LVL Handbook EUROPE

6. PERFORMANCE OF LVL IN FIRE

For timber surfaces facing a void cavity in a floor or wall

assembly (normally the wide sides of a stud or a joist),the fol-

lowing applies:

• Where the fire protective cladding consists of one or two

layers of gypsum plasterboard type A, wood panelling or

wood-based panels, at the time of failure tf of the cladding,

k0 should be taken as 0,3. Thereafter k0 should be assumed

to increase linearly to 1,0 during the following 15 minutes;

• Where the fire protective cladding consists of one or two lay-

ers of gypsumplasterboard type F, at the time of start of char-

ring tch, k0 is 1. For times t < tch, linear interpolation should

be applied, see Figure 6.6 (b).

The effective cross section should be used for the calculation of

the stiffness and fire resistance of an LVL member.

Note: The effective cross sectionmethod is recommended.

However, depending on the National Annex, the reduced prop-

erties method of Eurocode 5 may also be used.

3. Determination of design values of strength and stiffness

For the calculation of the design values of mechanical re-

sistance Rd,t,fi in a fire situation, the design values of strength

properties shall be determined by the equation:

f_(d,fi)=k_(mod,fi) f_20/γ_(M,fi)

(6.2) (EC5 2.1)

where

d,fi

is the design strength in fire;

is the 20 % fractile of a strength property at normal

temperature. It can be calculated as

∙

. For

LVL

is 1,1, so

is 1,1 times the characteristic

strength

;

mod,fi

is the modification factor for fire. It replaces the

modification factor for normal temperature design

mod

given in EN 1995-1-1. kmod,fi is 1,0 in most

cases, except when the method of annex C of

EN 1995-1-2 is used; and

M,fi

is the partial safety factor for timber in fire. The

recommended factor for material properties in fire is

M,fi

= 1,0. Information on national choice may be

found in the national annex.

For example, the design value of bending strength for LVL-

36 C:

f_(m,d,fi)=k_(mod,fi)∙(k_fi∙f_(m,k))/γ_(M,fi) =1,0∙(1,1∙36 N/

mm^2 )/1,0=39,6 N/mm^2

For stability calculations, the characteristic values of stiffness

properties at normal temperature are used.

4. Determination of the design values of actions

The design effect of actions Ed,fi for the fire situation is

determined in accordance with EN 1991-1-2:2002, including

the effects of thermal expansions and deformations. In typical

cases of timber structures where the own weight is relatively

low, the design values of actions

d,fi

are 0,2-0,4 times the ac-

tions

in normal temperature design.

5. Verification that design resistance is larger than design

action

It shall be verified for the required duration of fire expo-

sure

that:

d,fi

≤

d,t,fi

According to Eurocode 5, clause 4.3

Simplified rules for analysis

of structural members and components

, compression perpen-

dicular to the grain and shear resistance may be disregarded.

6.4.2 Charring rates of LVL

There are two different types of charring rates

and

. For

panels and wide cross sections one-dimensional charring rate

is used in the calculations. This is also used as the basis value

in some more advanced calculation methods. When the char-

acteristic density of LVL is

≥ 480 kg/m

, the one-dimension-

al charring rate

is 0,65 mm/min

The design charring depth for one-dimensional charring

char,0

[mm] should be calculated as follows when the surface is

unprotected throughout the time of fire exposure:

char,0

(6.3) (EC5 3.1)

where

[min] is the time of fire exposure and

[mm/min] is

the one-dimensional charring rate.

For all other structures that are exposed from multiple

sides, generally columns and beams, the notional charring rate

is used in the calculations of the notional depth

char,n

. When

the characteristic density of LVL is

≥480 kg/m

, the notional

charring rate

is 0,70mm/min

The design charring depth for notional charring

char,n

should be calculated as follows when the surface is unprotect-

ed throughout the time of fire exposure:

char,n

(6.4) (EC5 3.2)

where

[min] is the time of fire exposure and βn [mm/min] is

the notional charring rate

In the test report VTT-S-04746-16 the one dimension-

al charring rate of different wood products was evaluated in

120min fire exposure according to a standardized time-tem-

perature exposure curve (EN 1363-1:2012)

. According to the

report, wood products behaved predictably and, for LVL, the

one-dimensional charring rate

= 0,65mm/min can be used

for an extended fire exposure. The results were similar in both

face side and edge side exposure specimens. This gives the nec-

essary information and confidence for fire designers in assessing

d,fi

mod,fi

20 M,fi

(6.2) (EC5 2.1)

m,d,fi

mod,fi

∙

,k M,fi

= 1,0 ∙ 1,1 ∙ 36 Nmm

1,0 = 39,6 Nmm

162

LVL Handbook Europe