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Small Cabin Forum / Cabin Construction / Lag Screws for attaching floor to posts.
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dromedary
Member
# Posted: 15 Nov 2010 23:12
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I am afraid I may have built my house on sand so to speak. Instead of notching 3 inches out of my posts when attaching 2x12s as a band board for my 14x20 small cabin I instead used 3 lag screws for each connection (for a total of 30) on which the weight of everything else rests? I built this last year but I now fear it may collapse at any moment? These were 3/4 inch by 6 inch screws. Have i made a huge mistake? Can I fix it? The posts are 6x6 and are spaced at 10 ft intervals.

MikeOnBike
Member
# Posted: 15 Nov 2010 23:25
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Replacing the lag screws with through bolts might be a first good step. You could also bolt blocks under the 2x12 to help support them.

Someone else with a better building background might be able to tell you if you really have a problem.

hattie
Member
# Posted: 16 Nov 2010 00:07
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Quoting: MikeOnBike
Replacing the lag screws with through bolts might be a first good step. You could also bolt blocks under the 2x12 to help support them.


Hattie's Hubby agrees with MikeOnBike. Don't remove the existing lag screws, add 3 additional bolts. Bolt a 2 X 6 on the post that fits exactly from the bottom of your horizontal 2 X 12's to the foundation that the posts are sitting on. That will work like a doubler under the header of a door or window.

MtnDon
Member
# Posted: 16 Nov 2010 01:18 - Edited by: MtnDon
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I'm a little long winded...


I assume the band boards (twin 2x12 that I refer to as a beam) on the long sides support the floor joists and the weight of the walls and roof.

First let me say that supporting a structure on beams that are lag screwed or bolted to the sides of posts is not a really good system. It is better than usng a few nails in some ways though. The lag screws might have the shear strength to support the load, but that is assuming the wood is up to the task.

Ideally support beams should be placed on top of the post or knotched into the posts as second choice.

One issue I see though is that those 6 inch lags do not penetrate the post enough to develop full grip, full load carrying capacity. The connection calculator I have requires a lag length of 7" minimum when used to hold a thickness of three inches (two 2x12's) to the post. Replacing the lag with 7 inch lengths would be better. Replacement with bolts that go completely through the posts would be even better. Be sure to use washers and not to crush the wood.

Using southern pine as the post and plank wood species, x 7 lags yields a carrying capacity of 1000 lbs for each lag. With 30 lags that is most likely enough capacity to carry the weight of a typical 14 x 20 cabin. For comparison if through bolts were used, inch diamter, the carrying capacity of each is close to 1700 lbs.

The real concern though is about the wood, specifically the 2x12's. When bolts or lags are used close to the ends of the 2x's the carrying capacity of the wood is greatly reduced due to the possibility of splitting. A rule of thumb for maintaining best strength of the wood is to not have bolted fasteners any closer than 10-12inches to the end. Also such fasteners should not be placed closer than 2 inches to the edges of the 2x's.

The best peace of mind for the posts and beams would be to dig the ground away from the side of the post down to the footing. Then a 4x4 would be cut and be fitted tightly under the beam. Nail or screw the 4x4 to the 6x6 to keep it from wandering and backfill. Use a connector plate of some type to connect the 4x4 to the 2x12's. What sort of footing was used, how deep are the 6x6 set?


While we're looking at things, what is your snow load? If it is over 30 lbs/sq ft the doubled 2x12 may be undersized or spaced too far apart.

dromedary
Member
# Posted: 16 Nov 2010 09:40
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There is no snow load. We get about 2 inches of snow per year down south. I had tentatively planned to install 4x4 from the concrete pad up to the band board as well as doubling the number of posts (placing a new post in the center of each span). These new posts would be notched or perhaps even lie underneath the band board entirely. I also might place a beam down the center to add support to the span of the 2x12 joists. If I were to do these three things, do you guys think I would be in better shape? The other option would be to pour the concrete all the way up to the ban with a sono tube instead of using more 6x6 posts for my added supports.

MtnDon
Member
# Posted: 16 Nov 2010 10:03
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With 2x12 floor joists there is no need for a center beam. Span capability varies with species and grade as well as their on center distance. For example southern pine, 2x12, grade 2 on 24" centers can safely span 15'5"; on 16" centers 18'10"

A new post doubled up against the existing ones as described above would definitely be a good thing to do. PT wood and approved PT fasteners. Unless the ground does not have good bearing capacity there is likely no need for extra posts between the existing ones, but it would have more safety margin.

What is the soil type, size of footing used?

dromedary
Member
# Posted: 16 Nov 2010 10:56
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18 inch diameter footing. The 2x12s are 16" centers. I suppose I will dig to the footing and install new posts. What dimensions should the new posts be?

As far as soil type goes, its eastern Tennessee. The soil is very clay like, but there is excellent drainage.

MtnDon
Member
# Posted: 16 Nov 2010 11:20
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Extra posts. 4x4 would be fine as long as you double them up with the existing posts and leave the lag screws in place.

As long as the clay doesn't get more wet or dry things should be good. If you ever notice and settling you might want more footing area, which would mean more posts too at this stage. Or you could add now. Are there other cabins in the area? An older cabin that has been sitting there with no troubles for ten years or so would have a foundation that would be good to emulate.

dromedary
Member
# Posted: 16 Nov 2010 11:29
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I actually got this plan from a person with a cabin in the area. Apparently this soil is excellent for concrete footings and as long as it stays dry (under the roof) the soil is extremely hard. I have noticed this to be the case as well during the past year. I suppose I should bolt the reinforcing posts to the current posts?

MtnDon
Member
# Posted: 16 Nov 2010 12:06
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Anything to keep them in place. Metal angle brackets or just toe nailed. You simply want to keep them in place while you backfill. Good news on other area cabins being okay as long as it remains dry.

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