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Old 05-20-2008, 11:52 PM   #1
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Concrete Q: expansion and contraction after 30 years


From a separate thread (Deck Beams) I didn't want to takeover, re post bases/beams on a concrete patio:

Quote:
Originally Posted by thekctermite View Post
Putting it on the slab will only lead to a cracked patio and a deck that heaves and settles with the elements.
Can kc or someone else please expand on this: What are typical expansion and contraction measurements for concrete due to weather? I read on wikipedia that after 30 years concrete no longer expands and contracts. But it's wikipedia... and maybe I'm misinterpreting it. Regardless of the 30 year mark, there is still heaving and settling with frost? What are the measurements for that?

Quote:
Originally Posted by @wikipedia.org
As concrete matures it continues to shrink, due to the ongoing reaction taking place in the material, although the rate of shrinkage falls relatively quickly and keeps reducing over time (for all practical purposes concrete is usually considered to not shrink any further after 30 years).


I've got a 30+ year old patio attached to the back of my house. On this patio there is a section where I built a small 3x3 upper-landing for a ramp I'm making for my dog. (The lower landing is a 3x3 concrete pad). The upper-landing is freestanding with cross-braced posts. The post bases for the landing are secured directly to the patio via retrofitting.



About 12' away I've got a deck. I want to connect the deck to this free-standing upper-landing. I was going to run two joists probably 19.2" OC, and support by posts at 4' and 8'. Does it sound wrong to connect the upper-landing to the deck? Technically it wouldn't be freestanding any longer? It would be a deck attachment or something? I certainly don't want to build something that could affect the integrity of my deck.

Thanks :)
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Old 05-21-2008, 06:32 AM   #2
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Concrete Q: expansion and contraction after 30 years


Concrete will certainly continue to expand and contract. Heating and cooling will continue to make that happen forever. How much movement could depend on a lot of local situations. How much sun, ambiant temperatures, coatings, etc. Frost heaving would depend on local conditions such as type of base under the slab, amount of moisture present, drainage. Even the side of the house where it is located will have an effect.

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Old 05-21-2008, 07:54 AM   #3
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Concrete Q: expansion and contraction after 30 years


Concrete will always expand and contract due to heat and cold to some degree. This is why bridges have expansion joints.

However, thermal expansion/contraction alone likely won't cause noticeable cracks. Where you'll get the ugly cracks is from settlement or movement of the surface that your patio sits on. Frozen soil under the slab will expand and will take the slab with it, then will settle down again when it thaws. Over time, this can cause damage...Sometimes will, sometimes won't.

Frost heave/subsequent settlement of the bearing soil will occur with a slab forever. But don't confuse it with thermal expansion/contraction of the concrete (which will also occur forever)...Which isn't a concern I'd have if I were you...This isn't enough concrete to ever perceive whatever thermal expansion your slab has.

Due to freeze/thaw, you are required to have a frost footing for a house or deck. Otherwise your deck moves with your slab, and that isn't good.
Bearing even a small deck like the one pictured on a patio without ISOLATED piers is a bad idea.
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Old 05-21-2008, 05:04 PM   #4
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Concrete Q: expansion and contraction after 30 years


Hi, thank you both for your replies. I guess I misinterpreted the text. I'll adjust for this. I suppose I will run the freestanding structure almost right up to the deck, maybe 1/4" space in between the two, and the dog can walk to it from there.

This might sound crazy but do construction workers and engineers ever use galvanized springs between wood connections (or something else) to compensate for heaving and expansion? Have construction companies always dug really deep footings? Here in NY the footings are at 42" plus where I am you have to go up 6" above ground so that's 48".


Quote:
Originally Posted by thekctermite View Post
Bearing even a small deck like the one pictured on a patio without ISOLATED piers is a bad idea.
My deck is not pictured; are you referring to the porch or the 3x3 structure I have cross-braced (is that technically considered a deck?!)

The structure as it stands now, in the picture, does it look sound?


Also another Q about this 3x3 structure. I want to reinforce the cross-bracing like in this picture:



I was planning on 1/2" carriage bolts with a block or something in the middle. But to do that would require me to drill one hole 9/16" in the center of each 2x4. That's about 1.5" from either edge. Does that sound ok? I'm told the NDS has a requirement of 1.5D but it's only for the unloaded edge. I'm going to ask about that in another thread.
But which edge is the unloaded edge when the wood is used for crossbracing? Are they both unloaded edges? Does that even make sense? Thanks
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Old 05-21-2008, 10:26 PM   #5
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Concrete Q: expansion and contraction after 30 years


I answered in your text in bold print...

