An item to think about.... it kinda parallels Drew's comments .... the
majority of load for most long-span trusses is the dead load condition
(which creates a particular dead load axial stress in the member)....... the
addition of the reinforcement you speak of does nothing to lessen this axial
stress thus you have a given axial stress that is a certain % of the bucking
strength of the column (based on the kl/r ratio).......after you reinforce
the compression member, two states of axial load will exist... the
reinforcing that has little to no axial stress and the original member has a
lot..... it will definitely reach the critical buckling stress first and
then kick-into the "shell".....if you go with making the "shell" big enough
to do the entire job you may want to closely consider the additional dead
weight to a sloped member (diagonal) thus increasing the "beam-column"
affect....(i.e., its selfweight becoming large enough to induce a sufficient
"delta" that substantially lowers your buckling stress)..... also, bear in
mind that the buckling stresses discussed in the steel code are the result
of many factors (consider brushing up on that fact in Salmon & Johnson's
"Steel Structures" book)......just some random thoughts.....
Robert C. Rogers, PE
From: Paul Feather [mailto:pfeather(--nospam--at)san.rr.com]
Sent: Monday, May 07, 2001 9:19 PM
To: SEAOC List
Subject: Increasing Kl/r
I would like to bounce some thoughts by you.
I am looking at some existing trusses (wide flange chords, pipe webs) with a
130 foot span. For the new loading condition, everything checks out OK
except one diagonal compression element at the third panel from each end
(section changes at two end panels).
The sections have sufficient area to carry the required forces if the Kl/r
could be reduced. The area is sensitive to welding, and we would like to
create a bolted solution if possible.
The concept is to split a larger diameter section, add interior ribs at
approx. 24" like a plane fuselage and bolting flanges, and encapsulate the
existing member for the full length minus a foot ea. end. The two halves
would be attached with high strength bolts pulling the ribs uptight against
the existing member (design would allow sufficient gap to ensure the
interior member is clamped).
The resulting section would then be viewed as the original section only for
stresses, but as a combined section for buckling resistance. An r value
would be calculated for the combined section with both the existing and new
What opinions do you have? Do you see something I am missing?
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