Investigation of occupant induced dynamic lateral loading on exterior decks

James M. LaFave

Lateral loads on exterior decks caused by occupant movement can exceed those from extreme wind and seismic events. Occupant-induced dynamic loading is a function of the initial traction load, excitation frequency, and the stiffness and geometry of the deck system. A finite element modeling (FEM) modal analysis was used to characterize dynamic load amplification as a function of the deck diaphragm stiffness, substructure stiffness, and the deck aspect ratio. An occupant traction load of 4 psf and excitation frequency of 1 Hz were assumed based on previous laboratory testing of decks loaded perpendicular to the ledger. Design curves and tables were developed to allow a designer to determine the amplification factor for a wide range of deck constructions. A simplified design procedure was developed and implemented on a spreadsheet to calculate the unit shear demand on a deck diaphragm, as well as force demands on hold-downs and the deck frame. The predicted hold-down forces from the simplified procedure were compared to FEM analyses. For design adequacy checks, the predicted unit shear demand from the simplified method can be compared to the tabulated allowable design values published in Table 4.3D of the 2008 AWC Special Design Provisions for Wind and Seismic (SDPWS). Similarly, connection hardware solutions can be checked to meet the hold-down demand, and the deck substructure can be checked using the provisions of the 2012 AWC National Design Specification for Wood Construction (NDS). This study provides the tools necessary to perform lateral designs of decks, as well as inform the development of prescriptive design solutions for technical resources such as the Design for Code Acceptance-6 (DCA6). Dowel-type fasteners (screws or threaded nails) were assumed for the deck board attachments. Proprietary "hidden" fasteners are gaining popularity for attaching deck boards. Some hidden fasteners allow slip, which can work well to accommodate longitudinal shrinkage and expansion caused by moisture and temperature changes; however, this slip can dramatically reduce deck diaphragm shear capacity and stiffness. Further research is needed to investigate ways to reinforce exterior deck systems to increase lateral stiffness and load carrying capacity.

Investigation of occupant induced dynamic lateral loading on exterior decks

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