Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Lateral Brace Deflection

[Subject Prev][Subject Next][Thread Prev][Thread Next]

   >My reading of S&J says that  Kreqd = 2*Kideal does not require an
   >additional factor of safety.

The confusion between the editions is that 2ed uses a different definition
for Kideal.  In it, Kideal = Pcr/L = 2*P/L, hence the (second) factor of
safety of 2 is included.  My 2ed uses Kideal = P/L and Keqd=4Kideal.

I compounded the confusion by saying Pcr when I meant P.

Sorry about that, and thanks for the help.


The multiplier of 2 is a
   >function of the
   >initial crookedness and does not vary based on the loading.
   > An exception
   >would be if other loads increased the initial crookedness
   >of the member.
   >Kreqd is the translational stiffness at the point of
   >bracing and may be
   >provided by members acting in tension or bending.  In other
   >words if you
   >imposed a unit translational displacement at the point of
   >concern the
   >resistance should exceed Kreqd.  You should consider not
   >only the bending
   >stiffness of the beam and the diagonal brace but also any other
   >flexibilities in the bridge structure that would have a
   >significant impact
   >on this term.
   >Assuming that the transverse beams contribute to the
   >bracing of  chords for
   >each of the trusses I would be inclined to calculate the
   >stiffness by
   >imposing a unit displacement inword for both truss chords
   >at the same time.
   >If there were other transverse loads such as wind on the
   >truss chord or
   >vehical loads on the transverse beam that produced
   >deflections in the chord
   >consistent with the buckling shape you are trying to
   >suppress, then a
   >stiffer structure would be required.  Note that these loads could be
   >thought of as increasing the initial crookedness of the
   >member and would
   >not need to be considered when considering the availible stifness.
   >Hopefully you will be able to show that the transverse
   >deflection of the
   >truss chord is not significantly influenced by other loads
   >and that the
   >overall flexibilities of the bridge do not significantly
   >influence the
   >stiffness of your brace.  If this is not the case or if it
   >becomes too
   >involved you might want to consider performing an
   >eigenvalue buckling
   >Mark Gilligan
   >Message text written by INTERNET:seaint(--nospam--at)
   >   >Salmon & Johnson has a good treatment of point bracing for
   >   >beams.  You want
   >   >to make the brace stiff enough to inhibit undesirable
   >   >buckling shapes.  The
   >   >strength of the brace is then calculated as a function of
   >   >the crookedness
   >   >of the brace.
   >They mention members braced by transverse members, but they
   >only address
   >axially loaded braces...
   >By their method,
   >The required lateral brace capacity, Q, is:
   >Q = 0.004*Kideal*L
   >Kideal = 4*Pcr/L  (with beta=4), Pcr = 156k (service) and L
   >= 120" so,
   >Kideal = 5.2 k/in and
   >Q = 2.5k (conservative compared to 0.002*P, in this case)
   >2.5k is not hard to accommodate, but Kreqd:
   >Kreqd = 2*Kideal*FS  (FS = 2.12 for AASHTO) so,
   >Kreqd = 22 k/in
   >As I understand the derivation, Kreqd would simply be
   >directly related to
   >the bending stiffness of the beam in this case, rather than
   >the axial
   >stiffness of a brace for the case addressed in the book.  Given the
   >geometry, 1" vertical deflection of the beam = 1.7" lateral
   >deflection of
   >the top chord.  Therefore, the beam's stiffness must be at
   >least 22*1.7 =
   >37.5 k/in!
   >With a 6' cantilever, this is quite the beam.  If this is
   >correct, I have
   >some serious re-thinking to do!