Truss reactions

[ztengguy]

Anyone know online where i can get prelim truss reactions for a scissor truss, or do you figure them yourselves?

this is a new project to determine if a wall can take some horizontal force. Just need to get a prelim design going.

 

[Archie264]

A scissor truss shouldn't have much horizontal reaction...that's the point of using them - the horizontal components are resisted internally. Especially if you can find a way of having some type of slotted hole or simulated roller bearing on one side to mimic a theoretical simple span scenario so much the better.

 

[ztengguy]

I still would like to see the reactions, I dont have enough experience with them to ball park it either. Perhaps I can find a mfg webite with some prelim reactions.
Thanks

 

[rb1957]

except at the base of the scissors, no?

depending on the mechanism, but if you're pushing with an actuator on one leg there'll be two large horizontal forces.

if the actuator is mounted between the legs, with one leg fixed and the other sliding, then there shouldn't be any external horizontal reactions.

Quando Omni Flunkus Moritati

 

[Archie264]

Well, I was referring to the horizontal "component" from the vertical force, which should be zero, or thereabouts. Wind load is wind load; there's no way around that. But all that should be is the horizontal force (wind) imparted on the vertical projection divided by the two reactions. Yeah, you can't get into the pi^e*i*thetha permutations of wind load combinations as required by the latest code but the original request is for a preliminary design number.

 

[ztengguy]

When you say zero or thereabouts, is it insignificant, or a % of the span? I am about a 40 span, 2'-0" trib.

 

[msquared48]

When the truss deflects vertically under load, there will be horizontal lateral movement that will generate a horizontal lateral force at the support points if restrained. I think this is what ztengguy is talking about and needing.

Mike McCann
MMC Engineering

 

[jayrod12]

Depending on the slope wouldn't vertical load induce noticeable horizontal reactions(ourward thrust, same as rafters with no collar ties)? I would have a hard time assuming a roller end on a scissor truss considering the carpenters will nail both ends at a bare minimum?

 

[Archie264]

stengguy,

I'm sure no expert but I was just speaking theoretically. The reactions from a truss are analogous to those from a simply-supported beam. If there's a working slotted hole connection or some other allowance for expansion on one side then it really should be zero. Absent that there could be a little horizontal force but I'm guessing it would be less than that imparted by wind load. If it were my project I'd check to make sure the connection could take the wind load and not worry about the other until I had further information.

 

[Archie264]

jayrod,

You don't need collar ties with a scissor truss...or any other truss, for that matter. The truss chords take the lateral component internally. That's what they do.

 

[Archie264]

PS: The shop drawings I've seen from truss manufacturers are based off pinned-roller connections so don't expect to see a horizontal component from them. Maybe I've only seen a non-representative selection of such shop drawings but I'm just reporting on what I've seen.

 

[Archie264]

PPS: Be on the lookout for something like this. It's a scissor truss clip that should (ostensibly, at least) allow horizontal movement thus eliminating (ostensibly, at least) a horizontal thrust from a vertical load. Wind, then, would all be taken by the other support. Some skepticism and judgement should still apply, of course.

http://www.menards.com/main/store/20090519001/items/media/Hardware/usp/Prod_Tech_Spec/stc.pdf

 

[jayrod12]

But if you allow the one end of the scissor truss to move then how do you detail the finishes at the corners where it meets a wall. Wouldn't the drywall crack everytime the wind blew?

 

[tony1851]

Where I work, roof trussues are generally finished with concrete tiles. This causes the truss to deflect slightly once the load is on. To acommodate the slight lateral movement on the wall plate, we use glide shoes.

http://www.strongtie.co.uk/pdf/GS.pdf

This is nothing to do with wind loading, which is taken by the internal shear walls.

 

[woodman88]

All simple horizontal structural members have horizontal deflections at the reactions. For beams, etc. they are so minor no one bothers with the calculations.
Scissor trusses, curved GLB, etc. can have measurable horizontal deflections for pinned/roller reactions.
For scissor trusses spaced 24" oc, with a 1" horizontal deflection, placed near the corner of a building. Your roof sheathing would either resist (e.i. transfer the horizontal force to the end wall) the scissor deflection or allow that much movement of a scissor truss placed 24" from the end wall.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[ztengguy]

So would a truss supplier run a design and give reactions, or would they only do that for an order?

 

[msquared48]

If you did business with them regularly as a contractor, probably so, bit if you were a first time homeowner, probably not. Ask them, and let them know what you do. Communication is a good thing...

Mike McCann
MMC Engineering

 

[ztengguy]

Thanks.
As usual this is being put on my by the Architect, who is sorta doing a Design Build with the contractor. I asked him to get some reactions from the truss guy the contractor would use, but he said 'That would be like pulling teeth'. I think I will stick to my guns and let him sort it out with the contractor, I dont want to waste my time with a supplier that will never get the job. thats not right either.

 

[XR250]

They are modeled as pinned-roller. There are no horizontal reactions in theory. You can't resist it in normal wood construction. The walls simply push outwards to accommodate. Simpson sells a clip to allow the truss to deflect under dead load and then it is supposed to be nailed tight. I never spec them as I know that will never happen. I don't think Simpson really understands the physics of the situation.

FWIW, the same thing happens with a vaulted ceiling using a structural ridge. When it deflects, the walls push out or the roof sheathing temporarily prevents spread.

 

[woodman88]

Here is a link to a artical (Considerations in Specifying a Connection for Horizontal Movement) from Alpine about scissor trusses.

http://www.alpeng.com/index.php?option=com_content&view=article&id=31&Itemid=7

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.