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Figure 7
Isochronous curves for PET yarn.
the structure. In some cases, these strain requirements can be more restrictive
than the strength requirements. The values for the strain in the products can be
obtained from isochronous curves, which are available for most geogrids
(Figs. 7-
9
).
It should be emphasized that stress-rupture and creep-strain data are
product-specific. Even with flexible geogrids made of PET, knowing that they are
made of PET is not enough. In such a case, details about the type of yarn used
should be supplied and verified as the creep can vary for different products made
of the same polymer but in a different composition.
7 COMPARISON OF DESIGN METHODS IN THE UNITED
STATES AND EUROPE
In the past there were differences between the used design methods in the United
States and Europe, mainly in the calculation of the allowable design strength of
the geogrid. The new FHWA manual (1997), however, is now more in line with
the European practice and specifies the RF
cr
as creep reduction factor equal to the
ratio of the ultimate load to the maximum sustainable load within the design life.
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