CMU Retaining Wall Design Step by Step - Property Line Block Wall with Fence and Wind (TMS 402-22)

Intro

0:00Retaining wall design. A concrete block wall on a property line, with no toe, a fence on top, and wind, designed step by step in StructSuite.

The project: a property-line block wall

0:09Here is the project. A house on a hillside, with the neighbor's yard four feet lower, along the side property line. The owner wants a block wall right on the line. It retains the four feet of soil, rises two more feet above the high side as a garden wall, and carries a four-foot wood fence, for privacy. Because the wall face sits on the property line, the footing cannot extend under the neighbor's yard. So there is no toe: the footing is all heel.

0:33The backfill is a silty sand, drained, with no soils report, so we use the IBC's presumptive values. The wind analysis for this site gives twenty-one pounds per square foot on the wall and the fence. And the site is in Seismic Design Category D.

What you will get

0:50Here is what you will get by the end of this video. The earth pressure and the bearing pressure under an all-heel footing. And the reinforcement detail, with bars on both faces of the block stem. Now let's build it, input by input.

Design basis: codes & wind reversal

1:07The codes: the IBC 2024 and ASCE 7-22 for the loads and the stability. TMS 402-22 for the masonry stem, by allowable stress design. And ACI 318-25 for the concrete footing. The wall must not tip, slide, or overload the soil, with a safety factor of one and a half. Then the stem and the footing need the strength. And wind adds one more check: wind blowing back toward the soil can bend the stem the other way.

Step 1 — Geometry, CMU stem, no toe

1:36Step one, the geometry, per foot of wall. The stem is seven feet tall. The retained height is five feet: the four-foot grade change, plus one foot of soil over the footing on the neighbor's side. So the top two feet of the stem stand clear of the soil, as a garden wall. This stem is concrete block. Pick concrete masonry, solid grouted, designed to TMS 402-22. Then the block: eight-inch units.

1:57TMS designs on the specified thickness, seven and five eighths inches, three eighths less than nominal. The footing is twelve inches thick. And here is what makes this wall different: it sits right on the property line, and the footing cannot reach onto the neighbor's land. So the toe length is zero. The book icon on the toe explains what that means. With no toe, the resultant moves toward the front edge, so the edge bearing pressure grows, and sliding leans on base friction and any shear key.

2:24Trial one has a four-foot heel, about half the total height. One foot of soil over the footing in front, a level backfill, and a four-foot wood fence on top of the wall. Watch the cross-section draw the fence as you type.

Step 2 — Soil: IBC presumptive values

2:40Step two, the soil. No soils report, so the lateral soil load comes from IBC Table 1610.1. The backfill is a silty sand, SM: forty-five pounds per square foot per foot of depth, active. The other videos in this series explain active and at-rest pressure, and when each applies, in more detail. Soil unit weight, one hundred twenty pounds per cubic foot, with a drained backfill. Under the footing, IBC Table 1806.2, class four: two thousand psf of bearing, one hundred fifty psf per foot of passive, and a friction coefficient of zero point two five.

Step 3 — Wind on the stem & fence

3:14Step three, the loads. Because the stem stands above the soil and carries a fence, the app asks for wind. The book icon quotes ASCE 7-22 section 29.3.1: a freestanding wall takes the pressure from equation 29.3-1. For this site, the wind analysis gives a net pressure of twenty-one pounds per square foot, at strength level. The app uses your number; it does not calculate it. The solid wood fence takes the same twenty-one psf, over its full four feet.

3:42It weighs about twelve pounds per foot. That weight is permanent, so it helps resist overturning and sliding. The app checks the wind blowing both ways: toward the neighbor, and back toward the backfill. This site is in Seismic Design Category D. But section 1807.2.2 asks for the seismic earth pressure only when the wall retains more than six feet. Ours retains five, so it is not included. A wall that does need it is designed in the other videos of this series.

Step 4 — Masonry (TMS 402-22) & footing

4:09Step four now has a masonry section. The specified masonry strength, f prime m, is two thousand psi. The book icon shows where that comes from: TMS 602 Table 2, the unit strength method. Two-thousand-psi block, laid in Type M or S mortar, gives a masonry strength of two thousand psi. Prism tests are the other way. Grade sixty bars. In allowable stress design, that sets the allowable steel stress at thirty-two thousand psi.

