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Our existing floor joists and decking are over an 8" concrete foundation wall. The new work pictured below extends the existing 2x4 frames for more insulation space and we want to secure the new sill plate and original flooring to the concrete slab for Seismic Zone D (this will also become a shear wall, but thats another story).

  • What kind of concrete anchor should be used to tie the old joists as well as the new framing to the concrete?
  • If you suggest bolting all the way through from the top of the new sill plate into the concrete, what kind of drill and bit should be used and how deep into the concrete should it go?

Pictured from top to bottom:

  1. New framing for additional insulation
  2. New sill plate above old decking
  3. 3/4" tongue-n-grove solid wood decking from 1951
  4. A 2x10" floor joist
  5. 2x8" (?) original sill plate laid horizontally (2" showing)
  6. 4" of concrete above the garage floor
  7. concrete garage floor

sill plate over floor joist over concrete

This diagram describes the wall:

sill plate over floor joist over concrete diagram

KJ7LNW
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  • Can you get to the back of the very bottom sill to verify that it is adequately bolted down and possibly use a few tiedowns? – Aloysius Defenestrate Oct 19 '21 at 12:59
  • An impact drill is almost always what you use to drill through concrete like this. Looks like a power drill/impact driver, but with a perpendicular handle bar toward the front, and has forward hammer motion on the bit rather than torque (rotational) motion applied like an impact driver. – TylerH Oct 20 '21 at 19:36
  • Another resource for hold-down and shearing wall construction: https://www.preventionweb.net/files/7615_fplans07.pdf – KJ7LNW Oct 21 '21 at 01:21

2 Answers2

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The wall studs and sill plate should be fastened to the 2x12 joist using tie-down straps. The first picture shows the details (from woodworks.org; the second is the available strap from Simpson Strong-Tie. However, you need to consult with a structural engineer to determine the magnitude of forces the tie and the connection are to experience during an earthquake event.

enter image description here

enter image description here

ADD: For clarification:

Does the sketch below reflect your situation? Please make necessary corrections, which might/will help.

enter image description here

r13
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  • I agree 100%. The twist strap drawing, however, isn't clear to me. Does that indicate that the bottom part of the strap, below the sill plate, is to be _anchored to_ the concrete foundation, or should it be _embedded in_ the foundation? I know this isn't your drawing, but I'm hoping you, or someone, might have insight... – FreeMan Oct 19 '21 at 15:59
  • @FreeMan It is unclear to me too. But, since it is a tie-down for uplift, so I think it can be done either way, especially for retrofit works. For OP's case, I think it has to be fastened to the 2x12 joist to make the wall and the floor a whole unit. If it is possible, I prefer to at least double up the 2x12, so on top of the strap, the sill plate can be adequately nailed to the joist to gain shear strength. – r13 Oct 19 '21 at 20:30
  • I'm not quite sure I understood what you meant by "double up the 2x12". Do you mean we should place the metal strap on the existing 2x12 and then cover the strap with another 2x12? – KJ7LNW Oct 19 '21 at 20:36
  • @KJ7LNW Sister the 2x12 makes it double-wide to meet the high demand in nailing for a strong shear wall. – r13 Oct 19 '21 at 20:55
  • Good idea. Unfortunately it would then stick out further than the framing and the sheathing won't be flat across the whole surface. – KJ7LNW Oct 19 '21 at 23:13
  • Yes, just an idea if doing it from fresh, or have room for it. You do need the blessing from an engineer if it is going to be a shear wall in seismic zone D, which can't take it lightly. Good luck.. – r13 Oct 19 '21 at 23:31
  • The concrete sill only raises 4" above the concrete floor, so none of the suggested straps will fit on the concrete side because they need 1.5" clearance above the first screw and that leaves only 2.5" of concrete below the first anchor screw---but the concrete anchor length is about 5" . Ideas? – KJ7LNW Oct 20 '21 at 00:28
  • @FreeMan and r13 the one on the top right is intended to be embedded in the foundation as an alternative to J bolts. The top left one and bottom one are intended as after-construction add-ons. – TylerH Oct 20 '21 at 19:40
  • @KJ7LNW Wood is flexible (and foam is even more so). You shouldn't have much of a problem cranking down sheathing around these thin straps (they are really quite thin, like 1/8 inch thick or less, usually). Use a few screws if necessary to pull the sheathing in tight if necessary. – TylerH Oct 20 '21 at 19:41
  • @r13, about your drawing: actually the concrete that the wall sets upon is 8" wide foundation wall that goes down to the cellar another 10' or so. The garage slab floats next to the foundation wall, not connected: it was poured atop of lots of 2x8" lumber nailed together on their sides, and supported in the cellar by ~6x6 posts and ~6x10" beams! – KJ7LNW Oct 20 '21 at 23:44
  • @r13, see my OP edit with the new diagram. (Also note that the floor system is 2x10 not 2x12.) – KJ7LNW Oct 21 '21 at 01:29
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This video recommends using URFP "Universal Retrofit Foundation Plate" when there is not much concrete to tie into.

Since the shear wall will tie together #1-#5 in the question (with possible additional steel bracing between floor and framing) it becomes one unit for the UFRP to act upon:

Here is an image from the video showing the use either fastening the sheathing to a 2nd mudsill layer and the UFRP below the sheathing edge (left) or through the sheathing into the mudsill (right):

URFP through shear wall into mud sill

This document suggests that additional bracing is unnecessary in a shear wall if the cripple wall is less than 3-ft; in the original question it is about 11.5":

less than 3' cripple wall

Here is the UFRP:

URFP Universal Retrofit Foundation Plate

KJ7LNW
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  • Your 4" slab wouldn't be able to accept the anchor without damage/split it. One of my thought was to get rid of the 2x12 and cast an inverted concrete beam instead. Then you can properly embed the hold-down anchors. However, I don't know how your floor framing look like, so I didn't bother to mention it. The best you can do now is fasten the 2x12 the normal way, then tie the wall to the 2x12 to make them as one integral unit and move together. How you erect the 2x12 at the first place? Or it is not there yet? – r13 Oct 20 '21 at 01:38
  • @r13, see the OP picture above, I think it is described quite well. The 2x12 is old work. Minimum edge distance for the URFP fastener is 1.5" and the wall is ~8" thick, so might it work without damaging the slab? – KJ7LNW Oct 20 '21 at 03:13
  • If you have 8" deep concrete wall below the slab, yes, it can work. – r13 Oct 20 '21 at 12:16