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Re: Figuring load potential for deck

From: info@builderswebsource.com
Category: Decks
Remote Name: 63.198.181.43
Date: 08 Jun 2002
Time: 01:35 PM

Comments

We ran some quick calculations. Without knowing the type of wood, there can be significant variations in results. However, we assumed "Eastern Species" in our computations. Decks need to be designed for 40-60 pound/square foot live load and a minimum of 10 psf foot dead load. It seems that your current deck (including the 2x8 double beams @ 6' centers) and the 2x6 joists @ approx. 16"-17" centers are sized about right for the above conditions, excluding possible snow loads in the New York area.

However, when you add the pool to the deck, this adds 157 cubic feet of water (10' diameter x 2' deep). Water weighs approximately 62.25 pounds per cubic foot at typical outdoor temperatures. Your pool when filled will weigh about 9778 lbs (water weight alone), or roughly another 125 psf of dead load, distributed across part of your deck, somewhat centered over 4 posts. BeamChek shows that while the deflection is within acceptable limits, you may be severely overstressing the bending and shear strength of the wood members. Furthermore, due to the significant added weight on the deck, the sub-structure should be laterally braced to prevent side to side racking, which could cause it to collapse.

Based on preliminary calculations, we recommend at minimum, that you do the following: 1) Double up each 2x8, giving you a total of (4) 2x8 for each beam. You'll need to use longer bolts to secure these to the support posts. 2) Double up each 2x6 joist (so you have double 2x6). 3) Add diagonal lateral bracing to the deck in both directions between adjacent posts. This will help to strengthen the structure against lateral motion due to the top-heavy nature of the deck with the pool. Performing the above modifications may require serious reconstruction to the deck. However, it's clear the deck was not engineered to support this kind of weight, nor was it likely a design criteria at the time. These computations do not take into account the slight added benefit of the 2x6 members on top, which help to distribute the load and add some strength. Nor do these calculations reflect the actual wood in use. We simply chose a conservative species to get an order-of-magnitude look at your situation. The other option is to have a licensed structural engineer inspect the deck and run a quick computation. Other less disruptive options may be available depending on the actual as-built conditions. Our recommendations are guidelines and are for illustrative purposes only. Since we cannot see your structure first hand and don't know the type or condition of the existing wood, only a local, state licensed structural engineer can run proper calculations. We strongly urge you to take immediate action to mitigate any safety hazard to your occupants and guests. You can start by lowering the water level by 50%, which will go a long ways towards reducing the immediate danger.

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