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Truss Beam problem!
5

Truss Beam problem!

Truss Beam problem!

(OP)


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.

RE: Truss Beam problem!

Hi

I would still analyze it as though it were pinned, I've seen trusses which are bolted and still treated as pinned.

RE: Truss Beam problem!

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.

RE: Truss Beam problem!

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 ?

RE: Truss Beam problem!

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.

RE: Truss Beam problem!

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.

RE: Truss Beam problem!

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 ?

RE: Truss Beam problem!

See Commentary K.2 in AISC 13 (assuming U.S.) or AISC Design Guide 24. Both discuss the analysis portion of welded HSS trusses.

RE: Truss Beam problem!

(OP)
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

RE: Truss Beam problem!

Integrator62,

AISC 360-10 is available as a free download from the AISC website. That should allow you to use BadgerPE's information.

RE: Truss Beam problem!

Quote (OP)

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?

Quote (OP)

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.

RE: Truss Beam problem!

"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 ?

RE: Truss Beam problem!

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.

RE: Truss Beam problem!

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.

RE: Truss Beam problem!

Quote (KootK)

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.

RE: Truss Beam problem!

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.

RE: Truss Beam problem!

(OP)
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.

RE: Truss Beam problem!

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.

RE: Truss Beam problem!

(OP)
Ill do it a little days later I hope. smile Just made a little try for a very simple model and Stuck!
Tried to use min potential energy method, but can be done using simple calculation.

Iteration process is probably the only way, which is very time consuming. May be impossible without software for More complicated frame.

Assumptions,
Deflection is very little to affect Beam length.
Moment at point B C and D are not considered. Left for 2nd degree analysis, which may be too much lengthy process!

Thanks.

RE: Truss Beam problem!

I'm working through a similar problem....trying to determine the effective length of an unsupported compression chord. I just started doing some research into the topic, and this thread has given me a good start.

Integrator62, you should follow the link below...seems like a lot of great resources and they're all free if you register (takes 30 seconds to register).

http://www.cidect.org/en/Publications/

RE: Truss Beam problem!

your pic is indetminate (though solvable), but your assumptions of fixed end moments is causing the problem (not simple axial members).

axial members means the diagonal member is carrying the vertical load. this means the top member is in compression and the bottom member has zero load.

when you go to the next bay, the lower chord is reacting the horizontal component of the outer diagonal, the diagonal reacts any added vertical load, and the top chord reacts the outer top chord plus any diagonal component added in this bay.

a truss beam like yours, with single direction diagonals, is statically determinate.

now allowables, the compression allowable for the chords, ... that's a different question !

another day in paradise, or is paradise one day closer ?

RE: Truss Beam problem!

i think you've got it ... diagonal BD reacts the applied load P (the vertical component of the load in BD = P). the diagonal induces load in the top member AB (the horizontal component of the load in BD). BC and CD have no load in them.

just draw FBDs at each joint.

if the pic posted is real, then i'd worry about lateral loads.

another day in paradise, or is paradise one day closer ?

RE: Truss Beam problem!

(OP)
Yes I got the point! BC and CD will face very little I guess, so neglected. All is left is a 3 member frame! Load along BD later apply force on DC member, and again increase the Moment on AB by bending DC! Things get rough! I hate to do iteration! I am tired of it! Ill try some days later!

Thanks smile

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