Engineering and Construction

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The cruck (Figure 4.2), cut from a  bent tree, was a form of  rigid frame or  portal frame. Today's rigid frames are glue-laminated to shape and find wide use in longer-span buildings.  Standard configurations are readily available (Figures 4.22-4.24), or the  designer may order a custom shape.  Rigid frames exert a horizontal  thrust, so they must be tied together  at the base with steel tension rods, also  called  tie rods. In laminated wood construction, rigid frames are often called arches, acknowledging that the  two structural forms act in very nearly  the same manner.

Figure 4.2 European timber house forms generally followed a progression of development from crude pit dwellings, made of earth and tree trunks, to cruck frames, to braced frames. The crucks (curved timbers), hewn by hand from appropriately shaped trees, were precursors to the laminated wood arches and rigid frames that are widely used today.

Figure 4.22 Three-hinged arches of glue-laminated wood carry laminated wood roof purlins. The short crosspieces of wood between the purlins are temporary ladders for workers.

Figure 4.23 Typical details for three-hinged arches of laminated wood. The tie rod is later covered by the fl oor slab.

Figure 4.24 The shear plates in the crown connection of the arch, shown here in a larger-scale detail, are recessed into grooves in the wood and serve to spread any force from the steel rod across a much wider surface area of wood to avoid crushing and splitting.

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With large, old-growth trees no longer readily available, very large timber  beams are usually built up of laminations or strands rather than sawn. Such beams are stronger and more  dimensionally stable than sawn wood  beams and can be made in the exact  size and shape desired (Figures 4.20  and 4.21). Hybrid or FRP reinforced  glulam beams, can be useful when especially lon- ger span are needed or heavy loads  must be supported.

Figure 4.20 Installing tongue-and-groove roof decking over laminated beams and  girders.

Figure 4.21 This roof uses glue-laminated beams to support proprietary long-span trusses made of wood with steel tube diagonals.

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The heavy timber structure poses some special problems for the designer because the exposed framing and decking do not provide the concealed cavities that are present in light frame structures and other conventional building systems. Roof  thermal insulation cannot be hidden in spaces between ceiling joists  or roof rafters and, instead, must be placed on top of the roof deck. If the  roof is nearly flat, it can be insulated and roofed in the manner shown for  low-slope roofs in Chapter 16, or, if  the roof is steeply pitched, a nailing surface for the shingles or other roofing must be added on the outside of  the insulation. Electrical wiring for lighting fixtures that are mounted on the underside of the decking must  either run through exposed metal  conduits below the deck, which may  be visually unsatisfactory, or be channeled through the insulation above the deck. Ductwork for heating and  cooling must remain exposed. If  the walls and partitions are made of  masonry or stressed-skin panels, special arrangements are necessary for  installing wiring, plumbing, and me- chanical system components in the  walls as well (Figure 4.19).

Figure 4.19 When floors and roofs are heavy timber framed, lighting, communications wiring, ductwork, fi re sprinklers, and other services are exposed to view.

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A heavy timber frame building with an exterior masonry or concrete bearing  wall is normally braced against wind and seismic forces by the shear resistance of its exterior walls, working  together with the diaphragm action of its roof and floor decks. In areas of high seismic risk, the walls must be  heavily reinforced both vertically and horizontally, and the decks may have to be specially nailed or overlaid with plywood to increase their shear resistance, as well as specially anchored  to the perimeter walls. In buildings with framed exterior walls, diagonal bracing or shear panels must be provided. Seismic upgrades to historic  heavy timber and masonry buildings often require the insertion of new steel-braced frames or reinforced  concrete shear walls in order to meet contemporary lateral force resistance requirements.