Laterally supported top only Laterally-supported top and bottom Height of wall between supports (m) Chisel Stone Chisel Stone Shadow Stone Shadow Stone Lava Stone Lava Stone Length of wall between supports (m) 0 1 2 3 4 5 6 7 8 9 Chart 7 Wall is laterally supported from both ends and the top Height of wall between supports (m) Length of wall between supports (m) Laterally supported both ends and top Chisel Stone Chisel Stone Shadow Stone Shadow Stone Lava Stone Lava Stone Laterally-supported both ends and top Design Considerations Brick Technical Manual 73 Chart 8 Wall is laterally supported from both ends only Height of wall between supports (m) Length of wall between supports (m) Laterally supported both sides, top free Chisel Stone 76Chisel Stone 162Shadow Stone76 Shadow Stone162 Lava Stone76 Lava Stone162 Laterally-supported both ends, top free Chart 9 Wall is laterally supported from one end and the top

Where a solid masonry wall 12 in or more thick must be increased in thickness above a specific level, the increase should be achieved gradually by corbeling. In this method, successive courses are projected from the face of the wall, as indicated in Fig. 11.3f. The maximum corbeled horizontal projection beyond the face of a wall should not exceed one-third the wall thickness for walls supporting structural members. In any case, projection of any course of masonry should not exceed 1 in. Chimneys generally may not be corbeled more than 6 in from the face of the wall. In the second story of two-story dwellings, however, corbeling of chimneys on the exterior of enclosing walls may equal the wall thickness. 11.3.6 Openings, Chases, and Recesses in Masonry Walls Masonry above openings should be supported by arches or lintels of metal or reinforced masonry, which should bear on the wall at each end at least 4 in. Stone or other nonreinforced masonry lintels should not be used unless supplemented on the inside of the wall with structural steel lintels, suitable masonry arches, or reinforced- masonry lintels carrying the masonry backing. Lintels should be stiff enough to carry the superimposed load with a deflection of less than 1/720 of the

13.4.5 Mechanical Ventilation Mechanical ventilation is almost always preferred over natural ventilation because of reliability and the ability to maintain specific design requirements, such as air changes per hour and face velocities for exhaust hoods. Natural ventilation permits wide variations in ventilation-air quantities and uncertain durations of ventilation. (In critical areas, such as in carcinogenic research laboratories, natural ventilation is never relied upon.) For this reason, mechanical ventilation systems are almost always used where ventilation requirements are critical and must be highly reliable. Mechanical ventilation is often required by various building codes for various applications as follows: 1. Control of contaminants in the work area for health protection and compliance with OSHA standards for achieving the legal limits set on employee exposure to specific toxic and hazardous substances 2. Fire and explosion prevention for flammable vapors 3. Environmental protection 4. Reuse of valuable industrial materials 5. Human comfortremoval of heat, odors, and tobacco smoke 6. Humidity control 7. Corrosive fumes and noxious gases Mechanical ventilation may be a single system without heating, cooling, filtration, humidification, dehumidification, etc., or it may include various combinations of these functions. In other words, the systems can be heating-ventilating units or heatingventilatingair-conditioning (HVAC) units. In many complex and specialized buildings, certain functional areas will be required to have various degrees of positive pressurization, negative pressurization, or balanced atmospheric conditions. The ventilation air as a part of the system supply air is used to provide the positive and balanced pressures. An exhaust system is utilized to maintain the negative-pressure areas. In many designs, air lost by pressurization is exfiltrated from the system and does not become part of the returnair