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.

 

[slickdeals]

"If you design your members with the results from such an analysis (using all pinned connections), the members will just have to form plastic hinges to achieve that (still stable) state of equilibrium."

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

 

[slickdeals]

"Secondary Stresses in Trusses" - By Shankar Nair published by AISC gives me insight into what I was looking for.

The flexural stresses induced from continuous chords may be neglected if the axial forces from analysis were consistent with a pinned joint assumption.

 

[Qshake]

Yes, but even AASHTO has a limit at which secondary stresses must be considered and that limit is low, 4000 psi.

Regards,
Qshake

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

 

[slickdeals]

Folks,
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

 

[BAretired]

The gusset plate you show in your sketch has no eccentricity because all members meet at a point. The compression diagonal on the left and the tension diagonal on the right intersect at the centroid of the top chord.

If all bolts are centered on the c.g.of each member meeting at the joint, the bolts carry no eccentric moment.

 

[slickdeals]

@BAretired,

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

 

[BAretired]

There are five forces acting on the gusset plate, two from the top chord, two from the diagonals and one from the applied load which you have not shown. For equilibrium, the sum of these vectors must be zero. If they all intersect at a point, there can be no eccentricity on the gusset plate as a whole. If there was, the plate would not be in equilibrium.

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.

 

[BAretired]

I think I got my clockwise and counterclockwise reversed in the above. Sorry about that.

 

[slickdeals]

@BA,
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://ntsb.gov/Recs/letters/2008/H08_1_Design_Adequacy_Report.pdf

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.

 

[BAretired]

I have not read your reference, but at a glance it appears that the diagonals do not intersect at the work point. Sometimes it is necessary to spread members out a bit in order to make room for the connection. When this happens, there is an eccentricity applied to the joint.

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.

 

[BAretired]

I started looking through the Interim Report you referenced earlier. What is being done in the report is not entirely clear to me at the moment. I think I would like to reconsider my former response but will need a little time to do it.

 

[Lion06]

You shouldn't have to worry about moment in the gusset for your case. If you were coming into the flange of a chord and the workpoint was at the centroid of the WF then I would say you do, but the difference between where your gusset meets the chord and the workpoint is only tw/2. To be correct, you should make sure the weld from the gusset to the chord can take that moment out.

 

[slickdeals]

StrEIT:
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?

 

[BAretired]

Joint U10 of the Interim Report, contrary to my earlier post seems to have all members meeting at a point. The writer calculates for Section A-A that the shear, V in the gusset is 2,723k. This is the sum of the horizontal components of the two diagonals. It is also the algebraic sum of the forces from the top chord. That horizontal shear is constant between the work point and the end of the diagonals.

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.

 

[slickdeals]

@BA,
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