Virtually any species of wood can be used in the laminating process if the design values have been determined. Different species can be intermixed within the depth of a section to achieve optimum resource utilization. Higher-strength species are positioned in a beam in zones that will be subjected to high stresses under design loads. Lower-strength species can be placed in zones with lower in-service stresses. Similarly, manufacturer of glulam beams can be based on a graded layup concept. This requires that laminations with a higher lumber grading be used in zones subjected to high design stresses, and lower grades, in lower-stressed areas of the member. As a consequence, glulam members are a resource-efficient wood product, since varying grades and species can be used to achieve desired performance. Constant-depth members normally are a multiple of the thickness of the lamination stock used. Variable-depth members, because of tapering or special assembly techniques, may not be exact multiples of these lamination thicknesses. Nominal width of stock, in 4 6 8 10 12 Standard member finished width, in 3 or 31/8 5 or 51/8 63/4 81/2 or 83/4 101/2 or 103/4 Standard widths as listed above are most economical, since they represent the maximum width of board normally obtained from the stock used in laminating. Other widths, such as 31/2 or 51/2 in, which fit well with conventional 2  4 and 2  6 framing, are also available in many market areas. FIGURE 10.28 Finger joints: (a) fingers formed by cuts perpendicular to the wide faces of the boards; (b) fingers formed by cuts perpendicular to the edges. When members wider than the stock available are required, laminations may consist of two boards side by side. These edge joints must be staggered, vertically in horizontally laminated beams (load acting normal to wide faces of laminations), and horizontally in vertically laminated beams (load acting normal to the edge of laminations). In horizontally laminated beams, edge joints need not be edge-glued. Edge gluing is required in vertically laminated beams. Edge and face gluings are the simplest to make, end gluings the most difficult. FIGURE 10.27 Plane sloping scarf. Although no longer used in the glulam industry, a plane sloping scarf (Fig. 10.27), in which the tapered surfaces of laminations are glued together, can develop 85 to 90% of the strength of an unscarfed, clear, straight-grained control specimen. Finger joints (Fig. 10.28) are less wasteful of lumber and hence are widely used by the glulam industry. Quality can be adequately controlled in machine cutting and in high-frequency gluing. A combination of thin tip, flat slope on the side of the individual fingers, and a narrow pitch is desired. The length of fingers should be kept short for savings of lumber, but long for maximum strength. Typical finger joint lengths are 1
11/4
. The usefulness of structural glued-laminated timbers is determined by the lumber used and glue joint produced. Certain combinations of adhesive, treatment, and wood species do not produce the same quality of glue bond as other combinations, although the same gluing procedures are used. Thus, a combination must be supported by adequate experience with a laminators gluing procedure. The only adhesives currently recommended for wet-use and preservative-treated lumber, whether gluing is done before or after treatment, are resorcinol and phenolresorcinol resins. Melamine and melamine-urea blends are also used for highfrequency curing of end joints. Glued joints may be cured with heat by several methods. Radio-frequency (RF) curing of glue lines is used for end joints and for limited-size members where there are repetitive gluings of the same cross section. Low-voltage resistance heating, where current is passed through a strip of metal to raise the temperature of a glue line, formerly was used for attaching thin facing pieces. The metal could be left in the glue line as an integral part of the completed member. Printed electric circuits, in conjunction with adhesive films, and adhesive films impregnated on paper or on each side of a metal conductor placed in the glue line are other alternatives. Preheating the wood to ensure reactivity of the applied adhesive has limited application in structural laminating. The method requires adhesive application as a wet or dry film simultaneously to all laminations, and then rapid handling of multiple

Substitution in Eq. (8.44) with Fu
48 ksi yields the nominal tension load on the net section: P
[4  (2  11/16)]  3/16  48
23.63 kips n The allowable load is P
P /
23.63/2.22
10.64 kips a n This compares with the tensile strength of each sheet for tension member design: P
A F
[4  (2  11/16)]  3/16  33
16.24 kips n n sy The allowable load is P
P /
16.24/1.67
9.72 kips a n Use Pa
9.72 kips. TABLE 8.10 Nominal Tensile and Shear Strength for Bolts Description of bolts Tensile strength Factor of safety  Nominal stress Fnt, ksi Shear strength Factor of safety  Nominal stress Fnv, ksi A307 bolts, Grade A 1/4 in  d  1/2 in A307 bolts, Grade A d 1/2 in A325 bolt, when threads are not excluded from shear planes A325 bolts, when threads are excluded from shear planes A354 Grade BD bolts 1/4 in  d  1/2 in, when threads are not excluded from shear planes A354 Grade BD bolts 1/4 in  d  1/2 in, when threads are excluded from shear

