New Cob Cottage: Engineering notes

Footer plan: 8″ deep of 4000 psi gravel mix concrete (all concrete used in house is 4000 psi) with 2 #4 rebar placed 1/3 of the way up from bottom. Subsoil is 100% fine sand.


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Outer wall is 8″ CMU dry laid with Quikcrete Quikwall surface bonding cement (first row on footer in mortar bed). 1/2″x12″ J-hook at corners of footer (3 cores reinforced per corner) and every 4′ with same cores filled and #4 rebar in. Capped with 8″ bond beam with 1 #4 rebar.

Concrete columns 9.25″x16″ anchored to footer with 2 12″ J-hooks. 6 #4 rebar placed in column. 12″ overlap of rebar between bottom and top half of column. Clear span of beam is 75″ or less.


Columns are anchored to outer wall bond beam with 6 mm x 13″ Teplo ties (masonry ties made from extruded basalt embedded with sand with 1/20th the thermal conductivity of steel to eliminate thermal bridging) 4 at corners and 2 from each column to wall. Also columns are poured with 2 or 3 sided forms so the weight of the wet concrete during pour is squeezing against the 4-6″ of extruded polystyrene (“250” high compression board) so essentially wall and column are braced against each other.



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Arched beams are 9.25″ in width. Start at 36″ in depth and at thinnest point (top of arch are 15.5″) with 3 #4 rebar at bottom (3″ up) for an effective depth of 12″. Top of beam have 2 #4 rebar 3″ from top of beam. The posts support the post and beam frame, the beams only support floor joists for the living areas (40# live weight and 20# dead weight) and cob wall 8′ tall at +-100#’s/cu ft. The beams do not carry any roof loads. I made stirrups from #3 rebar and will have 5 approximately equidistant along the span. Control joints will be made with sheet metal placed to weaken an area of beam (along with cutting relief cut with concrete saw in line with sheet metal) which is over the column. From the top the sheet metal makes a “V” shape to make a key. See diagram below. Are these beams looking good?


Here are the beams with bottom row of 3 rebar placed:


Here are my stirrups and metal plates to make control joints:


Here are the beams with all rebar and stirrups placed:


I was going to use a central beam for the floor of the main room, but realize it is simpler and not that much more expensive to just use 2×12 joists.

#1 2×12’s 16″ o.c. says 19′-1″ with 40# live and 10# dead loads and I’m only going 18′-6″.

The house is a three aisled hall house. The tree trunks act as columns to order the space into bays and will weigh around 300# (10″ diameter, 12′ long oak at 45#’s/cu ft = 300#’s) I was going to put in rough sawn ash angle bracing and fabricate trusses, but think it would be better to use 3/4″ plywood in the lower area of the truss to make it rigid.

upper left: example of a hammer beam truss (I’m not using this plan)

middle left: darkened area needs to act as a truss

right: bottom shows 3/4″ plywood shear panel (also I will anchor to foundation properly)

lower left: detail of connection to post: flat area cut on post, 2x2s bolted to post with Spax screws (maybe 3/8″ screws 12 o.c.), 2×2 glued and screwed to posts, cross beam, and bolted through into floor framing on both sides (2×10’s). 5×5″ roughsawn ash, tree trunk, and flooring also notched and bolted together. Does this look like I’m on the right track?  If I put something like this together nice and tight does it look like it will work as a truss for wind bracing?




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