By: Javier Encinas, PE | February 21, 2016

ASDIP RETAIN is structural engineering software for design of retaining walls. Competent design of retaining walls can maximize land use and substantially increase property value.

This is a two-part blog post. In part one, Javier reviewed the basics of earth lateral pressure theories and overall stability checking. This second part of the article covers the typical loads in a retaining wall, and the structural design of the different components.

What loads act on a retaining wall?

Ordinarily, retaining walls are subject to surcharge loads at the top of retained mass. A surcharge may be a strip load. A strip load describes a distributed load within a limited width. The pressure distribution is calculated using the Boussinesq approach. The stem may also have concentrated loads at the top.

When the stem extends above backfill the retaining wall may be exposed to wind load. When retaining walls are located in seismic zones the seismic effects are considered by utilizing Mononobe-Okabe approach. ASDIP RETAIN is structural engineering software for engineering professionals to quickly model retaining wall loads. The image below depicts external loads of retaining wall.

How do you design the stem?

The stem is mostly subjected to bending and shear forces. It acts as a cantilever beam, so the maximum moment occurs at the top of the footing, and the shear must be checked at the critical section located a distance d above the top of the footing. The lateral earth pressure has a triangular distribution, whereas the surcharge pressure has a rectangular distribution. The seismic pressure is trapezoidal with the resultant applied at 0.6H, as shown below.

The maximum shear is then the sum of the horizontal loads, and the maximum moment equals the sum of the load times the corresponding lever arm. The picture below shows the different pressures on the stem of a typical retaining wall, sorted by load combination. Note the shear and moment diagrams generated by ASDIP RETAIN software where the shaded area is the structural capacity of the stem. If the stem is overloaded at any point, the problem can be immediately identified.

In ASDIP RETAIN the stem material can be concrete, masonry, or a combination of both. The required reinforcing steel is calculated per the latest design Codes provisions: ACI 318 for concrete and MSJC for masonry. To optimize the design, the stem can be tapered and alternate vertical bars can be cut-off at a certain height.

Is the footing design different?

The heel is subjected to the vertical loads acting on the backfill, including the backfill weight and any surcharge. Under the heel there is a bearing pressure acting upwards. However, it’s common practice to conservatively ignore this pressure and design the heel for the downward loads only. ASDIP RETAIN provides the option to either ignore or consider the upward pressure on the heel. The reinforcing steel at the heel should be placed at the top side of the footing.

The maximum soil bearing pressure is expected to occur under the toe. The toe acts as a cantilever beam subjected to an upward pressure, generally trapezoidal, from the soil reaction. The reinforcing steel at the toe should be placed at the bottom side of the footing. The picture below shows the ASDIP RETAIN calculation of a typical retaining wall footing.

The shear key may be controlled by shear or bending. The load is calculated as the passive pressure acting against the face of the key. The picture below shows the construction information of a typical cantilever retaining wall. Note that alternate vertical rebars have been cut-off in order to optimize the design.

You may want to read and comment the first part of this article. You may also be interested in our post ASDIP RETAIN (Version 3) Release.