×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Gusset Plate Design

Gusset Plate Design

Gusset Plate Design

(OP)
I am designing connections of a Special Truss Moment Frame. The top and bottom chords are continuous while the web members are connected by welding to the gusset plates. The joint is treated as rigid. As a result, the web members will carry axial force, shear force and moment. I can't find any book or anything on the net that tells me how to design the gusset plate of this type. Help me please.
 

RE: Gusset Plate Design

Is the moment resulting from eccentricities in the connections? Bending in the web members?  

RE: Gusset Plate Design

(OP)
The moment comes from treating all the joints as rigid. Yes, bending moment in the web members. I look at UFM but those web members only carry axial loads.

RE: Gusset Plate Design

I ask because usually when I do truss designs similar to yours I treat the top chords as continuous but model the web members as axial load members only.

ConnectEngr, where you been?
This seems to be your area of expertise.  

RE: Gusset Plate Design

(OP)
TJ: So have you dealt with bending moment due to eccentricity of the load in web members?

RE: Gusset Plate Design

In your exact case, no, I try to make sure there are no eccentricities which I believe is the same approach that the UFM uses.
With trusses I have dealt with (mill building type trusses and the like) the flexural stiffness of the web members and connections is so low that I never thought modeling them as rigid was appropriate.  

RE: Gusset Plate Design

...I would think (could be wrong here) that any moment in web members of a truss properly laid out to work as a classical truss, even with fixed connections, would only experience bending in the web members as a secondary force.
There has been papers written on this subject going all the way back to the early 1900's.
see attached....this is very old....probably outdated

RE: Gusset Plate Design

There is a more recent one on secondary stresses written by Shankar Nair.

We are Virginia Tech
Go HOKIES

RE: Gusset Plate Design

Typically designing truss joints as rigid is not economical in terms of fabrication and erection.  The design will produce a lighter truss, but the force combination will require additional connection material and welding. And in most cases reinforcement and stiffener plates.  The overall cost to the owner is typically more for the rigid design.  This is the reason for the lack of connection design examples.   

http://www.FerrellEngineering.com

RE: Gusset Plate Design

The connections can be designed for the force combination you stated.  But, more info is needed.  What member types are used for chords and web members?  HSS?  Wide Flange?  Web vertical or horizontal?

But, I do recommend releasing shear and moment in the panel points.  

 

http://www.FerrellEngineering.com

RE: Gusset Plate Design

Now that I think about it, I have modeled very heavy trusses for large, long span industrial conveyors with fixed joints at all locations. But, these trusses were VERY heavy and made entirely of heavy wide flanges (W14x193, for example) and had no gusset plates.
In this case I felt it made more sense given the high stiffness of the web members and their overall flexural rigidity.
For web members life small HSS or small pairs of angles I feel those are generally best suited as axially loaded members.  

RE: Gusset Plate Design

Modeling truss panel points as rigid is a phenomena of the software programs now available.  From the designer's standpoint the truss is lighter, due to reduced chord and web member sizes.  "Less weight = less cost".  This is a case where "sharpening our pencil" does a dis-service to our customer.  This does not account for connection design, detailing, fabrication, and erection.  As rigid members the web members are often sized to perform above 90% of their allowable capacity.  As TJ mentioned this is usually a wide flange truss with webs horizontal.  But, the model does not account for shear lag.  So if the web is not extended into the chord, the member is immediately inadequate 85% < 90%+.  If the web is extended, you have one or two diagonals and a vertical sharing the same workpoint.  If the chord webs are relatively thin, the intersecting webs will overlap and so with their welds.  The flanges of the diagonals must also be mitered and also clipped to avoid weld intersections.  If these flanges are large, welds must be sequenced to avoid deforming the chord or vertical flanges.  In order to control the welded framing the panel points are generally shop weldments with bolted splices in the chord diagonals.  With the force combinations this will require reinforcement of the net section.  If the designer has the foresight to consider these issues, the savings in truss weight are lost and the resulting member sizes may be larger than if selected for axial load only.  My comments come from experience with connection design of "rigid" truss joints.  Similar issues can occur in welded trusses with only axial force.  But, many of these can be avoided by using only 75-80% of the members axial capacity.    

http://www.FerrellEngineering.com

RE: Gusset Plate Design

(OP)
The truss we are making is for research purpose so we try to treat it as they are done in practice which is treating joints as rigid. Normally, they use double angles as chords, diagonal and vertical web. In our research, we are gonna be use 2C12x25 for all of the members. Cost and weight are not of our concerns. One panel of the truss is about 5.5" long and 4" high. Please help me figure out how to design the connection.

