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new lightweight steel truss/bridge design

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David Ratcliff wrote:

> Dear Sir/Madam,
>
> I am seeking like minded engineers to participate in a new lightweight
> steel truss/bridge design... I am presently assembling a 27 metre (90
> ft) span using only 25x25x2.5 (1"x1") as the main top chord.
>
> The truss is different in that it has three chords/double webbing
> compared to the conventional two chord/single layer webbing... The aim
> is to control horizontal deflection or 'buckling'... which cannot be
> done with conventional single layer methods.
>
> If you are interested in viewing plans and photos as work proceeds I
> will be happy to keep you informed... No catches!
>
> Kindest regards,
> David Ratcliff
> 19 Gladstone Street.
> West Wyalong NSW 2671
> Australia


David,
It sounds like you're describing a truss that's triangular in section.
I did some preliminary design of such trusses (about 400 feet long)
about 15 years ago, and recall some points that surprised me at the
time.

I first placed two of the three chords on the top (in compression), with
a single tension chord, reasoning that most trusses have more area in
their compression members.  Almost as an afterthought, I tried a design
with a single compression chord and two tension chords, and found that
it could be made far more economically, because the single compression
member, being bigger than either of the two smaller ones, required less
frequent bracing.  In other words, I could stretch out the panel point
spacing and reduce the number of diagonals and connections.

In addition the two-chords-down variant was obviously a lot easier to
set.  The two-chords-up design needed rather elaborate posts down to
bearing (mine were bottom bearing) at each end.

On these large trusses, I built up chords out of three plates made into
a triangular tube in an attempt to simplify the connection details for
the diagonals.  Whether it actually did or not is debatable, but that
was my intent.

If you're going 90 feet with basically a triangular bar joist, I would
assume a span-to-depth ratio of perhaps 15 to 20, putting you in the 5
foot depth range.  Any reasonable bracing length for that 1 inch chord
is going to leave you with a whole lot of diagonal members.  I'm
surprised if even a solid 1 inch square member will give you enough
chord area, especially for a bridge.  Also, what carries the deck?  How
do you make meaningful connection to such a small member?

I don't mean to sound critical--these are some questions that occurred
to me.  I'll be interested to hear more about your design as it
progresses.

Mike Hemstad, P.E., S.E.
TKDA
St. Paul, Minnesota
USA

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