Design and selection of plywood flooring

Floor Loadings

Floor live loads for a wide range of buildings and occupancies are given in Appendix B of AS 1170-1989 "SAA Loading Code - Part 1: Dead and live loads and load combinations". The loading requirements of the Building Code of Australia are deemed to be satisfied by using this code.

Designers should make allowance for reasonable future changes to the use of buildings or individual rooms. To facilitate this, Table 1 provides a summary of general load requirements for buildings.

It can be seen from Table 5 that structural plywood has excellent distributed load capacity, thus it is obvious after reference to Table 1 that point live loads will almost always control the flooring design process.

Occasionally, the high point live loads typical of small diameter hard wheels can cause localised abrasion of flooring surfaces. Hence it is good practice to use larger diameter and/or softer compound wheels to reduce the compressive stresses normal to the flooring surface.

Design Procedure

Table 2 has been calculated for point live loads in accordance with the Timber Structures Code. The Table has been developed to satisfy the strength requirements of AS1170.1 with an overriding serviceability limit of span/200 deflection at the required strength.

In cases where the flooring is to be subjected to both human and vehicular traffic it is recommended that the calculated vehicular point load be checked against the span/300 deflection criteria data given in Table 4. Finally, Table 5 provides the uniformly distributed live load capacity of structural plywood. Table 5 is again developed to satisfy the strength criteria of AS1170.1 with an overriding serviceability limit of span/200 deflection and need only be checked for flooring carrying exceptionally high U.D.L.'s. In cases where the loading is long term, i.e. dead load with j₂=2, then the recommended approach is to double the design load before making the selection from the tabulations.

DESIGN CRITERIA

Tables 3,4 and 5 were calculated using working stress design in accordance with AS1720.1 and using the following assumptions:

1. k₁=1.4 for uniformly distributed live load, 1.65 for point live loads and j₂=1.0.

2. The point live loads are treated as a line load with a distribution width (w¹) of: 400mm for 12mm plywood; 450mm for 15, 17 and 19mm plywood; 520mm for 21 and 24mm plywood; 600mm for 26mm and thicker plywood.

3. The face grain of the structural plywood runs perpendicular to joists.

4. The structural plywood is continuous over at least two spans.

5. A structural member must support all plywood end joints; however, nogging is not required to support plastic T&G edge joints for live loads up to 7.5kN.

6. The flooring is in a dry interior environment.

Point live loads, being transient, have a shorter accumulated duration than distributed live loads, justifying the differing values for k₁. Further, with the point loads and UDL's specified in the Loading Code, the important criteria is strength, thus a deflection limit of span/200 is reasonable. The span/300 criteria for point loads need only be applied to actual loads where human reactions to vehicular traffic are considered significant.

Table 3 has been calculated for point live loads in accordance with the Timber Structures Code. The Table has been developed to satisfy the strength requirements of AS1170.1 with an overriding serviceability limit of span/200 deflection at the required strength.

Finally, Table 5 provides the uniformly distributed live load capacity of structural plywood. The Table is again developed to satisfy the strength criteria of AS 1170.1 with an overriding serviceability limit of span/200 deflection and need only be checked for flooring carrying exceptionally high U.D.L.'s.

In cases where the loading is long term, i.e. dead load with j = 2, then the recommended approach is to double the design load before making the selection from the tabulations.

Fixing Requirements

Structural plywood flooring may be fixed to the sub-floor with hand or power driven fasteners or a combination of mechanical fasteners and structural elastomeric adhesive for a more rigid squeak free system. When mechanical fasteners are used without structural elastomeric adhesive the recommended fastener spacing is 150mm centres at panel ends and 300mm centres at intermediate joists. When elastomeric adhesive is used in conjunction with the mechanical fasteners the fastener spacing may be increased to 300mm centres at panel ends and 600mm centres at intermediate joists. Table 6 details the recommended fasteners.

Reference: Plywood Association of Australia brochure: "Structural Plywood for Commercial and Industrial Flooring"

Table 1: SUMMARY OF AS 1170.1 FLOOR LIVE LOADS;

Table 2: Maximum Allowable Joist Spacing For T&G Structural Plywood In Residential Building

NOTES

1. The above Table is based on tests and is included in AS1684 "SAA National Timber Framing Code".
2. The maximum allowable point live load is 2.7kN.
3. The structural plywood face grain runs perpendicular to the joists.
4. F11 plywood was the lowest stress grade on the market at the time of writing.
5. All of the thicknesses shown are not always readily available.
6. The joist spacing is the centre to centre distance between joists.
7. The plywood face veneers must not be thinner than any of the inner veneers.

Table 4 - Calculated Allowable Point Live Loads (kN) Deflection Limit - Span / 300

Table 5 - Calculated Uniformly Distributed Live Loads (kPa) Deflection Limit - Span / 200

Table 6: Minimum Fastener Specification

NOTES

1. Fastener coatings should be selected to suit the application e.g. hot-dip galvanised for chemical storage areas.
2. Plywood can be fixed within 10-12mm of its edges.
3. Structural elastomeric adhesive should be used where plywood is fixed to unseasoned timber joists.
4. When nailing to seasoned softwood sub-floor members deformed shank nails are recommended.

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Timber Research Unit
Department of Architecture
University of Tasmania