Pizza Slice Truss Connection

[KootK]

I've got a wacky steel truss connection to sort out (see attached sketch). I'm holding up a very heavily loaded truss by grabbing one of the vertical truss webs with a couple of outrigger plates. Additionally, the joint is pretty congested. I'm proposing to tie in one of the more lightly loaded webs (HSS) by attaching it to the neighbouring webs (ie. the web doesn't make it to the bottom chord). This results in a gusset plate resembling a pizza slice. Here are my, somewhat contradictory, thoughts:

1) If I make the applied HSS load and the two resisting plate shears pass through a common work point as shown, there should be at least one statically admissible load path that doesn't involve bending stresses at the gusset plate boundaries. The gusset boundary shear forces will still induce moments in the webs (V x d/2) which will need to be accounted for. However, by virtue of relative stiffness, I feel that the system will gravitate towards a load distribution that maximizes the resistance provided parallel to the truss webs and minimizes the resistance provided by plate boundary forces perpendicular to the truss webs. I like this feature if my logic is correct.

2) Viscerally, I feel as though the pizza slice will just plough through the two truss webs, spreading them apart, and inducing enormous web moment. Needless to say, I find this scenario much less appealing.

3) I've never seen anyone else do this. This makes me think that it's probably a bad idea.

I'm very interested to know what my engi-peers think of this proposal.

Thanks,

KootK

 

[BAretired]

You need to show the big picture, not just a joint detail. Why do you need the red members at all?

It doesn't look like the plates are supporting anything. They seem to be bolted to each flange of a vertical WF. Is there a horizontal splice in the WF? If so, I don't see it.



BA

 

[briancpotter]

If you draw the free body diagram, I suspect that balancing the forces will require a normal force at each gusset plate edge of the "pizza slice", not merely a shear. This will indeed induce a bending moment into each truss member as you guessed in case 2).

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[BAretired]

I missed the word "outrigger" when I commented yesterday. I'm still wondering what carries the outrigger plates, but if the truss reaction can be carried that way and the load in the HSS web member is light enough, the eccentricity might be acceptable. Its eccentric moment would be shared by all of the members meeting at the joint and the right hand outrigger would carry more load than the left.

Alternatively, you could slope the HSS a little more so that its work point coincides with the work point of the other members.

BA