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RE: Lateral Brace Deflection
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- Subject: RE: Lateral Brace Deflection
- From: "Ed Fasula" <tibbits2(--nospam--at)metro.lakes.com>
- Date: Sun, 15 Aug 1999 22:54:36 -0500
Mark, >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. Ed 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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- RE: Lateral Brace Deflection
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- RE: Lateral Brace Deflection
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