Jacking refers to raising or lowering a building so you can repair or replace defective framing or failed foundations or to level a house that has set­tled excessively. Shoring refers to a temporary system of posts, beams, and other structural ele­ments that support building loads. Temporary is the crucial word: Shoring supports the building between jackings. Once repairs are complete, you need to lower the house and remove the shoring as soon as possible. If repairs are extensive—say, replacing foundation sections—have a structural engineer design the new sections; specify jack size; and specify the posts, beams, and bracing needed to safely jack and shore the building.

Jacking a house is nerve-wracking. It requires a deep understanding of house framing and how structures transfer loads. It also requires superb organizational skills and a lot of specialized equipment. For that reason, foundation contrac­tors routinely subcontract house-raising to house movers who have crews who know what they’re doing and have on hand thousands of cribbing blocks, scores of hydraulic jacks, and cranes to lift steel I-beams for bigger jobs. Structural engi­neers will usually know qualified house movers. (By the way, these specialists are still called house movers even when the house stays on the site.)

Most foundations that fail were poorly designed, poorly constructed, or subjected to changes— especially hydrostatic pressure or soil movement —that exceeded their load-bearing capacities. Exact causes are often elusive.

for a span or by a failed or absent girder. If an existing girder seems sound, adding posts or new pads may fix the problem. Otherwise, add a girder to reduce the distance joists span.

Flooring that crowns above a girder, sloping downward toward the outside walls; doors and windows that are difficult to open; and cracking at the corners of openings are often caused by a failure of all or part of the perimeter foundation.

Foundation cracks often signal foundation fail­ure. Cracks may range from short surface cracks to through-the-wall cracks that should be exam­ined by a structural engineer. Here are some common symptoms and remedies:

► Narrow vertical or diagonal surface cracks with aligned sides are likely caused by foundation settlement or soil movement but are probably not serious. If water runs from cracks after a storm, fill cracks with an epoxy cement, and then apply a sealant.

► Wide cracks in foundations less than 2 ft. tall indicate little or no steel reinforcement, a common failing of older homes in temperate climates.

► Large, /і-in. or wider, vertical cracks through the foundation that are wider at the top usually mean that one end of the foun­dation is sinking—typically at a corner with poor drainage or a missing downspout.

► Large vertical cracks through the foundation that are wider at the bottom are usually caused by footings that are too small
for the load. You may need to replace or reinforce failed sections.

► Horizontal cracks though a concrete foundation midway up the wall, with the wall bowing in, are most often caused by lateral pressure from water-soaked soil. This condition is common to uphill walls on sloping lots.

► Concrete-block walls with horizontal cracks that bulge inward are particularly at risk because block walls are rarely reinforced with steel. If walls bulge more than / in. from vertical and there’s a chronic water problem, foundation failure may be imminent.

► In cold climates, horizontal cracks through the foundation, just below ground level, are usually caused by adfreezing, in which damp soil freezes to the top of the foundation and lifts it. If these cracks are accompanied by buckled basement floors, the foundation’s footings may not be below the frost line for your region.

Gaps between the chimney and the house are usually caused by an undersize chimney pad. If the mortar joints are eroded, too, tear the chim­ney down and replace it.

In many parts of North America, building codes don’t require steel reinforcement in concrete foundations, but steel is a cost-effective means of avoiding cracks caused by lateral pressure on foundation walls.

Steel reinforcement and fasteners. Steel re­inforcing bar (rebar) basically carries and distrib­utes loads within the foundation, transferring the loads from high-pressure areas to lower-pressure areas. It thereby lessens the likelihood of point failure, either from point loading above or from lateral soil and water pressures. Anchor bolts or threaded rods, tied to rebar, attach the overlying structure to the foundation. Steel dowels are usu­ally short pieces of rebar that pin foundation walls to footings, new sections to existing foun­dations, and so on.

There are also a number of metal connectors— such as Simpson Strong-Ties—that tie joists to girders, keep support posts from drifting, and hold down mudsills, sole plates, and such.

Several are shown in Chapter 4.

Quality. Concrete quality is critically important, both in its composition and in its placement. Water, sand, and aggregate must be clean and well mixed with the cement. Concrete with com­pressive strength of 2,500 psi to 3,500 psi (pounds per square inch) is common in residential founda-

THE FOUNDATION WltnlP

Most perimeter foundations have a companion foundation within, consisting of a system of girders (beams), posts (columns), and pads that pick up the loads of joists and interior walls and thus reduce the total load on the perimeter foundation. By adding posts, beams, and pads, you can often stiffen floors, reduce squeaks, avoid excessive point loading, support new partitions, and even avoid replacing a mar­ginally adequate perimeter foundation.

