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Footer question?

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#1 Footbag

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Posted 23 April 2013 - 11:15 AM

OK. I've been digging...

I have a question about my footers for the actual observatory. Can I move one footer or do the footers have to be in equally spaced corners. Due to the contour of my driveway, and a retaining wall if I could move the footer 1', I feel like I'd have a much better ground to put it in. Otherwise I need to disturb a lot of earth.

I would then make my beams of equal length and they would still be parallel.



o-----o
|-----|
|-----|
o-----|
|-----0

0 = concrete footer & post location
| = beam
- = joists

*Not to scale

#2 Midnight Dan

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Posted 23 April 2013 - 03:14 PM

When you extend a beam out farther than it's support point, it's called a cantilever. It's difficult to calculate how far you can safely cantilever depending on your materials, loads (snow, wind, live and dead loads), etc.

There are some rules of thumbs though. For example, for deck designs, beams can generally be cantilevered a distance of 1/4 the length between the next two supports. So if the distance between the two o's on the left side of your drawing is 8 feet, the cantilevered beam in the lower left can be 2 feet from the post support.

When cantilevering a room for a house, 2 feet is usually considered a safe limit. More than that and you should probably consult a structural engineer to evaluate your particular situation.

-Dan

#3 Footbag

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Posted 23 April 2013 - 04:20 PM

Well, I moved the hole. I still plan on cantilevering, just not to fix this problem.

The good news is I'm almost done digging. Just two shallow holes, and then hammer drilling some holes for rebar in the bedrock.

#4 roscoe

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Posted 24 April 2013 - 08:03 AM

Adam, you should be fine with your offset hole....it's an obs you're building, not a 2-story house, after all.....
The one thing I'd suggest is that you track down a piece of angle-iron, and bolt it to the framing going in both directions, with the steel on the inside of the corner.....something with two 1/2" bolts in each side would tie it all together well.
Russ

#5 Footbag

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Posted 24 April 2013 - 02:15 PM

Adam, you should be fine with your offset hole....it's an obs you're building, not a 2-story house, after all.....
The one thing I'd suggest is that you track down a piece of angle-iron, and bolt it to the framing going in both directions, with the steel on the inside of the corner.....something with two 1/2" bolts in each side would tie it all together well.
Russ


Thanks. I have already moved the hole and a retaining wall. I am pretty confident that when all is said and done, this would support a two story house. Every step of the way, I seem to be taking the overkill route.

Then again, I'm an astronomer. If it's not diffraction limited, then it's not good enough.

#6 JMW

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Posted 25 April 2013 - 10:38 AM

You may consider doubling up your beams since you only have 4 footers total. I would also upsize the diameter and depth of your footers to minimize the possibility of any settling.

#7 Footbag

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Posted 25 April 2013 - 10:58 AM

You may consider doubling up your beams since you only have 4 footers total. I would also upsize the diameter and depth of your footers to minimize the possibility of any settling.


That's already a given. The footers will be 12" sonotube. Since it lies on bedrock, it will be pinned using rebar.

The beams will either be 6X6's or 2 2X6's.

#8 JMW

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Posted 27 April 2013 - 10:31 PM

Sounds like it will be solid.






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