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dalepres 03-30-2011 12:25 AM

Laying out building footings
 
I bought an inexpensive Stanley optical level for laying out my house on the lot. It worked pretty well for getting grade drawings and placing the structure in rough position.

The bearing markers are not exactly consistent around the dial making it hard to precisely move to specific angles. In one test, I could move away from a point and then move back to the same half-degree angle as precisely as I could interpolate between the lines and found that, at 40 feet out, I was up to 3 or 4 inches from the previous reading.

That gets me close enough for getting the bulldozer in tomorrow to level a starting point but once he's gone, I need to lay out the footings exactly. 3 inches now will be an expensive error.

I know how to do 3-4-5 or 6-8-10 angle measurements or diagonals across a square or rectangle. And if our 22.5 degree turn in the front of the house turns out to be 22 or 23, it isn't the end of the world but the 90's have to be right. And with the 3-4-5 or diagonals, there is still some error for tape stretch or sag.

I uploaded our floor plan to http://www.debndale.com/images/house/firstfloorplan.jpg

So, if I want to have really square corners, I need help on how to do it. Please share your tips for precision layout - preferably on a budget but, if necessary, I can rent or buy a better quality level or transit. Suggestions on make/model would be great, if that is the key.

Thanks,

Dale

Daniel Holzman 03-30-2011 07:25 AM

You can certainly buy a better quality transit, or you can rent one from a U-rent place. An adequate transit for layout work would be accurate to within a few seconds of arc. A little perspective on accuracy. A 1 degree error is 1.7 feet per hundred, obviously unacceptable. A one minute error (1/60 of a degree) is an error of .03 feet per hundred, which may be acceptable for building layout, but is pretty poor. A 10 second error (1/360 degree) is an error of .005 feet per hundred, well within tolerance.

You can get similar results if you use a non-stretch calibrated tape using the 3-4-5 right triangle principle. You have to keep the tape level, which is a real pain over distance, hence I prefer using a 10 second or better instrument.

jomama45 03-30-2011 08:22 AM

On extremely cut-up foundations, we'll often have a surveyor "pin" the corners for us one the footings are poured.

Your layout isn't really that complicated and can be laid out easily & accurately with a plain ole' steel tape. A nylon coated steel 100' tape won't stretch like a fiberglass will in long lengths. The tape can be more accurate by using the 12" mark for your starting point, as the end of the tape is often bulky loose, and an add to more discrepincies.

As for the 90* corners, they're easy when you use the Pythagorean theorem, A squared + B squared = C squared. A construction calc with feet/inches is extremely helpful for this. I typically run these numbers in the office beforehand and just note them on the print to save time in the field.

Bondo 03-30-2011 09:35 AM

Quote:

Please share your tips for precision layout - preferably on a budget but, if necessary,
Ayuh,.... Batter boards, string lines, 'n careful meassurements,...

stuart45 03-30-2011 05:07 PM

1 Attachment(s)
There's also the Groma used for 1,000's of years and never fails.
Attachment 31451
Some people will tell you that plumb lines don't work in windy conditions, but if you let the plumb bob hang in a bucket with some oil in, it will work perfectly.

Willie T 03-30-2011 08:10 PM

Don't forget after you have turned your angles (no matter the method you choose to use) double check all your squares and rectangles by pulling a steel tape diagonally from corner to corner. The opposite cross measurements must read exactly the same.

dalepres 03-30-2011 08:13 PM

I went to both rental places in town today to see what they had. Both had sub-300-dollar optical levels - that was it. And they want 50 a day to rent them. Amazon has a David White theodolite that goes to 5 seconds for 861 dollars. If I bought one, I could probably sell it for 500 or 600 on eBay after the house is built so, to me, renting junk for 50 a day is just not an option. But, I don't want to spend the money for the David White either.

I am leaning toward the 1000s of years old Pythagorean theory method. Batter boards are a real problem here; the soil is so rocky that we can't pound more than a few inches in most places. I even bought the Rousseau batter board system with metal form stakes to try to get around it. As I pound them in, the metal form stakes turn and twist around the rocks - and many of them still won't go far enough into the ground to be stable.

I just realized, though, that even though I can't them to go in straight enough to use with the Rousseau units, I could use the metal form stakes and get a wooden board level across them. I'll put in some stakes as best I can before the excavator comes so I can get as close as possible to recreating a corner pin as a starting point when we actually start the layout for the forms. I have a bunch of 6 inch nails that I can use to mark points that I should be able to get into the ground accurately.

It's probably no surprise for anyone who has taken on something like this; I took off this week to be here when the excavator came - originally scheduled for Wednesday. Now he isn't coming until, hopefully, Friday. Possibly Saturday.

Willie T 03-30-2011 08:17 PM

Quote:

Originally Posted by stuart45 (Post 620213)
There's also the Groma used for 1,000's of years and never fails.
Attachment 31451
Some people will tell you that plumb lines don't work in windy conditions, but if you let the plumb bob hang in a bucket with some oil in, it will work perfectly.

Cool! Do you also use a chorobates?

All joking aside, I honestly do use a water level quite often.


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