Thank you.

RE: Gusset Plate Design

"treated as they are done in practice", probably not rigid

With back to back channels I don't have any suggestions for the force combinations in the connections.  Typically the channels are bolted through the web and gusset plate, or welded to the gusset plate. These connections are for axial force only.    

http://www.FerrellEngineering.com

RE: Gusset Plate Design

I agree with ConnectEngr here.

Connect-
Would you agree that with connections where the work points offer no eccentricity that there is no reason to design for moment?
My intuition tells me that the only moments on these connections will be secondary or the result of deflections?  

RE: Gusset Plate Design

TS
I don't think the moment in Chad73's panel point is due to workpoint eccentricity.  He is modeling the truss as a rigid frame, with the diagonals and verticals fixed at the truss chords.  In the model, this works fine.  And in your example of a welded wide-flange truss, it can work.  But in a double channel truss the forces must transfer through a single gusset plate at the centerline of the cross section.  Some moment can be taken in the channel web to the gusset.  But the majority of the moment is in the flanges.  The model sees lines with given properties and nodes.  The actual shapes and load local stresses are not considered.      

http://www.FerrellEngineering.com

RE: Gusset Plate Design

Connect-
I see.
I have done a fair amount of trusses made up of WT top and bottom chords with double angle webs. I always modeled the top and bottom chord continuous and the web members as "truss" or axial members.
Usually I did not need gussets at all as the top and bottom WT stems were large enough to make the connections. When I did need a gusset to extend the web I had the fabricator full pen weld a plate to the stem and grind it flush. I only considered axial force on this plate and I only make the plate as big as necessary to accommodate the welding or bolting of the web members.

 

RE: Gusset Plate Design

(OP)
Well, in my case, for the Special Truss Moment Frame, the company that has used this system treats all the joints as rigid. They use double angles welded to the gusset plate. I'm sure that using double angles will induce less moment in the web members. In my case, we simply just change from double angles to double channels and I have no clue as how to design the gusset plates to take this moment. :(

RE: Gusset Plate Design

I haven't designed a STMF, but I am not really understanding the logic behind forcing to design the joints for moment. It is a truss and does not need a moment connection (whether you get a partial moment connection due to connection fixity is another question).

Quote:

the company that has used this system treats all the joints as rigid
Is it really required?

We are Virginia Tech
Go HOKIES

RE: Gusset Plate Design

Are you by any chance using a Vierendeel Section in the middle span of your STMF?  

RE: Gusset Plate Design

...if this is the case I can see using a fixed joint for the "special" portion of the truss that is designed to dissipate seismic forces.
However, I still think the remaining portions of the truss should have joints that are pinned.
   

RE: Gusset Plate Design

A double angle member has very little moment capacity.  Basically the bending capacity of the back to back legs.  So the moment capacity of the angles can be developed in the welds to the gusset plate.  But, I don't thick the larger moment capacity of channels can be transferred in the same way.   

http://www.FerrellEngineering.com

RE: Gusset Plate Design

Connect-
I think you are getting at the point I am trying to make (albeit poorly so). Even if modeling members like double angles as pins, they probably in reality do develop some secondary moment but since their moment capacity is so low it can be ignored in design.  

RE: Gusset Plate Design

I have only designed one of these in my career (hopefully in 25 years, I will still be able to say that).  If memory serves, the the connections of the webs to the chords have to be sized to yield the web in tension + some factor that escapes me now as the intent of this truss is to absorb enery from seismic motion through the buckling of webs is the special segments.  I ended up with gusset plates that where rather larger with a large amount weld.  These connections ended up being extremely stiff.  Since these are designed almost exclusively for the resistance of lateral seismic forces (gravity loads are limited to something like 5% of allowable stress), I can see were th OP is coming from.  

