EUROPEAN CODES FOR TIMBER STRUCTURES DESIGN

2.2.1 Design and safety checks

As the European level for the design of timber structures reference can be made of Eurocode 5 (EN 1995-1-1) “Design of timber structures – Part 1-1:

Common rules and rules for buildings”, which was published by CEN (European Committee for Standardisation) in November 2004.

EN 1995-1-1 is a limit state design code which requires structural stability to be in accordance with two specifically defined state, within which the structure complies in relation to particular performance criteria. These limit states are:

- Ultimate limit state, associated with collapse or with other forms of failure which include loss of equilibrium, excessive deformation, transformation into a mechanism, rupture or loss of stability;

- Serviceability limit state, associated with deformation which affects the appearance or effective use of the structure, vibrations which cause discomfort to people or damage to the structure, damage or cracking which is likely to adversely affect the durability of the structure.

EC5 is intended to be used in conjunction with:

- EN 1990 (2004): Eurocode “Basic of design”, which sets out the principles and requirements for safety, serviceability and durability of structures”, which described the design and verification procedures and provides guidelines for related aspects of structural reliability;

- EN 1991 (2002): Eurocode 1 “Actions on structures”, which provides design guidance and actions for the structural design of buildings and civil engineering works, including data for densities of construction materials and stored material, self-weight of construction elements and imposed loading for buildings;

- EN 1998 (2004): Eurocode 8 “Design of structures for earthquake resistance – Part 1: General rules, seismic actions and rules for buildings”, which provides general requirements for assessment of seismic actions and combinations with other actions, together with general rules for earthquake-resistant design of buildings and specific rules for buildings and elements constructed with each of the various structural materials;

- EN Standards: in respect of construction products relevant to timber structures.

At National level, the following standards can be used for structural design in timber:

- German National Standard, DIN 1052 (2004): “Entwurf, Berechnung und Bemessung von Holzbauwerken – Allgemeine Bemessungsregeln und Bemessungsregeln für den Hochbau”; Deutsches Institut Fur Normung E.V.;

- British Standard, BS 5268 (2001): “Structural use of timber”; British Standard International;

Th e a n c i en t t i mb er st ru c t u re s o f t h e R o ya l Pa la c e of Na p l es :

d i a gn osi s, a n a l ysi s a n d rest o ra t i on 4 5

- French Norms, NF P21 (1993): “Structures en bois”; Association Francaise de Normalisation);

- Austrian Norms, ÖNORM B 4100-2: “Holzbau – Holztragwerke – Teil 2: Berechnung und Ausfuhrung”.

In Italy, until a few years ago, the structural codes do not officially include structural timber design. Recently, a proposal code, so-called N.I.CO.LE., acronym of Italian role for the wooden constructions (Norma Italiana per le COstruzioni in LEgno), has been approved in draft form by a special CNR committee (National Research Council), but actually this document does not exist in official version. At present state, a CNR document (CNR-DT 206,

“Istruzioni per la progettazione, esecuzione e controllo delle strutture in legno”, 2007) has been approved as technical guide for design of timber structures. However both European codes and the existing Italian codification refer to new wood constructions. Therefore, the use of such provisions to the ancient timber requires appropriate consideration based on the results of experimental investigations.

2.2.2 Rules for timber grading

A pre-requisite for the use of timber in load bearing constructions is that the strength and stiffness properties are known and can be controlled to stay within desirable limits. Therefore strength properties are the key to structural design although other attributes may well come into consideration when assessing the overall performance of a component or structure. However, as far as the mechanical properties of wood are concerned, the only realistic way of obtained quality within desired limits is grading. The present grading systems allow to determine the strength class of the timber to satisfy the design requirements. Strength grading takes into account timber defects, such as slope of grain, existence and extent of knots and fissures, etc.

All timber used for structural applications needs to be strength graded by either visual inspection or by an approved strength grading machine, aiming at assigning the elements to “Strength Classes”, defined in European standard EN 338 (2003) “Structural timber. Strength classes”. There are a total of 16 strength classes, C14 to C40 for softwoods and D30 to D70 for hardwoods.

The number in each strength class refers to its “characteristic bending

strength” value; for example, D40 timber hardwood has a characteristic bending strength of 40 N/mm² (Table 2.1). This is a value intended for use with the limit state Eurocode 5 (EN 1995-1-1, 2004).

Table 2.1. Strength classes for structural timber (hardwoods; EN 338, 2003).

D30 D35 D40 D50 D60 D70

Strength properties [N/mm²]

Bending fm,k 30 35 40 50 60 70

Stiffness properties [kN/mm²] Elasticity modulus

The visual method is a manual process carried out by an approved grader, which examines each piece of timber to check the size and frequency of specific physical characteristics or defects, as knots, slope of grain, rate of growth, wane, resin pockets and distortion. EN 518 (1995) “Structural timber

− Grading − Requirements for visual strength grading standards” provides the principles for visual grading that national standards should achieve.

Machine strength grading relies essentially on the relationship between the modulus of elasticity and the modulus of rupture (bending strength) of a particular species of timber. This relation is evaluated on a statistical population of pieces. The European standard EN 519 (1995) “Structural timber − Grading − Requirements for machine strength graded timber and grading machine” provides the required specifications for strength grading machine.

Th e a n c i en t t i mb er st ru c t u re s o f t h e R o ya l Pa la c e of Na p l es :

d i a gn osi s, a n a l ysi s a n d rest o ra t i on 4 7

For Italian structural timber, the required specifications for visual grading are given in UNI 11035-1 (2003) “Structural timber − Visual strength grading for Italian structural timbers: terminology and measurements of features” and UNI 11035-2 (2003) “Structural timber − Visual strength grading rules and characteristics values for Italian structural timber population” (Table 2.2).

Table 2.2. Strength classes for Italian structural timber (hardwoods; UNI 11035-2, 2003).

Chestnut Deciduous Strength properties [N/mm²]

Bending fm,k 28 42 26 27

Stiffness properties [kN/mm²] Elasticity modulus

2.3 ITALIAN STANDARDISATION ACTIVITY IN THE FIELD