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Thu 23 Aug 2007 - 5:11 am UTC
First of all if Redhoss is available I would like to ask if he could answer this question. I have traced him through a number of sites and questions am very impressed with his quality of answers and the satifaction of the questioners. No offense to others out there that might be equally qualified.
I have a house that is close to 100 years old. It is a 2 story balloon framed house with a full basement (6' high). House outside dimensions are 18' x 24'. It has a perimeter concrete foundation which is approximately 6" thick under the sill. Approximately 10" below the 2 x 6 (actual size) floor joists the foundations widens into the basement approximately 18" wide and stands about 5 feet tall. I am pretty certain that the outside 6" wide foundation is not poured as one with the inner 18" thick wall. At the center of the 18' width of the house a series of 2x6's nailed side by side support the center of the house. They run parallel with the length of the house.
The problems are:
1. One of the sides of the house (24' length) settled aproximately 2 1/2" (at it's midpoint)from the rest of the foundation perimeter. Approximately half way down the length of the wall there is a significant crack in the 6" foundation which allowed the house to settle. On the same wall approximately 12" from the end of the wall another significant crack exists. There is an abrupt drop in foundation height of approximately 1" there. The opposite end of the 24' wall is still intact with the adjoining foundation wall and therefore is creating a tapered sag from end to center of the wall. No cracks exist in the 18" wide portion of the foundation.
2. The center support beam which is made up of 2x6's was woefully undersized and has allowed the center of the house to sag approximately 1 1/2". To correct this I will jack up the center beam in the appropriate locations and add additional support to what is already there.
3. Between the center support beam and the foundation wall that has sagged exists an additional localized sag in the floor above. This is where a number of supporting walls are located on both the first and second floors which surround stairs going from the first floor to the second floor.
My specific goals are:
1. To correct the sag in the outside 24' long wall by adding 2 ~10-11' long I beams approximately 8" in from the 6" foundation wall, jacking them up to their correct elevation to eliminate the foor sag and then supporting them with a stack of solid 3" thick cement blocks and wood spacers at whatever interval is suggested. (There is one area of the foundation wall that has a window well which will not allow supports to be placed for a distance of 5' beginning at 2 1/2' from the center of the house.) My constraint is that the maximum height of the beam can only be 3" due to some space constraints. The ~10-11' lengths are required as that is the maximum length beam I can get into the basement.
2. To raise and support the localized sag by utilizing a 10-11' long I beam and supporting it with metal jackpposts commonly used for supporting I beams in basements. I am hoping two would be enough. Here again I would like to use as low a profile I beam as possible due to it's loaction in the basement and the low ceiling height.
My questions are:
1. If the 3" high I beam is properly supported is it strong enough to support the outside wall. If so what should the support size and interval be and what specific 3" high I beam should I use.
2. What is the lowest profile I beam that I can use to raise and support the localized sag.
I hope that I have not gotten too windy and yet supplied you with the info you need.
I look forward to your response.
Thu 23 Aug 2007 - 7:21 pm UTC
Hello knollwood, I can help you. Do you know what the live load/snow load is. We will need to know that to calculate beam loadings. Do you have a particular 3" beam in mind. The only 3" beams I have in my old AISC book are two weights of "S" beams. Unless you already have a 3" beam that is available, you might have a hard time getting one. Or, maybe you are thinking of using 3" channel.
Fri 24 Aug 2007 - 4:36 am UTC
Hi Redhoss, I am looking forward to working with you on this. As far as live load and snow load, unfortunately I do not know what they are. Are they a standard value or is it something I will need to calculate? If you could point me in a direction or let me know how I need to calculate it, I will get on it right away.
As far as the 3" beam goes, I am restricted in height between the top of 18" wide foundation and the bottom of the floor joists. When you consider the height of the jacks it just barely allows for a 3" high I beam, and to achieve that I need to jack the house partially before being able to insert the I beam. However, I don't want to get in a situation where I need to look for something (3" I beam) that is difficult to find. Your suggestion of a 3" channel would actually be ideal. Assuming it could be laid in an "n" orientation I could get the jacks in between the channel legs which would leave me lots of space height wise. Also, I have determined that the lengths of these beams would be ideally 10' long. Not only because of the space issue but also that the beams come in standard length of 20' which would allow me to get two 10' lengths out of one piece 20' long.
I hope I have clarified my question enough so that we can take the next step, whatever you determine that should be.
