47.5 MB - The Whole Building Design Guide

When using allowable stress design, asafety factor is applied to account forreasonable variations in material strengths,construction workmanship, and conditionswhen the actual wind speed somewhatexceeds the design wind speed. Fordesign purposes, the ultimate resistancean assembly achieves in testing is reducedby the safety factor. For example, if aroof assembly resisted an uplift pressureof 100 pounds per square foot (psf), afterapplying a safety factor of 2, the assemblywould be suitable where the design loadwas 50 psf or less. Conversely, if thedesign load is known, multiplying it by thesafety factor equals the minimum requiredtest pressure (e.g., 50 psf design loadmultiplied by a safety factor of 2 equals aminimum required test pressure of 100 psf).For structural members and cladding elementswhere strength design can be used,load resistance can be determined by calculations.For other elements where allowablestress design is used (such as most types ofroof coverings), load resistance is primarilyobtained from system testing.The load resistance criteria need to be providedin contract documents. For structuralelements, the designer of record typically accountsfor load resistance by indicating thematerial, size, spacing, and connection of theelements. For nonstructural elements, suchas roof coverings or windows, the load andsafety factor can be specified. In this case, thespecifications should require the contractor’ssubmittals to demonstrate that the system willmeet the load resistance criteria. This performancespecification approach is necessary if,at the time of the design, it is unknown whowill manufacture the system.Connections are a key aspect of loadpath continuity between various structuraland nonstructural building elements. In awindow, for example, the glass must bestrong enough to resist the wind pressureand must be adequately anchored to thewindow frame, the frame adequatelyanchored to the wall, the wall to thefoundation, and the foundation to theground. As loads increase, greaterload capacity must be developed in theconnections.Regardless of which approach is used, it is important that thedesigner of record ensure that it can be demonstrated, via calculationsor tests, that the structure, building envelope, andnonstructural systems (exterior-mounted mechanical, electrical,and communications equipment) have sufficientstrength to resist design wind loads.Step 3: Detailed DesignIt is vital to design, detail, and specify thestructural system, building envelope, andexterior-mounted mechanical, electrical,and communications equipment to meetthe factored design loads (based on appropriateanalytical or test methods). Itis also vital to respond to the risk assessmentcriteria discussed in Section 3.2.2, asappropriate.3-36 MAKING CRITICAL FACILITIES SAFE FROM High Wind