Even as early as the 18th century BC the importance of a structural building code was recognised. Article 229 in the Code of Hammurabi reads: “The builder has built a house for a man and his work is not strong: if the house he has built falls in and kills a householder, that builder shall be slain.”

While structural engineers may not be dealt with in such a manner now, it is their task to work out how to make a building safe and sound, by specifying load-bearing members that are sufficiently strong, stiff and stable, without going over the top. The Eurocode process, designed around the needs of structural engineers, started 30 years ago under the Construction Products Directive (CPD).

A design code like Eurocode 5 (EC5) is a rulebook for structural engineers. It sets out agreed calculation methods for checking the strength, stiffness and stability of buildings and other structures. And it has already been used to design a number of beautiful buildings in the UK, including the Sheffield Winter Garden and the Savill Garden Gridshell, Windsor Great Park.

Eurocode 5 is in three parts: the main code, Part 1-1; a section on fire design, Part 1-2; and a new code for the design of timber bridges, EN 1995 Part 2. The National Annexes to EC5 were published by BSI at the end of last year, so any designer who wishes to do so may now base their design on the Eurocode suite.

Adopting the Eurocode system will mean two major changes: firstly every European country will use the same design codes, and secondly all the structural materials will use the same design basis. No longer will some materials use limit state design and some permissible stress design, and no longer will different codes use different load factors for different load combinations. In those respects at least, life for the structural engineer will be simplified.

There are, inevitably, several differences between EC5 and BS 5268, the design code which it will eventually replace, probably in 2009. Limit state design enables the designer to account for the low probability of worst conditions and to determine the strength and stiffness required to achieve a required level of safety. The design procedure therefore involves adjusting the characteristic values of both actions and material properties to obtain design values, which will achieve a target level of reliability. In permissible stress design, however, all the safety factors for both loads and materials are built into the tabulated material properties, called grade stresses. Consequently the designer has very little flexibility to change them.

A second main difference between the codes is that BS 5268 has pages of tables giving the properties of timbers and wood-based panel products. EC5 has none of these: material properties must be sought in separate European standards. Again, much of BS 5268 consists of tables of pre-calculated values for connections, compression members and timber frame racking walls. EC5 provides only formulae; and it provides very little practical advice for designers.

Sheffield Winter Garden was designed using Eurocode 5

Whereas BS 5268 works with grade stresses, EC5 works with characteristic test values. These characteristic values have no in-built safety factors and apply only to five-minute test durations of loading: they must be reduced to a safe and appropriate level by the designer before they can be used.

The table left compares a few important properties of C16 and C24 timber. It is immediately apparent that the EN 338 characteristic values for bending and shear strength, shown in the lower half of the table, are much higher than the BS 5268 grade stresses, because, to make them equivalent to BS 5268 grade values, they must be reduced by partial safety factors for the timber and the loads, and then for BS 5268 long-term load duration.

The elastic stiffness properties, however, are very similar for both codes, because stiffness is not generally a safety issue, so no safety factors are involved.

Safety factors are based on the reliability of the data or consistency of the product.

Partial safety factors:

Actions

1.35 for permanent actions (material weights)

1.5 for variable actions (imposed, wind, snow)

Material properties (examples of values given in EC5)

1.3 for solid timber and particleboard

1.25 for glulam (glued laminated timber)

1.2 for LVL, plywood, OSB.

These are the normal values for the partial safety factors for actions and materials: 1.35 for permanent actions and 1.5 for variable actions such as wind and snow, a higher value because variable actions are known with less precision. For materials, the safety factors vary from 1.2 to 1.3. The lower values are specified for materials which are more consistent and are manufactured under quality control arrangements.

All the Eurocodes require designs to distinguish between ultimate limit states, when something breaks or becomes unstable, endangering safety, and serviceability limit states, when excessive deformation causes non-structural damage or discomfort, or an unacceptable appearance or a failure of functionality. One obvious difference in the design procedure for the two states is that, in EC5, no safety factors are applied to serviceability limit states.

Deflection is rarely a safety issue, so deflection limits in the Eurocodes are advisory rather than mandatory, and EC5 gives only suggested ranges.

Eurocode 5 sets out agreed calculation methods for a structure's strength, stiffness and stability

Another major difference concerns connections. In EC5 the design of connections is entirely formula-based. For timber-to-timber connections the formulae are the same as those in Annex G of BS 5268, but in addition there are special formulae for steel-to-timber connections, which are much more satisfactory than the current rough and ready method for steel-to-timber connections given in BS 5268. These formulae are very difficult to use but are far more flexible than the BS 5268 tables.

There will be plenty of help available to the aspiring EC5 user, from TRADA and others. In the autumn, the Institution of Structural Engineers (IstructE) and TRADA will jointly publish a 250-page manual to EC5 as part of the Institution’s new Eurocode series. Another useful document, when complete, will be BS PD 6693 Complementary information for use with Eurocode 5. This is being compiled by UK timber professionals to include all the practical information in various parts of BS 5268 that would otherwise be lost in time.

Keywords: Eurocode BS TRADA