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-   -   Spread footing excavation (http://www.diychatroom.com/f19/spread-footing-excavation-96770/)

WillK 02-27-2011 09:43 PM

Spread footing excavation
 
I want to see if I can get some guidance here please, my engineer has specified 24"x24" spread footings with a 10" depth. This is going under triple 2x10 beams he specified with posts at 7' spacing on center.

Before the project, the "foundation" was cement block under sill plate, the cement block was generally configured in irregularly spaced columns sitting on dirt with no footings of foundation wall. In some areas I've hit a 6" wide strip of concrete, the strip is about 2 inches thick and there is 1" of soil between the concrete blocks and the concrete strip.

So I'm building my forms at 12" deep, I'm planning to use high strength concrete and rebar with J-bolts to which post anchors will fasten.

My question is whether there is any typical guidance as to whether the spread footings can be partially above grade or do the top of the footing need to be flush with the dirt or does it matter as long as the bottom is on dense ground?

Daniel Holzman 02-28-2011 11:10 AM

OK, I gather your project is in Michigan, but perhaps you live in Michigan and the project is elsewhere? The frost line in Michigan is at least three feet, might be four feet in the UP. So I don't understand how you place a footer only 10 inches below grade. Is this an interior project, like under a heated basement? There is something missing in your project description, please try to fill it in, and perhaps the rest of your question will make sense.

WillK 02-28-2011 11:39 AM

2 Attachment(s)
Attached below is the post detail section from the drawing from my engineer. After I posted, I looked it over again and I'm reading the drawing as implying that the top of the 10" footing is to be at grade.

This is a conditioned (and vented) crawlspace over a dirt floor. As I said, columns of cement blocks support it "as-built" and I'm trying to implement something better.

Yes, this is in Michigan.

I can't imagine digging these all the way to 42". I'm working in a confined space here and I am digging this out with a garden trowel.

I'm also attaching a picture showing some typical cement block columns which this is intended to replace.

WillK 02-28-2011 11:56 AM

I'll also add some more details, but this information was left out before because it's only peripherally relevant and I intended to keep the topic focussed on the answer I'm looking for.

The house was built in 1917. The previous owner, who passed away and whose son inheritted and sold it to me on land countract, allegedly didn't do any structural modifications - according to his daughter. They did get a new furnace which is installed in the crawlspace, and the house originally had an oil burning furnace. Whether the current furnace replaced the oil furnace - I do not know, but I know that amongs crawlspace debris we've found oil furnace parts.

I know that there are 3 apparently original concrete footings supporting a triple 2x6 beam under the center of the house. I'll get back to that.

When we bought the house, the crawl space was littered with debris. The inspector didn't really get any further into the crawlspace than you could get dropping in from the openning in the floor. After purchase, the engineer spent a good half hour looking around, but I don't think he saw everything I saw and I know that he didn't see the back of the house where floor joists are perpindicular to the joists of the front.

I've spent a lot of time and cleaned out a lot of debris since we bought the house last August, I've done extensive plumbing work, but even I am still learning new things about the house.

Under the living room on the driveway side, I've known that the first 10 floor joists had rotted off the first foot or so on the bottom half. At some point prior to my buying the house, these joists had been sistered - but not all the way to the beam at the center of the house.

Until this weekend, it had not occurred to me that the rotted joists were not original. Once I got the tape measure out after I saw the appearance of the lumber under the front room, it occurred to me: Lumber in 1917 that is called a 2x6 would measure 2" by 6". I've seen this elsewhere in the house. None of the floor joists are 2" x 6", but rather they are 1.5" x 5.5". Apparently, floor joists have been completely replaced, then rotted, then sisterred.

Even the existing triple 2x6 beam doesn't seem to be original, adjacent to the furnace there is a concrete block column on dirt, this column is between two concrete block columns that sit on footings. Between the column on dirt and the center column on a footing, all 3 boards of the beam have lap joints that are nowhere near a column. This is a point where there is a major sag in our floor - on which the stairway to the second floor is resting.

Suffice it to say that the house has MAJOR structural issues. I'm aware of them, and I intend to deal with them. But I plan to do this systematically by dealing with one thing at a time, because heaven knows it's easy to get lost in contemplation if you try to take in the problems of this house in their entirety.

Daniel Holzman 02-28-2011 01:06 PM

You need to talk to your engineer about interpretation of those drawings. It looks to me like the horizontal line at the upper left is grade. The footing appears to be a 2 foot square concrete block, 10 inches thick, but set what looks to be about 3 feet below grade. Then a 6x6 post is attached to the footing, and connected to the beams above using a Simpson connector. Based on the photo, the bottom of the crawlspace appears to be below grade, but since I can't see grade, I don't know how far.

I would discuss with the engineer how exactly he thinks you should connect the PT post to the concrete footer, usually this is done using a galvanzied plate (standoff) such as the post base made by Simpson. The post base is generally attached to the footing using a J bolt. All of this should be specified on the drawings, maybe you have a detail drawing you did not post?

WillK 02-28-2011 01:37 PM

1 Attachment(s)
I was planning on Simpson ABA66Z for post base which would be bolted to a J-bolt set in the concrete. For the post attaching to the beam, he specified a strap but I was assuming that the use of LPC6Z would exceed the strap.

I'll admit I'm a little fuzzy over the definition of grade... This picture is a little more clear, as you can see the bottom of the boards enclosing the crawlspace which marks where grade is on the exterior. At this particular area, the crawlspace is dug out below grade. The area of the earlier picture is about the same as exterior grade. A very large area at the back of the house has appparently received the dirt that was dug out around the plumbing stack, in this area the crawlspace ground seems to be above grade.

The drawing would imply that the joists are less than 10" above grade, by way of the line on the left being above the bottom of the proposed beam. That is definitely not the case at any point around the building perimeter.

I think as far as the drawing, he's kept it least restrictive to leave leeway for me to make adjustments as necessary for things not accounted for... For example, the position of the beam with respect to the rim joist isn't specified... It's somewhat implied by the implied centering of the post on the 2'x2' footing which is shown near the perimeter, but I had earlier noted a problem with the beam going through the plumbing. His answer was to just move the beam inboard and get it as close to the perimeter as possible... I'm concerned about the amount of cantilever that would result so I'm planning to rework the stack with some 45 degree elbows so the stack goes around the beam. (Note, long-term plans include a bathroom relocation which will basicly involve a new stack, which can be placed where it won't interfere with the beam.)

mrgins 02-28-2011 03:38 PM

Grade is the horizontal line to the left of your drawing. If this was an exposed footing, i.e. for a porch, then you'd need to go down 42-48" minimum in your area. Because it is in a protected area that is warmer and is already down a couple of feet or so, you don't need to dig as deep, maybe only down as far as undisturbed soil. Talk to your inspector/engineer to confirm


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