Wood in the Landscape: Decks Part II
by Daniel Winterbottom
Last week, Part I of this series discussed the origins of wood decks and basic framing systems. This week our five part series continues with the fundamentals of foundations.
The structural integrity of a deck begins with the foundation, and, unlike a house with a continuous perimeter concrete foundation, deck foundations are typically footings that receive concentrated loads at specific isolated points. To create a stable structure, the framing must transfer the loads at the corners to the posts and to intermediate points along the beam's length.
Computing Loads on Foundations
The loads that are transferred include the live loads (people and movable items on the deck) and the dead loads (the weight of the structure itself). The live loads plus the increasing dead loads transfer from the decking through the joists, beams, and posts above grade, through the concrete pier and spread footing, and into the ground.
The footing is sized to carry the total loads, which is determined by combining the live and dead loads. If the loads are equally distributed, one can determine the "footprint" of the footing: Divide the total load on all the footings by the number of footings; then divide the load on each footing by the bearing capacity of the soil.
To determine the total load, multiply the total deck square footage by the design live and dead loads. Most codes require a deck to support an estimated live load of 40 pounds per square foot (psf) combined with a 10 psf dead load for a total of 50 psf. However some require a structure sized for 60 psf. Check with the applicable local codes for the locale of the project.
For estimating purposes, a beam carries the weight of the deck halfway between it and the next beam or ledger; this is called the tributary load.
This article is the second in a five-part series. It was excerpted from Wood in the Landscape: A Practical Guide to Specification and Design, with permission of the publisher, John Wiley & Sons.