How make wooden beam for construction use?

Answer 1

The best single source on all aspects of wood construction, preservation, coatings, maintenance and repair including laminated beam building is the nearly exhaustive, 496 page and free, "Wood Handbook, Wood as an Engineering Material" by the US Department of Agriculture, Forest Service, Forest Products Laboratory.

Wood beams built up from smaller pieces of wood can be made in several different ways depending on the way they are intended to be used and where they are to be used.

Beams that are to be used for decorative purposes are significantly different than beams that are to be used for structural purposes. Decorative beams include those used in cabinetry, furniture and other typical woodworking projects.

Structural beams carry loads where failure of the beam means failure of the structure it is part of. The proper sizing and design of beams in structures should always be checked by a licensed engineering consultant or engineer in a city building permit department.

A major difference between decorative and structural beams is the type of fasteners or glues to be used in making them. Many popular high strength glues, like Titebond III, that are very acceptable in decorative beams, have a warning that they are not to be used for structural purposes in their fine print, "so read the fine print!" If in doubt, don't!

Another major difference between beam applications is the use environment that a beam is exposed to. Beams used outdoors or in an interior application where they might accidentally become waterlogged need glues and fasteners that can withstand extended water contact. Outdoors applications will also need glues that resist mildew and other causes of wood rot.

A trusted adhesive for structural beams with all the above properties for exterior structural applications is resorcinol glue [ one such product is DAP's two part Weldwood Resorcinol Glue ]. While sometimes hard to find locally, it can be ordered over the internet.

The next issue is the wood itself. The type of wood and the quality of the boards being laminated, the orientation of the grain in laminated boards, their dimensional accuracy and the glue selection determine the strength of a laminated beam.

Laminated structural beams create some advantages over beams made of a single piece of wood. Defects in the individual laminated pieces may have less cumulative effect on reducing the beam strength than they would in a single piece beam.

When a structural beam is made of pieces with a glue line parallel with the direction of the load, the orientation of the wood grain is going to be important. This type of beam needs lumber with vertical grain.

Where the glue lines are perpendicular to the load, flat grain lumber is needed with the bark side oriented toward the direction the load is coming from. In flat grain lumber laminations, sometimes lower quality pieces can be used for interior layers with clear, parallel grain pieces used on the top and bottom layers of a beam loaded from the top.

Structural beams, longer than the pieces of wood that they are composed of, are created using finger or scarf joints between pieces in a single layer. While finger joints are shown in the free DOA book, most finger joint router bits available to "do it yourselfers", create joints that have a significant area of end grain butted against end grain and are unacceptable. Scarf joints are the best for DIY and should be angled at a ratio of 1:6 [ thickness to length ] or greater. Do not scarf joint boards with loads parallel to the glue line without professional advice and help. Plane the mating scarf joint surfaces accurately. It is very important for beam integrity.

Glue joints, as well as all surfaces to be glued, must not be sanded before gluing. Sanding destroys the surface cell structure and radically weakens the joint. It may also be necessary to plane the surfaces just before gluing to insure that the surfaces are unaffected by weathering and drying.

Before preparing wood surfaces for gluing, the wood must be dried and any crown or hump caused during drying must, eventually, be oriented toward the beam's load [ crown upwards if the beam is being loaded from above ].

Having prepared the pieces, resorcinol glues must be used and joints cured at temperatures above 70 degrees Fahrenheit. Temperature is very important. The glue will also become very hard when cured and allowing cured excess to accumulate can become dangerous during later sawing or finishing, so wipe excess off before it cures.

During lamination and cure you can use fasteners or clamps to compress the glue line during cure. Sometimes the use of fasteners should be avoided because they can cause the loads on the beam to concentrate in unexpected ways. Once again an engineering consultant or engineer in the building permit department is needed.

When using the completed beam, care must also be exercised not to install it in such a way that the connection and fasteners cause loads to defeat the beam lamination's integrity. For example, fastening lag bolts into a beam so that their load is carried by only part of the beam's layers. Sometimes using metal joint reinforcements that bolt to a piece connecting to the laminated beam and that hang over the beam for fastening are advised so the load is carried by the whole beam.

An example of these issues can be found in, "Building with Glue-Laminated Beams" by Kevin Hayes. It can be found at the Extreme How-To website,

I have built many beams like this as well as a large telescope mount. I am currently building laminated beams for my house's entryway pergola.

Answer 2

you can sandwich 2 x 8's together