Hy-land,
Re bowing :-
Bowed timber can be used to good effect, for example, if a tree is halved to form two cruck blades or storey posts then provided these are placed as a mirror image pair opposite one and other in a crossframe (bent) then the degree of bow will hopefully be the same over the whole length (height) of the crossframe and so all horizontal members fitting across the frame should still fit perfectly. Where a recognised difficulty will exist is in alignment along the length of the long wall frames where any deformation due to stress release will need to be accounted for in the layup of these frames. This is where employing scribe practice really comes into its own. Please consult the early Mortice & Tenon publications where Henry Russell provides a commentary covering this topic in respect of the layup of cruck frames. He uses the term - "let the timber dictate the frame" - good advice.
Re Boxed Heart :-
Provided the tree size is kept small (6" & below) then the opening up of large checks will probably be minimised however once conversion requires beam sizes above this then some drying heart checks should be anticipated. These will especially be promoted along a line of mortices cut into a beam i.e. as found in a summer beam or on the side of the timber nearest to an offcentre heart. Spiral grain timbers will develop a multiplicity of small surface checks and so these can be used in applications where large heart checks need to be avoided for example in mill drive shafts or in free standing columns. Boxed heart timbers are less prone to large knot defects and when encountered these will usually be on one face or aris only. All branches start growing from the heart and in one direction only. When knot whorls are encountered as ocurs with some coniferous trees (pine, fir, etc.) or certain hardwoods (cherry) then provided joints can be set out to avoid whorl locations then the live element of knots will tend to act like reinforcing bars and help prevent the timber from opening up along a continuous line.
Re FOHC :-
Where timbers can be obtained from really big trees then FOHC does become an option to help prevent large checks but this benefit might have to be traded with a degree of bowing and if the scantling cut from the FOHC timber is relatively small then the resulting timber can be significantly weakened by the presence of large knots, obviously branches grow bigger the further away from the heart they are encountered and these might appear on two oposing faces if a branch goes all the way through the timber or worse still all the way across a face as in quartered FOHC.
Regards
Ken Hume
