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.

 

[dhengr]

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.

 

[msquared48]

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

 

[Archie264]

dhengr,

Some good approaches and recommendations there, in my opinion.

 

[ztengguy]

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?

 

[msquared48]

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

 

[ztengguy]

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?

 

[ztengguy]

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

 

[Splitrings]

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.

 

[ztengguy]

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

 

[charliealphabravo]

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.