Top chord bracing

[Mixtli]

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

 

[ishvaaag]

If you do not place bracing atop, this is a pony truss. You without looking for a text may include in your 3D model the outwards (for example) initial prescribed maximum out of straightness (to be verified at work, and allowing for vandalism) and then proceed with the calculation with P-Delta and reduced stiffness if required by the level of compressive stress. This directly will give a safe structure if meeting maximum limit stresses for factored loads.

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.

 

[hokie66]

AISC does not apply to aluminum structures.

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.

 

[Lion06]

I've often wondered how pedestrian bridges like this work. I've seen many. Even if you consider only the bottom chord working in bending for strength, it would seem to me that the truss would want to buckle. The type I've seen have always been HSS sections. The only conclusion I've come to is that the verticals (most likely the end verticals) brace the top chord by acting as cantilevers off of the bottom chord.

If anyone else has ideas, I'd love to hear them!

 

[Mixtli]

Hokie,
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.

 

[Lion06]

The cantilevering I was referring to was of the vertical acting as a brace for the top chord. The vertical HSS would need to meet the strength and stiffness requirements per AISC to qualify as a brace for the top chord. The top chord is a compression member and needs to be braced accordingly. The only brace I can see for it is the end verticals cantilevering off of the bottom chord acting to brace the top chord as a cantilevered member. This has nothing to do with handrail forces. It has to do with brace forces and stiffnesses.

 

[msquared48]

I agree with Hokie here and have seen this method employed on many smaller bridges.

Mike McCann
MMC Engineering

 

[spats]

I'm no expert on this subject, but I believe the principle is to have fixed connections at cross-beams that are located at the intersection of truss verticals and the bottom chord that cantilever up to brace the top chord, similar to what StructuralEIT is saying. This forms a rigid "C" shape, if you will. These verticals are essentially analyzed as "columns on elastic foundation" (the bottom joint can rotate).

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.

 

[Mixtli]

hummmmm, interesting StructuralEIT, indeed......... any thoughts on that approach anyone?

 

[spats]

Just realized that I actually stated the principle wrong. The "columns on elastic foundation" are the top chords. The elastic foundations (supports) are the cantilevered web members, that deflect based on their bending stiffness AND the amount the bottom joint is able to rotate based on the stiffness of the cross beams.

 

[Mixtli]

Ok, spats, so any idea on how to get the thing evaluated properl? I mean, how to get the min distance without bracing? or the required strength of the bracing, that in your opinion would be the verticals?

 

[ishvaaag]

http://books.google.com/books?id=W_...esult&ct=result&resnum=1#v=onepage&q=&f=false

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.

 

[ishvaaag]

The other approach...

 

[BAretired]

This subject is treated in Chapter 2 of "Theory of Elastic Stability" by Timoshenko and Gere which can be downloaded from the internet.

BA

 

[JAE]

I agree with Spats and StructuralEIT. Typically for bridges like this the bottom deck structure is utilized by fixing it to the truss verticals. The truss verticals are then vertical cantilevers that are "braced" laterally at their tops - i.e. the top chord of the two main trusses.

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.

 

[Mixtli]

JAE, could you advise on the difference between Steel and Aluminum, besides E, Fy, Fy after welding, Fu, Poisson, and weight and the fact that elastic analysis is recommended over plastic?
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

 

[ChadV]

One other thing to consider (certainly not the only one but I'm not an expert in aluminum by any means) is fatigue since you're talking about a bridge structure. Steel has a fatigue limit, a pretty well-defined value of stress below which cycles aren't considered to be damaging. Aluminum doesn't have such a well-defined limit (check the Wikipedia on Fatigue Limit), so you need to consider fatigue more carefully than you would for a steel structure.

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!

 

[Lion06]

The bracing provisions are in appendix 6 of AISC 360-05.

 

[Mixtli]

Thanks ChadV, I am following CSA-S157

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

 

[Lion06]

What, in the text of App. 6, leads you to believe that the bracing requirements apply only to the bracing at support points?

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."