Modeling Truss Panel Points
Modeling Truss Panel Points
(OP)
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.
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.






RE: Modeling Truss 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
RE: Modeling Truss Panel Points
Thanks for that response.
RE: Modeling Truss Panel Points
RE: Modeling Truss Panel Points
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?
RE: Modeling Truss Panel Points
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.
RE: Modeling Truss Panel Points
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
RE: Modeling Truss Panel Points
RE: Modeling Truss Panel Points
Can you post a pdf or jpg figure?
RE: Modeling Truss Panel Points
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,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Modeling Truss Panel Points
PanamaStr: I think you are correct. I have members significantly bigger than they need to be for the chords.
RE: Modeling Truss Panel Points
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
RE: Modeling Truss Panel Points
RE: Modeling Truss Panel Points
RE: Modeling Truss Panel Points
RE: Modeling Truss Panel Points
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 :)
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\ | /\ |
\ | / \ | and so on....
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RE: Modeling Truss 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,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Modeling Truss Panel Points
RE: Modeling Truss Panel Points
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.
RE: Modeling Truss Panel Points
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.
RE: Modeling Truss Panel Points
I have a question regarding this. Assuming that I model the truss chords continuous, then there will be some moment induced at the panel point if there is a difference in T/C forces at the joint (there will be a vertical component to the T/C force).
However, due to the continuity, there will be some reduction in the axial force of the chord. However, will the P-M interaction cause a higher stress ratio in the member as opposed to one with a slightly higher axial force but no moment?
Your input is welcome.
Thanks
RE: Modeling Truss Panel Points
The flexural stresses induced from continuous chords may be neglected if the axial forces from analysis were consistent with a pinned joint assumption.
RE: Modeling Truss Panel Points
Regards,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Modeling Truss Panel Points
Please see attached a crude sketch of my gusset plate. I wanted to know if the bolt group attaching the gusset to the chord will need to be checked for eccentric shear in addition to the axial force due to difference in chord forces?
Is my eccentricity drawn correct? The moment in my gusset plate is equal to the horizontal shear force * distance from WP to last line of bolts. Isn't this the same moment that will be induced in the bolt group due to eccentric shear?
I am breaking my head with this and I would appreciate your inputs on this. I don't want to miss any checks on the gusset plate.
Any references or design check examples will also help.
Thanks
RE: Modeling Truss Panel Points
If all bolts are centered on the c.g.of each member meeting at the joint, the bolts carry no eccentric moment.
RE: Modeling Truss Panel Points
I agree that there won't be any moments in the members, but is it also true of the gusset plate?
Maybe I am complicating it way too much.
The way I see it is that all the force from the braces have been transferred into the gusset at the last line of the bolts. At the last line of bolts I have a tension and compression. When resolved into components I have a horizontal and vertical shear.
The horizontal shear is at a distance of "e" from the centroid of the chords. The shear has to be transferred into the chords as an axial force via shears/moments/axial force in the gusset.
Similarly the bolt group in the chord is subject to an eccentric shear (or axial force) and not a direct axial force.
Are my fundamentals completely wrong? Can someone please explain it to me with a sketch of how the loads are being transferred from the brace to the gusset and then to the chord?
Thanks
RE: Modeling Truss Panel Points
In the case of the left diagonal (compression), the clockwise moment caused by the horizontal component of force is precisely balanced by the counterclockwise moment of the vertical component. A similar argument applies to the tension diagonal on the right. Moment from horizontal component is balanced by an opposite moment from the vertical component. This is true only if the resultant force is directed through the common point, your work point.
The actual state of stress at any point within the gusset plate is not so easy to predict however. You would need to perform a finite element analysis on the plate to find how the stresses vary.
The gusset plates (one each side of the joint) must be adequate to transfer the horizontal shear from the two diagonal members to the chord member, but the moment is zero.
Hope that helps in your understanding.
RE: Modeling Truss Panel Points
RE: Modeling Truss Panel Points
I agree that there are no moments at the workpoint. But how does the force get into the chords from the gusset after the force is transferred into the gusset from the diagonal. I mean the force from the diagonal is not transferred into the gusset at the workpoint, but at some distance away from it. The eccentric force can't all be transferred in shear.
Incidentally, I found this during my web searches.
http:/
Folks, thanks a bunch for helping me get to the bottom of this. I may not be the smartest, but I don't want to leave things to chance. Your help is appreciated.
RE: Modeling Truss Panel Points
If the two diagonals in your sketch had intersected 'e' above the centroid of the top chord instead of at the work point, then a moment would be applied to the joint. That moment would be the sum of the horizontal components of the diagonals times 'e'. It would be distributed among the four intersecting members according to their stiffness and the applied moment to the gusset plate would still be zero. The bolts connecting each diagonal would be carrying axial load plus a normal force to resist the moment tributary to that member.
RE: Modeling Truss Panel Points
RE: Modeling Truss Panel Points
RE: Modeling Truss Panel Points
I agree with you in that I would need to check for moments if I were coming to the flange.
However, I still feel that the axial load has been transferred into the gusset at the last line of bolts from the diagonal to the gusset. The vertical and horizontal components of this axial load will have to get transferred to the centroid of the chords. Are you saying that this will happen with only shears and no moments?
RE: Modeling Truss Panel Points
The moment at the intersection of the two diagonals is 0, but Section A-A (the upper end of the diagonals) is 14" below the work point which gives a moment of 38,118k" at that section. That is an internal moment within the plate.
Returning to your sketch, you show a shear force in the gusset which would be the sum of the horizontal components of the diagonal members.
You show a curved arrow which suggests to me that the plate has a moment acting on it. It does not. It has a horizontal force acting at the work point. At the end of the diagonals, that horizontal force has a moment of H*e where e is the vertical distance from the work point to the last line of bolts in the diagonals.
The force from a diagonal to the plate is an axial load. The four bolts connecting a diagonal to the plate are acting in shear parallel to the direction of the diagonal.
The bolts connecting the top chord to the gusset plate are also transferring an axial load so they will not be subjected to eccentric shear.
RE: Modeling Truss Panel Points
Yes, you are right. The plate has an internal moment (which it should be checked for) that it should be able to transmit, in addition to the axial force and shear.
Thanks