in H2O per 100 ft Airflow, cfm Diam, in Velocity, ft /min Diam, in Velocity, ft /min Diam, in Velocity, ft /min Diam, in Velocity, ft /min Diam, in Velocity, ft /min Diam, in Velocity, ft /min * Based on data in ASHRAE HandbookFundamentals, American Society of Heating, Refrigerating and Air- Conditioning Engineers. The shape of ducts usually installed as rectangular, because dimensions can easily be changed to maintain the required area. However, as ducts are flattened, the increase in perimeter offers additional resistance to airflow. Thus a flat duct requires an increase in cross section to be equivalent in air-carrying capacity to one more nearly square. A 12  12-in duct, for example, will have an area 1 ft2 and a perimeter of 4 ft, whereas a 24  6-in duct will have the same cross-sectional area but a 5-ft perimeter and thus greater friction. Therefore, a 24  7-in duct is more nearly equivalent to the 12  12. Equivalent sizes can be determined from tables, such as those in the ASHRAE HandbookFundamentals, where rectangular ducts are rated in terms of equivalent round ducts (equal friction and capacity). Table 13.6 is a shortened

The impact insulation (or isolation) provided by floor-ceiling assemblies is usually expressed as their impact noise rating (INR) or impact insulation (IIC). Like STC, INR and IIC values are obtained by comparing the curve of the sound spectrum obtained in a test with a standard contour (except that for INR and IIC the sound pressure level is measured in the room below the noise source). The entire procedure is controversial and far from widely accepted; but its use is so widespread that designers and builders should be aware of it. (See ASTM RM 14-4.) Table 11.29 lists the impact isolation provided by several types of construction. Note particularly the enormous effect of certain floor coverings on the performance of the construction. (While IIC values are shown in Table 11.29, they can be converted to INR values by subtracting 51 points; for example, IIC 60  INR 9, and IIC 45  INR 6, etc.) TABLE 11.29 IIC of Various Floor Constructions Construction IIC Oak flooring on 1/2-in plywood subfloor, 2
10 joists, 1/2-in gypsum board

In most buildings, the superstructure structural system consists of floor and roof decks, horizontal members that support them, and vertical members that support the other components. The horizontal members are generally known as beams, but they also are called by different names in specific applications. For example: Joists are closely spaced to carry light loads. Stringers support stairs. Headers support structural members around openings in floors, roofs, and walls. Purlins are placed horizontally to carry level roof decks. Rafters are placed on an incline to carry sloping roof decks. Girts are light horizontal members that span between columns to support walls. Lintels are light horizontal beams that support walls at floor levels in multistory buildings or that carry the part of walls above openings for doors and windows. Girders may be heavily loaded beams or horizontal members that support other beams (Fig. 1.3). Spandrels carry exterior walls and support edges of floors and roofs in multistory