ASDIP RETAIN is a structural engineering software for the design of retaining structures. It includes the design of retaining walls based on the AASHTO LRFD-20 provisions. This article is a detailed step-by-step cantilever retaining wall design example per AASHTO, using ASDIP RETAIN software.
AASHTO Retaining Wall Example
As an example, a cantilever retaining wall supports 15'-0" of level roadway embankment measured from top of wall to top of footing. The wall will be built adjacent to the roadway shoulder where traffic load is 250 psf. The wall stem is 1'-6" thick to accommodate mounting a bridge rail to the top of wall. Design per AASHTO LRFD-20.
Enter the information given in the statement of the example, which are the stem height and the material properties. Go to the Geometry tab and the top enter the stem height as 15 ft. Set the stem thickness as 18" top and bottom. This value can be changed later in case that a taper section is required. Go to the Backfill & Cover tab and enter the backfill height as 15 ft at zero deg slope. We will modify the rest of the input values as required during the optimization of the design.
The At-a-Glance tab shows a summary of the design for a quick overview of your work as you go. The green check marks indicate that the checks passed.
Go to the Loads tab and enter the backfill material properties given in the statement of the example. In this case we will use the Coulomb theory to calculate the lateral soil pressures. Finally, go to the Surcharge tab and enter 250 psf live uniform surcharge.
Enter the material properties in the Materials tab, such as the concrete and reinforcing steel strengths. In this case we will use concrete with f'c = 4.5 ksi. In the Bearing Soil tab enter the ultimate soil bearing capacity as 7.5 ksf.
The Condensed tab shows the most relevant results organized by topic, for a quick overview of the design. The Detailed tab provides a step-by-step set of calculations, with exposed formulas and references to the AASHTO code.
To comply with the stability requirements, let's set the toe length to 2 ft and the heel length to 6 ft. With this configuration the Eccentricity ratio is 0.66 and the Sliding ratio is 0.95. The Bearing ratio is 0.92.
It should be noted that the AASHTO load combinations specify minimum and maximum load factors for some load cases. ASDIP RETAIN fully complies with the required application of these values for the different limit states. For example, the minimum gravity loads are considered for Eccentricity and Sliding, but the maximum gravity loads are considered for Bearing. Likewise, the live surcharge was fully applied for Bearing calculations, but only the horizontal effect was considered for the Eccentricity and Sliding calculations.
Go to the Graph > Stem tab, where ASDIP RETAIN generates the stem shear and moment diagrams, showing the effect of the development length of the rebars. This is useful to identify any problem with the stem structural capacity.
In the Graph > Footing tab ASDIP RETAIN shows graphically the load and bearing diagrams for the toe and heel design. It also provides the design ratios per load combination.
Note that the Graph tab includes separate tabs for each stage of the design process. This way you can visualize graphically the applied pressures, the resulting forces, and the design ratios per load combination.
Specify the rebars size and spacing in the Reinforcement tab, for the stem, the footing, and the shear key. ASDIP RETAIN provides visual tools such as check boxes and drop-down lists, to specify the required reinforcement easily. In this wall example we used #5 @ 6" for the stem and footing. This provided the requires strength and it also complies with the bar spacing limits.
In the Graph > Construction tab ASDIP RETAIN generates images of the construction wall in section and elevation views, showing the rebars specified during the design, as shown below.
ASDIP RETAIN includes the design of retaining walls per the AASHTO LRFD-20 provisions, with multiple options to easily optimize the design. This AASHTO retaining wall example shows that the task can be completed and optimized with ASDIP RETAIN in just a few minutes.
For our collection of blog posts about retaining wall design, please visit Retaining Wall Design. For engineering background please see the blog post Cantilever Retaining Walls: Overview of the Design Process.
Javier Encinas, PE
ASDIP Structural Software