basement column footings – traditional or thickened slab?
Hi
I am sure i asked a similar question before but am asking again as the design is waffling back and forth.
I am designing my place and plan on using a radiant slab in the basement. There are five columns, three of which are in a line. The slab is 4″ with 6×6 WWM on 2″ XPS. The two options for the column footings are:
1) Traditional box formed square footings (in this case 24x24x10thick). The footing would be poured 4″ short of slab elevation and later topped by the slab. Crack control saw cuts would likely be made in the slab above the outline of the footing below. I believe this method of footings is easier for the concrete contractor to do.
2) Thickened slab footings, single slab/footing pour. The footing would be poured mono-cast with the slab. Crack control saw cuts would be made more on a 10′ grid pattern. Potentially the three in a line could be one continuous “rib”. I foresee some potential problems with forming the sloping sides in the native or compacted fill. I really don’t want voids under XPS overhangs or whatever else can go wrong. I believe this method of footings is harder for the concrete contractor to do.
Could someone knowledgeable comment on the pros and cons of each technique? Originally i was leaning toward thickened slab but after the structural engineer did his first draft and didnt require as many columns as i though might be necessary, i am swinging back towards traditional.
Thanks
John
Replies
Thicken slabs are generally used where there is a bearing wall. Pre-poured piers or footings as you described are used for post with bearing beams on them.
Pour your post footings and then install the rigid insulation over everything. Build and install diamond shaped "boxed out" forms at the column footing loactions. Install the boxes with the points aligned along the column line and on top of the isulation. Pour the slab to the diamond forms, but do not fill them in. After finishing and some cure time, remove the forms at the post footings.
You now have a diamond shaped area at each colum footing with the points aligned in the direction of your crack control joint cuts. Cracks in concrete slabs will tend to run diagonaly away from any outside corner, so by using the diamond (square turned on a corner) forms aligned with this way you can control the crack into the saw joint.
You can now set the columns by removing a plug out of the continuous rigid foam. Then mix and pour the the open area around the forms. I ussually do this when I have a ready mix truck on site sometime after the framing has started. A couple of five gallon buckets of concret if about all you need to fill in each diamond aroud the columns.
When I did my new house recently, I did method #2 ... monolithic pour. I had a combination of bearing walls and column style footings and simple formed the excavation as you indicated. I put in my WWM and hung some rebar from that for extra reinforcing. My control joints were sawed like you say 10+ ft OC and at any interior corners. I also had a radiant slab. Method #1 seems like a PITA two step process that really isn't necessary.
Avoid voids under the insulation by cutting the insulation appropriately. Small voids are probably non issues, really, though, but most should be taken care of in the pour when they vibrate it into place. Pretty straight forward approach I thought. I did the WWM and a lot of the reinf (since I installed the tubing); the contractor did the pour and finish work. BTW ... I'm NOT a concrete expert.
Method#1
Method # 1 is done when the wall footings are poured, or when the walls are poured if you don't trust the layout off strng lines and batter boards.
No extra step. Just part of the first one.
The extra step could be pouring the diamond areas around the columns, but agian there is ussually more concrete work to be done after the slab is poured.
The diamond around columns has the advantage in sa cutting control joints because you can cut right to eac point of the formed out area. With monolithic pours and the columns set, you can cut to within about six inches of the post. That is if you set the post first and start framing before the basement floor is poured (not and uncommon practice around here.)
BTW, how could you have voids under your rigid foam?
I have formed and poured several insulated slabs and never seen that issue.
Having a little tough time following your train of thought.
Voids might happen if you layout your insulation and it overhangs the thickened area just a little and doesn't get filled during the pour ... although a good pour wouldn't really let that happen. You wouldn't actually ever see it ... as you really never know if it happens.
If the pour is monolithic, you saw cut the control joints w/in a day of the pour (or even hours), so, as a rule, there should be no post to work around with respect to the saw cut. If it is a monolithic pour, the basement floor IS poured w/ the column base.
Method #1 - diamond shaped blockouts
Dave
I really like the system you describe. The layout of the columns is critical to me and I've been on sites where the anchor bolts are beside the plate because they measured off the perimeter footing instead of the foundation wall or visa-versa. Who knows where the under slab column footing really is.
Just to clarify you say “diamonds” but I think these are actually true squares rotated 45 degrees. In my case with 24” square footings and likely 6x6 columns what size are the blockouts?
You wouldn’t have a photo you could share would you?
(This technique is attractive to me also because I am not really confident in the locally available column bases that rely on anchors drilled into the slab. We are in a seismic zone and I want the best! After the slab pour and removing the bit of XPS under the blockout I might drill and grout some rebar dowels to tie the plug pour to the actual footing.)
(As far as how voids could happen I can foresee our contractor cleaning out the unformed trench and undermining the XPS. Once undermined it’s impossible to put it back without a lot of rework which isn’t going to happen. On civil engineering projects we get voids all the time under horizontal formwork. We discover them when we pull the forms and they get filled. That too won’t happen on an insulated slab.)
Thanks
John
Forget the WWM
The welded wire mesh is almost useless. I have never done a demo where it was actually anywhere but in the bottom third of the slab, and generaly it is out of the slab and in direct contact with the gravel base.
Go with #4 bars at 12 or 16-inches oc, and enough chairs under them to limit the deflection to less than an inch when you stand on them. If the slab is going to be power screeded, then you probably need #6 bar at 12-inch oc to support the weight of the machine.
"I have never done a demo
"I have never done a demo where it was actually anywhere but in the bottom third of the slab, and generaly it is out of the slab and in direct contact with the gravel base."
Maybe that's why you're doing the demo ... it wasn't done right. :) If it was done right, you wouldn't be doing the demo, so you really don't know what was done right ... only what was done wrong.
There was the story during WWII that some academic type had mapped all the bullet holes in returning Allied bombers and suggested that the places where there were the most bullet holes were obviously the most vulnerable to German fighters and hence deserved the heaviest armor. But then some wise-arse at the back of the room popped up and suggested that they should armor the areas that had no bullet holes -- since the holes were in those bombers that made it back.
Have to be careful about the conclusions we draw from data sometimes. What seems obvious/logical may (or may not) be valid.