Basement Wall Design Step by Step - At-Rest Pressure, House Load and Adjacent Footing (ACI 318-25)

Intro

0:00Retaining wall design. An eight-foot basement-type wall that carries the house above it, with a neighbor's footing behind it, designed step by step in StructSuite.

The project: a basement-type wall under a house

0:10Here is the project. A house on a hillside gets a lower-level garage, cut seven feet into the slope. The concrete wall at the back of the garage carries the house wall above it, and the house's concrete floor slab ties into its top, so the wall cannot lean. Behind it, the neighbor's garage footing bears in the backfill. The soils report describes a clayey sand with cohesion, warns that water can perch behind the wall, and gives the seismic pressure for a restrained wall.

0:36The site is in Seismic Design Category D.

What you will get

0:40Here is what you will get by the end of this video. The at-rest pressure, the neighbor's footing, the house load, and the bearing. And the reinforcement detail. Now let's build it, input by input.

Design basis: at-rest, stem loads, restrained seismic

0:53The codes: the IBC 2024, ASCE 7-22, and ACI 318-25. A wall held at the top takes at-rest pressure, higher than active. Loads on the stem are entered by type, because each takes its own load factors, and only the dead load may resist. A footing behind the wall pushes on it. And in category D, the seismic pressure of a restrained wall comes from the geotechnical report.

Step 1 — Geometry

1:19Step one, per foot of wall. An eight-foot stem, retaining eight feet: the seven-foot cut, plus one foot of soil over the toe, under the garage slab. A twelve-inch concrete stem. Trial one: an eighteen-inch footing, a two-and-a-half-foot toe, a five-foot heel, and a level backfill.

Step 2 — At-rest pressure, groundwater, cohesion

1:42Step two, from the soils report. The house's concrete floor slab ties into the top of this wall, so it cannot lean: it is a restrained wall, and it takes at-rest pressure. For this clayey sand, the report gives sixty pounds per cubic foot, at rest. The book icon quotes section 1610.1: walls whose movement is restricted at the top are designed for at-rest pressure. The exception, active pressure for a foundation wall up to eight feet, needs a flexible diaphragm at the top, like wood framing.

2:10A concrete slab is rigid, so at-rest it is. The app still designs the stem as a cantilever, so the slab does not need to brace it. Soil unit weight, one hundred twenty. The report warns that water can perch two feet above the footing bottom after heavy rain. Tick groundwater, enter two feet, and a saturated unit weight of one hundred thirty. The app then adds the water pressure behind the wall and the uplift under the footing.

2:34The native soil is a clayey sand. The report gives a friction coefficient of zero point three, plus a cohesion of one hundred fifty psf. Cohesion multiplies the footing's contact area, and the code caps it at half the dead load. Passive, two hundred fifty psf per foot, but the report says use only fifty percent of it. Enter fifty here. And two thousand five hundred psf of bearing.

Step 3 — Neighbor footing, house load, seismic

2:58Step three. The neighbor's garage stands behind the wall, and its footing bears in our backfill. Tick footing behind the wall. Fifteen hundred pounds per foot, on a footing one and a half feet wide, its centerline six feet behind the stem, bearing one and a half feet down. The app spreads that strip load through the soil and adds its push on the wall. The house wall bears on top of the stem. Tick axial load, and enter it by type, because each one takes different load factors: dead, eight hundred pounds per foot; floor live, three hundred; roof live, two hundred; snow, two hundred.

3:28The dead part also resists overturning and sliding; the others never do. The wall sits two inches toward the toe, so the eccentricity is minus two, which adds to the earth moment. Seismic Design Category D, eight feet retained: the seismic earth pressure is required. For a restrained wall, the app has no built-in k h, so Mononobe-Okabe and Seed-Whitman do not apply. The seismic pressure must come from the geotechnical report.

3:54Reports give it in one of two shapes: a resultant with its height, or a uniform pressure. For this restrained wall, the report gives a uniform two hundred psf over the wall height. And a one-third bearing increase for seismic, one point three three.

