Civil Engineering Reference
In-Depth Information
but orthogonally to the sides of the beam ends, which have the appropriate angles. This
means that the test beam is also subjected to compression loading. Different eccentricities
can be configured by varying the angle.
Using the M/N test rig, numerous investigations of the load-bearing, deformation and crack-
ing behaviour of steel fibre concrete under the loading conditions of a tunnel lining have
been carried out at Ruhr University, Bochum, of which some results are described here.
Cracking behaviour of steel fibre concrete.
While under pure bending (
e/d
= ∞) it is normal
for just one crack to form, multiple crack formation can be observed under eccentric compres-
sion loading with a normal fibre content. Fig. 3-22 shows the relationships determined using
the M/N test rig between the crack width
w
and the strain of the tension side of the beam for
various steel fibre types and contents. As can be seen clearly, the crack widths of the individual
cracks in case of multiple crack formation under eccentric compression loading are less than
the crack widths in the case of simple crack formation under pure bending (Fig. 3-22).
Figure 3-22
Crack width w de-
pending on the strain at the tension
side of the beam, for e/d =
∞
(top)
and e/d = 0.5 (bottom).
The introduction of a limit strain into the stress-strain curve for the design of steel fibre
concrete is intended to limit the crack width at the limit state of load-bearing capacity. Since,
as shown, the equivalent strain of the cracked zone is distributed over numerous cracks, it is
justified to permit higher limit strains under eccentric compression loading. Similar depend-
encies can be derived for the strain parameter e
t2
. Fig. 3-23 shows the practical implementa-
tion of these qualitative relationships. The given values in this case correspond to the lowest
strains, at which a crack width of 2.0 mm was reached in the tests that were performed.
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