Building Homes or Extensions/Foundation..
bought Ark. creek property. Building 32 x 16 with loft, stick built. Lot is 350x1000 ft.. Slope of avg 15 degrees to creek. Rock shelf midway across width of property (350) that is limestone. Thought about building on that shelf or above it, but large flat boulders are broken up on top of the solid base. boulders about size of refridgerators with seams worn by weather and cracking between them up to 4 inches.Further above that you can't see the creek itself, so no go on building above this. Does not seems smart to build on the large boulders as movement likely over years. It would be really cool, and look awesome, but we have small quakes around here. One good 5.0 in the area could rip even floating piers apart.
Below the limsestone shelf is soil with large chunks of rock. LIkely to run into good soil and rocks from fist sized to engine block sized. Looking at pouring 5x5 treated posts in cardboard tubes for footings. I don't see any solution to get the footings in but a backhoe to deal with the rocks. The soil won't support "on top" application due to low density and slope. Leveling a lot does not fit financially nor practically given the slope.
Here are my questions: 1. Is there a smart way around my problem with excavating for footings (My wife, me, and my daughter are building this ourselves.) 2. If I use footings, how much space do I need to plan between footings? I was guessing I needed a footing every four feet. Have locals telling me go with three across the 16 foot span. 8 ft between supports didn't sound smart, even if I used 2x8 floor framing. So I am guessing with 16x24 being heated and cooled and the front 8 x 16 being porch, I should just go with 4 posts across and 8 posts back for a total of 32 holes. No special weight on this structure. Loft will be over one half of interior space.o, and will be 5ft high at it's peak. Typical load for stick build. Thanks ahead for feedback!
The first picture attached is of the shelf that runs the length that I was thinking of building on top of. You can see that it has seperation from the lower part of the rock. It also illustrates the soil I will be digging my footings un below that shelf, though the actual location does not have the very large stone in lower rigth corner of the pic. The second pic is the actual site I would like to use. Thanks! Andrew Schubert, Mt.View, Ark.
Your proposed approach is massively over sized. Obviously, you're free to build as strong as you want, but if you want to control costs I would suggest meeting code and sound engineering principles. The amount of rock might give you some grief when digging for footers, especially since you noted it is limestone (sometimes hard to take out with use of a rock hammer).
Here is my suggestion and the way I would approach this project. The first important point is to be sure the footers are sound. The way I would do that is to find a local concrete products supplier (In our area Gerdau Ameristeel is such a supplier) and ask them to help you select the proper size footer form and sonatube to use. The best footer form that I have used is the Big Foot foundation form. It is a conical plastic form, wider at the bottom and narrower at the top and comes in various sizes to carry various loads. This is a lot easier than digging then forming by hand each footer. The last time I bought them they were about $25-30 each. I'm sure they are higher now since all plastic products have soared with oil prices.
Since you have noted a concern about strength and stability I would not try to design to the least level but go with a middle of the road design criteria. A 32' x 16' building can be conservatively built on piers with only fifteen footers. Three rows of five each spaced 8' feet apart in all directions. The design loads for this type building are 40 lbs. per square live load per sf living space and deck/porch, 30 lbs. per sf sleeping areas, and 10 lbs. per sf dead load. Use all Southern pine No. 2 lumber for the subfloor. With this spacing you can use 2x10 joists 16" on center and headers 8' on center and be considerably above code and engineering requirements for both strength and stiffness. Run the headers down the length of the five foundation piers and use pressure treated lumber where in contact with the concrete piers, or lay a PTL plate over the tops of the piers then place the joists and headers on top of the plate; be sure to use PTL around the perimeter where the headers are exposed to the weather. I would also suggest using joist hanger instead of ledgers; some locals do not allow ledgers any more and hangers are a lot stronger and reliable. The following is a simple sketch of the footer layout.
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Need more help 865-531-9902 email@example.com. To give you an idea of where this design would fall on the strength scale--the hole thing could be done with 2x8 member and was just a hair shy of making it with 2x6 joists and 2x8 headers.