Weiher and Zilch - Lehrstuhl für Massivbau

Weiher and Zilch 2008 CBC
INTRODUCTION
Post-tensioned concrete structures combine the pros of steel and concrete. So, these
structures or members are stiffer and show less cracking than comparable RC members. As a
consequence, they can be designed more slender. Post-tensioning allowed in the past
bridging of very big spans with concrete structures. After WWII, post-tensioning was applied
to the majority of road and railway bridges 1,2,3 . Although the first pt bridge had unbonded
tendons, the huge stock of concrete brides was built using bonded post-tensioning according
to the ideas of Eugène Freyssinet. This did not change before the 90ies of last century, when
first pilot project have been realized using external cables. Another reason for that
development might have been the prohibition of building concrete bridges using segments in
Germany.
The first experiences in applying only external post-tensioning for road and railway bridges
have been published in a workshop at University of Karlsruhe in 1998 4,5,6,7,8,9 . Since 1999
post-tensioned concrete bridges with box-section must be built using external tendons (ARS
17/1999 10,6 ). They have to be arranged outside the concrete cross section and inside the box.
Moreover, the arrangement of tendons in the webs was forbidden for boxes because the
compact arrangement of tendons often lead to consolidation problems and poor corrosion
protection quality 11 . Using only external tendons with a restricted maximum force causes a
high number of tendons in the box. In addition to design problems when arranging concrete
deviators the concrete surfaces are difficult to reach for regular inspection (e.g. viaduct
Rumbeck, figure 1). That’s why combined post-tensioning was invented. Some tendons (e.g.
the primary tendons) are arranged internally with bond. During the last years some pilot
projects were realized using unbonded tendons instead of bond tendons as internal tendons.
Unbonded tendons transfer normal forces only at the anchorages. This also applies to
unbonded tendons that are arranged within the concrete cross section. By deflecting the tendons
it is possible to compensate for external stresses due to bending („load balancing“).
Anchor forces as well as deviation forces can be seen as loads to the structure.
Fig. 1 Rumbeck Viaduct: External tendons during construction and at final state (courtesy of Schäfer)
Fig. 2 RC T-Beam with external tendons 12
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