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AASHTO LRFD LOAD COMBINATIONSAASHTO LRFD 2010 Bridge Design Specifications utilize a number of partial safety factors on loads and resistances for the design of all structures. Our deepex software is the only software to include all AASHTO LRFD load combinations factors.
Our software program DeepeX is the only software to include all AASHTO LRFD load combinations factors! You would tend to think that AASHTO LRFD is always extremely conservative for deep excavations.
We have benchmarked six multi-level-braced excavations in the United States (Figure 1) against measured wall deflections with a non-linear elastoplastic solution. When the same projects were reanalyzed with the FHWA (1999) limit equilibrium method using benchmarked soil properties, the computed bending moments range from 19% to 68% of the benchmarked maximum wall moment. In some cases LRFD computed support reactions and wall moments calculated with limit equilibrium methods are only slightly greater than the corresponding unfactored load case.
While Eurocode 7 design approaches also exhibit scatter in the obtained results, the provision that allows results to be amplified appears to provide significantly more consistency when compared to AASHTO LRFD methodologies. The currently adopted design methods for AASHTO (2010) LRFD appear to produce inconsistent and possibly unsafe designs for many multi-level braced excavations. Selected design approaches from Eurocode 7 ultimate limit state methods appear to provide a more theoretically consistent framework and results. Earth pressure and retaining wall are one of the few subjects that may appear on both days of the structural engineer (SE) exam per the 2011 to 2014 SE exam specifications.
As an example for the lateral exam; steel or concrete special moment frames present a number of options that can be done in an hour.
The CRSI Handbook is kind of like the AISC Design Manual except it’s for the 2008 ACI building code. For the morning “multiple choice” session of the exam, tabulated design handbooks such as the AISC manual and CRSI handbook are invaluable. This is definitely true for any retaining wall structures, the CRSI handbook provides detailed tables for most cantilevered retaining wall structures you would see.
You can see that if we were asked to size the required width of base for a cantilevered retaining wall, the CRSI handbook would do a good job of getting us close and makes an excellent verification of our design. For the design specific calculations and details of each retaining wall design, I leave up to the reader for their own studying.
Retaining walls generally have little vertical load other than self-weight and weight of any soil on a footing. With the above in mind, let’s limit our discussion to non-bearing walls for the remainder of this article (the principles are similar for both bearing and non-bearing retaining walls).
You may also see some mentioning of piling walls or mechanically stabilized earth walls but they aren’t specifically called out in the exam specification so I believe you can reasonably ignore these during your studies.
HA is the total active earth pressure behind wall (HA may also include a hydrostatic component but note that any hydrostatic load will reduce the active earth pressure ).
Often you will be given the retaining wall geometry and the soil properties of the backfill.
The easiest way to speed up the analysis of a retaining wall is to break the vertical weights into rectangular sections, as the above image has done.
After that, you can then sum the moments about a “point” to obtain the total moment in the wall. I often choose the typical “point” such that the vertical loads will cause a clockwise moment (in the wall orientation shown above) and the soil pressures will cause a counter-clockwise moment. After tabulating these moments you can then calculate the required length of heel for overturning resistance, the soil pressure from the soil below the footing of the wall, and any anchorage forces required, depending on the wall type being designed. For cantilevered and other gravity walls, you have to first calculate the centroid of the required soil bearing force (see “e” in the picture above).
As far as I know you don’t lose points for an inefficient design unless they specifically mention it. Passive soil is a real force though so it can be included if required for a design to work.

