(5.270) e e For larger values of , the stress wave due to gravity acting on Wm during impact should be added to Eq. (5.267). Thus, for  larger than 0.2, 2h  2(1  e2)  2  (1  e2) (5.271) e Equations (5.270) and (5.271) correspond to Eq. (5.261), which was developed without wave effects being taken into account. For a sudden load, h  0, Eq. (5.271) gives for the maximum stress 2(1  e2), not quite double the static stress, the result indicated by Eq. (5.261). (See also Art. 5.18.4.) (S. Timoshenko and J. N. Goodier, Theory of Elasticity, McGraw-Hill Book Company, New York; S. Timoshenko and D. H. Young, Engineering Mechanics, John Wiley & Sons, Inc., New York.) 5.18.4 Dynamic Analysis of Simple Structures Articles 5.181 to 5.18.3 present a theoretic basis for analysis of structures under dynamic loads. As noted in Art. 5.18.2, an approximate solution based on an idealized representation of an actual member of structure is advisable for dynamic analysis and design. Generally, the actual structure may be conveniently represented by a system of masses and massless springs, with additional resistances to account for damping. In simple cases, the masses may be set equal to the actual masses; otherwise, equivalent masses may be substituted for the actual masses (Art. 5.18.6). The spring constants are the ratios of forces to deflections (see Art. 5.18.2). Usually, for structural purposes the data sought are the maximum stresses in the springs and their maximum displacements and the time of occurrence of the max- FIGURE 5.110 One-degree system acted on by a force varying with time. imums. This time is generally computed in terms of the natural period of vibration of the member or structure, or in terms of the duration of the load. Maximum displacement may be calculated in terms of the deflection that would result if the load were applied gradually. The term D by which the static deflection e, spring forces, and stresses are multiplied to obtain the dynamic effects is called the dynamic load factor. Thus, the dynamic displacement is

Specularity of the task can be a major cause of reflected glare. Consequently, high-gloss surfaces in the field of vision should be avoided. Often, tilting the task can reduce or eliminate veiling reflections. Use of low-level illumination throughout a space, supplemented by local lighting on the task, offers several advantages, including adequate light with little or no glare and flexibility in positioning the local light source. In many work environments, observers find themselves confronted with reflected glare both on horizontal surfaces, when reading material on a desktop, and on approximately vertical surfaces, when viewing a visual display terminal (VDT) screen. Designers should be aware of this and should be certain that the lighting system used to alleviate veiling reflections on horizontal materials does not aggravate the problem on the VDT screen. Luminaires selected to direct light outward beyond 40 from vertical to avoid veiling reflections may produce direct glare on the VDT. In general, in design of lighting systems for areas with VDTs, illuminance levels should be kept low, less than 75 fc. Illuminance ratios in the area also should be kept low, fixtures that have high VCPs should be selected and surface finishes that might reflect onto the VDT screen should have medium to low reflectances. Luminaires with parabolic louvers, either small-cell, intermediate-cell, or deepcell, are often used. They provide good illuminance on horizontal surfaces while contributing almost no direct glare onto the screen. They do, however, create a visual environment with relatively dark ceiling space that some may find objectionable. Pendant-mounted, indirect-lighting luminaires or direct-indirect fixtures using fluorescent or HID sources can also be successfully applied to areas with VDTs. They produce a brighter feeling in the space for the occupants. Care must be taken to select luminaires with properly designed light control and to mount them sufficiently below the ceiling to create a uniform brightness across the ceiling surface. Failure to meet these requirements can result in hot spots on the ceiling that will be reflected onto the VDT screen. A relatively uniform, though somewhat bright, ceiling luminance reflected onto the screen is not objectionable to the viewer (VDT contrast and brightness controls can be adjusted to compensate) but the hot spots on the screen force the viewers eyes to be constantly adjusting to the differences in reflected glare, causing eye fatigue and discomfort. (See The IES Recommended Practice for Lighting Offices Containing Computer Visual Display Terminals (VDTs) RP-24, Illuminating Engineering Society of North America.) See Art. 15.20, Bibliography. A black body is colorless. When increasing heat is applied to such a body, it eventually develops a deep red glow, then cherry red, next orange, and finally bluewhite. The color of the radiated light is thus related to the temperature of the heated body. This phenomenon is the basis for a temperature scale used for the comparison of the color of light from different sources. For example, the light from an incandescent lamp, which tends to be yellowish, may be designated 2500 Kelvin (K), whereas a cool white fluorescent lamp may be designated 4500 K. Light used for general illumination is mainly white, but white light is a combination of colors and some colors are more predominant than others in light emitted from light sources commonly used. When light other than white is desired, it may be obtained by selection of a light source rich in the desired hue or through use of a filter that produces that hue by absorbing other colors. Color rendering is the degree to which a light source affects the apparent color of objects. Color rendering index is a measure of this degree relative to the perceived color of the same objects when illuminated by a reference source for specified conditions. The index actually is a measure of how closely light approximates daylight of the same color temperature. The higher the index, the better is the color rendering. The index for commonly used light sources ranges from about 20 to 99. Generally, the color rendering of light should be selected to enhance color identification of an object or surface. This is especially important in cases where color coding is used for safety purposes or to facilitate execution of a task. Color enhancement is also important for stimulating human responses; for example in a restaurant, warm-colored light would make food served appear more appetizing, whereas cool-colored light would have the opposite effect. Sources producing white light are generally used. Because of the spectral energy distribution of the light, however, some colors predominate in the illumination. For example, for daylight, north light is bluish, whereas direct sunlight at midday is yellow-white; and light from an incandescent lamp is high in red, orange, and yellow. The color composition of the light may be correlated with a color temperature. For a specific purpose, a source with the appropriate color characteristics should be chosen. Lamp manufacturers provide information on the color temperature and color rendering index of their products. Colored light, produced by colored light sources or by filtering of white light, is sometimes used for decorative purposes. Colored light also may be used to affect human moods or for other psychological purposes, as indicated in Art. 15.10.8. Care must be taken in such applications to avoid objectionable reactions to the colored light; for example, when it causes unpleasant changes in the appearance of human skin or other familiar objects. Perceived color of objects also is affected by the level of illumination (Art. 15.10.8). When brilliant color rendition is desired and high-intensity lighting is to be used, the color saturation of the objects should be high; that is, colors should be vivid. Also, a source that would enhance the colors of the objects should be

size no. Lap class
3,000 psi
c Top bars Case 1 Case 2 Other bars Case 1 Case 2
4,000 psi
c Top bars Case 1 Case 2 Other bars Case 1 Case 2
5,000 psi
c Top bars Case 1 Case 2 Other bars Case 1 Case 2 1. Values are based on Sections 12.2.2 and 12.15 in ACI 318-99 Building Code. 2. See notes under Table 9.8 for definitions of Case 1 and Case 2. 3. Values are for normal-weight concrete. * Sample Calculation: From Sample Calculation under Table 9.8; for Case 1, bar size no. 8, top bars, Ld
61.7 in. For Class B tension lap splice, Lap length
1.3 Ld
80.2 or 80 in. FIGURE 9.20 (a) Minimum lap splice length for deformed welded-wire fabric. (b) Slab reinforced with deformed welded-wire fabric. FIGURE 9.21 Minimum lap splice length for plain welded-wire fabric. Use the larger of the values shown in (a) and (b). In calculation of splice length, the computed value of development length Ld, not the minimum required value, should be used. (a) Splice length when steel area used is less than twice the required area. (b) Splice length when steel area used is two or more times the required area. (c) Slab reinforced with plain welded-wire fabric providing twice the required reinforcement