<strong>LRFD</strong> MANUAL FOR ENGINEERED WOOD CONSTRUCTION11INTRODUCTION1.1 General In<strong>for</strong>mation 21.1.1 Load and Resistance FactorDesign 21.1.2 Load Combinations and LoadFactors 31.1.3 Resistance Factors 31.1.4 Time Effect Factors 31.1.5 Reference Conditions 31.2 Design Responsibilities 41.2.1 Bracing 41.3 Other Design Considerations 41.3.1 Serviceability 41.3.2 Designing <strong>for</strong> Permanence 41.3.3 Designing <strong>for</strong> Fire Safety 51.4 Products Covered in This <strong>Manual</strong> 91.4.1 Products Included 9AMERICAN FOREST & PAPER ASSOCIATION
2 INTRODUCTION1.1 General In<strong>for</strong>mationThis manual is organized as a multi-part package <strong>for</strong>maximum flexibility <strong>for</strong> the design engineer. All generaldesign in<strong>for</strong>mation, design equations, specification language,and commentary are organized by member type(tension, bending, etc.) and are included in this volume.Actual design values and design aids are packaged separatelyon a product-specific basis in a complete set ofdesign supplements and guideline documents.Each chapter in this manual follows certain conventions.Each chapter starts with a section entitled “GeneralIn<strong>for</strong>mation” that prepares the reader <strong>for</strong> the type of in<strong>for</strong>mationto be expected within that chapter. After thechapter-specific in<strong>for</strong>mation, the chapter ends with achecklist section that is used to identify reference conditions<strong>for</strong> this case, followed by one or more designexamples.The numbering of Chapters 3 through 9 correspondto chapters of the same numbers in AF&PA/ASCE 16-95,Standard <strong>for</strong> Load and Resistance Factor Design (<strong>LRFD</strong>)<strong>for</strong> <strong>Engineered</strong> <strong>Wood</strong> <strong>Construction</strong>.Chapter 10, Reference In<strong>for</strong>mation, includes not onlythe typical reference in<strong>for</strong>mation <strong>for</strong> a handbook of thistype, but also includes flowcharts of the design processesand alternatives in Chapters 3 through 9.AF&PA/ASCE 16-95 along with its Commentary isreproduced in its entirety following Chapter 10 of this<strong>Manual</strong>. This is provided <strong>for</strong> convenience to the user.The user will note that design values throughout this<strong>LRFD</strong> package are stated in ksi or kips per square inchrather than in the more familiar psi or pounds per squareinch. The reason <strong>for</strong> this departure from current units isto minimize confusion of design values between AllowableStress Design (ASD) and <strong>LRFD</strong>. The developers ofthe <strong>LRFD</strong> <strong>for</strong>mat debated other solutions, such as introducingcompletely different notation <strong>for</strong> <strong>LRFD</strong>-relateddata. However, the other solutions all proved to be muchless “user-friendly” <strong>for</strong> designers than the simple restatementof units.The user will also note that this design package definesseveral widely-used <strong>LRFD</strong> terms as follows:Resistance refers to the capacity of the member. Examplesinclude moment resistance (kip-ft), tensionresistance (kips), etc. Tabulated resistances are found inthe selection tables in the supplements, and are tabulatedas factored resistances (specific time effect factors, 8, andresistance factors, N, are included).Strength refers to the material property value -- thestrength values are the <strong>LRFD</strong>-equivalent of an allowablestress value. Example reference strengths (i.e., based onreference conditions) include bending strength (ksi), connectionlateral strength (kips), etc. Reference strengthvalues are also found in the supplements.1.1.1 Load and Resistance FactorDesignLoad and Resistance Factor Design (<strong>LRFD</strong>) hasevolved to become the preferred <strong>for</strong>mat <strong>for</strong> convertingstructural design standards to a so-called limit states approach.This section provides a brief discussion of <strong>LRFD</strong>and reassures engineers that this technique is simply analternative way of quantifying the concepts of safety factors.Although the underlying mathematics are fairlycomplex, none of these complexities are required in thedesign procedures. In fact, many of the design equationsare actually simpler to use than their Allowable StressDesign counterparts.Reliability-Based DesignTheoretical reliability-based analysis has been used<strong>for</strong> many years in the electronics and aerospace industries.In both of these industries the relative ease ofcomponent reliability assessment and reasonably low costof design redundancy made reliability-based design ahighly successful product development strategy.The extension of theoretical reliability concepts tobuilding applications has proven to be somewhat moredifficult. The primary source of this difficulty lies in therelatively uncontrolled nature of so many facets of constructedfacilities. The electronics engineer designs andbuilds a specific system out of precisely manufactured andassembled components that face well-defined bounds of“loading” over the product’s lifetime. Compare this withthe building designer who designs a facility <strong>for</strong> only theinitial occupancy type, using materials supplied by outsidemanufacturers and constructed by a broad range ofsubcontractors. The mathematics of reliability are thesame <strong>for</strong> both designers. However, many engineers believethat unknown and unknowable factors dominate theactual “in-place” reliability of a building more than thosefactors that can be quantified.With these thoughts in mind, the writers of AF&PA/ASCE 16-95 deliberately chose to develop a design procedurethat mixes some elements of theoretical reliabilitywith large quantities of engineering judgment.Specifically, AF&PA/ASCE 16-95 and its supportingASTM standards completely adopt the reliability refinementsembodied in the load factors of ASCE 7-93,Minimum Design Loads <strong>for</strong> Buildings and Other Structures.The procedures in ASTM D5457-93, StandardAMERICAN WOOD COUNCIL
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