Chap. XXII.
CARPENTRY.
1305
Tenon and Mortaise or Mortise ofthe most simple kind, in whichthe two timbers united are atright angles with each other; thefenon is on that which appearshorizontal, whilst the mortise is cutin the upright timber; the tenonis left one-third of the thickness ofthe timber, as shown in the upperpart of the figure.
The greatest strain upon thefibres of a girder is at the upperand lower parts, decreasing gra-dually towards the middle of thedepth, which is the best situation tomake the mortise. The form to begiven to the tenon requires consi-deration ; some carpenters introduceit at the lowest part of the girder,which in a great degree destroys itsstiffness:the lowest side.
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TENONS AND MORTISES.
being a sixth of the depth, it should be placed at one-third of the depth fromHorizontal timbers intended to bear great weights should be alwaysnotched on their supports, in preferenceto being framed in between them, andthis rule is applicable to inclined tim-bers, as common rafters and braces ; allthe pressures to which they are sub-jected should be brought to act in thedirection of their lengths, and the formof the joint should be such as to conveythe pressure as much as possible intothe axes of the timber ; when subjectedto a strain, a partial bearing is liableto very serious disadvantages, parti-cularly in bridges.
Where the mortise is to be madein the upright timber, and the tenonto be cut on another inclined, asin a brace to a partition, a bevelledshoulder is cut on the inclined piece,and a sinking made in the upright postto receive it, the pin which secures it inits mortise passing through the tenon. tenons.
The bevelled shoulder adds greatly to the strength of a mortise and tenon joint, andshould never be dispensed with; it renders the junction of the two pieces of timber moreexact, and makes the abutments of all the fibres stronger and more capable of resistance.
Fig. 2068.
BEVELLED SHOULDER JOINTS.