Designing Foundations and Different Design Thresholds

It is a good idea to begin the design of the foundations of a building at  the same time as architectural design  work commences. Subsurface conditions beneath a site can strongly influence fundamental decisions about  a building—its location on the site,  its size and shape, its weight, and  the required degree of flexibility of  its construction. On a large building project, at least three designers  are involved in these decisions: the  architect, who has primary responsibility for the location and form of  the building; the structural engineer,  who has primary responsibility for its  physical integrity; and the foundation  engineer, who must decide, on the basis of site exploration and laboratory reports, how best to support it  in the earth. More often than not,  it is possible for the foundation engineer to design foundations for a  building design dictated entirely by  the architect. In some cases, however, the cost of the foundations may  consume a much larger share of the  construction budget than the architect has anticipated, unless certain compromises can be reached on the  form and location of the building. It  is safer and more productive for the  architect to work with the foundation  engineer from the outset, seeking al- ternative site locations and building configurations that will result in the fewest foundation problems and the  lowest foundation cost.

In designing a foundation, a  number of different design thresh-olds need to be kept in mind. If the  designer crosses any of these thresholds, foundation costs take a sudden  jump. Some of these thresholds are:

•  Building below the water table. If the substructure and foundations of a building are above the water table, minimal effort will be required to keep the excavation dry during construction. If the water table is penetrated, even by an inch, expensive steps will have to be taken to dewater the site, strengthen the slope support system, waterproof the foundation, and either strengthen the basement floor slab against hydrostatic uplift pressure or provide for adequate drainage to relieve this pressure. For an extra inch or foot of depth, the expense would probably not be justified; for another story or two of useful building space, it might be.

•  Building close to an existing structure. If the excavation can be kept well away from adjacent structures, the foundations of these structures can remain undisturbed and no effort and expense are required to protect them. When digging close to an existing structure, and especially when digging deeper than its foundations, the structure will have to be temporarily braced and may require permanent underpinning with new foundations. Furthermore, an excavation at a distance from an existing structure may not require sheeting, while one immediately adjacent al-most certainly will.

•  Increasing the column or wall load from a building beyond what can be supported  by a shallow foundation. Shallow foundations are far less expensive than piles or caissons under most conditions. If the building grows too tall, however, a shallow foundation may no longer be able to carry the load, and a threshold must be crossed into the realm of deep foundations. If this has happened for the sake of an extra story or two of height, the designer should consider reducing the height by broadening the building. If individual column loadings are too high for shallow foundations, perhaps they can be reduced by increasing the number of columns in the building and decreasing their spacing.

For buildings at the scale of one-and two-family dwellings, foundation design is usually much simpler than for large buildings because foundation loadings are low. The uncertainties in foundation design can be reduced with reasonable economy by adopting a large factor of safety in calculating the bearing capacity of the soil.

Unless the designer has reason to suspect poor soil conditions, the footings are usually designed using rule-of-thumb allowable soil stresses and standardized footing dimensions. The designer then examines the actual soil when the excavations have been made. If it is not of the quality that was expected, the footings can be hastily redesigned  using a revised estimate of soil-bearing  capacity before construction continues. If unexpected groundwater is encountered, better drainage may have to be provided around the foundation or  the depth of the basement decreased.

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