Step 4 — Concrete & bars

4:08Step four: four thousand psi normal-weight concrete, grade sixty. Covers of two, three, and two inches. Stem bars, number six at eight; horizontal, number four at nine. Heel and toe, number six at twelve, both ending straight, and number four at twelve along the footing. Development, the simplified table, for now.

Seismic + groundwater refused → subdrain

5:08Step five is blocked. Seismic earth pressure with groundwater behind the wall is outside this module: a submerged backfill shakes differently, with a water component, and that pressure must come from the geotechnical report. The message gives the way out: drain the backfill. And that is what the report recommends anyway: a perforated subdrain at the footing, with free-draining gravel behind the wall. With it, the code lets us design for a drained backfill. Untick groundwater in Step two.

Step 5 — Trial 1 fails: sliding & toe bars

5:35Now Step five runs. Trial one fails sliding, static and seismic, and the toe bars do not develop. Sliding, one point two zero. Seismic sliding, zero point eight eight. And toe bar development. Open sliding, to see how the report's values work. Friction, two point seven. Cohesion, one hundred fifty psf times the eight-and-a-half-foot footing: one point two seven five kips per foot, within the half-dead-load cap. Passive gives zero point seven eight one, but only half is usable: zero point three nine one.

6:05Against it, the at-rest soil pressure, two point seven one, plus the garage footing's push, zero point nine four. One point two zero. The seismic case adds seventy percent of the report's pressure: two hundred psf, over nine and a half feet, is nineteen hundred pounds per foot. Zero point eight eight. And development: by the simplified table, a straight number six bar needs twenty-eight and a half inches past the stem; the toe gives twenty-seven.

Revision: deeper footing, longer heel, general ℓd

6:31Revision one: a twenty-four-inch footing and a six-foot heel, for more weight, more cohesion area, and a deeper passive block. Step three turned red. The heel now reaches under the garage footing, so its load bears on our heel, and the app asks: permanent, or variable? A building footing is permanent, so its weight counts with the dead load. A thicker footing needs more minimum steel. In Step four: heel and toe bars, number six at ten, and number five at twelve along the footing.

7:18And the toe bars. Rather than hook them, switch development to the general equation. The simplified table assumes a fixed confinement; the general equation uses this wall's real cover and spacing.

Step 5 — Final design passes

7:30Run the checks. Everything passes. Static overturning, four point six five, and sliding, one point five eight. Seismic overturning, three point zero eight, and sliding, one point one two. Bearing, one thousand six hundred fifty-four psf, with three quarters of the floor and roof live loads. Seismic bearing, two thousand one hundred twenty-four, against the increased three thousand three hundred twenty-five. Look at the stem: its governing combination has the roof live load leading, at one point six, because the house load bears on the stem with its eccentricity.

8:03And development. With the real cover and spacing, the confinement term reaches its cap of two and a half, and a straight number six bar needs only seventeen point one inches. The toe has twenty-seven.

Step 6 — Diagrams & rebar detail

8:14Step six draws the wall: the at-rest pressure, and next to it the garage footing's push, which peaks at eight hundred fifty-one psf just below its bearing level, where the strip load spreads into the soil. Pick the seismic stability case. Seventy percent of the report's uniform pressure, one point four kips per foot, sits on top. And the bearing rises to two thousand one hundred twenty-four psf. Switch to resultants and lever arms, to see each force and where it acts.

8:47And the reinforcement detail, to scale: the stem bars and hooked dowels, the heel and toe mats in the twenty-four-inch footing, and the longitudinal bars. Close the step. All six summaries are blue, and the wall prints as a complete calculation package.

Recap

9:03To recap. An eight-foot basement-type wall with the house on top and a garage footing behind. Seismic pressure with groundwater was refused, so the report's subdrain keeps the backfill drained. Trial one slid, and its toe bars fell short. A deeper footing, a longer heel, the bars to match, and the general development equation, fixed it all. Design your own wall, free, at StructSuite dot com.