4:36The block's unit density sets the wall weight. One hundred twenty-five pounds per cubic foot, fully grouted. Where do the bars sit in the cell? At the earth face, the face in tension, with the masonry cover of section 6.1.5.1: one and a half inches for a number five bar or smaller. That gives the deepest lever arm. And the seismic design category, D. Even with no seismic earth pressure, TMS still asks a masonry wall in category D for a minimum amount of reinforcement. Step five checks it.

5:06The footing is still reinforced concrete, to ACI 318: three thousand psi, grade sixty, one hundred fifty pounds per cubic foot. Three inches of cover at the bottom, two at the top. Stem bars: number five, one in every second cell, at sixteen inches. Because we entered wind, the app now asks for toe-face bars, for wind blowing back toward the soil. The hint says to leave it blank first, and Step five will tell us if they are needed. So we leave it blank.

5:33In the footing: number five at twelve across the top of the heel, number five at twelve along the bottom, and number four at twelve running along the wall. The heel bars end straight, and development uses the simplified table.

Step 5 — Trial 1 fails three ways

6:00Step five. Trial one fails, in three places. Overturning passes at three point five three. But sliding is one point three eight. The stem fails wind reversal. And the stem dowel hook does not fit in the footing. Open the sliding row. Friction under the footing gives zero point nine one nine kips per foot, and passive in front adds zero point three: one point two one nine of resistance.

6:23The soil pushes with zero point eight one, and the wind, at sixty percent for stability, adds zero point zero seven six. So the factor of safety is one point three eight. With no toe, there is no soil on the toe to add weight, so every foot of heel counts. Now wind reversal. This is the wind blowing toward the backfill. At sixty percent, the wind on the stem and the fence bends the stem with zero point six zero five kip-feet. The soil behind, also at sixty percent, pushes back with only zero point five six three. The wind wins, by zero point zero four two, so the stem bends the other way, and its toe face is in tension.

6:57There are no bars on that face, so the check fails. The app lists the fixes: add toe-face bars, center the bars, or reduce the exposure. And the development row. The stem dowels hook into the footing. A number five hook needs nine point six inches. A twelve-inch footing, less three inches of bottom cover and the bottom bar, leaves only eight point four. The footing must be thicker. Notice, too, the lap of the dowel with the stem bar in the grouted cell: forty-five point four inches, from TMS equation 6-2.

7:27Masonry laps are long, and the detail will draw it.

Revision: thicker footing, longer heel, toe-face bars

7:30The revision. Back in Step one, make the footing fourteen inches thick: that gives the hook room, and adds weight. And lengthen the heel to five feet, for more soil on the footing, and more friction.

7:44In Step four, add the toe-face bars: number four, every second cell, at sixteen inches.

Step 5 — Final design passes

7:58Run the checks again. Everything passes. Overturning, five point zero four. Sliding, one point six zero. Bearing, one thousand three hundred sixty-seven psf, against two thousand. Wind reversal and the dowel hook now pass. Open the stem flexure row, to see how a masonry stem is checked. TMS uses allowable stress design, at service loads, on the cracked section. The bar sits five point eight one inches from the toe face.

8:25The worst case is sixty percent of the dead load, with sixty percent of the wind, plus the soil. The masonry stress is three hundred sixty-one psi, against nine hundred allowed. The steel stress is fourteen thousand two hundred fifty-five, against thirty-two thousand. In the wind reversal row, the toe-face bars carry the small reversed moment easily.

Step 6 — Pressure diagrams & rebar detail

8:51Step six draws the wall. With wind on the wall, the stability case is dead load, plus earth pressure, plus sixty percent of the wind. The wind on the fence and on the stem above the soil: twenty-one psf, times sixty percent, is twelve point six. The active earth pressure, two hundred seventy-seven and a half psf at the footing bottom. And the bearing pressure. With no toe, the peak, one thousand three hundred sixty-seven psf, sits right under the stem, at the property line.

9:23Pick a load case on the left to draw it on its own, or switch to resultants and lever arms, to see each force and where it acts, the wind included. And the reinforcement detail, to scale: the block stem with its bars at the earth face, the hooked dowels in the fourteen-inch footing, and the heel mat. Notice the dowel lap in the grouted cells: three feet ten inches.

9:44Close the step. All six summaries are blue, and the wall prints as a complete calculation package.

Recap

9:52To recap. A block wall on the property line, with no toe, a fence, and wind. Trial one failed sliding, wind reversal, and the dowel hook. A fourteen-inch footing, a five-foot heel, and toe-face bars fixed all three. Design your own wall, free, at StructSuite dot com.