47.5 MB - The Whole Building Design Guide

schools are required to be designed with an importance factor of1.15, regardless of occupant load). Other facilities where occupantload controls the importance factor include certain healthcare facilities, such as nursing homes with 50 or more residents,for which a factor of 1.15 is required. For nursing homes with lessthan 50 residents, a factor of 1 can be used. Some critical facilitiesare not specifically addressed in ASCE 7. For example, variousbuildings on a hospital campus, such as medical office buildingsthat are integrally connected to the hospital and various types ofnon-emergency treatment facilities (such as storage, cancer treatment,physical therapy, and dialysis) are not specifically requiredby ASCE 7 to be designed with a 1.15 factor. This manual recommendsa value of 1.15 for all critical facilities.Wind directionality factor: The ASCE 7 wind load calculation procedureincorporates a wind directionality factor (k d). Thedirectionality factor accounts for the reduced probability of maximumwinds coming from any given direction. By applying theprescribed value of 0.85, the loads are reduced by 15 percent. Becausehurricane winds can come from any direction, and becauseof the historically poor performance of building envelopes androoftop equipment, this manual recommends a more conservativeapproach for critical facilities in hurricane-prone regions. A directionalityfactor of 1.0 is recommended for the building envelopeand rooftop equipment (a load increase over what is required byASCE 7). For the MWFRS, a directionality factor of 0.85 is recommended(hence, no change for MWFRS).Step 2: Determine Load ResistanceWhen using allowable stress design, after loads have been determined,it is necessary to select a reasonable safety factor inorder to determine the minimum required load resistance.For building envelope systems, a minimum safety factor of 2 isrecommended. For anchoring exterior-mounted mechanical,electrical, and communications equipment (such as satellitedishes), a minimum safety factor of 3 is recommended. Whenusing strength design, load combinations and load factors specifiedin ASCE 7 are used.ASCE 7 provides criteria for combining wind loads with other typesof loads (such as dead and flood loads) using allowable stress design.MAKING CRITICAL FACILITIES SAFE FROM High Wind3-35