Truss Question - Sanity Check 1

[Nick6781]

I'm designing something similar to the sketch below.


Under lateral load, the floor framing behaves like a truss, and I observe a significant amount of axial force in the chord members due to wind in the Y direction.


However, the chord is interrupted by interior columns, so it's not a continuous member.


If I'm using simple shear connections at the ends of all the beams, is there any issue with transferring the compression force through the middle beam (i.e., the middle segment of the chord)?


Since each simple shear connection can be idealized as a hinge, is there a concern with the compression force having to pass through two hinges before reaching the middle beam? Or will everything function correctly as long as the load path is properly designed and all members and connections are adequately detailed?

 

[hemiv]

I don't see any problem. Trusses often have idealized pins at the web-chord nodes, anyway! Should be good as long as the forces can be transferred through your connections, which may need to be strengthened a bit beyond a typical clip or shear tab.

 

[phamENG]

The thing to worry about with shear tabs and axial loading in the beams is slip in slotted holes. Most shear tab connections have slotted holes in either the beam or the tab to a facilitate fabrication and erection tolerances. Makes it a lot easier to get the beam into position if you have some wiggle room. But now, that beam can shift side to side when loaded in tension or compression. So you need to design your connection to both facilitate construction AND prevent movement after the building is up.

A few ways to do that - slip critical bolts and using a deck edge angle as the collector and creating a load path "around" the shear tabs" are two that I've seen commonly.

 

[Ron247]

Are your leftmost and rightmost column lines braced in the Y-direction, but the 2 interior column lines not braced in the Y? The springs are confusing me a little.

 

[Nick6781]

Are your leftmost and rightmost column lines braced in the Y-direction, but the 2 interior column lines not braced in the Y? The springs are confusing me a little.

All of them are braced in the y direction. The springs are there to show that there's stiffness along the column lines

 

[TRAK.Structural]

Depending on the stiffness of the "springs" at each column line you may have something that more closely resembles 3 individual trusses without much axial load going through the column joints at the chords.

If you have vertical frames at the 2 end column lines only than I would expect needing the axial load transfer for the chords as you have described.

 

[Ron247]

I agree with TRAK that you need to consider the smaller trusses and that the ends of each truss are possibly restrained to some degree in the X-direction.

 

[Nick6781]

Depending on the stiffness of the "springs" at each column line you may have something that more closely resembles 3 individual trusses without much axial load going through the column joints at the chords.

If you have vertical frames at the 2 end column lines only than I would expect needing the axial load transfer for the chords as you have described.

My model is telling my that there's compression in the middle beam. It's more like a continuous beam/truss with a very short middle span.

 

[phamENG]

where is that stiffness coming from at the columns?

 

[Ron247]

Is it showing single curvature across the middle 20' rather than several reverse curvatures?

 

[TRAK.Structural]

My model is telling my that there's compression in the middle beam. It's more like a continuous beam/truss with a very short middle span.

Is the compression continuous across the joints at the interior column lines?

A diagram showing the chord forces would be helpful to understand how your model is behaving.

 

[Nick6781]

Stiffness is provided by the braced frame below, which the column is part of.

The compression is continuous, and I don't think that's unreasonable. For example, in a three-span continuous beam under uniform loads with spring supports , if the middle span is significantly shorter, the moment may not reverse in that span.

 

[rb1957]

maybe worry about things not being perfectly aligned ? But then the buckling is based on a single bay length, so maybe this isn't something to worry about ?

 

[TRAK.Structural]

Stiffness is provided by the braced frame below, which the column is part of.

The compression is continuous, and I don't think that's unreasonable. For example, in a three-span continuous beam under uniform loads with spring supports , if the middle span is significantly shorter, the moment may not reverse in that span.

Seems like you've got your mind made up, I can buy it, design the connections to transfer the axial load.

 

[drlt5]

I think you are confused. The chords does not need to be continuous. You just need to provide the continuity via force transfer by introducing connections at the interrupted points or by introducing tensions bands, tension bars etc.
check your forces. most of the time the chords are interrupted as all conventional buildings are built by beams and columns ... nothing unique here...