Expert: Bruce E. Johnson - 11/11/2010

Question
Hello,
I am in the preliminary design phase of building a new breezeway and two story Cape style garage (second story formed by attic truss).

I am a civil engineer (only in field for a few years now), but have done little work in foundation design, and I am trying to figure out how to attach new CMU (or possibly concrete) foundation wall to existing CMU wall.  I also plan to open the existing wall with a doorway for access to new "basement" section.  I assume to use steel lintel or beam here for the short 6ft opening.

So, in your expereince and experience, how would I, or how have you pin/attach/connect new to existing.

Thanks a bunch.  I am in NY, so foundation wall will be about 9ft

Answer
Hi Adam, you have a couple of options and I will try to cover them the best I can without really knowing how your existing foundation is constructed, whether or not you have reinforced filled cells or the existing CMU wall is filled solid or has a concrete tie-beam or a reinforced grouted bond beam.  

First of all, you don't necessarily have to connect the walls structurally.  A foundation with properly sized footings and well compacted soil can stand alone next to the existing foundation.  If built this way then you leave a gap between the two walls and install a soft joint of backer rod and Urethane caulking inside and outside at this juncture.

If you want to have a positive connection between the old and new walls and the old wall has an open vertical cell at the point of connection you can punch fist sized holes in the existing open cell and insert bent reinforcing rods into the open cell and then pump the cell full with concrete grout.  The protruding ends of the reinforcing bars will then be attached to reinforcing bars of the new wall/s. This requires that you install horizontal bond beams every four feet in the new wall and the placement of the tie rod steel in the old wall at the same elevations to allow for the tie in.

If the old wall has a concrete filled vertical cell at the location of the tie-in then you can simply drill and install epoxy anchors into the old wall.  These are usually #5 or #6 bars and embedded at least 6" into the existing concrete using a good bonding epoxy and protruding  16-24" to be tied to the new concrete or masonry wall via bond beams or ties beams as mentioned in the previous paragraph

I always make sure that I have a double filled cell (two adjoining reinforced filled cells) at the ends of my addition walls essentially creating a concrete pilaster at these points.  Your protruding steel can also be tied crossing these filled cell reinforcing bars to provide you with  a good attachment.

Some people will just install masonry wall anchors at these junctures tapconned to the old wall and tied into the new courses of mortared block on the new wall.  This is probably adequate with a properly built supporting footing but I would at least epoxy some steel bars into the bond beam or tie beam of the old wall for added connection.

I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com