Timber Properties
Structural Engineering for Architects: A Handbook, by Peter Silver, Will McLean, Peter Evans page 49
Strength Class
Timber can be chosen by species, but is more commonly specified by stress grade for building purposes. Each piece of timber with a particular strength class will have similar bending, compression and shear capacities.
Orthotropic
Timber is an orthotropic material, and has varying structural properties in different directions. This is particularly relevant in shear design of timber beams as the shear capacity parallel to the grain is significantly lower than the shear capacity perpendicular to the grain; hence, when a beam is loaded in direction perpendicular to the grain it will generally fail in shear due to the complementary shear stress as opposed to the normal shear stress.
Natural Material
Timber being a naturally occurring material means that it contains imperfections and irregularities such as knots and can develop splits, know as shakes, as it dries out. In addition timer is a hygroscopic material meaning that it will give up moisture from the atmosphere depending on the relative humidity of its surroundings. Timber is unique among structural materials in these respects. Engineered timber products, such as glulam and cross laminated timber are manufactured from thin layers of timber glued together. This ensures enhanced mechanical properties in comparison to standard timber, as any imperfections are distributed across the length of the member as opposed to being concentrated in one area at a knot.
Creep
As with concrete, timber is subject to creep; the strain can increase by 60 per cent over ten years under permanent load. This is often allowed for in the design by the use of load duration factors, with higher factors being applied to loads applied for longer periods.
Service Class
Timber exhibits different properties when wet, and therefore the design must recognise the likelihood of the timber becoming wet and amend the material properties accordingly.
Timber can be chosen by species, but is more commonly specified by stress grade for building purposes. Each piece of timber with a particular strength class will have similar bending, compression and shear capacities.
Orthotropic
Timber is an orthotropic material, and has varying structural properties in different directions. This is particularly relevant in shear design of timber beams as the shear capacity parallel to the grain is significantly lower than the shear capacity perpendicular to the grain; hence, when a beam is loaded in direction perpendicular to the grain it will generally fail in shear due to the complementary shear stress as opposed to the normal shear stress.
Natural Material
Timber being a naturally occurring material means that it contains imperfections and irregularities such as knots and can develop splits, know as shakes, as it dries out. In addition timer is a hygroscopic material meaning that it will give up moisture from the atmosphere depending on the relative humidity of its surroundings. Timber is unique among structural materials in these respects. Engineered timber products, such as glulam and cross laminated timber are manufactured from thin layers of timber glued together. This ensures enhanced mechanical properties in comparison to standard timber, as any imperfections are distributed across the length of the member as opposed to being concentrated in one area at a knot.
Creep
As with concrete, timber is subject to creep; the strain can increase by 60 per cent over ten years under permanent load. This is often allowed for in the design by the use of load duration factors, with higher factors being applied to loads applied for longer periods.
Service Class
Timber exhibits different properties when wet, and therefore the design must recognise the likelihood of the timber becoming wet and amend the material properties accordingly.
Wood Joinery Techinques
Butt Joint:
This is one of the easiest joints to make. It consists of joining two pieces of wood at right angles and holding them together with nails, screws, dowels, or biscuits. A biscuit is a thin, flat, oval shaped disc of wood that is inserted into a pocket hole that is made in the surface of the two pieces of wood that are being joined together. Biscuits and dowels add strength to a joint. Glue should be used if using a dowel or biscuit. If the item is to be used indoors, then regular carpentry glue can be used. If the item is to be used outdoors, then a waterproof carpentry glue should be used. While this is not the strongest connection by itself, reinforcement can be added for strength.
Lap Joint:
If appearance is a factor, a lap joint may be in order. This type of joint has a recess cut in one piece of wood - equal in depth to the thickness of the crossmember - which will hold the other piece of wood.
A half-lap has a slightly different technique. A recess is cut in both the crossmember and the side member, but only half the thickness of the piece of wood. The appropriate glue should be used in all joints to add strength to the joint.
Dado Joint:
A dado joint is a way of suspending shelves from its side supports. To make a dado joint, draw two parallel lines equal to the thickness of the wood it is to engage. A cutaway will be made that is 1/3 the thickness of the wood.
First cut on the lines, then chisel out the wood to the correct depth. This can be done with a router, a bench saw or a radial arm saw.
A fancier type of dado - the stopped dado - cuts away only part of the wood and only part of the shelf is cut away.
First cut on the lines, then chisel out the wood to the correct depth. This can be done with a router, a bench saw or a radial arm saw.
A fancier type of dado - the stopped dado - cuts away only part of the wood and only part of the shelf is cut away.
Mark your guides for a stopped dado and chisel away the area to be recessed. Cut away on the connecting board for an accurate fit.
Mortise and Tenon Joint:
Mortise and Tenon Joint:
A strong joint, used in projects where no other reinforcement will be used, is the mortise and tenon joint. This requires a bit more skill but is worth the effort for a professional look.
There are two parts to the joint- the mortise and the tenon. The mortise is the recessed part of the joint, and the tenon is the part of that fits into the mortise. There are many kinds of mortise and tenon joints in frame construction, leg-and-rail construction, as well as other types of assembly. The blind or simple mortise and tenon is used in leg-and-rail construction. If you want square corners, the cut is made on a mortiser or mortising attachment. The mortise should be at least 5/16" from the outside face and at least 1/8" deeper than the tenon for clearance. The tenon should be about one-half the thickness of the stock. If you want rounded ends, the rounded ends are cut on a router.
A strong joint, used in projects where no other reinforcement will be used, is the mortise and tenon joint. This requires a bit more skill but is worth the effort for a professional look.
There are two parts to the joint- the mortise and the tenon. The mortise is the recessed part of the joint, and the tenon is the part of that fits into the mortise. There are many kinds of mortise and tenon joints in frame construction, leg-and-rail construction, as well as other types of assembly. The blind or simple mortise and tenon is used in leg-and-rail construction. If you want square corners, the cut is made on a mortiser or mortising attachment. The mortise should be at least 5/16" from the outside face and at least 1/8" deeper than the tenon for clearance. The tenon should be about one-half the thickness of the stock. If you want rounded ends, the rounded ends are cut on a router.
My Joinery Technique that Suits My Model
The first joinery technique I believe suits my model is- weaving timber, the method is simple. Fortunately, the strength is in overlapping. So the simple automatically reinforces itself.
Joinery Chosen for Model
Vector-works Timber Joint
Here I have added the reinforced nails, to make the joint connection extra secure and sturdy. I also included extra length to each end to add further detail, if needed.
Black and white final rendering
Coloured final rendering