REQUIREMENTS ON TENDONS FOR EXTRADOSED BRIDGES

fib Symposium PRAGUE 2011
Session 2B-2: Construction Technology
REQUIREMENTS ON TENDONS FOR EXTRADOSED
BRIDGES
Christian
Gläser
Abstract
Werner
Brand
Extradosed bridges are getting more popular in all countries. After a successful start in Japan also
in Europe a lot of projects are realized. For extradosed bridges the tendon layout is similar to cable
stayed bridges, but due to the increased stiffness of the superstructure and the flat inclination of the
tendon the actions on the tendons (especially fatigue loads) are much lower. Special care has to be
taken on the properties of the tendons used.
Due to the environmental exposure especially for the corrosion protection and leak tightness
identical requirements are essential for both bridge types. On basis of theoretical considerations it
was verified that a stay cable tested according to fib-bulletin 30 also fulfills the requirements for
extradosed tendons. As usually the fatigue resistance for external tendons is only shown for a stress
range of ∆σ=80 MPa, fatigue tests with higher stress range have to be performed on external
tendons to guarantee the usage in extradosed bridges.
In the article the requirements regarding static and fatigue behaviour, the transfer of load
differences to the pylon at the saddle, the exchangeability of tendons and durability aspects are
discussed.
Keywords: Extradosed, Tendon, Stay Cable, Durability, Testing
1 Extradosed Bridges
Post-tensioned bridges using external tendons are common for many years. The tendons are well
known as restressable, replaceable and inspect able. The disadvantage of the structures is the
reduced lever arm as the tendons cannot take the advantage of the full height of the cross section.
In order to reduce the self-weight of the superstructure and to increase the smaller inner lever
arm influenced by the external tendons in 1988 the French engineer Mathivat proposed for a bridge
the use of „deviators“ instead of increasing the height of the structure at the piers. This new
concept he called “extradosed prestress”[1].
2 Differences to Cable Stayed Bridges and PC-bridges with external
tendons
Extradosed bridges are something between cable stayed bridges and post-tensioned bridges with
external tendons. In extradosed bridges the tendons are not only arranged within the box girder but
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also outside of the box girder. The superstructure is much stiffer compared to usual cable-stayed
bridges and the pylon height is smaller related to the maximum span. Due to the gently inclined
tendons the main component of the tendon forces is introduced as longitudinal load into the girder
that is acting similar to a post-tensioned girder.
Compared to external tendons additional requirements are necessary for the corrosion
protection in the free length because the tendons are permanently exposed to outdoor weathering.
The requirements for the resistance against fatigue are more severe compared to external tendons,
too, but lower than for stay cables.
If the tendon layout requires saddles at the pylons it has to be guaranteed that the flexural
stresses at the ends of the saddles are minimized. Additionally it has to be verified that the
sheathing is not damaged due to friction. If the tendons shall be replaceable this must be
demonstrated, too.
3 Requirements on the components of the tendons
Currently no international standard for the design of extradosed bridges or international guideline
for testing and installation are existing. A fib task group is preparing an update of the existing
bulletin 30 [2]. The actual version is only covering stay cables and cable-stayed bridges. It shall be
extended for extradosed tendons and extradosed bridges. In the existing guideline the individual
components as well as the requirements on the total cable are described.
Due to the high exposure of the extradosed cables more restrictive requirements compared
tousual external tendons have to be fulfilled.
The corrosion protection of the strand itself has to be defined. As for stay cables mostly
galvanized waxed and PE-coated strands are used. It has to be discussed whether usual
monostrands (without zinc) can be acceptable, too.
As the fatigue actions are higher compared to post-tensioned concrete bridges a higher
fatigue resistance is necessary. Therefore, it is recommended to use the high fatigue level that is
also required for stay cable strands or wire. This can be verified in fatigue tests without anchorages
using an upper stress level of 0.65 Fpk, 2 ·10 6 load cycles and a stress range of 250 N/mm 2 for
strands resp. 300 N/mm 2 for wires. If fillers or coatings are used similar requirements as existing
for stay cable strand have to be defined.
For the outer sheathing of the cable mostly colored HDPE pipes are used. The stability
against UV radiation and color stability can be demonstrated in accelerated artificial weathering
tests.
4 Requirements on the tendons
In Europe external tendons – like bonded tendons - are usually stressed to 72% - 80 % of the
characteristic tensile strength. During the approval procedure tests with a stress range of 80 N/mm 2
at 2 · 10 6 load cycles are performed to define the minimum required level of fatigue resistance.
