Acoustics, derived from a Greek word meaning to hear, refers to generation, detection, transmission, absorption, and control of sounds. Acoustics is part science and part art, but modern studies are stripping away much of its mystique. As a result, it is possible to plan and predict the acoustics of finished spaces with reasonable certainty. Sound is a vibration in an elastic medium. It is a simple form of mechanical energy, and can be described by the mathematics associated with the generation, transmission, and control of energy. Almost any moving, vibrating, oscillating, or pulsating object is a potential sound source. Usually, though, vibratory sources radiate enough energy to be audible to humans or felt by them. Sound waves in air (or other gases or fluids) travel outward from the source, transmitted by air molecules, like a rapidly expanding soap bubble. Any particular group of molecules behaves like a pulsating balloon, moving only slightly, while the wave progresses swiftly to great distances. Transmission (flow), or prevention of transmission of sound, and conversion of sound energy to a nonaudible form are the function of so-called acoustical materials. Cycle. A complete single excursion of a vibrating molecule. Frequency. The number of cycles of vibration in a given unit of time, usually cycles per second (cps) or hertz (Hz). Sound Wave. The portion of a sound between two successive compressions or

The daily reports from the superintendent are a record that provides much essential information on the construction job. From these daily reports, the following information is derived: names of persons working and hours worked; cost code amounts; subcontractor operations and description of work being performed; materials received; equipment received or sent; visitors to the job site; other remarks; temperature and weather; accidents or other unusual occurrences. Figure 17.10 shows a typical daily report. Back-Charges. Frequently, either at the request of a subcontractor or because of the failure of a subcontractor to perform, work must be done on behalf of a subcontractor and the subcontractors account charged. If the work performed and the resultant back-charge is at the request of the subcontractor, then obtaining the information and the agreement of all parties to a back-charge order (Fig. 17.11) is easy. If, however, there is a dispute as to whether or not the work is part of the obligation of the subcontractor, then the task becomes more complicated. Backcharges in this situation should be used sparingly. Sending a back-charge to a subcontractor under circumstances that are controversial is at best only a self- FIGURE 17.10 Daily report, prepared by a construction superintendent. FIGURE 17.11 Back-charge order for work done by a contractor on behalf of a subcontractor. serving declaration, and in all likelihood will be vigorously disputed by the subcontractor. Issuance of a purchase order differs from issuance of a subcontract (Art. 17.10.1). A purchase order is issued for material on which no labor is expected to be performed in the field. A subcontract, on the other hand, is an order for a portion of the work for which the subcontractor is expected not only to furnish materials but also to perform labor in the field. An example of a purchase order form, front and back, is shown in Fig. 17.12a and b. For the specific project, a purchase order rider (like Fig. 17.7) and list of contract drawings (Fig. 17.8) should be appended to the standard purchase order form. The rider describes special conditions pertaining to the job, options or alternates, information pertaining to shop drawings, or sample submissions and other particular requirements of the job. Material-price solicitations are handled much in the same manner as subcontractprice solicitations (Art. 17.10.1). Material bids should be analyzed for complicated trades in the same manner as for subcontracts (Fig. 17.5). To properly administer both the subcontract and the purchase orders, which may number between 40 and 60 on an average building job, it is necessary to have a purchasing log (Fig. 17.13) in which is entered every subcontract and purchased order after it has been sent to the subcontractor or vendor. The entry in the log is copied from the file copy of the typed document, and an initial in the upper righthand corner of the file copy indicates that entry has been made. The log serves as a ready cross reference, not only to names of subcontractors and vendors but also to the amounts of their orders and the dates the orders were sent. Computers in Purchasing. On a large-scale construction project, which costs many millions of dollars, thousands of items may have to be procured. Also, information, shop drawings, and part lists may have to be distributed and revised, redistributed, approved, and distributed again. Systems have been developed to use computers to keep track of all equipment and materials and related purchase information, such as specifications, quotations, final orders, shipment, and delivery dates. The systems are based on the concept of critical-path items, and the various tasks that must be performed are assigned due dates. For example, a computer report could be by project and show all open purchase-order items for one project, or by buyer name, with all open purchase-order items for each buyer, including all

and not less than 5 in. For n  2.0, the thickness should not be less than L (0.8  /200,000) n y h
(9.69) and not less than 3.5 in. where Ln
clear span in long direction, in. m
average value of  for all beams along panel edges 
ratio of flexural stiffness of beam section to flexural stiffness of a width of slab bounded laterally by the centerline of the adjacent panel, if any, on each side of the beam 
ratio of clear span in long direction to clear span in short direction The computed deflections of prestressed-concrete construction should not exceed the values listed in Table 9.14. A one-way reinforced-concrete slab is a flexural member that spans in one direction between supports and is reinforced for flexure only in one direction (Art. 9.52). If a slab is supported by beams or walls on four sides, but the span in the long direction is more than twice that in the short direction, most of the load will be carried in the short direction; hence, the slab can be designed as a one-way slab. One-way slabs may be solid, ribbed, or hollow. (For one-way ribbed slabs, see Arts. 9.54 to 9.58.) Hollow one-way slabs are usually precast (Art. 9.100). Castin- place, hollow one-way slabs can be constructed with fiber or cardboard-cylinder forms, inflatable forms that can be reused, or precast hollow boxes or blocks. Oneway slabs can be haunched at the supports for flexure or for shear strength. Structural strength, fire resistance, crack control, and deflections of one-way slabs must be satisfactory under service loads. Strength and Deflections. Approximate methods of frame analysis can be used with uniform loads and spans that conform to ACI 318 Building Code requirements (see Art. 9.41). Deflections can be computed as indicated in Art. 9.51, or in lieu of calculations the minimum slab thicknesses listed in Table 9.15 may be used. In Fig. 9.22 is a plot of ratios of moments of inertia of cracked to gross concrete section for one-way slabs. These curves can be used to simplify deflection calculations. Strength depends on slab thickness and reinforcement and properties of materials used. Slab thickness required for strength can be computed by treating a 1-ft width of slab as a beam (Arts. 9.45 and 9.46). Fire Resistance. One-way reinforced concrete slabs, if not protected by a fireresistant ceiling, must have a thickness that conforms to the fire-resistant rating required by the statutory building code. Table 9.17 gives minimum slab thickness for various fire-resistance ratings for normal-weight and structural-lightweightconcrete construction. Providing a minimum 3/4-in. concrete cover for reinforcement in restrained construction is adequate under the Uniform Building Code and Standard Building Code for fire-resistance ratings up to 4 hours. Reinforcement. Requirements for minimum reinforcement for crack control are summarized in Art. 9.50. Table 9.18 lists minimum reinforcement when Grade 60 bars are used. Reinforcement required for flexural strength can be computed by treating a 1-ft width of slab as a beam (Arts. 9.44 to 9.46). Rebar weights, lb / ft2 of slab area, can be estimated From Fig. 9.24a for oneway, continuous, interior spans of floor or roof slabs made of normal-weight concrete. One-way reinforced concrete slabs with spans less than 10 ft long can be reinforced with a single layer of draped welded-wire fabric for both positive and negative factored moments. These factored moments can be taken equal to wuL2 / 12, where wu is the total factored uniform load and L is the span, defined in Art. 9.41, TABLE 9.17 Minimum Slab Thickness, in, for Various Fire-Resistive Ratings