unbraced length of the compression portion of a long member

I am designing a truss. the bottom chord in the first two panels at each end or at the supports are in compression and the rest of the bottom chord is in tension. there is no lateral bracing along the whole bottom chord. what do you think the unbraced length of the compression portion of the continuous bottom chord should be. I don't think we should use the whole length of the bottom chord since the tension part just would not buckle. should I use the length of the compression portion? but there is no lateral restrain where the compression portion meets the tension portion?

• Twice the length of the chord. – Solar Mike Nov 21 '20 at 10:28
• I am not quite catching the picture of it, perhaps some illustration or sketches will help? As far as I know, unless both sides of the compression member has pin end, you would need to consider factorizing the length of the member to obtain the effective length. – Disrudog Nov 21 '20 at 15:19
• The continuous bottom chord can be considered pinned at each end of the truss. Portion of the bottom chord is in compression. How do you factor that portion. More specifically the portion is by the support. Say couple of panels from the support. The middle part the bottom chord is in tension. It shouldnt be that hard to picture it in your mind. You can consider the whole length as unbraced but I think that is conservative. – DDD Nov 21 '20 at 15:28
• Or I think we can have a column pinned both top and bottom. Apply a downward force from the mid height that is high enough to have tension in the top half of the column. Is the unbraced length the length of the column or half of the column? – DDD Nov 21 '20 at 15:33
• Solar Mike. Just saw yours why? – DDD Nov 21 '20 at 15:38

Generally speaking the main application I can think of for trusses with unbraced compression chords is pony truss bridges. The AASHTO (ugh) Guide Spec for pedestrian bridges might have some information. I also believe the American Wood Council's NDS might have a paragraph with prescriptive requirements for unbraced wood truss chords in compression. It really depends on the material and what code you're designing to. I think the general idea of anything you find, though, will be that you need to either:

1. Go with the conservative full-length assumption.
2. Brace it.
3. Mathematically demonstrate according to some accepted methodology that the truss web members and connections are torsionally strong and stiff enough to brace the chord internally (i.e. looking at the truss globally as a single beam and checking it for lateral-torsional buckling).
4. Do a fancy advanced second-order analysis that explicitly models all system imperfections, any reductions in stiffness due to partial yielding, etc. so you can just throw the concept of effective length out the window entirely (most common for crazy curved structures drawn up by architects that defy any conceptual simplifications like "effective length").

I definitely agree that a sketch is needed, though. For example, it might help demonstrate why the ends of the bottom chord are in compression. Without a picture, right now the two possibilities I'm thinking of are:

1. The truss is restrained at both its top and bottom chords and is behaving as a fixed-fixed beam.
2. The ends of the bottom chord are only in compression in certain load cases due to some sort of applied axial load, like a braking crane, but the whole chord is in tension in the absence of such loads.

In scenario 2, it MIGHT be justifiable to treat the compression portion by itself as an axially-loaded, cantilever post.

• to make it simple you can just consider scenario 2. of course we can always brace it as we did but the client with no knowledge of it think the brace is nuisance and will take it out. it is not happening now it might happen in the future when he decide to make it prettier. without the brace we can always assume the whole length of course which i always do. but I need a correct answer of it. what do you mean by cantilever post. just be simple in scenario 2 it is L or L/2. I dont think it is L since it is not the same as you compress the whole length and it will not buckle the whole L. – DDD Nov 21 '20 at 18:45
• I kind of think in scenario 2 it is L/2 although there is not a brace at the mid height. but obviously if you just isolate the lower half of the post somehow it gets restrain at both ends of its own or it will not stand there. and the compression load only is going to buckle the lower part and definitely not the upper half. – DDD Nov 21 '20 at 18:51