why do standard modular brick units have 3 holes in them

huangyhg

why do standard modular brick units have 3 holes in them?
i looked on the bia website and didn't see anything...is it just to save weight?
thanks, just curious
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i think it may have to do with shrink crack control after firing.
steve wagner
because 4 was too many and 2 was not enough?
sorry, it probably has to do with the manufacture of these, it may be an extrusion process like for hollowcore slabs.
csd
'cause it worked 100 years ago when someone had the problem to solve, and we've done it that way ever since (and forgotten why) ?
seriously tho', i think to control shinkage sounds good; different question, how are they formed ? (dowels, cut out after the mound is filled ?)
it makes the material lighter and allows you to be able to reinforce it.  brick also comes solid as well.
don phillips
i've never seen reinforcing put down into one of the brick holes before.
the holes help the bricks to cool evenly after firing to avoid cracking.  more importantly they allow the mortar to squeeze into them for a doweling effect.  the holes are too small for any reinforcement.
old clay bricks used shallow depressions top and bottom instead for both reasons.  the shallow depressions were easy for manual casting.  modern bricks are extruded.
saves clay and suits the manufacturing process. the depressions in old bricks are called "frogs".  did once see someone try to reinforce in the holes, but that was with two hole bricks.  with three holes, don't think they would line up in running bond.
we have to re  
using the holes for reinforcement would only ever be advisable for protected (dry) locations.
even if you could get the bar through the holes the grout could not be relied on to provide adequate cover for durability. or rather, the grout could be relied on to not provide adequate cover.
an update from my post above.
what i assumed were hollow clay block because of size, are actually classified as hollow clay brick.
so that means products like interstate brick's 8x8x16 super atlas have to comply with astm specification for hollow brick, c 652. table 1 of this standard allows up to a maximum nominal width of 12 inches for class h60v hollow brick.