Curing the adhesive at room temperature has many advantages. Since wood is an excellent insulator, a long time is required for elevated ambient temperatures to reach inner glue lines of a large assembly. With room-temperature curing, equipment needed to heat the glue line is not required, and the possibility of injury to the wood from high temperature is avoided. 10.30.3 Prefabricated Wood I Joists Flanges of prefabricated wood I-shaped joists are either sawn lumber, visually graded or machine stress rated, or some type of structural composite lumber product, such as laminated veneer lumber (LVL) (Art. 10.30.4). The web members can be either plywood or oriented strand board (OSB) although OSB is most often used (Art. 10.12). Numerous manufacturers produce wood I joists, and the flange and web materials used depend on manufacturer preference. Inasmuch as these joists are proprietary products, the manufacturer provides design information, usually in the form of load/span tables, as well as installation and handling guidelines. Manufacturers establish design values for these products in accordance with the provisions of the Specification for Establishing and Monitoring Structural Capacities of Prefabricated Wood I-Joists, ASTM D5055. Each manufacturer uses the design values thus determined to obtain National Evaluation Service (NER) building- code approval from model code sponsors and to generate proprietary load/span tables. To establish consistent performance levels for I-joists, APAThe Engineered Wood Association promulgated APA Standard PRI-400 for I-joists used in residential floor construction. This standard has been recognized by all of the model code

Gas-fired unit heaters are often installed in kitchens and other premises where large quantities of air may be exhausted. When a makeup air system is not provided, and the relief air must infiltrate through doors, windows, etc., a negative pressure FIGURE 13.19 Pipe embedded in a concrete slab for radiant heating. FIGURE 13.20 Pipe embedded in a plaster ceiling for radiant heating. must result in the premises. This negative pressure will cause a steady downdraft through the flue pipe from the outdoors into the space and prevent proper removal of the products of combustion. In such installations, it is advisable to place the unit heater in an adjoining space from which air is not exhausted in large quantities and deliver the warm air through ducts. Since propeller fans on most unit heaters cannot take much external duct resistance, centrifugal blower unit heaters may give better performance where ductwork is used. Sizes of gas piping and burning rates for gas can be obtained from charts and tables in the ASHRAE Handbook Fundamentals for various capacities and efficiencies. The efficiency of most gas-fired heating equipment is between 70 and 80%. Radiant heating, or panel heating as it is sometimes referred to, consists of a warm pipe coil embedded in the floor, ceiling, or walls. The most common arrangement is to circulate warm water through the pipe under the floor. Some installations with warm-air ducts, steam pipes, or electric-heating elements have been installed. Warm-air ducts for radiant heating are not very common. A modified system normally called the perimeter warm-air heating system circulates the warm air around the perimeter of the structure before discharging the air into the premises

Chief Electrical Engineer STV Incorporated Douglassville, Pennsylvania Design of the electrical installations in a building used to be simple and straightforward. Such installations generally included electrical service from a utility company; power distribution within the building for receptacles, air conditioning, and other electrical loads; lighting; and a few specialty systems, such as fire alarm and telephone. There were, of course, some specialized installations for which this simple description did not apply, but such buildings were uncommon. Now, however, design of electrical systems has become more complex and sophisticated. This development has been driven by rapid advances in technology, availability of computers and computerized equipment, more enlightened life-safety and security concerns, and changes in the philosophical outlook of workers toward their workplace and their need for a comfortable environment. To meet these needs, a new building will likely include in its electrical installation an access control system, intrusion detection system, an extensive computer data network, Internet access, uninterruptible power supply, and numerous other systems not commonly installed in the past. Corollary to the advent of these new building systems is the need for suitable power quality to support them. Though highly sophisticated and capable, these systems can easily be disrupted or damaged by power system anomalies such as sags, surges, noise, and power outages. Electrical design elements to protect against these disturbances must be included and must be designed to be appropriately sensitive, fast, and robust. The introduction of electrical competition in some states adds further complexity to the electrical system design problem. Not only have these systems become common but the basic electrical systems have undergone drastic changes. Advances in electrical-power-distribution materials and methods, which have occurred at a nearly uniform rate since the turn of the century, have accelerated rapidly under the influence of computers and microprocessor controls. New light sources give designers added opportunities to improve lighting and energy efficiency. Microprocessor-based fire-alarm systems with addressable devices offer greatly improved protection, flexibility, and economy. And establishment of more local telecommunication operating companies and competition between them, encouraging innovation, has brought designers new choices and challenges with respect to telecommunication systems for buildings. Nevertheless, the basic principles of electrical design still apply, and they are described in this section. In addition, the section was developed to be helpful to those who must assume responsibility for applying, coordinating, integrating, and installing the many electrical systems now available for buildings. In many ways, transmission of electricity in buildings is analogous to water-supply distribution. Water flows through pipes, electricity through wires or other conductors. Voltage is equivalent to pressure; wire resistance, to pipe friction; and electric current, or flow of electrons, to water droplets. The hydraulic analogy is limited to only very elementary applications with electric flow like direct current, which always flows in the same direction. The analogy does not hold for alternating current, which reverses flow many times per second without apparent inertia drag. Direct-current systems are simple two-wire circuits, whereas alternating current uses two, three, or four wires and the formulas are more complex. Any attempt to apply the hydraulic analogy to alternating currents would be more confusing than helpful. The mathematical concepts are the only guides that remain true over the whole area of application. Ampere (abbreviated A) is the basic unit for measuring flow of current. The unit flowing is an electric charge called a coulomb. An ampere is equivalent to a flow of one coulomb per second. One source of direct current is the battery, which converts chemical energy into electric energy. By convention, direct current flows from the positive terminal to the negative terminal when a conductor is connected between the terminals. The voltage between battery terminals depends on the number of cells in the battery. For a lead-plate-sulfuric-acid battery, this voltage is about 1.5 to 2 V per cell. For high voltages, a generator is required. A generator is a machine for converting mechanical energy into electrical energy. The basic principle involved is illustrated by the simple experiment of moving a copper wire across the magnetic field between a north pole and south pole of a magnet. In a generator, the rotor is wound with coils of wire and the magnets are placed around the stator in pairs, two, four, six, and eight. When the coil on the rotor passes through the magnetic field under a south pole, current flows in one direction. When the same coil passes through the north-pole field, the current reverses. For this reason, all generators produce alternating current. If direct current is required, the coils are connected to contacts on the rotor, which transfer the current to brushes arranged to pick up the current flowing in one direction only. The contacts and brushes comprise the commutator. If the commutator is omitted, the generator is an alternator, producing alternating current. See also Conversion of AC to DC in Art. 15.3. Resistance of flow through a wire, measured in units called ohms (
), depends on the wire material. Metals like copper and aluminum have low resistance and are classified as conductors. FIGURE 15.1 Types of electric circuits: (a) series; (b) parallel. Resistance for a given material varies inversely as the area of the cross section and directly as the length of wire. Ohms law states that the voltage E (volts) required to cause a flow of current I (amperes) through a wire with resistance R (ohms) is given by Power P is measured in watts and is the product of volts and amperes: P
EI
(IR)I
I 2R (15.2a)