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Truss Beam problem! 5

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Integrator62

Aerospace
Jan 5, 2015
37
bg_super_beam.jpg


In the picture a space truss beam is shown.

Its not a truss because the Webs are welded, not pinned and the main four exterior members are solid Members. So they can resist bending moment where in pure truss they only can resist Tension and compression.

Its basically a open web complicated beam.

Is there any analytical way to solve this type of truss beam?

Thanks.
 
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Hi

I would still analyze it as though it were pinned, I've seen trusses which are bolted and still treated as pinned.
 
Trusses rarely fit the mathematical definition of a truss perfectly. I believe that most engineers would still consider this to be a truss. I also suspect that, for load in the plane of the diagonal webbing, this systems behaviour will be dominated by truss action. To account for the fixity that you have at your joints, simply model the system as a frame with the member ends rigidly connected to one another. This usually means software.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
i'd madel as a pinned member truss. you could FE it with fixed members (beams instread of rods).

another day in paradise, or is paradise one day closer ?
 
I just finished a class taught by Packer, a leading researcher on tubular steel structures. He wrote this book: Link

His recommendation is to model the chords as a continuous beam. The web members are then pinned to an extremely rigid vertical member with a length equal to the noding eccentricity. The vertical member is fixed to the beam. This will induce a realistic bending moment into the chord, and allow the the web members to remain pinned, which is reportedly how they behave according to current research.
 
All of the above, and,
Vertical webs are normally included to address a specific local loading item and I am wondering if this illustration is upside down...
The system looks like two 2D trusses with chord bracing between them rather than a 3D space frame truss, at least as I imagine them....
Just some observations.
 
agreed, as shown it wouldn't be that good at handling lateral load (across the structure, from 1 2D truss to the other) ... it should be ok at handling in-plane loads (vertical?, in the plane of the 2D truss).

if you've got lateral load, then you need lateral bracing ... discrete truss members, or continuous webs.

another day in paradise, or is paradise one day closer ?
 
See Commentary K.2 in AISC 13 (assuming U.S.) or AISC Design Guide 24. Both discuss the analysis portion of welded HSS trusses.
 
Thanks everyone. [smile]

Unfortunetly I cant buy books right now. So, things get tough for me!

However, DamsInc, will you please Describe a little about it? specially "noding eccentricity", I havnt heard anything about it. [smile]


I guess the Chords will act as pure beams, So there should be a big error considering it as truss desertfox [smile]

Yes, I use it just as an example rb1957 Triangled [smile]

Ill go for FE analysis later KootK [smile]
 
Integrator62,

AISC 360-10 is available as a free download from the AISC website. That should allow you to use BadgerPE's information.
 
OP said:
However, DamsInc, will you please Describe a little about it? specially "noding eccentricity", I havnt heard anything about it.

@DamsInc: this is good to know. Is the fictitious rigid link a vertical element extending to the interior sides of the chords? Does it change how one should physically detail the joint or is making the member centroids coincident still appropriate?

OP said:
I guess the Chords will act as pure beams, So there should be a big error considering it as truss desertfox smile

Not so. The chords will act primarily as axially loaded members. Any frame action will be a secondary effect.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
"noding eccentricity" is, i think, the distance between the true end of the (diagonal) truss element and the axis of the chord.

in the design world, the truss elements line of actions all intersect, but in the real world the diagonal truss element shears it's load into the chord member in a manner that'll introduce "kick" moments.

another day in paradise, or is paradise one day closer ?
 
Integrator62, I'm not sure if you meant your question as general or specific, but the image you used is of entertainment truss. Entertainment truss is governed by the relatively new standard PLASA/ANSI E1.2 - 2012 -Design, Manufacture and Use of Aluminum Trusses and Towers, which is freely available. I don't know how much that will help you. Most manufacturers of entertainment truss publish load/span tables as well. I believe that these are at least partially based on testing. One reputable manufacturer (among many) in the US is Tomcat.
 
Integrator62: Noding eccentricity is the vertical distance between the axis of the main chord member and the point of intersection of the axis of the web members. See the attached images I scanned from Packer's book (I don't think this violates copywrite?). If you have unbalanced forces coming into the chord from the webmembers, an eccentricity will induce a bending moment in the chord.

KootK: Yes, that is correct, but it can extend on either side of the chord depending on if the eccentricity is positive or negative.

Depending on the ratio of the noding eccentricity to the member dimensions, the effect of the eccentricity can often be ignored.

I really recommend this book. It is a few years old now and some sections are now out of date based on more recent research.
 
 http://files.engineering.com/getfile.aspx?folder=584c871b-c673-4cee-bd90-105a24551081&file=HSS_terminology.jpg
KootK said:
Does it change how one should physically detail the joint or is making the member centroids coincident still appropriate?

One major consideration is the size of the gap "g" between the web members. Too close and it won't be practical to weld. If the chord can handle the moment ok then the eccentricity isn't a big deal. Really it's just a fabrication/economics issue if the connection otherwise passes all the checks (there are 7 failure modes in the book). We spent a few weeks on the subject of these truss connections.

I got the impression that it is common in the industry to just consider the whole thing pin ended and be done with it.

That being said, I'm not a truss designer, I just finished his graduate class in December.
 
Your impression is correct. Assuming pinned members has served well for a long period of time, and trusses of this type can be easily analysed by hand. A bit conservative, but no big deal.
 
Thanks everyone. [bigsmile]

So I have got two ways.
1) Truss consideration:
i) Consider it as pure truss, 1st order analysis
ii) Consider the moments in joints, recalculate, 2nd order analysis

2) Beam consideration:
i) Consider chords as pure beam, 1st order analysis.
ii) Consider the web members as pinned and recalculate, 2nd order analysis

If 2nd order analysis of truss doesnt have much effect, I will go with simple truss calculation with pinned members. hokie66 KootK rb1957 [smile]

A lot thanks DamsInc for your help. [smile] If I take noding eccentricity as 0, then there will be no constant bending moment, rather an inclined axial force.

I also thought that Its testing and software work, Thanks 48v

Yes I have downloaded one fegenbush . Thanks.

 
Hi

Let us know how it works out, I would be interested to see how large the error might be, that was a concern you had if analysing as a simple pinned truss.
 
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