Quote:
Originally Posted by Deck View Post
This might sound crazy but do construction workers and engineers ever use galvanized springs between wood connections (or something else) to compensate for heaving and expansion?

NO

Have construction companies always dug really deep footings? Here in NY the footings are at 42" plus where I am you have to go up 6" above ground so that's 48".

IN ORDER TO BUILD A STRUCTURE CORRECTLY, YES THEY HAVE, OR AT LEAST THEY SHOULD HAVE.



My deck is not pictured; are you referring to the porch or the 3x3 structure I have cross-braced (is that technically considered a deck?!)

YES

The structure as it stands now, in the picture, does it look sound?

I'M NOT SURE WHAT I THINK OF IT...I'D LIKE TO SEE IT TOP TO BOTTOM. I'M NOT A FAN OF THE CROSS-BRACES...WITH PROPER POST BURIAL AND ATTACHMENT TO THE HOUSE, I SEE LITTLE NEED FOR THEM.


Also another Q about this 3x3 structure. I want to reinforce the cross-bracing like in this picture:



I was planning on 1/2" carriage bolts with a block or something in the middle. But to do that would require me to drill one hole 9/16" in the center of each 2x4. That's about 1.5" from either edge. Does that sound ok?

THE CROSS BRACES ARE IN TENSION AND COMPRESSION, NOT IN SPAN. I WOULD HAVE NO PROBLEM WITH DRILLING THAT HOLE AS YOU SEE FIT.

I'm told the NDS has a requirement of 1.5D but it's only for the unloaded edge. I'm going to ask about that in another thread.
But which edge is the unloaded edge when the wood is used for crossbracing? Are they both unloaded edges? Does that even make sense? Thanks
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Old 05-21-2008, 11:54 PM   #6
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Concrete Q: expansion and contraction after 30 years


THANK YOU FOR YOUR REPLY!

I appreciate the time you take to read and reply to my posts KC :)

The structure is free-standing, which is why I added the cross-bracing. I intend to leave it that way, and not attached to the house. I feel weird about calling it a deck ( I don't dispute what you are saying ). Its sole purpose is to be a landing for the dog ramp.

I'll add support to the cross-bracing by drilling for carriage bolts as described in the picture I attached.

Thanks again
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Old 05-22-2008, 12:09 AM   #7
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Concrete Q: expansion and contraction after 30 years


No problem. I forgot that this was a dog ramp landing. Yes, I think you're doing all you can to make it sturdy since it isn't being attached to the house.

I need to close this page before my dog sees it and gets jealous.
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Old 05-23-2008, 10:13 AM   #8
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Concrete Q: expansion and contraction after 30 years


Deck,

People that build boat docks deal with movement all the time. A simple dock hinge where your ramp connects to the deck will handle the slight up/down movement that might occur from frost heave. If your ramp only spans 12 feet from landing to deck then I don't think you'll need posts half way. Use 2x10s and you should be fine.
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Old 05-23-2008, 11:00 AM   #9
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Concrete Q: expansion and contraction after 30 years


Hi, thank you for your reply. Are you suggesting I can attach the 3x3 structure to my deck using a dock hinge to secure the joists to the deck?

I googled dock hinges and actually they might work to secure the dog ramp to the 3x3 structure. Otherwise I'll use adjustable joist hangers for that. How are ramps attached to decks anyway?

As far as the joist span to the deck, I have 2x6x16s I bought months ago at Home Depot. Some have warped but I have two good ones that I'm going to run from the 3x3 structure to within 1/4" of the deck. I have posts less than 2' away from the deck so it will cantilever by about 2'. I'll post pictures when I can, I'm going to work on it this afternoon.
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Old 05-23-2008, 11:21 AM   #10
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Concrete Q: expansion and contraction after 30 years


I'm suggesting putting the hinges at both ends of the ramp. If you only put them at one end you'll have problems when the 3x3 structure moves up or down because it will still cause a leverage on the unhinged end.

Posts will complicate it. If you put in posts holding the ramp then that will be another point that might raise or drop unless they are below frost line. If they are below frost line then you'll want to put a break in the ramp at the posts and hinges in each end of the ramp between the 3x3 and where it connects at the posts. No need for hinges on the ramp between the posts and the deck because neither will move.

If this will only carry a dog then the 2x6s will span 12' without posts but I'm sure it won't meet codes.

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