Modeling Downstand Beam in Timber Structures 2: Shear Coupling

Tips & Tricks

In order to represent the stiffness of the entire structure correctly, it is possible to consider shear coupling between the ceiling and the downstand beam by using the line release. In this way, you can define a spring constant, and thus avoid the replacement system by using coupling members. The spring constant results from the shift modulus of the fastener, which can be determined according to EN 1995‑1‑1 or ANSI/AWC NDS, for example.

In this example, there are hexagonal head wood screws with a diameter of 12 mm (1/2 in) used in a distance of 60 cm (2 ft). For softwood C24 according to Eurocode 5, this gives a spring constant of 7,498 kN/m².

To comply with ANSI/AWC NDS, it is necessary to multiply the determined shift modulus by the factor of 0.5 according to Chapter 11.3.6, since the shift modulus is defined by the standard in consideration of two shear planes. Thus, the result in accordance with NDS is a spring constant of 190,919 lbf / ft. The figure shows a schematic distribution of bending and shear stress for ‘solid’, ‘semi‑rigid’ and ‘loose’ coupling arranged from top to bottom.

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