Footings for Load-Bearing Posts
When is new footing needed when adding basement posts?
We are adding a beam to our basement to correct some floor bounce in the kitchen which is over an open area in the basement. Essentially we are shortening the span of the TJIs that run under the kitchen island area (they are currently side-loaded into a 4-ply LVL). The island is pretty big/heavy, approx 7×5′ and 4 of the TJIs below are doubled. The new beam will be parallel to the existing beam, directly under the area where the bounce is most pronounced.
The slab is a fiber-reinforced concrete approx. 4″ thick poured over 2″ of XPS foam over compacted gravel.
We are trying to decide whether we need to dig-out new footings under the 2 posts. Since the beam is addressing floor bounce and not carrying the primary load, the builder doesn’t think footings are necessary. What is the best way to make a determination?
Also, should we decide to do post-footings, do we just skip the foam, i.e excavate the foam and replace with concrete continuously all the way below frost line?
thanks in advance?
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Replies
Seems to me you need to diagnose the problem before you decide on the solution.
The symptom is you feel the floor is too bouncy? This is kind of a fuzzy complaint is there some way to measure it? I can’t read the numbers from the plan but the bed room and living rooms spans look to be almost the same as the kitchens span, do they seem bouncy as well?
Seems to me the smart move would be to talk to the engineer that designed the current system if at all possible and have him look at what was built and if it comports with the plans. If he thinks it is preforming as planed but not up to your expectations have him design the changes needed to meet your expectations.
I am afraid if you move ahead with your back of an envelope plan you will spend thousands and be disappointed by the resalts.
Walta
Thanks for the response, yes the spans in 12:00 to 3:00 quadrant are very similar to the spans at 6:00 to 9:00 in the drawing approx 16' And yes there is a bit of bounce in the lower left but much less pronounced and less bothersome as this is not a heavy foot traffic area (it's an office/bedroom above) whereas the kitchen area is open concept and high traffic. Also another key difference is the kitchen has a 7'x5' granite countertop and cabinets below so it's considerably more loaded. lastly both sides of the TJI's in the 6-9:00 zone are over either foundation wall or posted load wall whereas the 4 ply LVL supporting the kitchen area is posted on one end by a framed exterior wall (at 12:00) and on the other end supported by a massive 12 gauge hanger, no post below.
oldbungalow,
Unfortunately the beam doesn't know what you want it to do. So for example, if during the original build there was a beam under the joists at mid-span, structurally it would be taking up twice the loads that the beams on either end were carrying. Just because you put the beam in later doesn't change the loads involved, and there is no way to tell it "just help out a bit with the bounciness".
All beams need pad footings. If like these they are inside the foundation, they don't need to go down to the frost level, they just need to go down to undisturbed soil capable of supporting the loading. That may end up to be the same depth. The space between the top of the footing and the floor can be a smaller concrete pier (usually a sono-tube), which can have the foam replaced around it under the floor.
thanks this is a really helpful perspective, whereas in our minds it's providing supplemental support but the physics is indifferent to which came first or second, it's merely interrupting the span and potentially taking a proportionate load directly from that span.
oldbungalow,
That all changes if you don't support the beam. There are a few solutions that just reinforce or share the loading on the floor to reduce bounce. However unlike the beam, which you know will work, I don't know how effective these fixes would be.
- Sheath the TJIs in plywood.
- Sheath or strap the underside of the TJIs.
Note that putting in a new footing isn't that big of a job. The concrete floor in old houses tends to be thin and brittle, busting a hole in it isn't that hard. Then it's just a matter of digging out some dirt and putting back an equal quantity of concrete. It will need to be engineered though.
hi it's actually pretty new and fiber reinforced. Though i've often heard that concrete gets harder/stronger as it ages?
Yeah, it's not that the concrete gets any weaker, they just didn't think they needed more than 2" back in the day. There was a lot less mechanization back then, if you're mixing it in a wheelbarrow with a hoe you tend not to pour more than you think is necessary.
Rigid insulation can support a fair bit of weight. Say you take standard 20PSI foam, a 1 sqft area can support 2800lb. Now you don't want to put that much weight on it as it will deform the foam too much, but you can see it doesn't take all that much area to support a fair bit of weight.
I would figure out what this new beam would support assuming it is holding up all the floor load on that half of the kitchen and figure out how big of an area you need to hold it up. You can than pour a pad that size over the existing slab or use some 8x8 to spread the load out.
This is a good alternative. Most soil loads, at least that I deal with, default to 1500 psf and this is what I would size for. I would also make sure the new pier is anchored to slab with some doweled rebar.
But you don't know what's under that slab. It may not be undisturbed soil, it could be uncompacted fill.
This would be assuming it is built wrong. I suppose this could be an assumption though I think if you design for the 1500 the existing slab will disperse this even more. I think an engineer would be comfortable with this assumption, though you should have one sign off for sure.
Residential basement floors are built "wrong" all the time. Unless they know it's going to be a garage and cars are parked there, I'd say it's more common than not to skimp on the concrete slab.
A thin pour and uncompacted fill are par for the course. When you need to break them up for plumbing you can often do it with just hand tools.
I dont know the op specifically says it is over compacted gravel. It sounds like it was built recently. I think it’s a safe assumption and easy confirmation.
The PSI ratings for foam aren't what they can hold without deforming, but what they can hold without deforming permanently. A footing over foam will move under load if loaded to the PSI rating.
There is a smaller limit which is where the foam won't move appreciably.
I think it's easier to cut a hole and dig down to undisturbed soil.
Although it's interesting that where I live you're allowed to assume the soil can hold 1500 lbs/sf without soil testing, which is basically half of what foam can hold.
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I’m curious about the logic of the load on the new center post. It seems like if you don’t need to jack up the beam to address unacceptable sagging, a post just touching the bottom of the beam would have no load until the extra load that results in bouncing is applied. It should only absorb that extra load. It doesn’t seem like a bunch of the initial load on ends of the beam would magically move to the center. Is there something wrong with my logic?
Andy
andykahn,
" It doesn’t seem like a bunch of the initial load on ends of the beam would magically move to the center."
That's exactly what would happen as soon as any load was applied to the floor - say you had a party or you brought in a heavy sofa - the mid-span beam would take up twice what each end did.
It's important not to conflate deflection and bouncing though, although both are caused by an inadequate ratio of height to span on the joists.
Bouncing doesn't occur due to excessive loading or sagging. It can happen in a room with few loads, except someone walking through it. In fact one of the pieces of engineering advice to cure bouncy floors is to increase the loads within the space by adding heavy pieces of furniture. Deflection is sagging due to excessive loading.