Stay cables using prestressing steel are usually loaded to 45% of the characteristic tensile
strength of the single strand. To verify the high fatigue resistance the cables are tested with a stress
range of 200 N/mm 2 for 2 · 10 6 load cycles. Test specimens consist of two anchorages with the
original components and minimum free length of 0.5 m according to fib-bulletin 30. As in
extradosed bridges due to the stiffer superstructure lower stress ranges will apply a higher stressing
level seems to be acceptable (60% of the characteristic tensile strength of the strands). Besides all
discussion a S-shape tendon layout in the test as shown in the actual version of fib-bulletin 30
(α=10 mrad) is considering the influence of fatigue bending sufficiently for extradosed tendons
because more severe bending influences will not occur due to the stiff superstructure.
During the initial type testing of a new stay cable or extradosed tendon three axial fatigue and
tensile tests (if different sizes are used, a small, medium and large one) should be tested. On basis
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of studies of Nürnberger [3], [4] Smith diagrams for prestressing steel were created. Thus, it could
be shown, that all Stay Cable Tests according to fib-bulletin 30 can be accepted (fatigue test with
stress range 200 MPa at upper load level 45% GUTS) to verify the requirements on extradosed
tendons.
Existing tests with test layout according to fib-bulletin 30 with different stress ranges can be
used if the upper load (not the stress range) exceeds the tested one (see Tab. 1 resp. Fig 1).
Tab. 1 Characteristics on the sheathing of
the tendons
Upper
load level
45% F pk
50% F pk
Stress range for
strand tendons
200 N/mm 2
190 N/mm 2
Stress range for
wire tendons
200 N/mm 2
190 N/mm 2
55% F pk 180 N/mm 2 180 N/mm 2
60% F pk 170 N/mm 2 170 N/mm 2
65% F pk 160 N/mm 2 160 N/mm 2
70% F pk 150 N/mm 2 150 N/mm 2 Fig. 1 Relation between stress range and upper load
If new types of tendons are developed for extradosed bridges maybe reduced requirements
regarding the resistance against fatigue are sufficient. If it corresponds to the application maybe a
fatigue test with upper stress level of 0.65 Fpk, 2 ·10 6 load cycles and stress range 160 N/mm 2 is
possible (see Tab. 1).
For all tendon developments suitability testing and one leak tightness test according to fibbulletin
30 are necessary. If saddles are used one saddle test on a medium tendon size should be
performed. Project specific at least five static tensile and five fatigue tests on anchored single
strands with upper loads as foreseen for the project and stress ranges according to table 2 seems to
be sufficient.
5 Saddles
If the single strands are deviated at the pylon using special saddles also extradosed tendons have to
be tested according to fib-bulletin 30 [2], section 6.2.2 (see Fig. 2). If different tendon sizes shall be
used a medium size can be tested. The same stress ranges as described for axial testing (see table 2)
seems to be realistic. During the static tensile test on deviated tendons it shall be measured if load
differences in the tendon on both sides of saddles can be transferred to the pylon at the saddle.
After the test damages such as wear or cuts to the duct and sheathing of the tensile elements
shall be measured and recorded. The minimum remaining thickness of duct and sheathing can be
measured. Relative movements between tendon components, i.e. tensile element and duct, and
tensile element and sheathing shall be observed. If required strand replaceability can be
demonstrated after the test.
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fib Symposium PRAGUE 2011
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442
Fig. 2 Layout for saddle testing
If saddle solutions in steel are used where the strands are anchored on both sides of the pylons no
testing is required.
6 Conclusions
All stay cables which had been successfully tested according to fib-bulletin 30 can be used for
extradosed bridges, too. Only project specific tests on anchored single strand seems to be sufficient.
If external tendons shall be developed for the use in extradosed bridges special care shall be taken
to all durability aspects.
References
[1] Mathivat, J.: Recent developments in prestressed concrete bridges. In: FIP (Federation
internationale de la precontrainte) notes. Nr. 2, 1988, S. 15–21.
[2] Fédération internationale du béton (fib-bulletin 30): Recommendations for the Acceptance of
Stay Cable Systems using Prestressing Steels (2005).
[3] Nuernberger, U.; Patzak, M.: Metallische Verankerungen für dynamisch beanspruchte Zugglieder
(metallic anchorages for tendons with dynamic actions). SFB 64, University
Stuttgart, 1978.
[4] Nuernberger, U.: Dauerschwingverhalten von Spannstählen (Fatigue behaviour of
prestressing steels). In: Bauingenieur 56, page 311-319, Springer-Verlag, 1981.
Dr.-Ing. Christian Gläser
� DYWIDAG-Systems International GmbH
Regional Headquarter Europe
Siemensstraße 8
85716 Unterschleißheim, Germany
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URL http://www.dywidag-systems.com
Dipl.-Ing. Werner Brand
� DYWIDAG-Systems International GmbH
Technical Services Stay Cables
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85716 Unterschleißheim, Germany
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☺ werner.brand@dywidag-systems.com
URL http://www.dywidag-systems.com