CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18

Conclusions
It is obvious that, in general, different notional charring rates should be
used for modelling the effective section not only with respect to load capacity
and stiffness but also with respect to the state of stresses. In many
applications this distinction is not necessary as in the case of fire-exposure
on four sides or of very large sections. While in calculations of bending
load capacity a loss of strength should be taken into account, it is not necessary
to regard the loss of the modulus of elasticity when the flexural
stiffness of the member is determined. Therefore in design rules notional
charring rates should be linked together with rules about, whether, or how
much the strength and the modulus of elasticity has to be reduced. In the
case of the specimens of the investigation referred to here, it was possible
to use only two different charring rates in connection with different reductions
of strength and modulus of elasticity, see Figure 16. In other applications
this may be different.
Assuming a simple effective cross section of rectangular shape it is
possible to achieve good agreement between calculated and test results. It
is therefore unnecessary to introduce rounded edges in the model. In most
cases the increase in accuracy of the model should be illusory whilst its
use would be more complicated.
By using the notional charring concept it is possible to specify expressions
which are specific to different applications. Thus the concept is sufficiently
general and should be useful in future applications.
25-16-1 J König
The effect of density on charring and loss of bending strength in fire
Summary
The influence of density on charring of timber exposed to standard fire is
studied, evaluating test results by Norén. It was found that both the effective
and measured charring rates vary about 10 % in the density interval
between 290 and 420 kg/m 3 , representing characteristic densities of
strength classes C14 to C40 in EN 338. It was found that there was no influence
of density on the loss of bending strength.
Conclusions
The influence of density on the charring rate is considerably smaller than
given in the CIB Timber design code. Since it is small in the interval of
most used structural classes, it should he disregarded in practical applications.
25-16-2 F Bolonius Olesen, J König
Tests on glued-laminated beams in bending exposed to natural fires
Summary
A series of fire tests with so-called natural fire exposure of loaded glued
laminated beams was performed. The fire exposure of three sides during
the tests was governed by a temperature-time relationship determined according
to an energy balance method (opening factor method) with different
fire load densities and opening factors. The results confirmed the rate
of charring on the wide side of the member obtained by Hadvig. The average
charring depth on the lower side of the member was greater than the
average charring depth on the wide side when the width of the lower side
was smaller than about 180 mm.
Considering the mechanical behaviour, the tests showed that a loss of
strength and stiffness of the residual cross section occurred, and that it
CIB-W18 Timber Structures – A review of meeting 1-43 5 SPECIAL ACTIONS page 5.31