AASHTO traffic surcharge on MSE Wall
AASHTO traffic surcharge on MSE Wall
(OP)
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?
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!





RE: AASHTO traffic surcharge on MSE Wall
RE: AASHTO traffic surcharge on MSE Wall
Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!
RE: AASHTO traffic surcharge on MSE Wall
For internal analysis i.e. facing, connection, tension in reinforcement, I would say back of wall face.
EIT
www.HowToEngineer.com
RE: AASHTO traffic surcharge on MSE Wall
For internal analysis i.e. facing, connection, tension in reinforcement, I would say back of wall face panels.
www.PeirceEngineering.com
RE: AASHTO traffic surcharge on MSE Wall
EIT
www.HowToEngineer.com
RE: AASHTO traffic surcharge on MSE Wall
www.PeirceEngineering.com
RE: AASHTO traffic surcharge on MSE Wall
Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!
RE: AASHTO traffic surcharge on MSE Wall
RE: AASHTO traffic surcharge on MSE Wall
Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!
RE: AASHTO traffic surcharge on MSE Wall
www.PeirceEngineering.com
RE: AASHTO traffic surcharge on MSE Wall
@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!
RE: AASHTO traffic surcharge on MSE Wall
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).
www.PeirceEngineering.com
RE: AASHTO traffic surcharge on MSE Wall
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.
RE: AASHTO traffic surcharge on MSE Wall
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!
RE: AASHTO traffic surcharge on MSE Wall
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.
RE: AASHTO traffic surcharge on MSE Wall
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!
RE: AASHTO traffic surcharge on MSE Wall
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.
RE: AASHTO traffic surcharge on MSE Wall
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.
RE: AASHTO traffic surcharge on MSE Wall
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!
RE: AASHTO traffic surcharge on MSE Wall
There is no MSE wall provision for varying a standard uniform live load with depth since the load is considered an infinite condition in all directions and would not dissipate with depth as a strip or line load would. This situation does not happen in reality but the load model is very simplified (like the 2' of soil surcharge) and is just q x Ka for active earth pressure.
The culvert situation is different based on the size of the pipe/structure, depth of cover, etc. Soil arching action may occur that dissipates load with depth so the model for a buried pipe or structure code can have a special provision that accounts for these conditions. The reverse is true if a pipe/structure only has minimal cover and a wheel load can be right on top of the pipe/structure where the uniform surcharge and load reductions would not be appropriate from a structural sense.
RE: AASHTO traffic surcharge on MSE Wall
Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!
RE: AASHTO traffic surcharge on MSE Wall
Now if you are calculating the load acting on the back plane of the reinforced zone (what I'd do), the issue may be moot. Not sure if it'd be consistent with Boussinesq, but at least there is the 0.7H (or whatever the grid length turns out to be) that separates you from the point of no soil.
Not to confuse that is. . .
f-d
¡papá gordo ain’t no madre flaca!
RE: AASHTO traffic surcharge on MSE Wall
You are taking this thread past the point of simple uniform live load surcharges on MSE walls and entering the controversial area of when and where to apply and double Bousinesseq lateral loadings. The text book and design guide formulas would have the 2X built into them if it was as simple as "always double the load" and the simple formulas do not address an anisotropic reinforced mass (all classical earth pressure theories have this limitation). It is actually more a function of the type of retaining wall system, type of backfill material, relative density, and so on that seems to increase the stress on the wall from the formula values.
The AASHTO MSE wall sections actually have methods of distributing strip loads and footing loads that do not use Boussinesq formula and are based on simple Ka (and Ko conditions for inextensible reinforcement) and vertical/lateral stress distributions. I can't say that I agree with some of these methods but they are part of the AASHTO code for MSE walls referenced in this thread.
Not so sure TehMightyPirate asked for this discussion.
RE: AASHTO traffic surcharge on MSE Wall
From what I can tell I think Doc is correct in that the AASHTO code does not use Bousinesseq loadings when you consider strip loads or other vertical loads above the MSE wall. It definitely doesn't when you consider live load surcharge from vehicles.
Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!
RE: AASHTO traffic surcharge on MSE Wall
Q = a given point load
z = a given depth
r = a given horizontal distance, and
R = a given hypotenuse
The change in horizontal loading is given by,
Delta H = (3Q/2*pi)(z*r^2/R^5)
Please realize that the horizontal distance value "r" can be at any radial point surrounding Q (i.e., our
inclination is to consider only those points on a wall closest to the point load, but the equation is for any point).
Here's the line load equation:
Ql = a given line load (force/length)
Z = a given depth
X = a given horizontal distance perpendicular to the line load
R - a given hypotenuse
Delta H = (2Ql/pi)*(z*x^2/R^4)
When applying these formula in a spreadsheet to solve horizontal loads acting on a wall, the answers are doubled.
These are the Boussinesq equations (well two of them at least).
Hope this helps. . . Sorry if I confused.
f-d
¡papá gordo ain’t no madre flaca!
RE: AASHTO traffic surcharge on MSE Wall
Pictures will be a bit more helpful. You can find some of the Bousinesseq information in AASHTO LRFD 2012 also. Starts in Section 3.11.6 - Surcharge loads; ES and LS (pp 3-123 in my edition) and is followed by the MSE wall methods. There is also the trial wedge method which will give you different answers yet and is equally valid in its own way.
