methods Dynamic compaction Vibro-compaction Vibro-replacement Large impact loads applied by repeated dropping of a 5- to 35-ton weight; larger weights have been used Vibrating equipment densifies soils Jetting and vibration used to penetrate and remove soil; compacted granular fill then placed in hole to form support columns surrounded by undisturbed soil Cohesionless best; possible use for soils with fines; cohesive soils below groundwater table give poorest results Cohesionless soils with 20 percent fines Soft cohesive soils (su  15 to 50 kPa, 300 to 1000 psf) Simple and rapid; usable above and below the groundwater table; effective depths up to 60 ft; moderate cost; potential vibration damage to adjacent structures Can be efffective up to 100 feet depth; can achieve good density and uniformity; grid spacing of holes critical; relatively expensive Relatively expensive Vibro-displacement Similar to vibro-replacement except soil is displaced laterally rather than removed

Fabrication of rebars consists of cutting to length and required bending. The preparation of field placing drawings and bar lists is termed detailing. Ordinarily, the TABLE 9.7 Standard Hooks* Recommended end hooksall grades of steel, in or ft-in Bar size no. 180 hooks D A orG J hooks A or G D
finished inside bend diameter, in. Seismic Stirrup /Tie rebar supplier details, fabricates, and delivers to the site, as required. In the farwestern states, the rebar supplier also ordinarily places the bars, In the New York City area, fabrication is performed on the site by the same (union) workers who place the reinforcement. (See Details and Detailing of Concrete Reinforcement, Standard Hooks. The geometry and dimensions of standard hooks that conform to the ACI 318 Building Code and industry practice are shown in Table 9.7. Fabrication Tolerances. These are covered in Standard Specifications for Tolerances for Concrete Construction and Materials, ACI 117. Shipping Limitations. Shipping widths or loading limits for a single bent bar and an L-shaped bar are shown in Fig. 9.7. Bundles of bars occupy greater space. The limit of 7 ft 4 in has been established as an industry practice to limit the bundle size to an 8-ft maximum load width. (Manual of Standard Practice, Concrete Reinforcing Steel Institute.) TABLE 9.7 Standard Hooks* (Continued ) Stirrup and tie hook dimensions, in or ft-inall

as discussed in Art. 11.79.2, is that structural flanking can seriously degrade the performance of sound barriers. Additional transmission paths include pipes, ducts, conduits, and almost any solid, continuous, rigid member in the building. Impact and vibrations can be transmitted through the floor-ceiling assemblies quite readily, if proper precautions are not taken. Normally, in ordinary construction, a good carpet over a good pad will provide sufficient impact isolation against the sounds of footfalls, heel clicks, and dropped objects. Where carpet is not feasible, or in critical spaces, special floor-ceiling assemblies can be used. (See Art. 11.80 for various constructions and their impact isolation performance.) Isolation against vibration or impact produced by machinery or other vibrating equipment is usually provided by use of special resilient or mounting systems. Springs, elastomeric pads, and other devices are used in such work. For a detailed discussion of such isolation, refer to the current ASHRAE Guide, American Society of Heating, Refrigerating and Air-Conditioning Engineers, or other references on vibration and shock isolation. Noise and vibration transmitted via plumbing and heating pipes, ducts, and conduits can be objectionable unless proper precautions are taken. Resilient connectors for rigid members, acoustically absorptive duct linings, and similar approaches are described in detail in the ASHRAE Guide. 11.79.4 Damping of Vibrations As a panel or object vibrates, it radiates acoustical energy to the air surrounding it and to solid surfaces touching it or attached to it. If the energy of vibration could be dissipated, the radiation would be reduced and the sound and vibration levels lowered. One way to do this is to attach firmly to a vibrating panel certain lossy substances (those with high internal friction or poor connections between particles) or viscoelastic materials (neither elastic nor completely viscous, such as certain asphaltic compounds). These damp the vibrations by absorbing the energy and converting it to heat. In the assembly of barriers, damping can be accomplished with proper connections and attachments, use of viscoelastic adhesives, proper attachment of insulating materials, and similar means. Special viscoelastic materials for adhesive attachment or brush or spray application to panels are available. 11.79.5 Sound Absorption The best-known acoustical materials are acoustical absorbents (although actually all materials are acoustical). Generally, these absorbents are lightweight, porous, fuzzy types of boards, blankets, and panels. Acoustical absorbents act as energy transducers, converting the mechanical energy of sound into heat. The conversion mechanism involves either pumping of air contained within the porous structure of the material, or the flexing of thin panels or sheets. Most materials employ the first principle. The internal construction of most absorbents consists of a random matrix of fibers or particles, with interconnected pores and capillaries (Fig. 11.96). It is necessary that the air contained within the matrix be able to move sufficiently to create friction against the fibers or capillaries. Nonconnected-cell or closed-cell porous materials are not effective absorbents. Tuned chambers, with small openings and a restricted neck into the chambers, also are used for sound absorption; but they are somewhat specialized in design FIGURE 11.96 Cross section of sound absorbent (greatly enlarged). and function, and their use requires expert design in most instances. The surface of absorbents must be sufficiently porous to permit the pressures of impinging sound waves to be transferred to the air within the absorbent. Very thin, flexible facings (plastic or elastomeric sheets) stretched over panels and blankets do not interfere significantly with this pressure transfer, but thick, rigid, heavy coatings (even heavy coatings of paint) may seriously restrict the absorption process. Perforated facings, if sufficiently thin and with sufficient closely spaced openings, do not appreciably degrade the performance of most absorbents. The visible surface of absorbent panels and tiles may be smooth or textured, fissured or perforated, or decorated or etched in many ways. Figure 11.97 shows a typical commercial tile. Figure 11.98 illustrates an example of the use of both acoustical tiles and panels in a ceiling. Absorbents are normally produced from vegetable or mineral fibers, porous or granular aggregates, foamed elastomers, and other products, employing either added FIGURE 11.97 Fissured-surface acoustical tile with the appearance of travertine stone. FIGURE 11.98 Library ceiling comprising both acoustical panels and acoustical tiles, integrated with lighting panels. (U.S. Gypsum Company.) binders or their own structure to provide the structural integrity required. Because of their lightweight, porous structure, most such products are relatively fragile and must be installed where they are not subject to abuse; or they may be covered with sturdy perforated or porous facings to protect them. Absorbents are chosen for their appearance, fire resistance, moisture resistance, strength, maintainability, and similar characteristics. Their performance ratings in respect to these characteristics are usually published in advertising materials and in various bulletins and releases of trade associations. The acoustical property of absorbents most important to designers and builders is absorptive efficiency. [Sound transmission through most absorbents takes place readily. They are very poor in this respect and should never be used to attempt to improve the airborne sound isolation of a barrier. Sound transmission over the top of a partition, through a lightweight, mechanically suspended acoustical panel ceiling, is frequently a serious annoyance in buildings (Art. 11.79.7).] The absorptivity of a tile or panel is usually expressed as the fraction or percentage of acoustical energy absorbed from an impinging plane wave. If a perfectly absorptive plane surface represents 1.00 or 100%, the ratio of absorption of a given product to this perfect absorber is called the sound absorption coefficient of the product. The absorptivity of a material varies with its thickness, density, porosity, flow resistance, and other characteristics. Further, absorptivity varies significantly with the frequency of the impinging sound. Usually, very thick layers of absorptive material are required for good absorption of low-frequency sound, while relatively thin layers are effective at higher frequencies. Little or no absorption, however, can be obtained with thin layers or flocked surfaces, textured but nonporous surfaces, or other products often mistakenly thought to be absorptive or claimed to break up