47.5 MB - The Whole Building Design Guide

In the past, design professionals seldomperformed load calculations on thebuilding envelope (i.e., roof and wallcoverings, doors, windows, and skylights)and rooftop equipment. These buildingcomponents are the ones that have failedthe most during past wind events. Inlarge part they failed because of thelack of proper load determination andinappropriate design of these elements.It is imperative that design professionalsdetermine the loads for the buildingenvelope and rooftop equipment, anddesign them to accommodate such loads.Providing at least two means of site egress isprudent for all critical facilities, but is particularlyimportant for facilities in hurricane-proneregions. If one route becomes blocked by treesor other debris, or by floodwaters, the other accessroute may still be available.3.3.1.2 Building DesignGood wind performance depends on gooddesign (including details and specifications),materials, installation, maintenance,and repair. A significant shortcoming inany of these five elements could jeopardizethe performance of a critical facility againstwind. Design, however, is the key element toachieving good performance of a building against wind damage.Design inadequacies frequently cannot be compensated for withother elements. Good design, however, can compensate for otherinadequacies to some extent. The following steps should be includedin the design process for critical facilities.Step 1: Calculate LoadsUplift loads on roof assemblies can alsobe determined from FM Global (FMG)Data Sheets. If the critical facility is FMGinsured, and the FMG-derived loads arehigher than those derived from ASCE 7 orthe building code, the FMG loads shouldgovern. However, if the ASCE 7 or codederivedloads are higher than those fromFMG, the ASCE 7 or code-derived loadsshould govern (whichever procedureresults in the highest loads).Calculate loads on the MWFRS, the building envelope, androoftop equipment in accordance with ASCE 7 or the localbuilding code, whichever procedure results in the highest loads.In calculating wind loads, design professionalsshould consider the following items.Importance factor: The effect of using a 1.15importance factor versus 1 is that the designloads for the MWFRS and C&C are increasedby 15 percent. The importance factor formost critical facilities is required to be 1.15.However, ASCE 7 permits a factor of 1 forsome critical facilities. For example, schoolswith an occupant load of less than 250people are permitted to be designed with animportance factor of only 1 (provided theyare not used as a shelter—if used as a shelter,3-34 MAKING CRITICAL FACILITIES SAFE FROM High Wind