Oak butt weight

[Bogoak]

There have been a few references to boxing the heart recently which has made me think about how relevant it is. Perhaps it was just the traditional way of squaring off a log by hewing the sides with an axe before bandsaws were invented and so now not needed but we continue to do it because it is the way it has always been done?

 

In terms of strength, visual grading is based on the visible defects on the outside of a beam or column and since the most significant defects tend to originate in the heart, a boxed heart section with the pith at the centre, milled from a small/medium sized log is likely to have fewer visible defects on the outside and hence, I would say this is likely to achieve a higher strength grade compared to a section milled with the pith on one face for instance. So that is an advantage.

 

Nowadays large diameter logs can be easily handled with machinery and milled so there is no need to be restricted to small/medium diameter logs. The outer parts of large logs are likely to have more closely spaced growth rings and might be completely clear of defects such as knots, wane and fissures. So I would speculate that this timber will have a high strength grade, maybe even higher than the boxed heart section. But as it dries, the original sawn squared section will distort into a diamond shape whereas the boxed heart section will remain nearer to a square shape. This probably doesn't matter for floor joists or beams, but in a timber frame, the beam to column joints need to remain tightly together, held by the oak pegs. So if the column distorts into a diamond shape the joints may open excessively. But this will be more of an issue on columns with a large cross section so not always relevant.

 

Another other issue that I think might be a problem is a beam or column warping along its length as it dries. I feel that a boxed heart column which has no intermediate restraints between each floor level is more likely to stay straight compared to a timber cut from the outer part of the log. A banana shaped column is obviously weaker than a straight one. But a beam or joist cut from the outer part of a log and used as part of a floor will be restrained by floorboards or other timbers which hold it in a straight line as the frame dries.

 

Andrew

 

Perhaps it's also worth mentioning, that because of the way timber moves as it dries, boxed heart can potentially split in half (or quarters) lengthways.

[ucoulddoit]

Perhaps it's also worth mentioning, that because of the way timber moves as it dries, boxed heart can potentially split in half (or quarters) lengthways.

 

A boxed heart column will almost certainly develop long splits along its length as it dries, but surprisingly, it's more of an aesthetic issue than a concern about strength, unless it split in half along the full length, but I've not heard of that happening. John Boddy's had a method of drilling a largish hole the full length along the pith/centre which apparently reduced the splitting very significantly. I've also heard of a technique where a saw cut is made the full length of a column on the least visible side (e.g. the side against a wall) and this minimises random drying splits developing on the other sides.

 

I realised my comment about a beam/column drying and distorting into a diamond shape if milled away from the heart doesn't apply for those that are quarter sawn.

 

Andrew

[agg221]

I've also heard of a technique where a saw cut is made the full length of a column on the least visible side (e.g. the side against a wall) and this minimises random drying splits developing on the other sides.

 

Andrew

 

Funnily enough I have literally just installed one of these (finished about 15 minutes ago). The beam section is 9" x 9" to line up with the wall and give headroom. The span is 3.3m so it required a steel flitch plate let in to the top.

 

I milled the original beam and immediately cut the slot with a mini-mill, using the narrowest kerf saw I could get my hands on, which turned out to be my neighbour's McCulloch on a 1/4" narrow kerf bar. I offset the cut 6" - 3" so it lines up properly with the joists, which are notched in, and with the upper beam which is 5.5" wide.

 

Shrinkage in trees is always around the ring, meaning that the slot widened, becoming wedge shaped. I have now trued it up to take a 12mm steel plate which will be bolted through, and made a wedge which will drop in to fill the slot.

 

The first picture shows the side and underside - you can see that this is clean without splits. The second picture shows the slot from the top, which will obviously be under the floor.

 

Alec

[Big Beech]

1 M3 of green Oak weighs around 900kg