Top chord bracing
Top chord bracing
(OP)
Hi everyone,
I am designing a aluminum pedestrian bridge: two trusses with the deck in between the two bottom chords. The bridge has a 3' camber and is simply supported.
The question is in regards of the bracing for the top chord. App 6.3 of the AISC states that trusses can be considered as beams in regards of the lateral bracing, but I think this is at the supports, and the critical point would be at the center of the span, so what would be the way to obtain the min spacing for lat bracing along the truss?. Should I calculate Lp as if it was a beam?
Thanks in advance
Tony
I am designing a aluminum pedestrian bridge: two trusses with the deck in between the two bottom chords. The bridge has a 3' camber and is simply supported.
The question is in regards of the bracing for the top chord. App 6.3 of the AISC states that trusses can be considered as beams in regards of the lateral bracing, but I think this is at the supports, and the critical point would be at the center of the span, so what would be the way to obtain the min spacing for lat bracing along the truss?. Should I calculate Lp as if it was a beam?
Thanks in advance
Tony






RE: Top chord bracing
If you are using top chord bracing you have a through bridge and you can proceed in the same way, with the benefit of initial deformations not being as critical (i.e., you wouldn't need to model them explicitly, yet the members would be assumed within industrial production tolerances), what would be even more true if a horizontal bracing plane truss is at top chord level.
For pony truss proper you have more or less closed form statements of the top chord critical loads and you can find them I think in both Galambos' fifth and fourth editions.
RE: Top chord bracing
The top chord of your truss will be a compression strut, braced vertically by the web members, but unbraced laterally. For bracing in this direction, you can extend the deck members and provide diagonal braces to the top chord.
RE: Top chord bracing
If anyone else has ideas, I'd love to hear them!
RE: Top chord bracing
I know AISC doesn't apply to sluminum structures DIRECTL?Y, but if you take into consideration the mechanical properties and have things in the elastic range then there shouldn't be a problem.
I thought on those diags, I just don't know what would be the spacing and the strenght.
Hey StructuralEIT, the verticls acting as cantilievers off the bottom chord is like if it was a handrail, but it is not, so the consideration is different. Actually there has to be some provisions as per the truss acting as handrailing, in regards of some lateral load, but that's the next issue, lol.
Ishvaaag's post provided me some guidance that I am going to expplore right now.... any thoughts would be muchly appreciated, though.
RE: Top chord bracing
RE: Top chord bracing
Mike McCann
MMC Engineering
RE: Top chord bracing
There are many papers and books around that explain how to apply this principle to determine the buckling load for members with elastic lateral supports.
RE: Top chord bracing
RE: Top chord bracing
RE: Top chord bracing
RE: Top chord bracing
In this chapter of Galambos' the pony truss thing is at hand.
Follows a printout of my port of what in Galambos' and in the following entry another.
RE: Top chord bracing
RE: Top chord bracing
BA
RE: Top chord bracing
The key here is that you need to ensure that these vertical cantilever-braces are stiff enough to laterally brace the LTB behavior of the top chord. AISC has guidance on this for steel structures, but like hokie66 correctly points out, AISC doesn't apply to aluminum structures. There is an aluminum code to use in the US - Aluminum Design Manual . Not sure what is used in Europe, elsewhere.
RE: Top chord bracing
If that difference doesn't interfere with the guidelines of AISC, then, I can proceed with AISC, no?
Another question JAE, where are this guidelines you mentioned? I couldn't find them on in AISC 360/05
BAretired, I will follow that lead too, thanks.
Greetings
RE: Top chord bracing
I had the opportunity to work briefly on reviewing some codes for aluminum structures. I believe the relevant ones are Eurocode 9, ADM, some parts of AASHTO bridge design documents, CSA S157 and others. For welds, one source is the International Institute of Welding's Fatigue Design of Welded Joints and Components which has a more detailed set of fatigue categories than CSA S6 does for steel.
Another good source on aluminum is Kissell and Ferry, "Aluminum Structures: a Guide to their Specifications and Design." Well-written and actually interesting to read because it's written a bit informally. For example: "We begin by introducing you to aluminum, and we hope that by the end of Part I you are sufficiently well acquainted to be ready to get serious about the relationship."
Hope this helps!
RE: Top chord bracing
RE: Top chord bracing
StructuralEIT, I think the provisions there are for bracing at support points, or at least it is not clear for me where they mention it applies all along the beam, could you please clarify?
Greetings
RE: Top chord bracing
Read The last paragraph on page 16.1-19. It says "Where elements are designed to function as braces to define the unbraced length of columns and beams, the bracing system shall have sufficient stiffness and strength to control member movement at the braced points. Methods of satisfying this requirement are provided in Appendix 6, Stability Bracing for Columns and Beams."