Concrete Retaining Wall Design Step by Step - 6 ft Cantilever Worked Example (IBC 2024, ACI 318-25)

Intro

0:00Retaining wall design. A six-foot cantilever concrete wall, from the site to the reinforcement detail, designed step by step in StructSuite.

The project: a 6 ft backyard wall

0:10Here is the project. A new house on a sloping lot. The owners want a flat backyard, so the hill behind the house is cut back, leaving a six-foot drop at the back of the yard. A cantilever concrete retaining wall holds up the higher ground. Above the wall, the ground is a level terrace, with no driveway or building near it. Behind the wall, a gravel drain and a perforated pipe keep water from building up. It is a small residential job with no soils report, so we will use the IBC's presumptive soil values.

0:38And the site is in Seismic Design Category C.

What you will get

0:43Here is what you will get by the end of this video. The earth pressure, the passive resistance, and the bearing pressure under the footing. And the reinforcement detail, drawn to scale, with every check passing. Now let's see how each input produces it.

Design basis: how a retaining wall fails

1:01We design to the IBC 2024, ASCE 7-22, and ACI 318-25. A retaining wall usually fails by moving, not by breaking. It can tip over, slide forward, or press too hard on the soil under its toe. So first, the wall must be stable: a safety factor of one and a half against overturning and against sliding, and a bearing pressure within what the soil allows. Then, the strength of the stem, heel, and toe, and the anchorage of every bar.

Step 1 — Geometry

1:31Step one is the geometry, and everything in this module is per foot of wall. The stem height is seven feet, from the top of the footing to the top of the wall. The retained height is also seven feet: the six-foot grade change, plus the one foot of soil over the toe, because it is measured down to the top of the footing. Every input has a book icon. Open it for the guidance and the code text behind that input, quoted word for word. Here it is IBC section 1807.1.2, the unbalanced backfill height, and section 1807.2.2. Keep that second one in mind: in Seismic Design Category D, E, or F, a wall retaining more than six feet must include seismic earth pressure. Ours retains seven.

2:12The stem is reinforced concrete, ten inches thick, on a twelve-inch footing. Ten inches leaves room for the bars and their cover, and it is easy to form. How wide should the footing be? The book icon on the toe length gives the usual starting point: a footing about half to seven tenths of the total height, with roughly a third of it in the toe. Our total height is eight feet.

2:32So trial one has a one-and-a-half-foot toe and a three-foot heel. The heel matters most, because the soil sitting on it is what holds the wall down. One foot of soil over the toe. The backfill slope is zero, because the ground behind the wall is level. There is no fence on the wall, and no shear key for now. The cross-section draws itself as you type, so you see the proportions right away.

2:54Close the step, and its summary lists every value. Red means a step still needs input; blue means it is complete.

Step 2 — Soil: IBC Table 1610.1 & 1806.2

3:00Step two: soil and earth pressure. First, where the lateral soil load comes from. With no soils report, IBC section 1610.1 makes the Table 1610.1 values the minimum design lateral loads. ASCE 7-22 Table 3.2-1 says the same thing, and you can use either one. With a geotechnical report, you would choose the third option and enter the report's equivalent fluid pressure. The backfill is a well-graded clean sand, SW.

3:28Table 1610.1 gives thirty pounds per square foot, per foot of depth, for active pressure, and sixty for at-rest. Click the row to use it. Active, or at-rest? A cantilever wall is free to move and rotate a little at the top, and section 1610.1 permits active pressure for exactly that case. At-rest pressure is for walls held at the top, like a basement wall. So this wall is designed for thirty.

3:51The soil unit weight is one hundred twenty pounds per cubic foot. It gives the weight of the soil on the heel and over the toe. Groundwater stays off, because this backfill is drained by gravel and a perforated pipe. If water can build up behind your wall, tick this box, and the app adds the water pressure and the uplift. You will find a wall with groundwater in the other videos of this retaining wall series.

4:13Now the soil under the footing. Without a report, IBC Table 1806.2 gives presumptive values. Our native soil is sand, so it is class four: two thousand psf of allowable bearing, one hundred fifty psf per foot of lateral bearing, which is the passive resistance, and a friction coefficient of zero point two five. Footnote a: that coefficient multiplies the dead load. The passive resistance of the soil in front of the toe counts toward sliding.

4:42If that soil could be dug out later, say for a utility trench, you would neglect it here.

Step 3 — Loads & seismic

4:47Step three: loads on the wall. This backyard has no driveway, no building near the top of the wall, and nothing bearing on the stem. So the surcharge, the footing behind the wall, and the axial load on the stem all stay unticked. When a wall does carry one of these, tick its box and enter it. The other videos in this retaining wall series walk through each of them.

