47.5 MB - The Whole Building Design Guide

3.2.2 Evaluating Critical Facilities forRisk from High WindsThis section describes the process of hazard risk assessment. Althoughno formal methodology for risk assessment has beenadopted, prior experience provides a sufficient knowledge baseupon which a set of guidelines can be structured into a recommendedprocedure for risk assessment of critical facilities. Theprocedures presented below establish guidelines for evaluatingthe risk to new and existing buildings from wind storms otherthan tornadoes. These evaluations will allow development of avulnerability assessment that can be used along with the site’swind regime to assess the risk to critical facilities.In the case of tornadoes, neither the IBC nor ASCE 7 requiresbuildings (including critical facilities) to be designed to resisttornado forces, nor are occupant shelters required in buildingslocated in tornado-prone regions. Constructing tornado-resistantcritical facilities is extremely expensive because of theextremely high pressures and missile impact loads that tornadoescan generate. Therefore, when consideration is voluntarilygiven to tornado design, the emphasis istypically on occupant protection, which isachieved by “hardening” portions of a criticalfacility for use as safe havens. FEMA 361includes a comprehensive risk assessmentprocedure that designers can use to assistbuilding owners in determining whether atornado shelter should be included as partof a new critical facility. See Section 3.5 forthe design of tornado shelters and otherrecommendations pertaining to critical facilitiesin tornado-prone regions.3.2.2.1 New BuildingsWhen designing new critical facilities, atwo-step procedure is recommended forevaluating the risk from wind storms (otherthan tornadoes).In this manual, the term “tornado-proneregions” refers to those areas of the UnitedStates where the number of recordedF3, F4, and F5 tornadoes per 3,700square miles is 6 or greater per year(see Figure 3-2). However, an owner ofa critical facility may decide to use otherfrequency values (e.g., 1 or greater, 16or greater, or greater than 25) in definingwhether the building is in a tornado-pronearea. In this manual, tornado shelters arerecommended for all critical facilities intornado-prone regions.Where the frequency value is 1 or greater,and the facility does not have a tornadoshelter, the best available refuge areasshould be identified, as discussed inSection 3.5.MAKING CRITICAL FACILITIES SAFE FROM High Wind3-29