b'THE OFFICIAL NFBA MAGAZINEKNEE BRACES IN POST-FRAME BUILDINGS WITH DIAPHRAGM-ACTION DESIGNRESEARCH & TECHNOLOGYDimitry A. Reznik, P.E. 1 Kifle G. Gebremedhin, Ph.D. 21 Timber Tech Engineering, Inc., 22 Denver Road, Suite B, Denver PA 175172 International Professor Emeritus, Biological and Environmental Engineering, Cornell UniversityIntroduction all three-slip joints is time N ot too long ago, installing knee braces betweenconsuming and may not be postsandtrusseswasastandardpractice.necessary.The purpose of a Buildingdesignersappliedlateralloadstoknee brace is to modify the structural framing without consideration of diaphragmpost-to-trussconnection action and the wood frame was designed to resist alltobehaveasamoment-resistingjoint.Moment lateral loads.In 1983, Hoagland and Bundy publishedis a rotational force, and an article on diaphragm design procedures for post framerotation is associated with buildings.Subsequently,in1986,Gebremedhinanda circle.The relationship of Woeste published an article on the effects of diaphragmthe slip-joints can be betterFigure 1: action on a post-frame building with knee-braces withunderstood by comparingKnee Brace with SlipRing Analogandwithoutfastenerslippageatknee-bracejoints.a knee-brace triangle to a Fast forward to today, design with diaphragm actioncircular ring where three short segments, representing provided by light gauge corrugated metal panels, orthe joints, are cut out and replaced by internal springs or structural wood panels, has become standard practice.members with significantly smaller axial stiffness (Figure 1). In some cases, diaphragm action is the only mechanismThe ring analogy helps to recognize that there is only one providing lateral stability and resisting lateral loads. Theload path to transfer rotational forces (moment) between diaphragm is the main component for resisting lateral forces.the rafter (truss) and the post.This can be validated by However, builders or end users sometimes install kneeobservation: if the ring in Figure 1 is the only mechanism braces even when such braces are not specified in the designfor transferring moments between the rafter and the column documents.This raises several questions. Do knee braces(assuming that individual joints do not have moment-make a post-frame building stronger or more efficient?Theresisting capabilities), and the continuity of the ring is objective of this study is to model the effects of knee braces,severed at any of the three springs or at any other location with and without fastener slippage, on a post-frame buildingalong the circumference of the ring, the ability of the post-including diaphragm action using modern design tools. to-rafter connection to transfer moments is lost entirely (no alternative load path).Multiple springs arranged along a Modeling knee braces with fastener slippage single load path behave independently of each other and haveKnee brace forms a closed loop in the shape of a triangle.an equivalent spring constant calculated using Equation 1. Fastener slippage occurs at every one of the three joints ink eq= (1/k 1+ 1/k 2+ 1/k 3 )-1[1]the triangle, each independently contributing to the overallwhere, k eq = equivalent stiffness of the knee brace, reduction of stiffness in the knee brace frame.Modelingk 1 ,k 2 andk 3 arestiffnessofJoints1,2and3.16 / FRAME BUILDER - MAY2023'