Fri 24 Aug 2007 - 1:19 pm UTC
I know that you probably don't want to give any specific info about the location of this house, but can you give me a general area so that we can estimate what the snow load might be. Also, I don't think that you would want to use 3" channel in the weak orientation. I will do some ballpark calcs using a 30 psf live load and see what might work.
Fri 24 Aug 2007 - 1:53 pm UTC
One thing is clear to me after taking a first look. I don't know what column spacing you had in mind along the 24' walls, but 3' spacing is probably the limit with the limitation of using a 3" deep section. That sure is a bunch of columns (9 per side) plus what you will put under the existing center beam. Is this anything like what you expected.
Fri 24 Aug 2007 - 11:33 pm UTC
The house is located in Central Minnesota. We have on average 36-48" of snow per winter. I don't know if it matters but it is a Gambrel style roof. Also to clarify it will only be one of the 24' walls that needs the correction. The other side of the house is fine.
Once I have the floor level, I will place wood spacers/shims under the exterior wall and the existing 6" foundation. I plan to have that wall supported the best that I can given the slope of the 6" foundation wall and the abrupt drops. I don't know if that can be factored into your calculations. That's something I probably should have mentioned earlier. Sorry about that.
I am not confident that the floor will stay level on that existing 6" foundation wall and for that reason I wanted to go with the beam over the 18" wide foundation which is structurally much more sound. With the support of the outside foundation, the beam support is not quite as critical as if it were the only thing supporting that entire side of the house.
Regarding the 3' spacing, that is closer than what I was hoping for. Are there heavier gauge channels available that would allow for longer spacings.
Also we need to remember the one area of localized sag located 1/2 way between the outside wall and center beam. A 4" I beam would be ideal for that location if it can be supported with only 2 columns.
Sat 25 Aug 2007 - 12:25 pm UTC
It dawned on me we have been talking about a 3" channel. The 3" constraint is in the vertical direction when the channel is placed in position on it's side. The space I am working in allows for up to a 6" channel. I don't believe the 2 legs of this size channel are higher (wider) than 3" and therefore will not violate my 3" constraint.
Sat 25 Aug 2007 - 2:22 pm UTC
Wow, Minnesota has some serious snow. We are probably looking at a 50 psf snow load. What you said about the 6" channel is correct. However, the heaviest 6" channel only has a section modulus of 2.48 in^3. How about using 3 x 6 rectangular tubing. It looks like you could go with 4' column spacing and use 3" x 6" x 1/2" wall rectangular tubing. It has an s of 5.63 in^3 and weighs 24.28 # per foot. As for the center sag. The center beam supports twice the load that the 24' wall supports. You would have to assume that the sag beam could actually take the whole load even though it probably won't. If you were to use a W4x13 (4" wide flange beam @ 13 # per foot), you could space the columns on 30" centers. I know that this sounds ridiculous and you can probably get by with much greater spacing. Exactly how much of the sag you lose and how much of the load the sag beam actually supports will have great influence on the sag beam load. I would suggest that you could experiment with this beam and add columns as needed to achieve the desired results. Possibly 3 coulmns would work. There is no danger of failing the sag beam. It might just deflect more than you want. Does that make sense to you. Are we getting close to something that is possible for you to work with.
Sat 25 Aug 2007 - 4:23 pm UTC
If I was took take out the 3" height constraint, what height I beam would I need, given a heavier gauge. My goal was to get the jacks under the beam but I am having second thoughts given the weight of the rectangular tubing, and the difficulty I would have getting it in place. Rather, I could get several jacks (8 or so) spaced closer together and use smaller channel beams to elevate the house into position and then set the large I beam in place and use several supports to hold it to the correct height and then lower the house on to the beam. Let me know what you think might work that way.
Regarding the W4x13, I like that idea. That should work fine.
Sat 25 Aug 2007 - 6:42 pm UTC
Another thought. Does the fact that we are spanning the 24' length of wall with 2 11' lengths of structural steel affect the load capacity? Or do you already have that factored in?
Mon 27 Aug 2007 - 3:25 pm UTC
I think we are getting to a point where the answer is very close. If this isn't the final solution, you can certainly ask for a clarification. First, you are right about the fact that a continuous beam does calculate to require a smaller section modulus. However, the difference between continuous and simply supported is not very much and we want to be somewhat conservative with our beam choice. If we use 5 columns on the 24' side, that will put a column right under the splice between the two 11' beams. That makes a W12x14 work out just fine. The 14 # per foot is about as light as you are going to get and is a popular beam size. How does this sound to you.
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