CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL ...

harmonization of glulam standards that are being developed in countries
such as China and Taiwan as well as for ISO standards currently under development.
Conclusions
With a large number of projects underway in ISO/TC165, Timber Structures,
to develop ISO standards for glued laminated timber, it is important
that countries involved in the process understand various national standards
and how the provisions of those national standards need to be accommodated
in the ISO process. This is also true when countries such as
China and Taiwan move toward developing their own national standards
for products such as glulam and must consider how other national standards
have been developed. In virtually all cases some compromises are required
by the various countries involved to be able to work towards harmonization
of glulam standards through the ISO process while acknowledging
national differences.
40-12-6 M Frese, H J Blass
Bending strength of combined beech-spruce glulam
Abstract
This paper is the third part of an easily understandable series dealing with
the bending strength of beech glulam published in CIB-W18 since 2005. It
describes how to handle combined beech-spruce glulam with bottom and
top lamellae of beech and middle lamellae of spruce.
The main topics of the paper are particularly stress distribution in the
cross section, characteristic bending strength, MOE, beam lay-up and demands
on both the strength grading of the lamellae and the characteristic
finger joint bending strength.
In comparison with combined beech glulam it is to be expected that
substituting middle spruce lamellae for beech decreases the cost of materials
due to the higher price of beech. But the lower MOE of spruce compared
to beech has to be considered causing an increase in tensile or compression
edge stress, respectively, if the cross section is to be subjected to
the same bending moment as a combined beech glulam beam. The use of
the calculation model for the characteristic bending strength which is originally
related to combined beech glulam does, therefore, necessitate some
further considerations to define strength classes for combined beechspruce
glulam.
Conclusions
Substituting beech by spruce in the middle zone of a combined beech glulam
beam causes an increase in edge stress in the beech-spruce cross section
of less than 3% in comparison with the combined beech cross section.
The important thing about it is that the portion of the middle zone must not
exceed 60 % of the beam height. By defining marginally higher demands
on the characteristic finger joint bending strength than in case of combined
beech glulam it is possible to compensate for the decrease of about 3% related
to the characteristic bending strength. Hence, the same strength classes
are possible as in case of combined beech glulam. Table 5 gives a survey
of the results and the demands on both strength grading and finger
joints.
Table 5 Strength and stiffness values and requirements; reference beam
height 600 mm
strength values (N/mm 2 )
GL28hyb GL32hyb GL36hyb GL40hyb GL44hyb GL48hyb
fm,k 28 32 36 40 44 48
stiffness values (N/mm 2 )
E0,mean 13200 13200 13200 14000 14700 14700
E0,05 12400 12400 12400 13300 14200 14200
beech: requirements outer zone (>h/5)
DEB 2 48 >54 >59 >64 >71 >72
spruce: requirements middle zone (