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






RE: Truss reactions
RE: Truss reactions
Thanks
RE: Truss reactions
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
RE: Truss reactions
RE: Truss reactions
RE: Truss reactions
Mike McCann
MMC Engineering
RE: Truss reactions
RE: Truss reactions
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.
RE: Truss reactions
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.
RE: Truss reactions
RE: Truss reactions
http://www.menards.com/main/store/20090519001/item...
RE: Truss reactions
RE: Truss reactions
http://www.strongtie.co.uk/pdf/GS.pdf
This is nothing to do with wind loading, which is taken by the internal shear walls.
RE: Truss reactions
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.
RE: Truss reactions
RE: Truss reactions
Mike McCann
MMC Engineering
RE: Truss reactions
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.
RE: Truss reactions
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.
RE: Truss reactions
http://www.alpeng.com/index.php?option=com_content...
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.
RE: Truss reactions
Then, tell the Arch. to sit down and smile while you pull a couple of his teeth with a vice gripe pliers; and an oh shucks, it can’t hurt much, I’m standing right here and I don’t feel a thing. Use the phone and not a damn e-mail, and call the last guy who supplied trusses for one of your jobs, remind him of the job. They are usually willing, and it doesn’t take them long to run a generic 40' scissors truss. For simplicity of analysis, they assume one reaction pinned and the other to be free to slide, and they give you a lateral deflection. This lateral deflection is actually split btwn. the two support walls and you pretty much have to detail to tolerate this potential movement. You really don’t have one sliding end. You’ve nailed each end to the top pls., as a min., and likely applied some hold down clips too, so the tops of the walls do tend to move out. As Garth suggested , near the end walls (gable ends) most of this movement, and the forces, will be taken to the end walls by the roof sheathing when its nailed properly. But, I’ve also seen the mid length of a long bearing wall bowing out and showing almost all of that lateral deflection at its top. You can’t make a strong enough light framed wall to prevent this movement.
RE: Truss reactions
Correspondingly, especially for flatter roof diaphragms, the roof diaphragm's beaming effect will limit any lateral movement from the wind.
The lateral deflections will still be there, just less than you think, or we calculate, unless we include the whole lateral resisting system in the picture.
Mike McCann
MMC Engineering
RE: Truss reactions
Some good approaches and recommendations there, in my opinion.
RE: Truss reactions
I will put it back on the Architect, I really dont need the headache this job is going to be, but either way, want to be diligent and do it right. The Architect said why not use a bond beam to resist the force, I said thats fine, but your building is 100' long, not sure its going to take the load. I convinced him we needed to put the load into the existing pilasters at the existing trusses, and strengthen them.
Sometimes its best just to not call an architect back right?
RE: Truss reactions
Mike McCann
MMC Engineering
RE: Truss reactions
ugh. What do they pay at walmart to stock shelves?
RE: Truss reactions
Thanks for all your help.
RE: Truss reactions
RE: Truss reactions
RE: Truss reactions
The problems arise when the truss is installed with a pinned connection at both ends so that the thrust is restrained. Then the horizontal thrusting load can be many hundreds/thousands of pounds if the wall is not able to deflect sufficiently.
In the OP's case it looks like the main thing is to ensure that the scissor trusses are supported as pinned/slider to ensure that the horizontal reactions are as expected. The close proximity of the more rigid bow trusses may prevent the parapet from having the flexibility to accommodate much horizontal deflection in the pin-pin case.