The figure shows the cross-sections of the most commonly made nailed and glued beams and girders. Rectangular and I-section layered sections (Lynx. b, c) is used in beams, arches or frames. I-section and box sections (Lynx. d, e, f) they are also used in girders and frames.
Sections of beams and nail or glued board girders: a), b), c) layered I-and rectangular, d) I-girder, e), f) box; 1 - web made of boards or plywood, 2 - shelves made of boards or logs.
In beams or girders of greater length than the commercial length of the lumber - boards, logs and square timber are joined along their length. In the case of nailed elements with a layered cross-section, the boards are butt jointed, in other elements, butt joint with a cover plate, while in glued elements it is joined with a butt joint or diagonal joint.
Sandwich beam: a) arrangement of board contacts, b), c) glued wedge joints, oblique, d) frontal contact.
The butt joint is assumed in these glued elements, which work in compression or are located in this zone of the beam, in which there are compressive stresses.
Board joints (contacts) it is staggered along the length of the element. There should be no more contacts than in one section of a sandwich element 25% all boards. Contact distance in adjacent layers (bet) – resulting from the possibility of including an adjacent board in cooperation - it should not be less than 20 δ or 50 cm (δ - board thickness).
Planks are used to make nailed structures, bali, battens and square timber, wooden plywood and wood-based materials and nails. Plywood and wood-based materials are used for the webs of I-beams and girders and box girders.
Round and square nails are used as fasteners. The diameter of the nails in the joints of wooden elements should be from 1/6 do 1/11 thickness of joined elements, while in joints made of hard boards and plywood with a thickness of up to 8 mm, the diameter of the nails is taken within 2,0-4,0 mm. The length of the nail should not be less than 1 = δ + 12 (δ— thickness of the connected element, d - Nail diameter).
The ways of placing the nails in the elements connected in parallel and at an angle are presented in the drawing.
Arrangement of nails in connected elements: a), b), c) parallel, d), e) at an angle.
For the production of glued structures, sawn timber with a thickness of up to 5,0 cm. Elements of greater strength are obtained from thinner sawn timber, more resistant to delamination and weather conditions. However, this is associated with the consumption of more adhesives and greater wood losses due to planing. Thinner lumber is used for curved elements, e.g.. frame nodes. If the radius of curvature is R = 300δ, boards of thickness δ = are assumed 4,0 cm, while for R = 200δ it is assumed δ = 3,0 cm.
Glues are used to glue wood: casein, urea and resorcinol. Casein glue is gradually being replaced by other glues, supplied by industry.