1996 LRFD Manual for Engineered Wood Construction

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GUIDELINELRFD MANUALTO LRFDFORFORENGINEEREDSTRUCTURALWOODCOMPOSITECONSTRUCTIONLUMBER 3Specification for Computing the Reference Resistance ofWood-Based Materials and Structural Connections forLoad and Resistance Factor Design, are consistent withother standards in their use of reliability concepts only inthe background calculations. The design equations includethe end result of these calculations in the load factorsand the resistance factors.Nearly all of today’s (i.e., ASD) standard design formulaeand adjustment factors are directly applicable foruse in LRFD, leading to maximum consistency betweenthe familiar design concepts of Allowable Stress Designand the new LRFD procedures.Basic LRFD EquationsThe basic design equation for LRFD, as with all engineeringsafety checking equations, requires that thespecified product strength or resistance meet or exceedthe stress or other effect imposed by the specified loads.In ASD, the permissible stress levels are set very low andthe load magnitudes are set at once in a lifetime levels.This combination produces designs that maintain highsafety levels yet remain economically feasible. In LRFDthe basic design equation follows a similar format, in whichthe factored resistance must be greater than or equal to thefactored load effects.From a user’s standpoint, the design process is similarto ASD. The most obvious difference between LRFDand ASD is that both the resistance and load effect valuesin LRFD will be numerically much higher than in ASD.The resistance values are higher because they are verynear test magnitudes rather than being reduced by a significantinternal safety factor.The load effects are higher because they are multipliedby load factors in the range of 1.2 to 1.6.1.1.2 Load Combinations and LoadFactorsThe load combination equations for use with LRFDare given in AF&PA/ASCE 16-95 Sec. 1.3.2:1.4 D (1.3-1)1.2 D + 1.6 L + 0.5 (L ror S or R) (1.3-2)1.2 D + 1.6 (L ror S or R) + (0.5 L or 0.8 W) (1.3-3)1.2 D + 1.3 W + 0.5 L + 0.5 (L ror S or R) (1.3-4)1.2 D + 1.0 E + 0.5 L + 0.2 S (1.3-5)0.9 D - (1.3 W or 1.0 E) (1.3-6)Refer to AF&PA/ASCE 16-95 and its commentary foradditional details about application of these equations.The load factors in these equations are intended toprovide a consistent level of reliability across a range ofratios of the various load types.1.1.3 Resistance FactorsTo provide additional flexibility in achieving consistentreliability across a range of product applications,resistance factors are applied to the reference resistancevalues. Resistance factors (N) are always less than unity.The magnitude of a resistance factor represents the relativereduction required to achieve comparable reliabilitylevels.AF&PA/ASCE 16-95 provides the following resistancefactors for wood-based products and connections:Compression φ c= 0.90Flexure φ b= 0.85Stability φ s= 0.85Tension φ t= 0.80Shear/Torsion φ v= 0.75Connections φ z= 0.65These factors provide roughly equivalent reliabilityamong different stress modes for a given product type.1.1.4 Time Effect FactorsThe time effect factor (8) is the LRFD-equivalent ofthe load duration factor in Allowable Stress Design. Timeeffect factors are tabulated in Table 1.4-2 for each loadcombination equation. The factors were derived based onreliability analysis that considered variability in strengthproperties, stochastic load process modeling and cumulativedamage effects. Because reference strengths are basedon short-term test values, time effect factors equal unityfor load combinations in which no cumulative damageoccurs. Time effect factors range in value from 1.25 for aload combination controlled by impact loading to 0.6 fora load combination controlled by permanent dead load.Examination of Table 1.4-2 from AF&PA/ASCE 16-95reveals that common building applications will likely bedesigned for time effect factors of 0.80 for gravity loaddesign (AF&PA/ASCE 16-95 Eq. 1.3-2 under occupancyfloor load and 1.3-3) and 1.0 for lateral load design(AF&PA/ASCE 16-95 Eq. 1.3-4, 1.3-5 and 1.3-6).1.1.5 Reference ConditionsReference conditions have been defined such that amajority of wood products used in interior or in protected1INTRODUCTIONAMERICAN FOREST & PAPER ASSOCIATION
4 INTRODUCTIONenvironments will require no adjustment for moisture, temperatureor treatment effects.Moisture reference conditions are identical to thosein Allowable Stress Design. These include moisture contents19% or less for sawn lumber products. Theequivalent limit for glued products (glulam, structural compositelumber, I-joists, panel products) is defined as 16%MC or less.Temperature reference conditions are also identicalto those in Allowable Stress Design. These include sustainedtemperatures up to 100 o F. Note that it has beentraditionally assumed that these reference conditions alsoinclude common building applications in desert locationswhere daytime temperatures will often exceed 100 o F.Examples of applications that may exceed the referencetemperature range include food processing or other industrialbuildings.1.2 Design ResponsibilitiesStructural wood products are provided to serve a widerange of end uses. Some products are marketed throughcommodity channels where the products meet specificstandards and the selection of the appropriate product isthe responsibility of the user.Other products are custom manufactured to meet thespecific needs of a given project. Products that often fallinto this category are prefabricated trusses and customglulam members. Design of the individual members isbased on criteria specified by the architect or engineer ofrecord on the project. Manufacture of these products isperformed in accordance with the product’s manufacturingstandards. Engineering of these products normallyonly extends to the design of the products themselves.Construction-related issues such as load path analysis anderection bracing remain the responsibility of the professionalof record for the project.1.2.1 BracingDesign considerations related to both temporary andpermanent bracing differ among product types. Specificdiscussion of bracing is included in the product supplementor guideline.1.3 Other Design ConsiderationsMuch of this manual focuses on building design fromthe perspective of structural engineering design. Whiledesign against structural collapse remains the primarypurpose of structural design, engineers must also considerhow their design will perform from the perspective of serviceability,durability and fire safety. While each of thesetopics could easily fill a book themselves, this section providesan introduction to each topic and some brief guidancefor the designer.1.3.1 ServiceabilityIn addition to designing buildings for strength limitstates, designers must determine whether any special serviceabilitylimit states must be considered for a givenapplication. The most common serviceability limit usedin the design of typical wood-framed buildings is a limitationon the deflection of roof or floor members. Thebuilding codes have traditionally defined these limits as aratio of the member span. For example, limits of R/360computed under live load or R/240 under total load arecommon for floors.While the traditional static deflection limits were originallyintended to limit cracking of brittle finish materials,they have served equally well in short span applicationsto limit vibration problems. As engineered wood productshave evolved to span longer distances with lighterweight members, it has become increasingly common formanufacturers to recommend more stringent deflectioncriteria. The user is directed to the product supplementsand guidelines for additional information on specific products.For applications that might be particularly sensitiveto vibration considerations, the user is directed to AF&PA/ASCE 16-95 Chapter 10 Commentary.1.3.2 Designing for PermanenceWood has a feature that is unique among constructionmaterials. It retains its structural integrity indefinitelywhen maintained in a dry condition, and becomes com-AMERICAN WOOD COUNCIL
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1996 LRFD Manual for Engineered Wood Construction

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