Where 11/8-in roof panels are desired, such as for heavy timber construction, shear values for 19/32-in panels are used. Blocked shear values for 11/8-in panels may be obtained by specifying stapled tongue-and-groove (T&G) edges. Staples should be 16 ga, 1 in long, with a 3/8-in crown. They should be driven through the T&G edges 3/8-in from the joint so as to penetrate the tongue. Staples should be spaced at one-half of the boundary nail spacing for Cases 1 and 2, and at one-third the boundary nail spacing for Case 3 through 6, as illustrated in Table 10.27, which gives panel and fastening recommendations for roof diaphragms. Panels and framing are assumed already designed for perpendicular loads. To design a diaphragm, follow these steps: 1. Determine lateral loads and resulting shears. 2. Determine the nailing schedule (Table 10.27). Consider the load direction with respect to joints. 3. Compute the chord stress due to bending moment. Provide adequate splices. Check deflections. Check the anchorage of boundary framing, for example, to

Building designers also should use judgment in determining. the degree of protection to be provided against specific hazards. Costs of protection should be commensurate with probable losses from an incident. In many cases, for example, it is uneconomical to construct a building that will be immune to extreme earthquakes, high winds of tornadoes, arson, bombs, burst dams, or professional burglars. Full protection, however, should always be provided against hazards with a high probability of occurrence accompanied by personal injuries or high property losses. Such hazards include hurricanes and gales, fire, and vandals. Structures containing extremely valuable contents or critical equipment justifying design for even the most extreme events may require special hardened rooms or areas. 3.1.1 Design Life of Buildings For natural phenomena, design criteria may be based on the probability of occurrence of extreme conditions, as determined from statistical studies of events in specific localities. These probabilities are often expressed as mean recurrence intervals. A mean recurrence interval of an extreme condition is the average time, in years, between occurrences of a condition equal to or worse than the specified extreme condition. For example, the mean recurrence interval of a wind of 60 mi/ hr or more may be recorded for Los Angeles as 50 years. Thus, after a building has been erected in Los Angeles, chances are that in the next 50 years it will be subjected only once to a wind of 60 mi/hr or more. Consequently, if the building was assumed to have a 50-year life, designers might logically design it basically for a 60-mi/hr wind, with a safety factor included in the design to protect against low-probability faster winds. Mean recurrence intervals are the basis for minimum design loads for high winds, snowfall, and earthquake in many building codes. 3.1.2 Safety Factors Design of buildings for both normal and emergency conditions should always incorporate a safety factor against failure. The magnitude of the safety factor should be selected in accordance with the importance of a building, the extent of personal injury or property loss that may result if a failure occurs, and the degree of uncertainty as to the magnitude or nature of loads and the properties and behavior of building components. As usually incorporated in building codes, a safety factor for quantifiable system variables is a number greater than unity. The factor may be applied in either of two

thickness, in Nailing* Size Spacing, in Panel edge Intermediate 5/16 to 1 8d 6 12 11/8 8d or 10d 6 12 * In general, use common smooth or deformed-shank nails for panels up to 1 in thick. For 11/8-in-thick panels, use 8d ring- or screw-shank or 10d common smooth-shank nails. Other approved fasteners, however, may be used. For stapling asphalt shingles to panels 5/16 in or more thick, use staples with a 15/16-in minimum crown width and a 1-in-long leg. Space the staples in accordance with the recommendations of the shingle manufacturer. For spans of 48 in or more, space the nails 6 in c to c at all supports. Deflection limits are 1/180 of the span for total load; 1/240 for live load only. Nailing requirements for preframed panels are the same as for roof sheathing. See APA Design/Construction GuideResidential and Commercial, APAThe Engineered Wood Association, for recommended maximum roof loads. 10.12.13 Panel Diaphragms With only slight design modifications, any panel roof-deck system described in Arts. 10.12.11 and 10.12.12 will also function as an engineered roof or floor diaphragm to resist wind and seismic loading. The ability of a diaphragm to function effectively as a deep beam, transferring lateral loads to shear walls, is related to the quality of the connections. Nailing is critical, since shears are transmitted through these fasteners. Common nails provide required strength. Other nail types may be used when their lateral bearing values are considered in the design. Load-carrying capacity is highest when the diaphragm