LOAD-DURATION CLASSES

There is nothing here that you have not learnt so far except that if a structure is loaded by loads which have different load-durations, the kmodfactor for the shortest load-duration should be used. For example, if the structure is loaded by dead (permanent load-duration) and imposed loads (medium term load-

duration), then kmodfactor should be chosen from medium-term load-duration.

3.1.4 DEFORMATION MODIFICATION FACTORS (Kdef) FOR SERVICE CLASSES

Please note that, based on the ‘Application rule’ (4), the value of kdef given in Table 3.2 should be increased by 1.0 when the timber installed is very wet (near fibre saturation point) and is likely to dry out while under load.

3.2 Solid timber

(1)P Solid rectangular timber should comply with EN 14081 – Parts 1 to 4 (strength classes of timber are given in EN 338) and round cross-sectional timber should comply with EN 14544.

(2) & (3): Dimensions of timber (depth for bending, width and length for tension) have an effect on the strength of timber. It must be noted that the characteristic mechanical properties values obtained and given in relevant standards (EN 338 for solid timber, EN 1194 for glulam, and EN 14374 for LVL) have been based on certain dimensions (called ‘reference dimensions’). Therefore, when member sizes differ from those used for obtaining characteristic mechanical properties, modification factors are given in this section of the code for increasing or decreasing the mechanical properties in order to take account of the variation in dimensions.

For rectangular solid timber with a characteristic density ρk≤ 700 kg/m3 , the ‘reference depth’ is 150 mm. Therefore, for depths in bending or widths in tension of less than 150 mm, the characteristic bending and tension strengths may be increased by the following factor kh:

where

fm,k= is characteristic bending strength

ft,0,k= is characteristic tension parallel to grain strength

kh = minimum of (150/h)0.2or 1.3 where h is depth for bending case and width for tension case in mm.

(4) The value of kdefgiven in Table 3.2 should be increased by 1.0 when the timber installed is very wet (near fibre saturation point) and is likely to dry out while under load.

(5)P Finger joints shall comply with EN 385.

3.3 Glued laminated timber

(1)P Glulam shall comply with EN 14080 and its strength and stiffness mechanical characteristic properties with EN 1194.

(2) & (3) For rectangular solid glulam, the ‘reference depth or width’ is 600 mm. Therefore, for depths in bending or widths in tension of less than 600 mm, the characteristic bending strength (fm,k) and tension strengths (ft,0,k) may be increased by the following factor kh:

fm,k= kh . fm,kfor bending, depth less than 150 mm ft,0,k= kh . ft,0,kfor tension, width less than 150 mm where

fm,k= is characteristic bending strength

ft,0,k= is characteristic tension parallel to grain strength

kh = minimum of (600/h)0.1or 1.1 where h is depth for bending case and width for tension case in mm.

(4)P: Large finger joints complying with ENV 387 shall not be used in products in service class 3 where the direction of grain changes at the joint.

(5): This ‘Application rule’ says that the effect of member size on the tensile strength perpendicular to grain shall be taken into account, but it does not say how!

3.4 Laminated Veneer Lumber (LVL)

LVL is a material which consists of veneers like plywood, but where the grain direction of veneers are the same. LVL can be manufactured for different sizes and thicknesses.

It should be noted that there are two harmonised standards for LVL; one for LVL as structural member and one for LVL as panel/boards.

(1)P: Structural LVL shall comply with EN 14374 and LVL as panels with EN 14279.

(2)P & (3): For rectangular LVL, the ‘reference depth’ is 300 mm. Therefore, for different depths in bending, the characteristic bending strength (fm,k) shall be multiplied by the following factor kh:

fm,k= kh . fm,kfor bending, depth other than 300 mm where

fm,k= is characteristic bending strength

kh = minimum of (300/h)Sor 1.2 where h is depth in mm for bending case and ‘s’ is the size effect exponent which can be obtained from EN 14374. (4): For rectangular LVL, the ‘reference length’ in tension is 3000 mm.

Therefore, for lengths in tension other than 3000 mm, the characteristic tension strength (ft,0,k) shall be multiplied by the following factor kl:

ft,0,k= kl . ft,0,kfor tension, length other than 3000 mm where

ft,0,k= is characteristic tension parallel to grain strength

kl = minimum of (3000/l)s/2or 1.1 where ‘l’ is the length in mm and ‘s’ is the size effect exponent which can be obtained from EN 14374.

(5)P: is already dealt with above.

(6)P: Large finger joints complying with ENV 387 shall not be used in products in service class 3 where direction of grain changes at the joint.

(7)P: This ‘Application rule’ says that the effect of member size on the tensile strength perpendicular to grain shall be taken into account, but it does not say how!

3.5 Wood-based panels

(1)P & (2): It must be noted that for the design and application of wood-based panels, designers are required to refer to EN 12871 as there is not much guidance for the structural design of panel products. All specified wood-based panels must comply with the harmonised standard EN 13986 and its supporting

3.6 Adhesives

Different adhesive types are being researched currently. The choice of adhesive is vital as it is required to maintain structural integrity for various conditions throughout the life of the structure. However, these are allowed:

3.7 Metal fasteners

All metal fasteners shall comply with EN 14592. All metal connectors shall comply with EN 14545.

2.11 ‘Section 4: Durability’ (page 30)

The following clause numbers are kept the same as those in EN 1995-1-1 for clarity:

4.1 Resistance to biological organisms

This part of design is very important for timber, wood-based panels and the connections used in the timber structures in different environments called ‘Hazard Classes’. There are usually two options which must be chosen:

– Design by natural durability (EN 350-2) for a particular hazard class (EN 335- 1, EN 335-2 and EN 335-3).

– Design by preservative treatment (EN 351-1 and EN 460).

4.2 Resistance to corrosion

This part has the utmost crucial importance in the design of connections (joints) as the majority of timber structures depend on their joint design. Ensuring the durability of connections is a must, either by inherent corrosion- resistance or by protective measures against corrosion. Table 4.1 gives the MINIMUM specifications for material protection against corrosion for fasteners (related to ISO 2081):

Adhesive type Compliance with Environment used in (ie Service class) Type 1 EN301 All service classes

Type 2 EN301 ONLY in Service classes 1 and 2 and NOT exposed to prolonged temperatures in excess of 50°C

2.12 ‘Section 5: Basis of structural analysis’