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- [Instructor] Steel box straddle caps

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are categorized as fracture critical,

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meaning that they may
require stringent designs

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and hands-on biennial inspections.

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Researchers at the Center
for Transportation Research

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worked with TxDOT

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to develop a comprehensive
design methodology

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to remove this fracture critical
designation on the caps,

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- Straddle bents are supported
by two columns on each side

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and are commonly used in
congested urban environments.

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And steel straddle bins are fabricated

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by welding plates
together to create a box.

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These types of structures
are classified as

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non-redundant steel tension members,

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which require a federally mandated

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in-depth hands-on
inspection every two years

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for the life of the structure.

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- Every two years, the state
has to put a lot of money

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and time into investigating these.

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So what we were trying to
do is, is develop details

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that gave us added
redundancy in those systems

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and can then be labeled as a,

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call 'em a non fracture critical.

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But the term we use now is related more to

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whether it's a redundant member.

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And that's what we were able
to kind of do in our research

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is look at different details

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that we can use on these
steel box Strava caps

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to try to get that
redundancy in the system

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and thereby give a much more
efficient structural system

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from the long-term
maintenance perspective.

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- Nationally, there's
been a lot of research

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on non-redundant steel
tension member bridges,

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but there was not any
research on developing

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these types of redundant
details for straddle bits.

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- [Todd] These were what we
call full scale laboratory tests

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and we also did parametric
finite element analysis

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that then develop design methodologies.

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But the laboratory tests
were somewhat unique

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in that they were full scale
steel box straddle caps,

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but we wanted to simulate a
brittle fracture on these.

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So the way we can get a material,

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steel is a very ductile material,

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but in order to get it to
behave in a brittle fashion,

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we can cool it down.

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And what we do is use
liquid nitrogen to do that.

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So we were able to cool
down the steel box caps

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in a region where we introduced a defect

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by grinding a defect
in cycling it to grow,

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we call a fatigue crack.

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Then we cooled it down

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to about minus 200 degrees Fahrenheit,

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loaded it very quickly to
initiate a brittle fracture.

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And then we looked at the performance

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of these different
details and their ability

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then to support what we
would call the design load

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or the redundancy load
that we would have to carry

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in a safe fashion.

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Those tests then provided valuable data

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that allowed us to then validate

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detailed or refined finite element models

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that we could simulate that
behavior computationally

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and look at a much wider
range of geometries then.

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That allowed us to then go in and

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develop a comprehensive design methodology

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so that designers can actually take this

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and actually relatively simply
incorporate this into design.

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- [Jamie] I think the most
important research finding

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came from the study of
two different concepts

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for these steel straddle bents.

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So one concept for the straddle bent

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was fabricated with post tension bars

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and then the other concept was fabricated

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with a bolted bottom flange.

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So this project demonstrated

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that if the steel straddle bent

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is fabricated with that bottom
flange bolted to the webs,

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that it provides sufficient capacity

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in the faulted condition.

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So due to that bolted
bottom flange connection,

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the simulated fracture didn't propagate

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through the entire member.

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And so this approach allows
these types of structures

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to be designed as re internally redundant,

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therefore eliminating that
costly hands-on inspection.

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- [Instructor] For more information

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and to find the publication
for this project,

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please visit the TxDOT Research Library

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at the link shown below.