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37-7-8 R Steiger, E Gehri, R Widmann
Glued-in steel rods: A design approach for axially loaded single rods
set parallel to the grain
Introduction
Steel rods bonded in glulam elements are very efficient to introduce high
forces into timber structural members as well as to strengthen timber perpendicular
to the grain. Research on bonded-in rods started in the late
eighties of the last century and attempts to develop design methods and to
optimize the application were intensified within the last 10 years. A good
compilation of existing knowledge, including lists of basic literature can
be found in the proceedings PRO 22 of the 2001 RILEM Symposium on
"Joints in Timber Structures" and in the proceedings of the CIB W18-
Meetings 28 and 32 -34.
Several design approaches and code models have been published. By
comparing these models and approaches, some discrepancy and partly
even contradictions between the models, especially regarding the treatment
of isolated parameters, can be found. On this background, a small
test program was initiated, to study the influence of a selection of these parameters,
known or supposed to be determinant on the pull-out strength of
single, axially loaded steel rods with a metric screw-thread, bonded with
an epoxy-type adhesive in glulam made of spruce. The tests were focused
to determine the influence of timber density ρ, rod-to-grain angle (0° or
90°), length l and diameter d of the rod (or the corresponding drill-hole dh
respectively), represented by the slenderness ratio λ = l/dh on the pull-out
strength of glued-in rods.
Important objectives of the test program were that it should be based on
practical situations and dimensions and that it should enable a comparison
with similar test series. These objectives could only be reached by permitting
certain compromises regarding the test layout. Although for example
in practice the use of one single rod will not or hardly ever be the normal
case, all tests described here were carried out on connections with one single
rod, because the examination of such a connection provides a good basis
to study the influence of isolated parameters. The tested GSA®-system
was "optimized" (type of adhesive and rod, geometry) in a way that failure
was forced into the timber. The test results and conclusion are therefore
specifically valid for the tested system and loading configuration.
A generalisation of the conclusions will not be possible and a comparison
to other test series has to be done with some restraint. Nevertheless it
is possible to quantify the influence of the parameters focused by the present
study on the pull-out strength of axially loaded rods and to propose an
adequate design model.
Conclusions
Based on the test results it can be stated, that for the used GSA®-system:
– the pull-out strength of rods bonded in glulam made of spruce depends
on the density of the timber around the anchorage zone. For rods set
parallel to the grain, the influence of the density is stronger and can be
covered by a power function with an exponent of c0 = ½. Rods set perpendicular
to the grain exhibit a smaller dependence on the timber density.
There the influence of the density could be taken into account with
an exponent c90 = 0.25 or could be disregarded.
– there is a difference of about 20 to 40 % in pull-out strengths according
to the rod-to-grain angle (0° or 90°), which should be considered by design
models.
– the influence of the anchorage length is marked more for rods set parallel
to the grain, than for those set perpendicular. In the case of rods set
parallel to the grain, an adjustment based on l -1/3 is a good approach.
– the 0°-series show a dependence on the slenderness ratio λ = l/d, which
can be quantified by λ -1/3 . If a design model is based on λ, the influence
of the drill-hole diameter on the pull-out strength, which is about dh 1/3 ,
is automatically taken into account.
– with regard to an optimal load transmission capacity steel to timber,
rods with smaller diameter should be given preference.
37-7-10 C Faye, Le Magorou. P Morlier, J Surleau
French data concerning glued-in rods
French Professional Guide for Glued-in Rods
French Professional Guide entitled 'Guide professionnel - Assemblages
bois: tiges ou goujons colles de grandes dimensions' was published in
1999.
– It contains (in 40 pages) the following topics:
– description of the glued-in rod technology,
– specifications and mechanical requirements for wood, adhesives and
rods used for this technology,
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.54