1996 LRFD Manual for Engineered Wood Construction

GUIDELINELRFD MANUALTO LRFDFORFORENGINEEREDSTRUCTURALWOODCOMPOSITECONSTRUCTIONLUMBER33333.3 Special Considerations3.3.1 Net Section CalculationDesign of tension members is often controlled by theability to provide connections to develop tensile forceswithin the member. In the area of connections, one mustdesign not only the connection itself (described in detailin Chapter 7) but also the transfer of force across the netsection of the member.3.3.2 Radial Tension in CurvedMembersStresses induced in curved members under load includea component of stress in the direction of the radiusof curvature. This stress is traditionally called radial tension.Radial tension is a specialized design considerationthat is covered in AF&PA/ASCE 16-95 App. A2 and isexplained in detail in the American Institute of TimberConstruction (AITC) Timber Construction Manual.3.4 Checklist: Using Tension Member Selection TablesTension member selection tables provide values for factored tension resistances (λφ t T′) for common gradesand sizes of tension members. Tabulated values apply to tension members that satisfy the following conditions:3TENSION MEMBERS√ “dry” service condition (C M = 1.0)√ “normal” temperature range (C t = 1.0)√ untreated material (C pt = 1.0 ; C rt = 1.0)√ time effect factor based on “live” (L or L r ) or “snow” (S) load combination (λ = 0.80)For members that do not satisfy all of these conditions, review the design equations in this chapter andmodify tabulated values as necessary.To compute the factored resistance for a specific condition, apply the design equations directly (productspecificdesign adjustment factors and reference resistance values are provided in each supplement).3.5 Design ExamplesExample 3-1: Truss Bottom ChordDesign the bottom chord of a sawn lumber commercial/industrialtruss to support a factored tensile force (T u )of 36.0 kips. Assume a dry moisture service condition,untreated material and a time effect factor of 0.80.Practical ConsiderationsEfficient choice of a trial section requires practical,as well as engineering, considerations. For example,choice of lumber species, grade and even commonly avail-able sizes may differ among geographic regions of thecountry. Consult your local supplier for assistance. Inaddition, other considerations include dimensional compatibilitywith the other members of the truss or minimumsizes required to adequately connect the truss members(while meeting fastener edge distance requirements).Engineering CalculationsUsing Selection Tables: Select a member from thetension member selection tables in the Structural LumberSupplement that is adequate to resist 36.0 kips factoredtensile force (T u ).AMERICAN FOREST & PAPER ASSOCIATION