where M
bending moment, in-lb R
radius of curvature at centerline of member, in b
width of cross section, in d
depth of cross section, in When M is in the direction tending to decrease curvature (increase the radius), tensile stresses occur across the grain. For this condition, the allowable tensile stress across the grain is limited to one-third the allowable unit stress in horizontal shear for southern pine for all load conditions, and for Douglas fir and larch for wind or earthquake loadings. The limit is 15 psi for Douglas fir and larch for other types TABLE 10.12 Bearing Area Factors Cb Bearing length, in 0.50 1.00 1.50 2.00 3.00 4.00 6 or more Bearing area factor 1.75 1.38 1.25 1.19 1.13 1.10 1.00 of loading. These values are subject to modification for duration of load. If these values are exceeded, mechanical reinforcement sufficient to resist all radial tensile stresses is required. When M is in the direction tending to increase curvature (decrease the radius), the stress is compressive across the grain. For this condition, the allowable stress is limited to that for compression perpendicular to grain for all species. (K. F. Faherty and T. G. Williamson, Wood Engineering and Construction Handbook, and D. E. Breyer, Design of Wood Structures, 2d ed., McGraw-Hill Publishing Company, New York.) 10.5.10 Form Factor Design values for bending Fb for beams with a circular cross section may be multiplied by a form factor C
1.18. For a flexural member with a square cross section loaded in the plane of the diagonal (diamond-shape cross section), C may

The characteristics of a luminous environment that determine quality are contrast, diffusion, and color rendering. Contrast, created by shadows or by relatively bright areas in a field of vision, affects visibility, mood, comfort, and eyestrain (Art. 15.10). Diffusion, the dispersion of light in all directions, may be produced by transmission or reflection. Diffuse transmission occurs when light from a bright source passes through a material that disperses the incident light, with consequent reduction in brightness. Translucent materials or specially constructed lenses are often employed for this purpose in lighting fixtures. Diffuse reflection occurs when incident light on a surface is reflected almost uniformly in all directions by tiny projections or hollows. Such surfaces appear nearly equally bright from all viewing angles. Diffusion tends to reduce contrast and promote uniformity of lighting. Brightness Ratios. For good quality of lighting, the degree of contrast of light and dark areas in the field of vision should be limited to provide for viewing angle changes. The reason for this is that the eye adapts to the luminance of a task after a period of time. When the eye leaves the task and encounters a field of different brightness, the eye requires an appreciable time to adapt to the new condition, during which eyestrain or visual discomfort may be experienced. To promote quick, comfortable adaptation, brightness ratios in the visual environment should be kept

Lamps are constructed to operate at a specific voltage and wattage, or power consumption. In general, the higher the wattage rating of a specific type of lamp, the greater will be its efficacy, or lumen output per watt. 15.16.1 Considerations in Lamp Selection Greatest economy will be secured for a lighting installation through use of a lamp with the highest lumen output per watt with good quality of illumination. In addition to lumen output, however, color rendering and other characteristics, such as lighting distribution, should also be considered in lamp selection. Information on these char- acteristics can be obtained from lamp manufacturers. Latest data should be requested, because characteristics affecting lamp performance are changed periodically. The following information usually is useful: Lamp life, given as the probable number of hours of operation before failure. Lamp efficacy, measured by the lamp output in lumens per watt of power consumed. Lamp lumen depreciation, as indicated by tests. Curves are plotted from data to show the gradual decrease in light output with length of time of lamp operation. The decrease occurs because of both aging and dirt accumulation (Art. 15.16.2). The latter can be corrected with good maintenance, but the possible effects nevertheless should be considered in lighting design. Lamp warm-up time, which is significant for some fluorescent lamps and all high-intensity-discharge lamps, for which there is a delay before full light output