Engineering and Construction

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The strength of a piece of wood depends chießy on its species, its grade, and the direction in which the load acts with respect to its grain.

For example, in both tension and compression, wood is typically several times stronger parallel to grain  than perpendicular to grain. With its usual assortment of defects, it is  stronger in compression than in tension. Allowable strengths (structural  stresses that include factors of safety) vary tremendously with species and  grade. For example, allowable com- pressive strength parallel to grain  for commercially available grades  and species of framing lumber varies  from 325 to 1700 psi (2.24 to 11.71 MPa), a difference of more than five times. Figure 3.21 compares the average structural properties of framing lumber to those of some other  common structural materialsÑbrick  masonry, steel, and concrete. Of the  four materials, only wood and steel  have useful tensile strength. Defect- free wood is comparable to steel on  a strength-per-unit weight basis, but  with the ordinary run of defects, an  average piece of lumber is somewhat  inferior to steel by this yardstick.

Figure 3.21 Comparative physical properties of four common structural materials: wood (shaded row), brick masonry, steel, and concrete. Wood has signifi cant strength in both tension and compression. The ranges of values of wood strength and stiffness refl ect differences among species and grades of lumber.

When designing a wooden struc- ture, the architect or engineer determines the maximum stresses that are  likely to occur in each of the structural  members and selects an appropriate  species and grade of lumber for each.

In a given locale, a limited number of  species and grades are usually avail- able in retail lumberyards, and it is  from these that the selection is made.

It is common practice to use a stronger  but more expensive species (Douglas  for or Southern pine, for example) for  highly stressed major members and  to use a weaker, less expensive species  (such as Eastern hemlock) or species  group for the remainder of the structure.  Within each species, the designer se- lects grades based on published tables of allowable stresses. The higher the  structural grade, the higher the allowable stress. But the lower the structural  grade, the less costly is the lumber.

There are many factors other  than species and grade that inß  uence  the useful strength of wood. These include the length of time the wood will  be subjected to its maximum load, the  temperature and moisture conditions  under which it serves, and the size  and shape of the piece. Certain fire retardant treatments also reduce the  strength of wood slightly. All these factors are taken into account when engineering a building structure of wood.

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Because of the public safety considerations that are involved, building codes contain numerous provisions relating to the design and construction of excavations and foundations. The IBC defi nes which soil types are considered satisfactory for bearing the weight of buildings and establishes a set of requirements for subsurface exploration, soil testing, and submission of soil reports to the local building inspector. It goes on to specify the methods of engineering design that may be used for the foundations. It sets forth maximum loadbearing values for soils that may be assumed in the absence of detailed test procedures (Figure 2.5). It establishes minimum dimensions for footings, caissons, piles, and foundation walls and contains lengthy discussions relating to the installation of piles and caissons and the drainage and waterproofi  ng  of substructures. The IBC also requires engineering design of retaining walls.

In all, the building code attempts to ensure that every building will rest upon secure foundations and a dry substructure.

Figure 2.5                                         Presumptive surface bearing values of various soil types,                                                                 from  symbols  the 2006 IBC. Classes 3, 4, and 5 refer to the soil group in Figure 2.2.

Figure 2.2 The Unifi ed Soil Classifi cation System, from ASTM D 2487. The group symbols are a universal set of abbreviations for soil types, as seen for example, in Figure 2.8.

Figure 2.8 A typical log from a soil test boring indicating the type of soil in each stratum and the depth in feet at which it was found. The abbreviations in parentheses refer to the Unifi ed Soil Classifi cation System, and are explained in Figure 2.2.

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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.