After you’ve calculated the forces on the wall you can then design the individual components for the loads on them.
From here, you will design the components typically as reinforced concrete, reinforced masonry, or steel structural members, whichever is applicable.
Further details about design of the wall components is highly dependent of the material of the retaining wall and I leave it up to the reader to study those further. Note that the governing section of the codes for masonry and concrete will be the wall and foundation sections of those codes.
Bearing pressure and other service related design aspects are typically done with un-factored loads under LRFD.
Finally, remember that retaining walls are just spread footings that are trying to tip over. I had the good fortune of being required to write a spreadsheet for retaining wall design in my undergrad Geotech II class.
Each factor relates to a limit state as described in the AASHTO LRFD page within our webpage.
The results for the different load combinations or design approaches are summarized in Table 1. This behavior can be explained by the fact that high water tables are present in all projects but AASHTO (2010) LRFD does not directly factor water pressures (which forms a theoretically consistent practice since vertical effective soil stress depends on the water pressure). This finding appears to undermine the current application of LRFD for braced excavations which is primarily calibrated against support reactions and not wall bending moments. In such cases AASHTO LRFD methodologies tend to produce more consistent results, albeit very conservative when compared to traditional allowable stress design methods. Most officials do not have the authority to superceed or understand the limitations inherent with LRFD. Limit-equilibrium analyses combined with LRFD methods appear to severely underestimate benchmarked wall bending moments. Thus, this analysis exercise strongly suggests that the application of AASHTO (2010) LRFD design procedures for braced excavations needs to be officially revised so that safe and consistent braced excavation designs are produced. That said, as best as I can tell, you will only have to design a full retaining wall during the vertical portion of the exam.
Since retaining walls generally would take about an hour to design, it makes an attractive option if I were designing the SE exam so it’s a good idea to know it well.
Below is an example of one of the tables from the CRSI handbook for cantilevered retaining walls. Since both masonry and concrete retaining walls are likely to appear on the exam, I will only focus on the general design requirements for retaining walls.
If you’re taking the SE exam, you should make sure to practice at least one design of each type of retaining wall.
There are plenty of other good references on retaining wall design that will do a much better job than I can on explaining the detailed design of a retaining wall. Either way, the design will likely be one of the four structures shown above: gravity wall, piling wall, cantilever wall, and an anchored wall. I’d just be aware that they exist and understand the fundamentals of how those retaining structures work prior to the exam. You then usually are tasked with finding the resulting bearing pressures under the retaining wall. See the picture below from the 2008 CRSI Handbook for details, note they have separated the overturning moment Mo and the resisting moment Mr as you are often given one and must design for the other. Anchorage loading for an anchored wall should be much simpler to calculate so I’ll leave that for another time. For example, if a shear key is added below the wall for sliding resistance then passive soil is almost certainly being used. Frictional forces below a wall can often be sufficient to resist sliding and are very quick to calculate. See the previous blog post from Andy regarding design of reinforced concrete members for a quick refresher as much of that will apply to a concrete retaining wall.

I will conclude this by broadly addressing some of the other items the reader should be familiar with regarding retaining walls.
Glance over Table 3-1 Design Lateral Soil Loads on page 7 and be familiar with that table and the footnotes given there. Some other service failures that must be checked are sliding of the wall, lateral deflection of the wall, lateral tilt of the wall due to differential settlement, and crack control.
Much of the same design applies to both, and both will likely be encountered in either a large or small portion of the exam. It was a great refresher for me because it has been a while since I have calced a cantilever wall by hand. For earth retaining structures many combinations do not make significant difference unless considerable external loads are applied to the earth retaining structure. The main issue with LRFD is that it's recommendations have their origins directly in a structural engineering approach of a non-linear elastoplastic problem. The benchmarked excavation models were then reanalyzed with AASHTO (2010) LRFD combinations and Eurocode 7 design approach ultimate limit state methods,  with a non-linear solution and with a limit-equilibrium method as proposed by FHWA (1999).
The above observations are alarming as they reveal that currently proposed AASHTO (2010) LRFD procedures may produce unconservative and inconsistent designs for multi-level-braced excavations. From all design approach cases, EC7* appears to be the most consistent and comprehensive as it allows only one main analysis to be performed. For this reason, analyses in this paper suggest that limit-equilibrium methods should not be used to design the wall bending resistance for multi-level-braced excavations. In the author's experience, LRFD practice can benefit by adapting the best parts of current Eurocode 7 design methods to US realities. One of the references that I used often during the exam was the 2008 CRSI Design Handbook, 10th Edition. Round and square columns, beams, development and lap splice lengths, slabs, footings and pile caps, and cantilevered retaining walls all have tabulated design sections in the CRSI Handbook. If the retaining wall is, for example, the basement foundation wall of a building, then it likely has a beam or other lateral support at the top as well as the cantilevered support at the bottom.
Or perhaps you have to find the length of the heel of the retaining wall (LH in the image above).
In the case of a shear key, excavation is unlikely and the weight of the wall confining the soil will help ensure that the passive soil pressure is likely to be present. Note that shear in the walls will likely not control but don’t forget to check it as some minimal shear reinforcement may be required. Shear in the footings, anchorage of the reinforcement, flexure in the walls, temperature and shrinkage reinforcement, these are all going to be similar in both retaining walls and typical footings and foundations. AASHTO LRFD refers to FHWA apparent earth pressure diagrams that have been primarily calibrated against measured support reactions.
Thus, in current LRFD multi-level braced excavation designs what appears to save the day is extreme conservatism and not consistent design-analysis methods.
The main criticism though for EC7* is the extreme conservatism on the passive soil resistance, which should be revised to obtain more reasonable wall embedment safety factors.
My point is that handbooks, flowcharts, design guides, and other quick reference materials are essential to rapid problem solving during the exam, use them wherever possible.
For example, a counterfort retaining wall spans continuously between the counterforts, assuming no construction joints. For example the increased vertical load could cause bearing pressure issues or require additional reinforcement. For instance, if the wall toe is excavated for repairs or future construction then there will be no passive soil resistance to sliding or overturning.
Thus, being able to rapidly solve these sorts of problems through familiarity with the design steps is crucial to success.

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