Roof, Floor and Wall Diaphragms Values for Douglas Fir sheathing grade ply, wind loading.
All sheet edges blocked, nails 6" centres on edges, 12" centres intermediate framing.
Ply
Allowable
thick- Nail Gauge shear in lb/ft
Пess
size
run of wall
2*
11
250
รี
21-
3"
10
360
425
4
5
as given in BS 476. Where improved fire properties are needed, retardant treatment by surface application or pressure impregnation can be used to bring the plywood up to Class one.
Durability
The bonding of plywood manufac- tured to the Canadian, Finnish and ABPVM specification is unaffected by moisture, temperature, fungus or in- sect attack but the durability of the material is dependent on the consti- tuent wood and will vary according to the species.
The durability of all types will be adequate for normal short-term ex- posure on site and where subject to long-term exposure to weather or other adverse conditions the durability can be made adequate by surface coating or by the use of preservatives with dip or pressure application. An example is illustrated of one of several farm buildings designed by the author and erected more than ten years ago using Douglas Fir plywood sheets for the roof cladding.
These can be untreated but are liable to severe checking of the surface
veneer and subsequent moss growth. Treatment with creosote would appear to be a cheap and satisfactory way of improving the durability, inhibiting moss growth and decay and, by im- proving the water repellent properties of the surface, reducing checking.
Except for farm buildings the plywood decks are covered with a weatherproof membrane (roofing felt, etc.) and the deck has of course been quite satisfactorily left exposed to the weather for long periods until con- venient to apply the finish. Applications-formwork
The high rigidity and strength/ weight characteristics of plywood make it an ideal material for efficient concrete formwork. With high impact resistance and dimensional stability under the conditions of alternate wet- ting and drying, it has become well established in this use.
Used both flat and curved, and on wood or steel frames, the economy of multiple re-uses and of rapid erection of large units makes it a widely used material for this purpose. Where a very large number of re-uses is required
plywood manufactured with an over- lay of phenolic resin impregnated paper is used.
Lightweight
construction with modular components is a type of construction becoming increasingly used for houses, schools, clinics and offices. Stability under wind force is economically obtained by the dia- phragm action of roof and areas of external wall and internal partitioning, by nailing plywood sheathing to the framework.
A normal specification is a nail spacing of 6in. centres along ply panel edges. Because of deflection the height/width ratio limit of three for diaphragm action panel is suggested. The panel must be adequately secured to the ground slab to transmit the shear force.
Plywood for roof and floor decking also provides calculated disphragm ef- fect at little increase in cost. Shear stress in a roof diaphragm is usually extremely low. The edges of the sheet may not therefore, need to be blocked and the use of H section roof clips will be sufficient. Tests show that some T
Far East BUILDER, November 1969
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