the increase in vertical stress ( v) due to the proposed building weight exceeds the maximum past pressure ( vm). For the second case, there will be virgin consolidation of the cohesive soil. Given the same cohesive soil and identical field conditions, the settlement due to the second case will be significantly more than the first case. As previously mentioned, primary consolidation is a time-dependent process that can take many years to complete. The rate of consolidation can be estimated using the Terzaghi theory of consolidation (see K. Terzaghi and R. B. Peck, Soil Mechanics in Engineering Practice, John Wiley & Sons, Inc., New York): Secondary Compression. The final component of settlement is due to secondary compression, which is that part of the settlement that occurs after essentially all of the excess pore water pressures have dissipated (i.e., settlement that occurs at constant effective stress). The usual assumption is that secondary compression does not start until after primary consolidation is complete. The amount of secondary compression is often neglected because it is rather small compared to the primary consolidation settlement. However, secondary compression can constitute a major part of the total settlement for peat or other highly organic soil (see R. D. Holtz and W. D. Kovacs, An Introduction to Geotechnical Engineering, Prentice-Hall, Inc., Englewood Cliffs, NJ). The final calculation for estimating the maximum settlement (max) of the insitu cohesive soil would be to add together the three components of settlement, or:   s  s  s (6.23) max i c s where max  maximum settlement over the life of the structure si  immediate settlement sc  primary consolidation settlement ss  secondary compression settlement 6.5.7 Settlement of Granular Soil A major difference between saturated cohesive soil and granular soil is that the settlement of cohesionless soil is not time dependent. Because of the generally high permeability of granular soil, the settlement usually occurs as the load is applied during the construction of the building. Many different methods can be used to determine the settlement of granular soil, such as plate load tests, laboratory testing of undisturbed soil samples, equations based on the theory of elasticity, and empirical correlations. For example, Fig. 6.23 shows a chart that presents an empirical correlation between the measured N value (obtained from the Standard Penetration Test, see Art. 6.2.4) and the allowable soil pressure (tsf) that will produce a settlement of the footing of 1 in (2.5 cm). As an example of the use of Fig. 6.23, suppose a site contains a sand deposit and the proposed structure can be subjected to a maximum settlement (max) of 1.0 in (2.5 cm). If the measured N value from the Standard Penetration Test  10 and the width of the proposed footings  5 ft (1.5 m), then the allowable soil pressure  1 tsf (100 kPa). FIGURE 6.23 Allowable soil bearing pressures for footings on sand based on the Standard Penetration Test. (From K. Terzaghi and R. B. Peck, Soil Mechanics in Engineering Practice, 2d ed., John Wiley & Sons, Inc., New York. Reprinted with permission of John Wiley & Sons, Inc.) For measured N values other than those for which the curves are drawn in Fig. 6.23, the allowable soil pressure can be obtained by linear interpolation between curves. According to Terzaghi and Peck, if all of the footings are proportioned in accordance with the allowable soil pressure corresponding to Fig. 6.23, then the maximum settlement (max) of the foundation should not exceed 1 in (2.5 cm) and the maximum differential settlement () should not exceed 0.75 in (2 cm). Figure 6.23 was developed for the groundwater table located at a depth equal to or greater than a depth of 2B below the bottom of the footing. For conditions of a high groundwater table close to the bottom of the shallow foundation, the values obtained from Fig. 6.23 should be reduced by 50%. A bearing capacity failure is defined as a foundation failure that occurs when the shear stresses in the soil exceed the shear strength of the soil. Bearing capacity FIGURE 6.24 General shear foundation failure for soil in a dense or hard state. (Adapted from A. B. Vesic, Bearing Capacity of Deep Foundations in Sand, Highway Research Record, no. 39.) failures of foundations can be grouped into three categories (A. B. Vesic, Bearing Capacity of Deep Foundations in Sand, Highway Research Record, no. 39): 1. General Shear (Fig. 6.24). As shown in Fig. 6.24, a general shear failure involves total rupture of the underlying soil. There is a continuous shear failure of the soil (solid lines) from below the footing to the ground surface. When the load is plotted versus settlement of the footing, there is a distinct load at which the foundation fails (solid circle), and this is designated Qult. The value of Qult divided by the width (B) and length (L) of the footing is considered to be the ultimate bearing capacity (qult) of the footing. The ultimate bearing capacity has been defined as the bearing stress that causes a sudden catastrophic failure of the foundation. Note in Fig. 6.24 that a general shear failure ruptures and pushes up the soil on both sides of the footing. For actual failures it the field, the soil is often pushed up on only one side of the footing with subsequent tilting of the structure. A general shear failure occurs for soils that are in a dense