GETTING THE

Learn what you can about local soil conditions before hiring a soils engineer. Start with local builders—especially those who’ve worked on nearby properties. Next consult with building and land-use departments, for many have maps indicating watersheds, slide zones, contaminated soil, and the like. Finally, the U. S. Department of Agriculture (USDA) has extensive soil maps. And the U. S. Geological Survey (USGS) topographic (topo) maps show streams, lakes, flood plains, and other natural features that could have an impact on your site.

tions, yet there are many ways to achieve that strength, including chemical admixtures. Discuss your needs with a concrete supplier who’s famil­iar with soil conditions in your area, and read "Ordering Concrete,” on p. 222.

Drainage. The drainage system is not technically a part of the foundation, but the flow of water alongside and under a foundation is important to its success. In some soils, it’s essential to mediate water flow. At the very least, water seeping through foundations can cause damp basements and encourage mold. Worse, excessive water can rot framing, undermine footings, and cause un­reinforced foundations to crack, bulge inward, or fail altogether. Often moisture problems can be mitigated simply by keeping gutters and downspouts clear, grading the soil away from the foundation, and improving drainage around basement window wells. Beyond that, the "cures” are increasingly expensive, such as excavating along the outside of the foundation to add gravel and perimeter drainpipe and to apply water­proofing treatments.

Foundation issues

before starting extensive remedial work, such as replacing failed foundation sections or adding a second story to your house, have a structural engi­neer evaluate your foundation. In addition, bring in a soils engineer if the site slopes steeply or if the foundation shows any of the following dis­tress signs: bowing, widespread cracking, uneven settlement, or chronic wetness. Engineers can also assess potential concerns such as slide zones, soil load-bearing capacity, and seasonal shifting.

An Overview

A foundation is a mediator between the loads of the house and the soil on which the foundation rests. A well-designed foundation keeps a house’s

ably adaptable. On a flat site in temperate regions, a shallow tee foundation is usually enough to support a house, while creating a crawl space that allows joist access and ventilation.

Where the ground freezes, foundation footings need to be dug below the frost line, stipulated by local codes. Below the frost line, footings aren’t susceptible to the potentially tremendous lifting and sinking forces of freeze-thaw cycles in moist soil. (Thus most houses in cold climates often have full basements.)

When tee foundations fail, they often do so because they’re unreinforced or have too small or too shallow a footprint. Unreinforced tee founda­tions that have failed are best removed and replaced. But reinforced tees that are sound can be underpinned by excavating and pouring larger footings underneath a section at a time.

Slab on grade is a giant pad of reinforced con­crete, poured simultaneously with a slightly thicker perimeter footing that increases its load­bearing capabilities. Beneath the slab, there’s typically a layer of crushed gravel and sheet plastic over that to prevent moisture from wicking up from the soil. Slabs are generally installed on flat lots where the ground doesn’t freeze because, being above frost line, shallow slabs are vulnerable to frost heaves. Although shallow, the large footprint of a slab sometimes makes it the only feasible foundation on soils with weak load-bearing capacity. Because slabs sit on grade, their drainage systems must be meticulously detailed.

Drilled concrete piers in tandem with grade beams are the premier foundation for most situations that don’t require basements. These foundations get their name because pier holes are typically drilled to bearing strata. This foun­dation type is unsurpassed for lateral stability, whether as replacement foundation for old work or for new construction. Also, concrete piers have a greater cross section than driven steel piers and hence greater skin friction against the soil, so they’re much less likely to migrate. The stability of concrete piers can be further enhanced by concrete-grade beams resting on or slightly below grade, which allow soil movement around the piers, without moving the piers.

The primary disadvantages of drilled concrete piers are cost and access. In new construction, a backhoe equipped with an auger on the power takeoff requires 10 ft. or 12 ft. of vertical clear­ance. Alternatively, there are remote-access portable rigs that can drill in tight quarters, even inside existing houses, but they are labor inten­sive to set up and move, increasing the cost.

Driven steel pilings are used to anchor founda­tions on steep or unstable soils. Driven to bedrock and capped, steel pilings can support heavy vertical loads. And, as retrofits, they can stabilize a wide range of problem foundations. There are various types of steel pilings, including helical piers, which look like giant auger bits and are screwed in with hydraulic motors, and push piers, which are hollow and can be pushed in, strengthened with reinforcing bar, and filled with concrete or epoxy.