2012 ASD/LRFD Manual for Engineered Wood Construction

ASD/LRFD MANUAL FOR ENGINEERED WOOD CONSTRUCTION41A third method uses vertically oriented short studs,often called squash blocks or cripple blocks, on each sideof the joist and cut to a length slightly longer than the depthof the joist. This method should be used in combinationwith some type of rim joist or blocking material whenlateral stability or diaphragm transfer is required.The use of horizontally oriented sawn lumber as ablocking material is unacceptable. Wood I-joists generallydo not shrink in the vertical direction due to their paneltype web, creating the potential for a mismatch in heightas sawn lumber shrinks to achieve equilibrium. Whenconventional lumber is used in the vertical orientation,shrinkage problems are not a problem because changes inelongation due to moisture changes are minimal. FigureM7.4-11 shows a few common methods for developingvertical load transfer.Figure M7.4-11 Details for Vertical Load Transfer7Web HolesHoles cut in the web area of a wood I-joist affect themember’s shear capacity. Usually, the larger the hole, thegreater the reduction in shear capacity. For this reason,holes are generally located in areas where shear stressesare low. This explains why the largest holes are generallypermitted near mid-span of a member. The requiredspacing between holes and from the end of the member isdependent upon the specific materials and processes usedduring manufacturing.The allowable shear capacity of a wood I-joist ata hole location is influenced by a number of variables.These include: percentage of web removed, proximity toa vertical joint between web segments, the strength of theweb to flange glue joint, the stiffness of the flange, andthe shear strength of the web material. Since wood I-joistsare manufactured using different processes and materials,each manufacturer should be consulted for the proper webhole design.The methodology used to analyze application loads isimportant in the evaluation of web holes. All load casesthat will develop the highest shear at the hole locationshould be considered. Usually, for members resistingsimple uniform design loads, the loading condition thatdevelops the highest shear loads in the center area of ajoist span involves partial span loading.Web holes contribute somewhat to increased deflection.The larger the hole the larger the contribution.Provided not too many holes are involved, the contributionis negligible. In most cases, if the manufacturer’s holecriteria are followed and the number of holes is limited tothree or less per span, the additional deflection does notwarrant consideration.M7: PREFABRICATED WOOD I-JOISTSAMERICAN WOOD COUNCIL