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RE: Lateral Brace Deflection
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "INTERNET:seaint(--nospam--at)seaint.org" <seaint(--nospam--at)seaint.org>
- Subject: RE: Lateral Brace Deflection
- From: Mark Gilligan <MarkKGilligan(--nospam--at)compuserve.com>
- Date: Sun, 15 Aug 1999 13:45:48 -0400
Ed My reading of S&J says that Kreqd = 2*Kideal does not require an additional factor of safety. 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 analysis. Mark Gilligan --------------------------------------------------------------------------- ----- Message text written by INTERNET:seaint(--nospam--at)seaint.org > Mark, >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! Ed <
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