Cracking of Decks on Deep Plate and Tub Girders
concrete decks supported by steel girders has been a perennial problem in the
industry. Despite all research performed
to date, this problem seems to be eluding a concrete (no pun intended) and
satisfactory solution. The industry has seen a clear pattern of transverse deck
cracks on several Tub and Plate Girder Bridges.
There needs to be a solution.
seems to be even more pronounced in the case of deep and stiff girders such as
steel tub or deep steel plate girders. It seems the stiffer the girders, the
bigger the problem. This research would propose to study deck cracking to identify
at least the structural part of the problem. This would compare actual
performance of bridge decks that have cracked with the analytical models to
determine if we are capturing the actual stresses at hand. Other structural aspects to be researched would
- Excessive shrinkage of the concrete
- Curvature of structure
- Stiffness differential: Deep steel girders are significantly stiffer than the deck
- Thermal effects: Girder is expanding and deck is resisting the expansion
- Effects of eccentricity: Girder is expanding due to temperature rise, the deck is resisting but at fibers that are further away from the neutral axis in these types of structures. Therefore, the stress they would see would be larger, potentially taking them above the modulus of rupture threshold.* Differential of the coefficient of thermal expansion: Typically, approximately 10%. New studies sponsored by Caltrans show the concrete coefficient of thermal expansion as low as 0.0000055 or lower, increasing the differential to approximately 12%.
- Differentials in heat conductivity and ambiance differentials such as temperature and sun exposure: Steel is a very good heat conductor and can get heated faster and way beyond the temperature of the deck. Then the stiffness of the deep girders expanding would carry along the deck forcing it to crack for compatibility along the interface surface.
- Longitudinal and Transverse Bending (for more flexible bridges), etc.* Placement, curing, and workmanship and specifications issues
- Creep and shrinkage
The main objective is to
understand the problem fully, the real causes of cracking, which might be
singular but might also be a combination of all the mentioned basic cases. What
causes a deck that has been designed to not see any tension in the deck from
dead and live load to end up in the first few months of service with evenly
spread cracks at every 2’-4’ frequency.
Once the problem is
understood, it should be easier to mitigate the causes accordingly.
Research should be mostly
analytical, combined with ambiance data from specific projects, but it may also
involve some experimental testing/monitoring of new bridges.
Concrete decks in the USA crack. We have not figured it
out yet as to what would be the right solution. Billions of dollars are spend
replacing decks within 35-45 years, when their life should match that of the
bridge to at least 75 years. Even when special mixes and protection are
specified for the concrete mix, they usually assume concrete models with no
cracks. That would indicate that even with special concrete mixes, and for
bridges where decks have been designed for zero tensile stress based on the conventional
service combinations of basic loads, the decks still crack and in frequencies
that are similar from structure to structure, and are not easily explained from
conventional practice of load combinations.
has been performed on the subject. It is believed that the structural portion
is usually limited and the problem is assumed to be more linked to workmanship,
and proper assurance of just bending behavior and assurance from the engineer
that the concrete does not see any tension, at least from Dead Load and
especially in the conventional positive moment region. Experience show that
cracking in deep stiff girders does not distinguish between positive or
negative moment regions, cracking is almost evenly distributed. So the problem
should be also investigated as a structural problem linked to axial performance
of the two materials and all resulting effects.
A combination of field measurements, lab
testing and a lot of finite element studying of the issues noted above.
This issue could be implemented by all DOTs
through bridge design manuals and specifications based on the research
recommendations. This may also be implemented through specification changes by
the AASHTO Committee on Bridges and Structures.
All bridge owners
|Sponsoring Committee:||AKB20, Steel Bridges
|Research Period:||24 - 36 months|
|RNS Developer:||Tony F. Shkurti, PhD, PE, SE|
|Index Terms:||Bridge decks, Girder bridges, Cracking, Stiffness, Shrinkage, |
Maintenance and Preservation
Bridges and other structures