roof framing connections in conventional residential construction

Therefore, it can be assumed that the use of the material properties at the target limit state allowsfor calculating the connection strength at the corresponding limit state. For example, the yieldequations used with the ultimate dowel bending strength and the ultimate dowel bearing strengthpredict the ultimate connection strength, i.e. connection capacity. Knowledge of capacity can becritical in certain types of construction (i.e., breakaway walls) [16] or in balancing the designstrength of various components and connections in a wood-frame assembly such that prematurefailure is circumvented and more favorable failure modes occur when ultimate strength isachieved. Determination of the 5 percent offset limit is ambiguous, in terms of structural safetyobjectives, whereas a capacity based approach provides a known reference point relative tostructural safety [17]. Moreover, capacity-based design is the most favorable method for accurateanalysis of seismic response of structures.2.1.2 Deflections of Lateral Dowel ConnectionsDesign for a capacity limit state does not address structural failure modes and serviceabilityissues associated with excessive joint deformations. For example, a joint slip in a roof assemblycan be geometrically magnified resulting in large deflections that are perceived unacceptable bythe occupants or exceed deformations tolerated by finish materials. Large deflections can alsopromote “P-delta” effects and contribute to the failure modes caused by structural instability.Therefore, another limit state should be introduced, as a part of a capacity-based designmethodology, to analyze the effect of joint deformations and to ensure adequate serviceability ofthe structure. To incorporate the deflection limit state into the design procedures, a method forpredicting the connection load-slip relationship is required as a separate design check.Modeling of the load-slip relationship for lateral dowel wood connections is a complex,nonlinear problem that involves analysis of the interaction of the body of the dowel and thesurrounding wood material. Theories that simulate nail connection as a beam on elastic orelastic-plastic foundation [11] can provide accurate predictions, but require solution of highorderdifferential equations. Wood Handbook [18] presents one such model which predicts theinitial slope of lateral dowel connections. Development of empirical equations is limited by thelarge number of combinations of variables which affect the connection performance includingjoint geometry, fastener geometry, wood specific gravity, direction of loading, fastener bendingyield strength, etc. Most of the analytical models developed to date are complicated for practicalengineering design applications and none of the models are referenced in the current designspecifications.The finite element method is another analytical tool that can be used to model the load-sliprelationship. A three-dimensional finite element model of a single bolted connection developedby Patton-Mallory [19] is an example of this approach. Although finite element analysis canprovide valuable insights into the response of wood connections, it is impractical for mostengineering design purposes due to complex and time-consuming operations involved in themodel formulation and results interpretation.Aune and Patton-Mallory [5][6] used the yield equation format to predict joint slip by assuminga forth-root curve relationship between wood embedment stress and joint slip. If this relationshipis used in place of the constant dowel bearing strength value, the yield equations can be solved todetermine the joint slip as a function of the lateral load. Analytical predictions were in good4