Poured Concrete & Concrete Block Walls Forces

Basement Walls

Many parts of the nation enjoy soil and topography which favors the installation of basements or partial basements (crawl spaces). Areas which have very little soil cover, marshy land, or high water tables make frequent use of concrete slabs. However, those people or builders who do build basements often overlook an important point. Basement walls, be they poured concrete or concrete block, are also retaining walls. They retain, or hold back, the dirt which surrounds your house.

Retaining walls which exceed four feet in height often are engineered. They require special footers, specific wall thickness, and the prolific use of steel reinforcing. Many residential basement walls often have seven feet of backfill against them! However, there is a difference between a free standing retaining wall and a residential foundation wall. A residential foundation wall has two things that help it structurally. The basement slab and the wood subfloor assembly which is bolted to the top of foundation walls are important structural elements. The basement slab stops the bottom of the wall from sliding inwards, while the subfloor assembly helps to keep the top of the wall from tilting inwards.

However, these two elements are not enough to totally withstand the forces of soil and water against a poured concrete or foundation wall. These walls must be constructed in such a way as to resist the forces which will be placed against them. To further complicate the issue, no two building sites are exactly the same. For example, a foundation constructed at the bottom of a hillside will have greater forces against it than a foundation built at the top of the hill. A foundation which will last 100 years in areas of little or no seismic activity might crumble in a moderate earthquake in southern California. All foundation system plans should be checked for the particular building site by a competent structural engineer. The $200 - $300 fee may equate to a lifetime insurance policy. It is money well spent.

Tension Headaches

The strength of concrete and concrete masonry products (block) is usually measured in the amount of weight they can support before failure. This weight squeezes or compresses the concrete or masonry materials. Thus it is called compressive strength. However, concrete or concrete masonry is not always squeezed. Often it is stretched or pulled. These forces are referred to as tension. Non-reinforced concrete or concrete masonry products generally are only 1/10th as strong in tension than in compression. That means that if a concrete wall is rated for 3,500 pounds per square inch in compression, it will only be able to withstand 350 pounds per square inch when subjected to a tension force.

Foundation walls are subjected to tension forces as they try to hold back the soil around them. The dirt wants to push in or bulge the wall. You can illustrate this very easily with a thin piece of cardboard. Stand the cardboard up straight just like your foundation wall. Using your finger, push the center of the cardboard. The cardboard bows inward. When this happens to a foundation wall, the inside face of the wall is actually being stretched or pulled. This is tension. The resulting crack is a measurement of just how much it was stretched!

Steel, on the other hand, can withstand tremendous tension forces. In fact, the average steel used in reinforcing rods for residential construction will fail only after 60,000 lbs per square inch of force is used to pull it apart! Fortunately, steel is very inexpensive. If you decide to build anything with concrete or concrete masonry and you do not include reinforcing steel, I guarantee that you will develop a massive headache when that first jagged crack appears! Be smart, insist on the use of reinforcing steel anytime you build with concrete or concrete masonry products. You will not be disappointed.

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