Understanding the lateral strength, load path, and occupant loads of exterior decks

Brian Jared Parsons

The magnitudes of occupant lateral loads are needed to safely design decks. Two decks, with different lateral stiffnesses, were constructed to measure lateral loads. Load was measured with two load cells attached at each end of the ledger. The exterior deck beam was supported on rollers, simulating negligible lateral resistance from columns. Occupants performed two different types of dynamic actions, in two orthogonal directions, at occupancy levels of 10, 20, 30, and 40 psf. The first was a cyclic side-to-side sway motion which simulated synchronized movement (e.g. dancing or aerobics). The second was an impulse type load which simulated “horseplay” type activities. The difference in stiffness between the decks caused a significant difference in the human-structure interaction. Inertial effects were present when the flexible deck was loaded parallel to the ledger. If sufficient lateral bracing is provided to minimize lateral displacements, a design lateral load of 7.6 psf of plan area is recommended. If lateral bracing is not provided, a design lateral load of 12.1 psf of plan area is recommended due to additional inertial forces. Two additional decks connected to a diaphragm (simulating attachment to the house floor diaphragm) were constructed to investigate deck lateral strength and stiffness, as well as the load paths from the deck to the house diaphragm. One deck was constructed with a lateral hold-down device similar to that depicted in International Residential Code Section R507.2.3 and one without. Joist hangers were installed with screws and the ledger board was connected to the house rim joist with lag screws in a staggered pattern according to IRC Section R502.2.2.1.1. A monotonic force along a line parallel to the ledger board passing through the centroid of the deck surface was applied. Forces in each lag screw and hold-down were recorded using specially fabricated load cells. In both tests, the connection between the deck ledger and the house rim joist remained structurally intact up to a maximum load of approximately 7000 lbs, at which point the tests were terminated due to damage to other parts of the deck. Splitting of the joists occurred due to a couple formed by each deck board-to-joist connection which induced tension stresses perpendicular to the grain. Hold-downs provided little impact on short-term strength and stiffness of the decks as constructed with screws substituted for nails in all joist hangers. It is likely that hold-downs would provide much-needed structural redundancy for actual decks subjected to moisture cycling, environmental exposures and constructed with nails in joist hangers. Additional research is needed to examine deck performance subjected to environmental loadings and construction methods.

Understanding the lateral strength, load path, and occupant loads of exterior decks

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