Civil Engineering Reference
In-Depth Information
Figure 13:
Steel fatigue failure: test specimen (left), FE-analysis of stresses due to local
and global loading (middle and right)
In general tension stiffening has to be considered in the stress determination for regions
where concrete cracking is expected. On the safe side the influence of tension stiffening
(cracked section) can be neglected for the determination of stresses due to global bearing. For
stresses due to longitudinal shear transfer the more unfavorable value from calculations with
a cracked and an un-cracked section should be used. The design rule in EN 199411, which
allows for the conservative determination of the fatigue stress in the composite joint with an
un-cracked section, can be unsafe for composite dowels under hogging moment.
(ii) Concrete fatigue
Two types of concrete fatigue failure are known from tests:
the loss of bearing capacity due to trickling of crushed concrete out of the composite
joint;
cyclic concrete pry-out of composite dowels with insufficient concrete cover subjected
to high upper loads.
The first failure mode can be prevented by a limitation of the crack width to 0.15 mm in
regions, where the composite dowel is in the concrete tension zone. This has to be considered
for bending action in the concrete chord in longitudinal as well as perpendicular direction.
Cyclic pry-out can be prevented by limitation of the upper load to 70 % of characteristic
static bearing capacity according. to equation 1 and 2. Up to this load level the bearing be-
havior is mainly elastic.
(iii) Securing rigid shear connection
The basic requirement for the determination of stresses for the composite cross-section based
on elastic theory is the assumption of a rigid shear connection between steel section and con-
crete section. However, results of cyclic beam tests show that this assumption is not justified
a priori, which can lead to higher geometric stresses in the steel section (Fig. 14).
The loss of rigid shear connection is caused by a degradation of the concrete dowel due
to cyclic loading. The consequence is a forceless slip before the composite dowel is activated
(Fig. 14 bottom). This can be avoided by a limitation of the upper force. As criterion for a
relevant degradation of the composite joint, the strain shift between steel and concrete section
is used. Zero shift is equal to full shear connection; the maximum strain shift corresponds
to a composite beam without shear connection. The threshold for an unacceptable degrada-
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