truss end portal frame: non-moment connections as moment connections
truss end portal frame: non-moment connections as moment connections
(OP)
Is there any way to prove/verify that a non-moment connection (in reality) in a(n) (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.






RE: truss end portal frame: non-moment connections as moment connections
RE: truss end portal frame: non-moment connections as moment connections
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.
RE: truss end portal frame: non-moment connections as moment connections
RE: truss end portal frame: non-moment connections as moment connections
RE: truss end portal frame: non-moment connections as moment connections
RE: truss end portal frame: non-moment connections as moment connections
RE: truss end portal frame: non-moment connections as moment connections
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.
RE: truss end portal frame: non-moment connections as moment connections
With a standard base plate, "fixed" is not accurate.
RE: truss end portal frame: non-moment connections as moment connections
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.
RE: truss end portal frame: non-moment connections as moment connections
Just a thought.
Mike McCann
MMC Engineering
RE: truss end portal frame: non-moment connections as moment connections
This is mounted on steel, not concrete.
I'll just be sure to check the anchor bolts and base plate.
Thanks.
RE: truss end portal frame: non-moment connections as moment connections
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.
RE: truss end portal frame: non-moment connections as moment connections
RE: truss end portal frame: non-moment connections as moment connections
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.
RE: truss end portal frame: non-moment connections as moment connections
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?
RE: truss end portal frame: non-moment connections as moment connections
RE: truss end portal frame: non-moment connections as moment connections
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.