Structural applications and strength properties

While the bulk of American hardwood is used for furniture and joinery, it may on occasion be considered for structural work, such as the stringers of a free-standing staircase, or the edge beam of the floor which supports it. This in turn creates a need for the strength properties of the species to be available in a form which can be used by engineers in conjunction with an appropriate design code, to validate a particular design.

Structural design in Europe

Over the last fifty years many European countries have developed national codes. For example, the UK code (BS 5268) gives ‘permissible stresses’ for various species, which embody the total factor of safety against failure. Other codes are in ‘limit state’ format, in which the total factor of safety is split between the material strengths and the applied forces.

In 1994 the first Eurocode for timber was issued (in limit state format), and although still a draft in 1998, it is accepted in some countries (including the UK) as an alternative to the national code. It was accompanied by Euronorms, which standardised test procedures (EN 408) and defined strength classes for softwoods and hardwoods (EN 338). As the use of the unified European codes and standards increases, it will be easier for designs, and designers, to cross national boundaries.

A programme of testing

Design information for softwoods is readily available, due to their general structural use, but there is currently no equivalent information relating to the American hardwoods. For this reason, AHEC have commissioned a programme of testing to be undertaken in Europe by the Building Research Establishment, UK. The tests will be carried out, and the characteristic values determined, in accordance with the relevant European Standards. It is proposed to test five species in all: red oak, white oak, ash, tulipwood and hard maple, which would then be allocated to a strength class of EN 338. The results will be published when available, but in the interim, design information for three species is given below, based on testing work already carried out. They are related to material which passes the TH1 grade of BS 5756, the UK hardwood grading standard complying with the requirements of EN 518, which lays down the principles for visual grading standards.

Structural grades

Timber is a naturally variable material, and as a result, all commercial supplies are related to a particular grade from a set of published rules. Almost all the American hardwood available in Europe is graded to National Hardwood Lumber Association (NHLA) rules, mainly grades ‘Firsts and Seconds’ (FAS) and ‘No 1 Common’ (#1C). Specific details of NHLA grading is contained in AHEC’s “An Illustrated Guide to Hardwood Lumber Grades”. All grading systems are designed for an end use. As would be expected, the NHLA rules relate to the appearance of the timber, for use in joinery and furniture. Therefore, to use this timber for structural work, it must be re-graded to an appropriate structural grading standard. The rules of BS 5756 are straightforward, and relate to the defects covered by most grading standards, principally knot size and slope of grain. In practice, most FAS grade material would achieve a 90% pass rate when re-graded to TH1 grade of BS 5756.

Structural properties for red oak, white oak, and hard maple for timber of TH1 grade, BS 5756

Grade stresses for use with BS 5268, a permissible stress code

Source BS 5268, Table 7

Characteristic values for use with a limit state code

Source EN 338

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