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JMHdesigns
Joined: 16 Dec 2004
Posts: 9
Location: Maryland
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 Posted: Sun Dec 19, 2004 11:11 pm Post subject: Footing is too small for residential addition! |
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I have a situation where my client wants to add an entire second floor to his existing home. The home is a single story rambler built in the 70's with a 16" wide footing.
Per IRC2000, the footing will have to be at least 19" wide. When I apply the PLF loading from the new second floor, my calc's say the footing needs to be 24" wide.
How do we add the additional 8" (16" + 8" = 24") to the existing footing?
Thanks..
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cast
Joined: 15 Dec 2004
Posts: 9
Location: seattle
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| Posted: Mon Dec 20, 2004 7:16 pm Post subject: |
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| i've widened footings by doweling the new into the old footing with rebar. Also, you may be able to just pour the additional width on one side. As long as it is bonded with the old foundation, you will be able to us the entire footing width. |
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JMHdesigns
Joined: 16 Dec 2004
Posts: 9
Location: Maryland
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| Posted: Tue Dec 21, 2004 5:50 pm Post subject: |
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cast,
I am not sure how to calculate adding concrete with rebar dowels. I believe the solution would be underpinning with helical piers. Any suggestions using these piers? |
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cast
Joined: 15 Dec 2004
Posts: 9
Location: seattle
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| Posted: Tue Dec 21, 2004 7:13 pm Post subject: |
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| i am not a structural engineer so I can't really help you with the calcs, but my engineer has detailed this for various 2nd story additions in the past, typically when we are adding a large point load to an existing perimeter foundation. I haven't used helical piers, but sounds expensive : ) |
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RWL
Joined: 10 May 2004
Posts: 399
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| Posted: Wed Dec 22, 2004 4:53 am Post subject: |
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I don't feel comfortable with the "re-bar dowel and side additions" solution-- may be viable, but....? You seem to really to come-by three footings which may not act in concert.
Helical piers for a residence would be a costly solution. May I propose that you underpin the footing with incremental [spaced] new footings, properly sized under the old footing [excavate and pour sections perhaps 6-feet long with a 6-foot space before the next new footing]. Once the first set is in place, you go back and do the same thing for filling in the "spaces" left the first time. Here you can dowel the footings, one to another along the length. |
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doug aleshire
Joined: 07 Sep 2004
Posts: 9
Location: Royal Oak, MI
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| Posted: Sun Jan 02, 2005 8:09 pm Post subject: |
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RWL,
Do you think it would be a good time to suggest to the client that you could always do a full basement - if location is permissable to do so? Underpinning isn't cheap and as long as we are digging more out, the option seems a good choice for added space.
By the time you try and widen the exitsting footing, inside and out, it may be best to do a full basement. What is proposed, less underpinning is labor intenstive.
Just a thought
_________________
Doug Aleshire
Residential Building Designer
Softplan Ver 13.3.3 |
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pbrown
Joined: 12 Jan 2005
Posts: 2
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| Posted: Wed Jan 12, 2005 9:30 am Post subject: Footings |
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First, unless you are a licensed Architect I would be careful moving forward with structural designs for this issue. If you are looking for remedies to improve the bearing capacity. Absolutly, there is a solution.
I had similar situations like this and a structural engineer is a must.
But, again if you looking for thoughts on what methods will work best and cost the least. Heres what I say. Additional concrete can be added to the footing in such a way as to improve bearing. It could be done similar to a underpinning situation. Sections would be done at different areas at a time
A concrete foundation contractor should be consulted to determine cost for underpinning
For Helical piers. I've used them. They can be less intrusive to the exterior around the building. In my case I was doing a second floor on a residence at the beach. This was the cheapest solution for us. Search helical piers on the web. Call the company the makes them and they'll give you names of qualified helical pier installers |
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rjeci
Joined: 21 Jan 2005
Posts: 1
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| Posted: Fri Jan 21, 2005 11:07 am Post subject: |
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Depending on the type of soils underlying your existing footing, helical piers are probably the more cost-effective and least disruptive solution. HP are regularly installed from inside the existing basement through a small window cut in the slab concrete and dirt excavated from below the existing footing. The pier is driven using a drill-press type machine that can be wheeled down stairs and into place, the hydraulic lines are run out though a window to a power source (usually a skid steer). A special bracket is installed on the top of the pier after it has reached it's designed depth and secured vertically and horizontally to the existing footing/stem wall. Cost for piers can run from $800-1,500 each with the bracket. A general spacing (rule of thumb) suggested by most manufacturers is 5 feet on center (change if you have point loads). Some key components to use of this tool - soils, existing footing condition, loading.
I would look at the disruption necessary for the footing width increase vs. the piers - adding the width to the footing will require BOTH interior demo and exterior excavation. Depending on landscape/sprinkler/utilities on the outside and the mess of removing the entire slab on the inside, some value can been added for a smaller work foot print. Hope this helps! |
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Donald Mallow
Joined: 31 Jan 2005
Posts: 1
Location: Leonia, N.J.
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| Posted: Mon Jan 31, 2005 9:06 pm Post subject: footing too narrow??? |
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| According to IRC table R403.1 a 2 story conventional light frame construction on the poorest soil in that table of 1,500 psf requires only a 15" wide footing. Check further in R403.1.1.. You may not need any additional footing width unless there are conditions you have not described. Good Luck. |
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Greg18k
Joined: 02 Apr 2005
Posts: 1
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| Posted: Sat Apr 02, 2005 4:21 pm Post subject: Footing width |
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You don't have to add additional footings if your second floor will rest on the existing wall that already being supported by the footing.If you have a 8 inch block wall,your footing should have 4 inch projections on each side.So you should be ok unless there is already a crack in your footing.
Most situations existing footing will carry new loads.
Good luck. |
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