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Truss reactions

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.

RE: Truss reactions

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.

RE: Truss reactions

(OP)
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

RE: Truss reactions

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

RE: Truss reactions

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.

RE: Truss reactions

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

RE: Truss reactions

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

RE: Truss reactions

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?

RE: Truss reactions

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.

RE: Truss reactions

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.

RE: Truss reactions

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.

RE: Truss reactions

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.wink

http://www.menards.com/main/store/20090519001/item...

RE: Truss reactions

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?

RE: Truss reactions

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.

RE: Truss reactions

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.

RE: Truss reactions

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

RE: Truss reactions

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

RE: Truss reactions

(OP)
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.

RE: Truss reactions

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.

RE: Truss reactions

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...

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

Ztengguy:
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

I agree with dhengr. However, if the pitch of the top chord of the truss is 1:1 or steeper, the roof diaphragm will tend to act more like a deep beam, limiting the lateral movement of the walls due to any vertical load. This effect has been discussed before in other threads.

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

dhengr,

Some good approaches and recommendations there, in my opinion.

RE: Truss reactions

(OP)
Im bearing on CMU, and its a rehab, new trusses over a bowstring truss roof, so the CMU will be cantilevered up a couple feet, thus any horizontal reaction is of concern.

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

Let alone the inherent problems with old Bowstring trusses, having to frame over them with a new roof truss system would be a nightmare. I can visualize all sorts of structural issues to deal with - none good.

Mike McCann
MMC Engineering

RE: Truss reactions

(OP)
They are steel bowstring trusses @ 20' on center, and a 12" joist between them at a bout 2' spacing. the contractor doesnt want to tear off the old roof and replace, would rather overframe it, and add a bunch of load to existing structure. I said remove the joists, and frame inbetween the steel truss with a beam spanning between the old trusses/pilasters. That was too much steel work, they want masonry since they will have a mason on site.

ugh. What do they pay at walmart to stock shelves?

RE: Truss reactions

(OP)
I received some reactions from a truss company. The horizontal reactions are not too bad, 0.130K each side.
Thanks for all your help.

RE: Truss reactions

I have a 32' scissor truss design in front of me with a 10:12 pitch. The max horizontal reaction is 465#. The truss is designed with a GSL of 60 psf.

RE: Truss reactions

(OP)
@ Splitrings...thanks. I am 20 ground snow. Seems about right then, not quite your pitch though.

RE: Truss reactions

The horizontal reactions that come from the manufacturer for scissor trusses are to resist external lateral applied loads and are usually pretty nominal for lower slope roofs.

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.

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