- [Instructor] Steel box straddle caps
are categorized as fracture critical,
meaning that they may require stringent designs
and hands-on biennial inspections.
Researchers at the Center for Transportation Research
worked with TxDOT
to develop a comprehensive design methodology
to remove this fracture critical designation on the caps,
- Straddle bents are supported by two columns on each side
and are commonly used in congested urban environments.
And steel straddle bins are fabricated
by welding plates together to create a box.
These types of structures are classified as
non-redundant steel tension members,
which require a federally mandated
in-depth hands-on inspection every two years
for the life of the structure.
- Every two years, the state has to put a lot of money
and time into investigating these.
So what we were trying to do is, is develop details
that gave us added redundancy in those systems
and can then be labeled as a,
call 'em a non fracture critical.
But the term we use now is related more to
whether it's a redundant member.
And that's what we were able to kind of do in our research
is look at different details
that we can use on these steel box Strava caps
to try to get that redundancy in the system
and thereby give a much more efficient structural system
from the long-term maintenance perspective.
- Nationally, there's been a lot of research
on non-redundant steel tension member bridges,
but there was not any research on developing
these types of redundant details for straddle bits.
- [Todd] These were what we call full scale laboratory tests
and we also did parametric finite element analysis
that then develop design methodologies.
But the laboratory tests were somewhat unique
in that they were full scale steel box straddle caps,
but we wanted to simulate a brittle fracture on these.
So the way we can get a material,
steel is a very ductile material,
but in order to get it to behave in a brittle fashion,
we can cool it down.
And what we do is use liquid nitrogen to do that.
So we were able to cool down the steel box caps
in a region where we introduced a defect
by grinding a defect in cycling it to grow,
we call a fatigue crack.
Then we cooled it down
to about minus 200 degrees Fahrenheit,
loaded it very quickly to initiate a brittle fracture.
And then we looked at the performance
of these different details and their ability
then to support what we would call the design load
or the redundancy load that we would have to carry
in a safe fashion.
Those tests then provided valuable data
that allowed us to then validate
detailed or refined finite element models
that we could simulate that behavior computationally
and look at a much wider range of geometries then.
That allowed us to then go in and
develop a comprehensive design methodology
so that designers can actually take this
and actually relatively simply incorporate this into design.
- [Jamie] I think the most important research finding
came from the study of two different concepts
for these steel straddle bents.
So one concept for the straddle bent
was fabricated with post tension bars
and then the other concept was fabricated
with a bolted bottom flange.
So this project demonstrated
that if the steel straddle bent
is fabricated with that bottom flange bolted to the webs,
that it provides sufficient capacity
in the faulted condition.
So due to that bolted bottom flange connection,
the simulated fracture didn't propagate
through the entire member.
And so this approach allows these types of structures
to be designed as re internally redundant,
therefore eliminating that costly hands-on inspection.
- [Instructor] For more information
and to find the publication for this project,
please visit the TxDOT Research Library
at the link shown below.