truss end portal frame: non-moment connections as moment connections

[stuctural101]

Is there any way to prove/verify that a non-moment connection (in reality) in a (unbraced) truss end portal acts as a moment connection (as modeled)? In order for the model to be stable, I either had to 1) make the top chord (strut/beam) to portal leg connections moment connections, or 2) make the support conditions at the portal legs fixed. I went with option 1 since the truss is mounted on a steel bent, and not directly to a rigid concrete foundation.

 

[stuctural101]

I forgot to say that the concern is transferring lateral loads (wind/seismic) from the top chord of the truss down to the support via the end portal verticals.

 

[ToadJones]

this sounds like a typical mill building frame.
truss connections to the columns are usually modeled as pins and the column bases (with proper detailing paramount) are modeled as fixed.
The couple formed b/t the top and bottom chords at the connection to the column are analogous to a giant moment connection with stability from frame action.

 

[ToadJones]

note that the column needs to extend to the top chord elevation with the bottom chord framing into the column lower.

 

[stuctural101]

Actually, this is a conveyor truss with walkway outriggers.

 

[ToadJones]

same principle ?

 

[dhengr]

The moment is imparted to the column top as axial loads in the top and bottom chords of the trusses, with a lever arm equal to the truss depth. Then the question becomes can the chord members tolerate these forces, and can the two non-moment connections btwn. the chords and the column transmit these forces. In the orthogonal direction bracing btwn. the two column legs provides the lateral load resistence. The column bases may be either pinned or partially fixed, probably not totally fixed.

 

[stuctural101]

Please refer to the attached sketch showing the cross section of the truss end portal.

Typically, I would use sway bracing (alternating) to stiffen up the cross section at every panel point. Also, typically I would put some lateral/sway bracing at the end portals to take the lateral loads. Unfortunately, this is an existing design that I am reusing, which does not have any sway (cross-sectional) bracing at all.

From a design perspective (simple statics) and also to make, in order for the end portal (frame) to be stable (and not a "mechanism"), either joints (C & D) OR (A & B) must be assumed to be fixed.

I'm trying to justify that either joints (C & D) OR (A & B) are fixed or at least have enough fixity/moment resistance to take the lateral loading down to the supports.

 

[ToadJones]

With large brackets and extended anchor rods, the bases can be accurately modeled as fixed.
With a standard base plate, "fixed" is not accurate.

 

[stuctural101]

dhengr,

I think what you're saying is treat the base/support as fixed, and the portal posts/vertical (on one side only) as a cantilever (flagpole) laterally loaded at the top. This would assume that the horizontal strut is just a tie and not taking any moment (shear connection only) (takes very little load in my model). The moment in the post would need to be taken by the anchor bolts at the base. This is a simple (simpler than assuming that joints C & D are moment connections) and reasonable approach. I may have been overthinking this in the first place. Thanks.

 

[msquared48]

Also, with the proper conveyor clearances that do appear to be available here, short knee braces could be added to the sketch with pinned connecgtions for the most part, obtaining the same moment effect, but not with the code complexity of a moment connection.

Just a thought.

Mike McCann
MMC Engineering

 

[stuctural101]

ToadJones,

This is mounted on steel, not concrete.

I'll just be sure to check the anchor bolts and base plate.

Thanks.

 

[stuctural101]

msquared48,

That would be the simplest solution. Unfortunately, as I said, this is a "duplicate" of a previous job (heard that one before). As such, we were told to reuse the design for the second phase of the project. This really isn't how my company would design a truss from scratch. I believe these trusses are already in detailing/fab so adding the kickers would bring up a lot of questions (can of worms). There are a LOT of these trusses. Anyway, I know for a fact that this type of design has been in operation and that can be our justification. Thanks.

 

[dhengr]

Since this is an existing structure, I like Mike’s knee brace idea, since you probably won’t have to work as hard making the existing anchor bolts or connections check for this new loading. Your sketch is really not exactly what I was thinking of as I wrote my first post. What you are showing, in elevation, is what I called the orthogonal direction. In any case, for the lateral loads you show, I would certainly bring both vertical/posts into play, with the horiz. members acting to transfer half the lateral load to the far post, thus half the canti. base moment if that’s the way you try to go. Are the anchor bolts and base pls. able to take this moment?

 

[paddingtongreen]

Almost any configuration can be designed as fixed or pinned as long as it is sufficiently strong and stiff to hold the resulting forces and moments.

Assume that all four of the joints are fixed then check the joints for the resulting moments and loads. If the top ones don't work, try again with only the bottom ones being fixed.



Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[stuctural101]

Wrench in the works...I just remembered that these trusses sit on bearing pads that allow for allow for rotation...thus acting like a pin. So the base as fixed idea is out.

Funny thing is, I got my FEM model to be stable even without any sway bracing. I've assumed welded connections to be fixed, bolted connections to be pinned. The sides of the truss are shop welded frames, and the top & bottom chord bracing are field bolted.

I think what's going is that the truss verticals (being fixed-fixed) have end moments, giving rigid box/frame behavior. Thus the truss just doesn't flop over like a mechanism.

I know some recommend designing trusses as ideal trusses (all pin-pin connections). What do you folks think of the end release conditions I'm assuming?

 

[hokie66]

There is fixity at all four corners of your cross frames, just not full fixity. Analyze the moment capacity of these connections, then back figure how much lateral load the frame can take. Compare the two. Based on your sketch, it looks like the top connection could be strengthened by adding bolts on the outside of the flanges (on the conveyor side).

 

[csd72]

Structural101,

from the description you first gave I was picturing something completely different to what you have. In order to get valid comments you need to ensure that you either give a clear description or post a drawing/sketch.

Anyhow, I agree with Hokie, there definitaly is some fixity to those connections particularly if they are tensioned bolts.