• Technical Blogs
  • Apr 12, 2018

Which Diameter Do I Enter into the Yield Model Equations?

MTC Solutions

The dowel bearing resistance and dowel moment resistance of laterally loaded fasteners is a function of the diameter (parameter “D”). The Yield Model Equations are modeled on smooth dowels like nails and bolts which have a consistent diameter along the embedded lengths. Self-tapping screws, on the other hand, can have up to 3 diameters.

Depending on fastener size and connection geometry, different dowel bearing areas (D⋅L) and plastic section moduli (D3/6) can characterize connection dowel bearing resistance and dowel moment resistance in different locations along the fastener. For example, a connection made with partially threaded screws with the threads embedded only in the main member can have Ds characterizing dowel bearing resistance and moment resistance in the side member, while Dm characterizes bearing resistance and moment resistance in the main member.

Tabulated reference design values for threaded fasteners in the NDS are based on the minor diameter for simplicity and conservatism. According to Technical Report 12 (AWC, 2015) the smooth shank diameter can be used where the smooth shank penetrates sufficiently far into the main member such that moment resistance is developed in the smooth shank, some distance away from the shear plane. Detailed analysis may follow procedures in Technical Report 12, but for the sake of simplicity, NDS 2018 instructs that the smooth shank diameter for threaded full-body fasteners (such as lag screws) may be used when it penetrates at least ¾ of Lm (Cl. 12.3.7.2).

In CSA O86, standard practice is to use the smooth shank fastener diameter as parameter df for lag screws (at present, CCMC 13677-R specifies that lateral resistance of Assy screws is to be calculated in accordance with Cl. 12.6.6). In EC5, if the smooth shank penetrates at least 4D into the main member, Ds may be used for laterally loaded conventional wood screws [Cl. 8.7.1 (2)]. Otherwise, the additional bearing resistance and moment resistance provided by the thread wings of laterally loaded wood screws is recognized through the use of an effective diameter, 1.1⋅Dm [Cl 8.7.2 (3)].

European Technical Approvals (ETAs) for self-tapping screws such as ETA- 11/0190 generally specify that the outside thread diameter (D) is to be used to calculated dowel bearing resistance. Specified moment resistances for self-tapping screws in Europe are based exclusively on bending tests in the threaded section and are not expressed as a function of the plastic section modulus or assumed bending yield stresses.

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