the concepts regarding application of live load may not apply. The conservative assumption is to design a MSE wall for full surcharge regardless of exact position of the load. The formulas provided in AASHTO do not account for offset loads so anyone checking your work will apply the AASHTO formulas that includes live load.

It is not correct but hard to argue as it is "more conservative" and that is desirable in highway work.

 

[TehMightyEngineer]

Sorry, yes, I meant CDR ratios.

Hmmm, I will double check the MSE wall section of AASHTO and see if it overrides the chapter 3 live load surcharge in any way. I would agree that it makes more sense to leave it in.

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[Doctormo]

Chapter 11 is for retaining walls and Chapter 10 is for foundations so you have to work them together with Chapter 3.

The chapter differences have been a minor problem for a long time but are magnified a bit under LRFD criteria.

For example, the current MSE wall code as written does not apply the LL factor of 1.75 in the internal load calculation nor the horizontal earth pressure factor of 1.5 to the internal lateral soil pressure but applies a factor of 1.35 (EV?) to both internal loads. This factor is proportioned to work with the resistance factors to provide the same internal answers as the previous allowable stress design methods until "the MSE wall model is calibrated for LRFD". The external stability analysis uses the normal load factors though. This drives most structural engineers crazy but it is what it is.

There is nothing in Section 11 that directly addresses an offset live load unless it is behind the reinforced zone and inside of the 2*Hgt zone measured from the wall face. If is outside the 2*Hgt zone, then it can be ignored and treated as an infinite slope condition.

 

[TehMightyEngineer]

Thanks Doctormo, that was a great summary of the reasoning behind the LRFD MSE wall design per AASHTO. If I understand you correctly you're saying that there is a provision in Section 11 that discusses live load in the 2H zone behind the wall? I have yet to see that in AASHTO but I will look for it in the morning.

So, in conclusion, this is what I've gathered from all this:

AASHTO is trying to absorb a lot of the traditional MSE wall design methodologies but it still isn't consistent with other literature nor even within itself (such as the load factors). The conservative and general consensus seems to be that the for applying the lateral live load surcharge we must consider the back face of the wall to be the back face of the mechanically stabilized soil mass. Or, even more conservative, one can simply include the live load (if present) regardless of location. Obviously live load shall be ignored if it provides some stabilizing effect.

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[Doctormo]

There is MSE wall figure for a broken-back slope somewhere, could be in Chap 3 or 11, but it is intended to define the equivalent slope for a broken back condition within 2H and does not address the live load directly. One has to infer that if the live load is applied within this zone, it should be accounted for.

Otherwise, there is only a level and infinite slope case shown with formulas for each. This is pretty typical of all retaining wall types since the equation solutions only work for the simple cases. Offset loads and broken back slopes are not really covered in the code except for Bousinesseq formulas for offset strip loads, etc.

 

[Doctormo]

Some additional info from work:

In 2012 AASHTO LRFD, the three earth pressure load cases (external stability) are described in Chapter 3, section 3.11.5.8.1. The third case is the broken back condition behind the reinforced zone and the cases are just for external earth pressure calculation not addressing surcharges.

The internal stability design typically includes live load in some manner if the load is over the reinforced volume. The external design usually includes the live load in some manner if it falls between the end of the reinforced zone and the 2H zone defined in the Chapter 3 above. The exact manner of treating these loads is not defined except for the level surcharge condition where it is treated uniformly over the mass in the figures.

Chapter 11 discusses external stability and references Chap 3 as needed but goes into more depth regarding the internal analysis for level and infinite slope conditions. The problem is still the fact that equation solutions are only good for the level and infinite slope conditions so any variation from those conditions are not clearly defined in the MSE code except the broken-back external condition noted above.

I believe the concept of ignoring live load if greater than 1/2 the height behind a wall was in the code long before the MSE wall sections were added (1992?) and based on a fixed Rankine failure plane for a level granular material behind a cantilever wall. This goes along with the concept of traffic surcharge being treated as 2' of soil surcharge vs. a more conventional 250 psf loading that structural engineers and building codes would apply. (does the traffic surcharge actually vary with the weight of the soil? only if it is dump trucks carrying the same soil around?)

However, this simplified assumption ignores the other parts of earth pressure theory such as Rankine and Coulomb failure planes that are flatter and Bousinesseq & trial wedge theory which would indicate that surcharges affect a wall design at greater offsets (closer to a 1:1 influence line from bottom of wall). It gets much more complicated than the MSE code allows for so the simple models are much easier to understand and the code is left that way.

 

[TehMightyEngineer]

"There is MSE wall figure for a broken-back slope somewhere, could be in Chap 3 or 11, but it is intended to define the equivalent slope for a broken back condition within 2H and does not address the live load directly. One has to infer that if the live load is applied within this zone, it should be accounted for."

Ah! That makes a lot of sense, obviously if the soil can interact with the wall in the 2H zone then the live load should as well. I believe this is what the various design examples I have are doing as well (though they annoyingly don't state it explicitly that I have found).

"The exact manner of treating these loads is not defined except for the level surcharge condition where it is treated uniformly over the mass in the figures."

I was wondering if the live load surcharge gets reduced or terminated at a depth (such as the 8' limit for live load and live load surcharge on buried structures such as culverts). Do you know of any prevision for this?

"I believe the concept of ignoring live load if greater than 1/2 the height behind a wall was in the code long before the MSE wall sections were added (1992?) and based on a fixed Rankine failure plane for a level granular material behind a cantilever wall."

I assumed as much, that provision doesn't seem to mesh well with the MSE design standards of chapter 11 or the various examples.

"This goes along with the concept of traffic surcharge being treated as 2' of soil surcharge vs. a more conventional 250 psf loading that structural engineers and building codes would apply. (does the traffic surcharge actually vary with the weight of the soil? only if it is dump trucks carrying the same soil around?)"

I'm glad I'm not the only one who dislikes that provision for extra soil height for live load surcharge. I always assumed that they kept that in there to keep older designs valid or some similar reason. It never made sense that in a code that is as strict as AASHTO and tries to do everything to such a high safety factor that it would have such an arbitrary load.

Going forward I agree that your interpretation of live load within the 2H zone makes the most sense and seems to be practical and conservative.

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!