U.S. Department of Transportation Federal Highway Administration
Improving Inspection Access to Post-Tensioned Tendons in Spliced I-Girder Bridges
Notice: Personal protection equipment (PPE) should be worn during inspection and construction operations.
A series of videos has been produced to encourage the design and detailing of post-tensioned
concrete bridges to accommodate nondestructive evaluation.
This video considers enhancing inspection access to post-tensioning tendons used in
the construction of spliced I-girder bridges.
The bridge shown in this video is comprised of approach spans and a three-span continuous
unit crossing a navigational channel.
The approach spans are built using simple span prestressed concrete I-girders.
Spliced girder construction is used to construct the three-span navigational unit.
Post-tensioning tendons are used to join precast concrete I-girder segments
into three-span continuous girders.
The 3-span navigational unit has side span lengths of 175 feet
and a main span length of 250 feet.
The 47 foot wide cross section of this bridge is comprised of four girder lines.
78 inch deep I-girder segments are used in the constant depth portions of the unit.
Haunched girder segments varying from 78 inches to 144 inches are used over the channel piers
to provide appropriate negative moment capacity.
A typical construction sequence of the 3-span unit begins with the erection of the haunched
girder segments on the channel piers.
Temporary supports are required to stabilize these girder segments.
The side span girder segments are erected next
with one end supported on the expansion joint piers.
The other ends of these girder segments are suspended from the haunched girder segments
using strong-back beams.
The main span drop-in girder segments are then erected, being suspended at both ends
by strong-back beams.
Closure joints, usually 2 feet in length, are cast between the ends of the girder segments.
Post-tensioning tendons are pulled from one end of the unit to the other
through ducts cast into the girder segments.
The tendons are then stressed, making the girders continuous.
The profiles of full-length post-tensioning tendons in spliced girder bridges typically
follow a series of reversing parabolic curves.
Tendon anchorages are located in end blocks, distributed over the heights of the girders.
The tendon profiles move to the bottom of the girders near the middle of the side span
and then move to the top of the girder over the channel pier.
Leaving the channel pier, the tendons again travel to the bottom of the girder
at mid-span of the main span.
From that point to their other end, the profiles are symmetrical
with respect to the center of the 3-span unit.
Difficulty accessing the anchorages of the post-tensioning tendons arises from the close
proximity of the approach span girders to the ends of the 3-span continuous girders.
Here the tendons of the navigational unit have been stressed, permanent grout caps installed,
and tendon grouting has been completed.
A concrete pourback is cast over the anchorages to provide another layer of corrosion protection.
The girders of the approach spans are then erected.
A small gap, sufficient to accommodate expansion of the superstructure,
is typically provided between girder ends.
This small gap limits access to the ends of the post-tensioning tendons
for inspection and evaluation.
Access could be obtained by increasing the gap between the ends of the girders.
This would require, however, a very wide pier cap and side curtain walls
to offset negative aesthetic impacts.
Access to inspect the tendons of the 3-span unit can be improved by using
alternate post-tensioning tendon details.
Rather than using full-length tendons, the continuous unit can be constructed using individual
tendons in each span.
These tendons overlap at the channel piers to provide continuous prestressing.
These tendons are often called "Laced Tendons" as they lace the spans together.
The laced tendons anchor at the ends of the continuous girders similarly to full-length tendons.
The first span tendons move to the bottom of the girders near the middle of the side
and then to the top of the girder over the channel pier where they deviate horizontally
and anchor on transverse faces of the haunched girder web stiffeners.
The main span tendons begin at anchorages on the side span faces of the web stiffeners.
The tendons drape to the bottom of the girder at midspan,
and anchor on the far side of the next haunched girder web stiffener.
The tendons of the third span are symmetric to those of the first span.
A primary detail to be developed when using laced tendons is the haunch girder web stiffener.
Full-length tendons require web stiffeners that are sized to stabilize and strengthen
the web as vertical reactions are transferred to the bearings.
When laced tendons are used the web stiffeners must also be designed to accommodate tendon anchorages.
The length of the web stiffeners must be increased in order to develop the horizontal deviations
of the tendons from the centerline of the webs to anchorage locations in the web stiffeners.
Protective pourbacks covering the anchorages are required, but can be detailed to permit
a variety of non-destructive evaluation methods.
This approach to improving access to post-tensioning tendon anchorages has been championed by the
Washington State Department of Transportation.
The haunched girder segments shown here are those of the Manette bridge.