You will have to look at the various geotechnical text books such as Bowles to get background information on lateral pressure distribution as different methods are appropriate for different situations. Earth reinforced structures violate the basic assumptions of Coulomb, Rankine, and Bousinesseq so the internal analysis with surcharges is a bit of a mystery still.
RE: AASHTO traffic surcharge on MSE Wall
www.PeirceEngineering.com
RE: AASHTO traffic surcharge on MSE Wall
Sorry I can't be more precise. I can look further into this however.
f-d
¡papá gordo ain’t no madre flaca!
RE: AASHTO traffic surcharge on MSE Wall
Boussinesq doesn't really have a 'line or strip load' equation. He only has an elastic solution for a point load. You can discretize an area and sum the results of the individual point loads. However this will not match the results of the "Terzaghi modified" line load equation.
So I have heard of using the Boussinesq (elastic) equation for a point load and discretizing an area (Bowles does this in his fourth addition) adjusting poisson's ratio and applying a factor of saftey.
I have also 'heard' of using the "Terzahi modified" equations and applying a FS for rigid walls however I agree with PEinc that I do not see it frequently in texts.
One text that has a FS of 2 when using modified line and strip loads but not for point loads is Principles of Geotechnical Engineering" 2nd Edition by Braja M. Das. He also gives the resultant force from integration siting Jarquio (1981). I should note that he incorrectly uses H opposed to pi in the denominator of the strip load equation. Also he does not have this section in his more current editions.
Gotta love the topic of lateral earth pressures, even as a structural.
EIT
www.HowToEngineer.com
RE: AASHTO traffic surcharge on MSE Wall
www.PeirceEngineering.com
RE: AASHTO traffic surcharge on MSE Wall
f-d
sorry to the OP if I got this thread off-topic. Sometimes I just need somebody to talk geek with, sigh. . .
¡papá gordo ain’t no madre flaca!
RE: AASHTO traffic surcharge on MSE Wall
I use the 2.0 when a strip load is within 1.5H of a retaining wall, even if it is a private job. Eventually, it is your level of understanding and comfort level. See attached.
RE: AASHTO traffic surcharge on MSE Wall
www.PeirceEngineering.com
RE: AASHTO traffic surcharge on MSE Wall
I thought that you were referring to the 2 in the equation as doubled. Azizi "Engineering in Geotechnics" says that the strip load value is doubled to account for rigid wall effects and displays the equation as FixedEarth shows in his attachment. So this would lead me to believe that the 2 in the numerator is doubling what the equation would normally result in. I believe Poulos and Davis also give the equation w/out the 2 in the numerator.
Also see here:
http://www.ejge.com/iGEM/Articles/FactorOf2/Factor...
Another tangent (and also one of my favorite threads) the last post by miecz mentions a book published in Poland and I always wanted to know that reference and if anyone else thought his equation was an acceptable solution.
http://www.eng-tips.com/viewthread.cfm?qid=150849
EIT
www.HowToEngineer.com
RE: AASHTO traffic surcharge on MSE Wall
www.PeirceEngineering.com
RE: AASHTO traffic surcharge on MSE Wall
Look at it this way. If strip load surcharge is 0.51H away, Bousinnesq theory misses the shored height so it does not apply (1:2 H:V distribution). If that surcharge is 0.61H away, it misses Rankine's pailure plane so it does not apply. But with the 2 in the numerator, the lateral stresses are felt by the wall until you are greater than 1.5H away.
Same issue with sheetpile design. The Swedish method gives very liberal values yet in Sweden they have worked for 100 years. So I guess we agree to each have a preferred method for surcharges and be o.k. with other research findings.
RE: AASHTO traffic surcharge on MSE Wall
www.PeirceEngineering.com
RE: AASHTO traffic surcharge on MSE Wall
RE: AASHTO traffic surcharge on MSE Wall
Now where is this Swedish research?
EIT
www.HowToEngineer.com
RE: AASHTO traffic surcharge on MSE Wall
The rule of thumb I've always followed is soil design is always +/- 50% anyway so maybe that's where the 2x factor came from? :P
Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!
RE: AASHTO traffic surcharge on MSE Wall
In elastic theory the calculation of horizontal loads at some distance removed from a vertical point, strip or limited-extent areal load assumes that there is elastic media on the opposite side of the point load. If there is not (i.e., it's the other side of the retaining wall, non-free-field elastic theory would say to double the horizontal loads obtained from free-field theory.
When asking the question how will a point load, strip load or limited-extent areal load affect my calculated retaining wall loads (i.e., as calculated by geotechnical engineering earth pressure equations) you are using elastic theory. Sure, you can adapt it for various values of Poisson's ratio, but friction angle is not in the equation.
I'm sorry to belabor my point, but if folks are extrapolating this free-field v. non-free-field distinction to "being conservative," safety factor or other such explanation, it's just not the point I was striving to make.
f-d
¡papá gordo ain’t no madre flaca!
RE: AASHTO traffic surcharge on MSE Wall
That said, wouldn't the soil under the wall footing/MSE section be the "other side" of your elastic theory? It strikes me as an elastic medium which resists the overturning load. Or does your elastic theory require something to be directly opposite of your lateral force?
Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!
RE: AASHTO traffic surcharge on MSE Wall
Or so I think. . .
I don't want to sound like some expert and we are off the OP.
f-d
¡papá gordo ain’t no madre flaca!