Engineering and Construction

Tweetear

Building codes require that Type IV Heavy Timber buildings have floors and roofs of solid wood construction without concealed cavities. Figure 4.18 shows different types of decking used for these purposes. Minimum permissible thicknesses of decking are given in Figure 4.7. To meet code requirements, floor decking must also be covered with a finish  floor consisting of nominal 1-inch (19-mm)  tongue-and-groove boards laid at right angles or diagonally to the struc- tural decking. In some circumstances,  ½-inch (13-mm) plywood or other composite wood panels are also per- mitted as the finish layer.

Figure 4.18

Tweetear

Where heavy timber beams join masonry or concrete walls, three problems must be solved: First, the beam  must be protected from decay caused by moisture seeping through the wall.  This is done by leaving a ventilating  airspace of at least ½ inch (13 mm) between the masonry and all sides of the beam except the bottom, unless the beam is chemically treated  to resist decay. The second and third  problems have to be solved together:  The beam must be securely anchored  to the wall so that it cannot pull away under normal service, yet it must be able to rotate freely so that it does not pry the wall apart if it burns through and collapses during a severe fire (Figure 4.12). Traditional and more contemporary methods of accomplishing these dual needs are shown  in Figures 4.11 and 4.13, respectively.

Figure 4.11 Two alternative details for the bearing of a beam on masonry in traditional Mill construction. In each case, the beam end is fi recut to allow it to rotate out of the wall if it burns through (Figure 4.12), but it is also anchored against pulling away from the wall by means of either lag screws or a lug on the iron bearing plate.

Figure 4.12 A timber beam that burns through in a  prolonged fi re is likely to topple its sup- porting masonry wall (left) unless its end  is fi recut (right). In this illustration, the beam is anchored to the wall in each case  by a steel strap anchor.

Figure 4.13 An example of contemporary heavy timber construction, based on steel plate connectors and an insulated cavity wall.

Tweetear

Where the edges of ß oors and roofs of a heavy timber frame are supported on concrete or masonry, special attention must be given to the potential for differential shrinkage between  the outer walls and the interior wood  column supports. Wood, compared  to masonry or concrete, expands  and contracts more with changes in  moisture content, particularly in the  direction perpendicular to its grain.

These changes occur over a period of  years as large timbers gradually dry  and seasonally with changes in ambient conditions. A heavy timber frame  building is detailed to minimize the  effects of this differential shrinkage  by eliminating cross-grain wood from  the interior lines of support. In traditional Mill construction, cast iron

Figure 4.7 Minimum sizes for solid wood members used in Type IV Heavy Timber construction, as specifi ed in the IBC.

Tweetear

In addition to the issues of sustainability of wood production and use that were raised in the previous chapter,  there are issues that pertain especially to heavy timber  frame construction:

It is wasteful to saw large, solid timbers from logs:
In most instances, only one or two timbers can be  obtained from a log, and it is often difficult to saw  smaller boards from the leftover slabs.

Glue-laminated timbers and composite timbers utilize  wood fiber much more efficiently than solid timbers.

Recycled timbers from demolished mills, factories,  and barns are often available. Most of these are  from old-growth forests in which trees grew slowly,  producing fine-grained, dense wood. As a result,  many have structural properties that are superior to  those of new-growth timbers. Recycled timbers may be  used as is, resurfaced to give them a new appearance,  or resawn into smaller members. However, they  often contain old metal fasteners. Unless these are  meticulously found and removed, they can damage  saw blades and planer knives, causing expensive mill shutdowns while repairs are made.

Continuous bending action of beams may be created  by splicing beams at points of inß ection rather than  over supports, as shown in Figures 4.15, 4.20 and 4.21.  This reduces maximum bending moments, allowing  timber sizes to be reduced substantially.

Timbers do not lose strength with age, although they  do sag progressively if they are overloaded. When a  heavy timber building is demolished at some time in the future, its timbers can be recycled, even if they  were obtained as recycled material for the building  that is being demolished.

A heavy timber frame enclosed with foam core  sandwich or stressed-skin panels is relatively airtight  and well insulated, with few thermal bridges. Heating  and cooling of the building will consume relatively  little energy.

The glues and finish coatings used with glue-laminated  timbers may give off gases such as formaldehyde that  can cause indoor air quality (IAQ) problems. It is  wise to determine in advance what glues and coatings  are to be used, and to avoid ones that may cause  IAQ problems.