scissor truss support

[sponcyv]

Anyone ever had pre-fab wood scissor trusses supported by wood columns or bundled studs? I know the concerns about lateral kick-out of scissor trusses and this concerns me being that they would be supported by columns.

 

[BAretired]

Scissor trusses are usually supported on a wall or beam which offers no lateral restraint, i.e. hinge and roller support. The lateral deflection can be calculated and if it is unacceptable, the truss geometry must be changed as required.

BA

 

[KootK]

My thoughts:

1) The kick out should be evaluated in terms of deformation compatibility with surrounding construction.

2) The trusses should be positively attached to the top of the columns in most cases. No slip bearing nonsense.

3) The top of the column would need to be braced in the plane perpendicular to the trusses, particularly if a high truss heel is employed.

Other than that, I'm fine with it.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sponcyv]

Thanks for the input. The scissor trusses are spaced at 15' o.c. and bear on columns that are within walls. Does a bearing height of 24' scare you? This makes for a very tall column. In addition, there are windows on either side of the columns. The columns are formed because the bottom of scissor truss is lower than the top plate of the wall. The top plate is disconnected at the column leaving the column being the only element resisting any lateral forces. At the least, p-delta would need to be checked, but if a wood column (bundled studs) is used, that's an awful flexible column at that length.

 

[jayrod12]

Ya that's going to need to be an extremely large column. Can you post a framing plan? This feels like a pole shed for some reason.

 

[sponcyv]

jayrod12

I don't have a framing plan yet - just went over the concept that the arch was planning today. It's going to be a nice building once all said and done. It's not a pole barn, but I see where you are coming from on that. I don't think there is any way to allow one end to slide free with it bearing on a column. I would think you have to make a connection that is rigid enough to not slip and design the column for that horizontal force at the top of the column. You would need to analyze it to see how much it moved and look at what that would do to the structure. I'm thinking we'd need to drop in steel columns. Maybe even fix them at the base.

 

[BAretired]

You will not be able to generate sufficient horizontal reaction in a 24' high column unless the column is monstrous.

The truss will not be permitted to "slide free" on the column but the flexibility of the columns supporting the scissors truss will be equivalent to a hinge-roller support for the truss from an analysis point of view. The reactions of the truss will be vertical with little or no horizontal component.

Steel, glulam or Parallam columns would be a better choice than bundled studs. Dimension lumber is not easily obtained in 24' lengths.

BA

 

[XR250]

Might also want to see what the horizontal load at the top of the column does to the scissors truss. May make it bow up or down excessively. Have the truss manufacturer run that load case.

 

[sponcyv]

BAretired

Based on your feedback, the column will move no matter what type of connection you have because of the flexibility of the column. This was my thought exactly. These are the checks that I believe I would need to make:

- check the horizontal deflection at the top of the column against recommended allowable out of plane movement
- check the column for p-delta effects from that movement - combined stresses on column
- check the connection to make sure it can take the force into the column - determine the force needed to move the top of the column by the amount of horizontal deflection and check for this load
- check the column base to make sure that it is anchored properly to the foundation - steel would be easy because it is bolted - wood not as simple
- check the horizontal reaction as XR250 recommended for effects on truss - coordinate with truss designer

any other checks anyone can think of?

Thanks for the feedback!

 

[jayrod12]

With these columns being 24 ft tall at 15' centres I expect the vertical reaction to be sizable, 10 kips or more, you'll be hard pressed to find a wood column that will do that for you. What's the intended connection at the base of the column? Foundation type?

 

[KootK]

In all likelihood, you'll go with a pin ended colun and therefore not need to check any of those things. Just design the column for axial and wind and make sure that you're okay with the lateral movement at the top of the column that will be imposed by the scissor truss.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sponcyv]

jayrod12 - agreed - I think steel is the way to go

kootk - with steel, we'll have the possibility of modeling either fixed or pinned bases. I will probably check both just to make me sleep better at night.

thanks

 

[jayrod12]

The problem with that is providing the fixing at the base to validate your modelling.

 

[jike]

I agree with previous comments. Here are a few other considerations: What is providing stability for the system? Does the roof diaphragm carry load over to the end shear walls? What are the overall dimensions of the building?

 

[sponcyv]

jike,

Yes, the roof diaphragm distributes the lateral forces to shear walls. The footprint is 60'x100' roughly. The scissor trusses will have framing running over the top of the scissor trusses. This will be decked with structural sheathing. Roughly 60% of the building is 2 story and 40% is full height to bottom of scissor trusses. We will have 3 sides of shear walls on the 2 story portion and 1 side of shear wall on the full height portion. Of course I need to check aspect ratio at the full height because those walls are 24' tall.

 

[jayrod12]

If there is a roof diaphragm and lateral system (adequate shear walls) I'm less leery about wood or steel columns in the wall.

 

[sponcyv]

jayrod12,

Are you saying this because you you believe the roof decking will help keep the scissor trusses from deflecting horizontally?

 

[jayrod12]

precisely, that depends on the diaphragm stiffness obviously, and the shear wall stiffness. But I don't feel as concerned overall. I still don't think you'll have a good time getting a wood column to work for those loads at that height, but my overall stability concerns are lessened.

 

[KootK]

If your column occurs between adjacent windows and doesn't make it to the to plate, I worry that you may have a stability issue on your hands. It might be prudent to post a couple of details that we can help you vet.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sponcyv]

kootk - will do - it may take me a day or so to get the details posted, but I appreciate you being willing to look over!