The Compass DeRose Guide to Strengthening Basement Walls

This page was written by Steven J. DeRose on 2003-03-30, and was last updated on 2003-04-23.

Despite some publications stating it is flatly impractical to strengthen existing basement walls, I disagree. It may be impractical to make them as good as if you were designing them from scratch; but that's quite a different question. This page explains some steps you can take.

There are two main issues involving basement wall strength:

1. What happens if there is a sudden pressure or blast wave hits? This could come from a tornado or hurricane, a car slamming into the wall, a nearby conventional explosion, or a fairly distant nuclear explosion.
2. Can the foundation hold up the weight of any additions you want, such as ceiling shielding?

I speak here only of basement walls, because most above-ground residential walls have close to zero value as protection against wind, pressure, projectiles, fire, or radiation. Concrete, stone, or brick walls do have some value, but not as much as a typical poured concrete basement wall, most of which is protected by being underground, too.

The blast wave

A blast wave is a big deal -- the 30 pounds per square inch overpressure that can easily occur from a nuclear blast can shear off objects like telephone poles, pipes, and the like with no trouble. It can also blow right through a solid door (it won't even notice windows) and topple a house frame.

Even a major storm like a tornado, can break things off, pick up debris, and hurl it at high enough speeds to threaten solid walls

First things first

Basement walls will not be the first point of failure. Take care of any other critical points first. For example, do you know what anchors (if any) hold your house down onto its foundation? Houses are heavy, so often the answer is "very little". In that case your whole house may get lifted off from over your shelter room; if your shelter's ceiling is also the house's first floor, you then have a roofless shelter.

Check this issue first. If there is not big, obvious harware holding the house down, add it. You may wish to consult an engineer for expert advice (I am not an engineer); a few enhancements to consider include:

• Add steel "hurricane ties" to connect each basement rafter to the sill plate that sits on top of the concrete wall (I've done this already -- about $20 and a couple hours' work).
• Put diagonal cross-braces ("bridging") between the joists over your shelter -- this will massively strengthen it against horizontal forces that could otherwise topple the joists onto their sides.
• Drill through the sill plate and well down into the concrete wall, and put in expansion bolt anchors, or threaded rods fastened into place with setting cement. We're not talking dinky 1/4" anchors here -- more like 1/2" or more. Unless you have a seriously macho drill (with an angle attachment), or are praying to develop patience, don't try drilling and placing these yourself. Hilti makes superb drills for doing this kind of work.
• If you can't get in to drill (the Hilti right-angle attachment costs almost as much as the drill itself), you can drill into the wall horizontally, and bolt in a piece of 2" angle steel stock. Then run 3/8" lag screws into the sill plates to hold them down. If you do this adjacent to joists, you can screw them into the joists as well, as more vertical strength and having the same effect as bridging to protect against torque.
• Cut 2x4s to fit between the joists, and run 3/8" x 3 1/2" screws upward: Through your new 2x4, through the subfloor, and into the sole plate of the floor above (the sole plate rests along the floor, and is what the bottom of the wallboard, paneling, etc. is fastened to. Use wide "fender" washers under the screw head to spread the force out. If you cut the 2x4s to fit snugly between the joists, and attach them to the adjacent joints as well, they'll help as cross-bracing at the same time.

• You can combine the last two steps if you're willing to tear out some plaster on the floor above. Then you can drill straight down through the sole plate, the subfloor, some air, the sill plate, and into the concrete. This will require a long drill bit (sole plate, subfloor, joist height, sill plate(s), then concrete -- in my house I have to go 18.5" before the bit even touches the concrete, so a 24" (usable length) bit is the minimum). You can get long "SDS" (hammer drill) bits from Cummins Industrial Tools, from MyToolStore.com, Coastal tool, and other place.). I've found one place that sells SDS extension rods; item 015512 12inch SDS Plus Extension Rod from unifix online in the UK.

  Then you run one anchor all the way down -- say, at least a 1/2" bolt (more likely a threaded rod, much easier to find in long enough lengths). Tensile strengths of such rods are available here) or locally here. Hilti makes a high-quality setting epoxy to hold the rod in the concrete. you can then put a nut and shoulder washer on the top and you've got a serious hold-down.

• If the subfloor above isn't already glued down to the joists (as well as nailed or preferably screwed), run a big bead of Liquid Nails ™ along each joist/subfloor seam.

All these steps are easy, fairly cheap, and shouldn't bother anything else.

Now, consider the basement walls

Most of your basement walls are not at issue, since they're underground. If you're house is set on a hill, the shelter should be on the fully-buried side of the basement. The concern is for the foot or two or wall that extends above ground.

Think for a moment about what happens when that blast wave comes screaming in. Clearly if the roof of your whole house was level with the ground, horizontal blast waves would have relatively little effect (not none, but way less). The concrete basement walls of a typical house are 6-10" thick. A wall like that can take a lot of force, and the underground part is not in much danger (except from earthquakes, but that's another story).

The main risk is right where the concrete emerges from the ground and exposes itself directly to the blast.

Concrete is brittle. Where wood will bend and take a lot of force before breaking, concrete will not give: it either holds or suddenly shears. It actually takes a martial artist less energy to break a concrete block than to break a single pine board. More force, but less energy because as soon as the critical force is reached, the block shatters (see Michael S. Feld, Ronald E. McNair, and Stephen R. Wilk, "The Physics of Karate", Scientific American Volume 240, Issue 4, Pages 150-158 (1979)).

If the wall is going to give, it will likely be sheared off right at ground level. That would be unpleasant, especially because residential basement walls typically have no steel reinforcement bars ("rebar"). Thus if it cracks, there is nothing left to keep it from separating completely.

So the point of emergence from the ground is the place to focus attention: Take care of this weakest link first.

Basic mechanical engineering tells us that forces concentrate in narrow, angular corners. So if a wind or blast is coming in and suddenly hits the right angle where the ground stops and your concrete starts, the pressure there will be greatly amplified. This is why cars and planes are not flat on the front, and why trucks commonly add those curved wind-directors on top of the cab.

The good news is that you can cut down on the pressure amplification simply by rounding off the corner: bank up earth to make a curve that directs the blast more gradually from horizontal to vertical. A whole bunch of dirt will make a whole bunch of difference.

The next step up is not much harder: dig a trench around your foundation, and pour a second layer of concrete to back up the first. Note these details:

• Since the stress is concentrated near ground level, you don't have to dig way down; go a little past frost line to prevent shifting and cracking, and you should be fine.
• Since you're pouring on the outside, you can shape the new concrete to make a curve from ground to wall, instead of a corner. The bigger the radius the better, but even a small radius is massively better than a true corner.
• the new concrete can have all the rebar and all the thickness you want. You could even make it into a decorative raised planter, thus adding a few feet of earth shielding too.
• if you want, you can tie it in to the old concrete by drilling holes into the old, installing steel pins or rebar with setting cement, and then pouring. I'm not sure it this is worth the extra effort, and it causes extra stress as the new concrete settles into place.

None of this will make your house "bomb-proof" or absolutely safe. But it seems to me a far cry from not being able to do anything to strengthen an existing house. None of it is very expensive, either. But if you're going to do it do it now. Like most preparations, if you wait til you need it it's too late.

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