Not a lot is known about the effects of lateral loads on decks. Since this information is vital for creating safe and efficiently-engineered designs for decks and balconies, Don Bender's team at Washington State University recently explored lateral loads—imposed by wind, seismic forces, and occupancy—on a 12-foot by 12-foot deck. Within the specific constraints of their example deck, they found that neither wind nor seismic loads pose much of a design challenge except in hurricane and special wind regions. You can read the wind study here and the seismic study here.

Lateral loads due to occupant movement are another matter. Building codes are silent on the subject of these loads, with the exception of grandstands, bleachers, and stadium seating. So the WSU team performed laboratory experiments on full-size decks with two types of occupant loadings: cyclic side-sway, and impulse (run and jump stop). What they found is that lateral loading from occupants can exceed the worst-case design loads from either wind or seismic forces. A key point is that extreme occupant loading can occur anywhere and on any deck, while extreme wind and seismic loads are limited to smaller geographic regions.

Armed with a better understanding of lateral loads, researchers then examined how these loads are transferred from decks to the house floor framing and diaphragm. They tested two 12x12 decks—one with tension hold-down connectors, and one without—to determine their ultimate strengths and stiffness when laterally loaded (each lag screw in the deck ledger was instrumented to monitor loads). The study yielded interesting results, including the importance of screwed vs. nailed joist-hanger connections, and the effectiveness of ½-inch-diameter lag screws when installed per IRC deck ledger connection provisions. Hopefully, data from this testing will be used to guide new design solutions and products to resist lateral loads.

To read the actual studies on occupant loading and lateral load paths, click on the PDF link above.