Back to the original question.  I believe that if you size the gussets as required by AISC 341, the gussets will end up being sufficiently stiff to be considered moment connections.  

Chad73 you did mention above that you are using C shapes for all memmbers.  AISC 341 requires that the webs in the special section be constructed of flat bars.  If you follow this, there it the webs would likely not have much moment capacity.   

RE: Gusset Plate Design

Chad73, moments in truss web members is very rare and should be avoided, however, there are times when fixed ends are required and you simply have to deal with the resulting moments.  It is true gusset bending, especially weak-axis bending, will result in thick gusset plates, so if in-plane moments can not be released in your model, try like hell to release your out-of-plane moments which are producing torsion on your chords.  Otherwise it's simply a matter of P/A + Mc/I.  Your normal stress will add directly to your Mx and My stresses, and then your shear stress is applied.  It's the old unity equation we've all seen since we were kids.  The result will be if using LRFD(P/A + Mx/Zx + My/Zy)/.9Fy + (Hshear/A)/.6Fy <= 1.0.
Good luck.    

RE: Gusset Plate Design

Sage advice from connectegr, as usual.  The chords of trusses need to be designed for moments, but the webs should be strictly axial members.  Except for rigid frames, often referred to as Vierendeel trusses.

RE: Gusset Plate Design

Connectegr:

Agree with your assumptions about pinned connections for trusses and everyone else's comments about how to model the chords, but this discussion has raised another question that I have always struggled with.  Would you still assume pinned connections if you are designing a HSS truss with all-around welded connections?  I don't want to hijack this thread, but I thought it might be an appropriate question given the theme of this thread.

Thanks!

JWB

RE: Gusset Plate Design

Jberg
Yes. HSS welded panel point is a pinned connection.  Similar to a bracing connection with gusset on top an bottom flange.  The beam is still considered a simple beam.  But ductility limits on endplate or angle thinks would not apply.   

http://www.FerrellEngineering.com

RE: Gusset Plate Design

jberg,
My initial answer was brief, because I was typing on my cellphone.  

IMHO
Welded HSS trusses with staggered member sizes have pinned panel point connections.  As an example of staggered, I mean 10" chords, 8" verticals, and 6" diagonals.  Unless the walls are extremely thick, the connection flexibility is found in the ductility of the HSS walls.

If the members are all the same width, so the welds are directly in alignment with the supporting HSS wall...
The connections have less flexibility.  But, you cannot obtain the full strength of the walls with the welds.  An AWS BTC-P10 (J-Grove PJP) has a maximum effective size of 5/8 x the wall thickness.  And there is still ductility in the walls along the fillet welded sides.

This is another case, where the modeling software has no problem analyzing these rigid panel points.  But, developing the welds and checking the HSS walls may be impractical.  And may require backing up and changing the model or member sizes.   

http://www.FerrellEngineering.com

RE: Gusset Plate Design

Thanks connectegr:

That is exactly what I thought, but recently there has been a discussion at the office, debating whether truss connections in general behave as "pinned" or "fixed".  I have always thought, in reality, truss connection behavior was somewhere between pinned and fixed, but everyone assumes pinned for the reasons previously stated.  Once again, thanks for the prompt responses.

RE: Gusset Plate Design

While I agree with most of the posts above regarding treating the joints as pinned for typical truss design, I must point out that the question was specifically for Special Truss Moment Frames.  These are trusses that are designed for the resistance of lateral seismic forces and not for gravity loads.

When you look at the actual requirements for the design of the joints and web members in the special segment of the truss you will see that the gusset plates and connections will be large enough to transfer moment.  Since the web members must be flat bars, they have very little bending capacity.  Thus, for the truss specifically asked about by the OP, it is more accurate to model the joint as fixed.  

Since the intent of the truss is to allow for significant elastic deformation in the special segment, moments created at the joint due to the webs should not be discounted.

RE: Gusset Plate Design

As has been stated, in practice truss joints are usually, and should be, assumed pinned both during the design of the main members and during the connection design. I have posted some thoughts regarding the design of trusses here:

http://larrymuir.building.officelive.com/Trusses.aspx

Near the end of the page there is a discussion of the consideration of moments at truss joints with the opinions of several experts listed.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources