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Old 08-30-2017, 10:18 PM  
Chris
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Default Lumber strength

A while back I had a thread about building a deck off the second floor of my shop. It came up on how I needed to use 6x6 instead of 4x4 for my supports and it got me thinking of why? As I look around town about 99 percent of decks and the very large one on my house all have 4x4 posts yet that is somehow taboo now days. I am wondering why? I looked up the verticle strength of a 4x4 and it is somewhere around 4400 pounds. My large deck has two in the center and the outside edges sit on a stem wall, the deck is about 28 feet x 12 feet. It's sturdy and has no movement. I am wondering why the new code would call for such an over build on posts?


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Old 08-31-2017, 12:08 AM  
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Years ago we saw all kinds of crap like 4x4 beams on 4x4 posts. The big problem is the strength for beams and warping and twisting for posts.
To counter the twisting and warping of the beams you can no longer use one piece beams, they want multiple of 2x nailed together.
They want the top of the post notched for the double beam and bolted there. The want the notch to be 3" deep so you get full bearing for the beam and they want 3" of the beam for the 1/2" bolts.. to counter any twisting in the beam and or post. Same with the saddle at the pier is about countering twist.
I don't think you will find the 4x4 against code, you just can't make it work with in code for connections.
I prefer 4 x 2x6s with opposing crown straighten and you will have very little trouble with warping and twisting


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Old 08-31-2017, 05:16 AM  
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The strength of a post is based on two things Chris the first is the compression rating of the material it is made from and the second is the column strength depending on the length of the post. A 4x4 has more than enough compressive strength for a deck but that amount is de-rated depending on how long it is and if it has mid support, In the case of my deck I have a 6000 pound hot tub and another 1000 pounds of deck and another 1000 pounds of people resting on (4) 4x4 post columns, but they are only 24” long. If they were 8’ long the numbers would have crunched much different. Building codes don’t want to get into all kind of number crunching so they require something large enough to cover typical needs and it is sometimes overkill. As Neal mentioned the industry then reacts to the code and comes up with products to work around materials people are required to use.

My deck being low and not using the space below it for anything I cross braced half of it between every post in both direction. It looks like a wooden roller coaster under half of it and that is the side with the hot tub. The cross bracing then gives stability to the non-braced side and that is the side I crawl under to get to the supply lines and power lines.

The thing about column strength is the cross sectional area is what determines the compressional loading but the extreme fiber is what factors into column strength. So many times a pipe or box tube is as strong as a solid shape with a lot less weight and expense. And if you compare solid to hollow where they both have the same area and weight the hollow is much stronger. Same is true for I beams only they go horizontal the extreme fiber is in the flanges and the web just spaces them apart. Bar joists like you see holding the roof at Walmart don’t even have a solid flange they just have crisscross rods, Super lightweight and carry high loads.

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Old 08-31-2017, 07:41 AM  
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I guess my question is more, it has been working fine for all these years, why is it not ok now? The posts on my deck are about 9 feet tall and straight. I have been looking around town and it seems most are tall and doing well. Heck there are hundred of pole barns around here with 4x4 posts and heavy heavy roofs on them. It just seems odd that what has been normal building construction that has worked well for over a hundred years is now crap and shouldn't be done?
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Old 08-31-2017, 07:45 AM  
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Code gets changed to solve a problem so the 4x4 must have been a problem somewhere.
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Old 08-31-2017, 08:18 AM  
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The code changes were based on accidents where decks collapsed and people were hurt or killed. My first house, a town house build in 1984, had a deck about 10 feet off the ground. It had 4x4 posts if I remember correctly the rim joist was nailed into the posts. Decking was nominal 1x6 (fence boards) not the preferred 5/4 x 6 decking and no cross bracing. The footer was a cinderblock buried in the ground. This deck was built by the builder of the town house and actually passed inspection. I shudder to think about that deck today. 33 years later and a lot of deck accidents in the meantime and the standards are a lot tighter now. Cross bracing, 5/8" bolts with washers on both ends, no carriage bolts, 6x6 posts with notches cut into them for the beams to rest in. 5/4" or larger decking. Codes change based on experience. They aren't just there to drive up the costs.
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Old 08-31-2017, 08:19 AM  
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This is getting a little old now, so don't know it it still applies.



http://www.deckmagazine.com/design-c...r-a-4x4-post_o
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Old 08-31-2017, 08:34 AM  
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Quote:
Originally Posted by Sparky617 View Post
The code changes were based on accidents where decks collapsed and people were hurt or killed. My first house, a town house build in 1984, had a deck about 10 feet off the ground. It had 4x4 posts if I remember correctly the rim joist was nailed into the posts. Decking was nominal 1x6 (fence boards) not the preferred 5/4 x 6 decking and no cross bracing. The footer was a cinderblock buried in the ground. This deck was built by the builder of the town house and actually passed inspection. I shudder to think about that deck today. 33 years later and a lot of deck accidents in the meantime and the standards are a lot tighter now. Cross bracing, 5/8" bolts with washers on both ends, no carriage bolts, 6x6 posts with notches cut into them for the beams to rest in. 5/4" or larger decking. Codes change based on experience. They aren't just there to drive up the costs.
I think there is a difference between hack work and just using 4x4 lumber. I get what you are saying but I am referring to 4x4's that are mounted properly.
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Old 08-31-2017, 09:19 AM  
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Quote:
Originally Posted by Chris View Post
I think there is a difference between hack work and just using 4x4 lumber. I get what you are saying but I am referring to 4x4's that are mounted properly.
They look at the whole picture and try to solve all problems at the same time.
Codes are bad enough they could have givin the what ifs and what have you's
For lower quality lumber that still fits in the grading parameters. If lumber is all 2 and better and some have to much heart wood or something, it is easier to just code up to the next size.
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Old 08-31-2017, 09:43 AM  
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Quote:
Originally Posted by Chris View Post
I think there is a difference between hack work and just using 4x4 lumber. I get what you are saying but I am referring to 4x4's that are mounted properly.
My deck was built to the standards at the time. Over the years decks that passed inspection and were built to the proper standards failed, so the codes were tightened. With a second story deck, I'd err on the side of over engineering it rather than under-engineering it. For the beams they want them to be resting on wood and secured with 5/8" through bolts. To do that with a 4x4 post you're cutting away too much wood. The codes don't want you to be relying on just the strength of the bolt to hold your beam up. In the giant scheme of things, the cost difference between a 4x4 and a 6x6 won't raise the price of your deck by more than a couple of hundred dollars unless you have a lot of posts. A 4x4x10 retails for about $11 and a 6x6x10 for about $31 here. How many do you need? Five of them would add $100 to your deck.


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