(electronic tones)
- [Narrator] Lean-on bracing applications are being used
at a greater rate in steel bridge systems.
This has highlighted a need for improved rules
of configuration and design expressions.
The Center for Transportation Research,
the Texas A&M Transportation Institute,
and the Texas Department of Transportation
monitored a recently designed bridge
with lean-on bracing to come up with guidelines
for bracing design and improve accuracy in stiffness
and strength components.
- If you've ever gone underneath a steel bridge,
you may have noticed these Xs underneath,
steel cross frames underneath.
The function of those is for stability,
primarily during construction,
to keep the girders from buckling.
However, because they're these cross frames
and they're welded together and they're hard to fit up,
they look small as compared to a bridge,
but compared to a man trying to put it up there,
they're quite heavy.
So this lean-on bracing concept
removes a lot of the diagonals,
simplifying the construction and fabrication dramatically.
However, the cross frames that you were removing
were doing something before.
So now there has to be a new design methodology
to make sure that the cross frames that you do have
are sized appropriately
and then also placed throughout the structure appropriately
so that you still have the stability that you need.
- What happens with cross frames and diaphragms
is that there are regions
where there's usually concerns about fatigue.
And also, when we have cross frames and diaphragms,
they usually represent one of the most expensive components
on a per pound basis
because there's lots of fabrication associated with them.
So in lean-on bracing, what we did 20 years ago
is we developed a concept
where we were able to selectively go into a bridge
and eliminate some of these full cross frames,
and instead, just make use of some much more simple,
we call them lean on struts,
that allow several girders to lean on a single cross frame.
And while some of the suggestions we had done
and the systems we had designed 20 years ago,
they started to be implemented into design.
It was found that we probably left too much of it
for interpretation by a designer,
where some designers were having a hard time
applying some of those concepts,
and a number of questions came up.
So to try to get in on those design concepts
we developed 20 years ago
and really try to develop some refined procedures
that make it easier for designers to apply,
but also try to answer some of the questions
and so on that came up. - The benefits of this project
are really stem from the fact
that cross frames are some of the most expensive components
on a steel bridge, though they are obviously essential.
They're difficult to fabricate, difficult to install,
they're also difficult to inspect,
and can have trouble spots in them.
So being able to safely reduce
the number of these cross frames
really leads to a more economical structure,
both in the short term and long term.
- And then over the last six years,
there have been some applications where TxDOT
has essentially utilized these methods on the design
of a few different bridges.
One was up in Fort Worth, which was a heavily skewed bridge,
which is just a geometrical configuration
that happens with the bridge.
Then we had a bridge over the Brazos River
that had normal support.
So it's been used on a couple of different geometries.
So it's already starting in implementation,
but these concepts actually
are seeing a lot more widespread usage,
because the National Steel Bridge Alliance
has been pushing these concepts,
and there was a design guide
that was based upon our original work that was put out.
So we're gonna start seeing a lot more applications
of these, not just within Texas,
but actually around the country.
- We plan to take the findings of the report
and put directly into our design guidance,
so that engineers can use it readily and quickly.
- [Narrator] For more information
and to find the publication for this project,
please visit the TxDOT research library
at the link shown below.