Prestressed/post-tensioned concrete bridges - Computational Analysis and Design of Bridge Structures - page 134

Expansion joint Continuous unit Expansion joint

Detail A
(typical)

(a)

L Pier L Pier

Closure joint Closure joint

(b) Typical interior span

B

B
Detail A (c) Section B-B

Figure 5.7 (a-c) Interior span post-tensioning for span-by-span construction. (Data from
FLDOT/Corven Engineering, Inc., New Directions for Florida Post - Tensioned
Bridges , Volume 1: Post-Tensioning in Florida Bridges, Florida Department of
Transportation, Tallahassee, FL, February 2002.)

5.2 PrinciPle and Modeling of Prestressing

Any modeling method that satisfies the requirements of equilibrium and
compatibility and utilizes stress-strain relationships for the proposed mate-
rial can be used in the analysis. As it is commonly known, the prestressing
force used in the stress computation does not remain constant with time.
The collective loss of prestress is the summation of all individual losses,
which may be examined individually or considered a lump sum loss. The
four most critical conditions in the structural modeling of tendons are
(Fu and Wang 2002) the following:

• Immediate loss of stress in tendon —Friction between the strand and its
sheathing or duct causes two effects: (1) curvature friction and (2) wob-
ble friction. The retraction of the tendon results in an additional stress
loss over a short length of the tendon at the stressing end. Loss will also
happen due to tendon slip before full grip of the anchorage. The com-
bined loss is commonly referred to as the friction and seating loss.
• Elastic shortening —The elastic shortening of the concrete due to the
increase in compressive stress causes a loss of prestressing force in tendons.
• Long - term losses —Several factors cause long-term losses: (1) relax-
ation of the prestressing steel, (2) shrinkage in concrete, and (3) creep
in concrete. In grouted (bonded) post-tensioning systems, creep strain