Modeling Truss Panel Points 1

[slickdeals]

Folks,

I am modeling a deep floor truss that will carry floor beams at each panel point and one in between each panel point. The panel points are at 16' on center and the beams are 8' on center.

I am modeling the truss with moment releases (minor and major) on the diagonal web members. I am unsure about the releases at the top and bottom chords.

At the panel point where the diagonal frames in, should I release the moments as well? Because the top chord of the truss will have studs and concrete will be placed on it, wont the top chord act as a continuous beam supported on panel points?

The bottom chord will act as a simple beam spanning between panel points.

Are these assumptions right? I would appreciate your suggestions.

 

[msquared48]

If the truss top and bottom chords are continuous, unbroken members, then, where the bottom and top chords intersect the panel points, they need to be fixed to properly model the truss for member stresses and deflections, especially since you have intermediate loading between the panel points.

As for any diagonal and vertical members, they are usually pinned at the ends.

If the top and bottom chords have splice points, then these locations need to be treated as pinned joints too. Just remember though that at any panel point, one joint must be fixed to avoid spiining in space, ultimately resulting in no solution. The fact that one of the member ends is fixed though, with all the rest pinned at the same panel point, will still give a solution as if all the members are pinned at the joint. Such is the typically the condition at the ridge of an open web truss.

Mike McCann
MMC Engineering

 

[slickdeals]

Mike,
Thanks for that response.

 

[WillisV]

You can model the truss with either continuous top and bottom chords or pinned top and bottom chords. Either is statically admissable. Utilizing the continuity and designing for the moments in the chords will generally result in lighter sections and certainly less deflections within your analysis. Traditional true "truss" behaviour assumes pure axial behaviour (pinned top and bottom chords). If you design your members with the results from such an analysis, the members will just have to form plastic hinges to achieve that (still stable) state of equilibrium.

 

[slickdeals]

WillisV,

I have a condition where I am using WF members (web horizontal). I am getting high moments due to intermediate panel point loads, even though I have a vertical there.

I am unsure whether to model one side of the top chord pinned (like Mike suggested) or to design for that moment.

Any ideas?

 

[JAE]

slickdeals,

The key to modeling structural systems in a program is to approach it with the goal of replicating the actual configuration and composition of the system as close to reality as possible.

What Mike was discussing had to do with how typical structural programs deal with global stability in the matrix (i.e. a node must have one member not-pinned).

If you add a pin, then you must account for the presence of that pin in your design - by detailing the connection not to take moment.

If you have a vertical in the truss at the point load, why are you getting "very high moments" there if the vertical is also connected to a diagonal? It seems to me that a proper lay-out of truss would negate any significant chord moements.

 

[slickdeals]

JAE,
I totally agree with you in that the idea of modeling is to replicate the exact situation.

Please see the attached ETABS file. It is a simple truss with panel point loads. I have a vertical for loads in between panel points.

In Elevation 2, the truss top chord and bottom chord are modeled continuous. I still get moments in the chord and it looks like the chord is spanning between the diagonals and not between the diagonal and intermediate vertical.

Any help with the model is appreciated.

Thanks

 

[JAE]

slickdeals - can't open your file - can you post a pdf of it?

 

[PanamaStrEng]

I can´t see your model, but probably the vertical element is just acting as a tie that "shares" the point load between the top and bottom chords. If there are no diagonals forming a triangle, it may happen. It may also have to do with the relative stiffness of the top and bottom chords. Are they similar in size?
Can you post a pdf or jpg figure?

 

[Qshake]

The beauty of a truss is that it's members take only axial load and concentrated loading is at the connections only. Doing so ensures that you will always have a relative high ratio of axial load to bending.

Why would you want to add significant loads between panel points? THis suggests to me that the panels need to be closer together or the loads moved out to the panels.....

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[slickdeals]

QShake:My truss is 8' deep and my panel points are spaced 16' apart for a 45 degree angle. That forces me to have a load in between panel points (beam spaced at 8' o.c.)

PanamaStr: I think you are correct. I have members significantly bigger than they need to be for the chords.

 

[msquared48]

Slickdeals:

What I am hearing you say is that you have no interior vertical members, only diagonal members. Not the typical Pratt or warren Trusses. If you can add the verticals too, then the moments you mentioned will go away, or at least become extremely manageable.

Mike McCann
MMC Engineering

 

[hokie66]

If your truss is 8' deep and the diagonals are sloped at 45 degrees, your panel points are at 8' centres horizontally, not 16', but perhaps not at both top and bottom chord. If you can't post a file we can read, maybe you can describe the truss generically. Pratt, Howe, Warren, modified Warren. In your third post, you said that you have verticals at the intermediate load. Sounds like modified Warren, which would, as Mike said, deal with the moments.

 

[haynewp]

I also think you should model the top and bottom chords as they would be in real life,--if continuous then model as continuous, if not, then don't. The continuity of the chord should affect the allowable axial buckling load of the chord (think of a column that is continuous between floors rather than individual separate column segments between floors).

 

[Dinosaur]

Based on your description, I think you should consider a warren truss with panel points 8 ft oc in both chords. Good luck.

 

[slickdeals]

I think I may have not explained it right. I have a 8' deep truss. The intersection of the diagonals are 16' apart at the top and bottom chords. I have a vertical member at 16' on center (or in between diagonals at the top chord).

I am trying to eliminate the bending in the top chord by using the vertical member, however, I am still getting some bending. Are there ways to minimize this?


My truss profile looks so, I am just being lazy to sketch and scan. It's a Sunday morning
-------------
\ | /\ |
\ | / \ | and so on....
\|/ \|
----------

 

[Qshake]

Have you considered sub-dividing the diagonals and lower chord member to provide additional panel points?

As noted above, individual truss members shouldn't be subject to bending or you wind up with problems such as you're having....

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[kslee1000]

I would first try to model the top chord as a composite (with concrete slab) continuous beam with intermittent diagonals and verticals pinned to it.

 

[hokie66]

slickdeals,

What you have drawn is a modified Warren truss. There should be negligible moment in the chords if the loading is all at the panel points in the top chords, including over the verticals. If you have substantial bending of the top chord, your model is incorrect. Suggest you analyze it by hand using method of joints or sections, or graphically if you know how. Compare results with your computer output, and then figure where the difference is.

 

[hokie66]

slickdeals,

Forgot to say, first thing to look at is the force in the verticals. Compression in each vertical should match load at that panel point.