5:06Last, the seismic question. The book icon spells it out: in Seismic Design Category D, E, or F, a wall retaining more than six feet needs the seismic earth pressure increment. This site is in category C, so we answer, not included. A wall with seismic earth pressure is designed in the other videos of this series.

Step 4 — Concrete, covers & bars

5:24Step four: concrete and reinforcement. Three thousand psi concrete, grade sixty bars, normal-weight concrete at one hundred fifty pounds per cubic foot. Concrete cover comes from ACI 318-25 Table 20.5.1.3.1. Three inches where concrete is cast against the ground: the bottom of the footing. For number five bars and smaller exposed to earth, the minimum is one and a half inches; we use two, on the stem and on top of the footing.

5:54The bars. Number four at eight inches on the earth face of the stem, the face in tension. Number four at ten, horizontal, for shrinkage and temperature. Number five at twelve across the top of the heel. Number five at twelve along the bottom of the toe. And number four at twelve running along the footing.

6:26How do the heel and toe bars end? Straight, for now. A straight bar needs enough length past the face of the stem to develop its strength. We will see if it gets it. Development length uses the simplified table. And this live check compares every bar layout with the code minimum steel and maximum spacing, as you type.

Step 5 — Trial 1 fails: sliding & toe bars

6:43Step five runs every check, and trial one fails. Overturning is fine, at a safety factor of five point three one. And bearing is fine. But sliding is one point four five, below the one point five that IBC section 1807.2.3 requires. And the toe bars fail development. Open the sliding row for the full calculation. Friction is zero point two five, times the weight of the wall and the soil:

7:09one point zero nine four kips per foot. Passive pressure on two feet of soil and footing adds zero point three. That is one point three nine four of resistance. Against it, the soil pushes with zero point nine six. One point three nine four over zero point nine six is one point four five. Not enough. The development row says why the toe failed. A straight number five bar needs twenty-seven point four inches past the face of the stem, and a one-and-a-half-foot toe has only fifteen.

7:36The app suggests the fix: a standard hook on the toe bars, or a longer toe.

Revision: longer heel, hooked toe bars

7:41Revision one: lengthen the heel by six inches, to three and a half feet. More soil sits on the footing, so there is more weight pressing down, and more friction. It is the cheapest fix: a little more footing, and nothing else changes. Revision two: hook the toe bars. Try a ninety-degree hook first. Sliding now passes, but development still fails.

8:06The derivation shows why: a ninety-degree hook on a number five bar stands ten inches tall, and a twelve-inch footing has only seven inches between its top and bottom covers. It does not fit. A one-hundred-eighty-degree hook is only about five inches tall, so it fits. Switch to that.

Step 5 — Final design passes

8:27Run the checks again, and everything passes. Overturning, six point three nine. Sliding, one point five eight. Bearing pressure, nine hundred five psf, against two thousand allowed. The stem, heel, and toe all have strength to spare, and every bar develops. Every row opens to its derivation. Overturning first. The soil pushes with zero point nine six kips per foot, at one third of the eight-foot height. That is an overturning moment of two point five six.

8:56The stem, the footing, and the soil on the heel and toe resist with sixteen point three seven. Sixteen point three seven over two point five six is six point three nine. Stem flexure, at the base of the stem. The factored moment is one point six times the earth-pressure moment: two point seven four kip-feet per foot. The number four bars at eight inches give a design strength of ten point zero seven.

9:20A ratio of zero point two seven. And the section is tension-controlled, as the code requires.

Step 6 — Pressure diagrams & rebar detail

9:30Step six draws it. These are the service pressures for dead load plus earth pressure. The active pressure on the back of the wall, two hundred forty psf at the bottom. The passive pressure in front of the toe. And the bearing pressure under the footing: nine hundred five psf at the toe, well under the two thousand allowed. Pick any load case on the left to draw it on its own.

9:52And switch to resultants and lever arms to see each force, and where it acts. And the reinforcement detail, drawn to scale from your inputs: the stem bars, the hooked dowels and their lap, the heel mat on top, and the toe mat at the bottom with its hook. Notice the toe bars stop under the heel. They only need a full development length past the stem, and the bottom of the heel is in compression, so that is all the code requires. Many engineers still run both mats the full width, to keep the field simple.

10:30Close the step, and all six summaries are blue. The wall is designed, and it prints as a complete calculation package.

Recap

10:41To recap. A six-foot concrete wall on presumptive soil values. Trial one failed sliding, and its toe bars could not develop. Six more inches of heel, and a one-hundred-eighty-degree hook, fixed both.

10:55Design your own wall, free, at StructSuite dot com.