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scissor truss horizontal movement at bearings?

scissor truss horizontal movement at bearings?

scissor truss horizontal movement at bearings?

(OP)
If a scissor truss design comes back from the fabricator with a requirement for allowance for horizontal movement at the bearing point, say 0.5 inches, what is wrong with alowing movement of the top of the wall 0.5 inches to accomodate this required movement?  Assuming that this movement of the top of the wall is acceptable.  If this requirement for horizontal movement is meet. ie the truss pinned to the top of the wall moves out 0.5", does this relieve the horizontal truss reaction at this point?

RE: scissor truss horizontal movement at bearings?

I would think the fabricator is telling you that the truss will spread (or contract) .5" horizontally, with the assumption that there is a roller at one end.  But that is not a condition which will necessarily occur in the real world.  Your roof diaphragm/bracing may limit the horizontal deflection.

RE: scissor truss horizontal movement at bearings?

Yes this does relieve the horizontal reaction at the roller end.  Remember this is a max reaction most likely from either snow or gravity loads.  Both of which are typically pretty conservative.  The truss will most likely never see the amount of load that produced that .5".  Horizontal deflections on scissor truss will be seen in the field.  From my experience, depending on the pitch difference in the top and bottom chord, you'll see the deflection before wall finishing is applied.  Long spans with steep vaults WILL deflect horizontally.  Typically you wont see it in shorter spans with moderate vaults

RE: scissor truss horizontal movement at bearings?

The horizontal reaction you are viewing may be only the wind lateral forces and not the deflection horizontal thrust reaction.
Assuming a two bearing truss:
PIN-ROLLer horizontal reactions are due to wind
PIN-PIN horizontal reactions may also include horz. thrust
Confirm with the designer bearing conditions.
PIN-PIN designed truss conditions should be allowed to vertically deflect with dead loads applied prior to PIN connection being made.

RE: scissor truss horizontal movement at bearings?

I've often wondered about this.  I would think that if the fabricator is telling you to account for horizontal deflection, then you need to allow for it, not prevent it.  If you prevent it, you will be putting a bottom chord that was designed for tension into compression - definitely not a good thing.  If you want to not allow the movement, then tell that to the fabricator up front so he can design as pinned-pinned instead of pinned-roller.

RE: scissor truss horizontal movement at bearings?

I agree that the deflection from the truss manf will not be seen in the amount they are saying.  Top of wall will push out under significant loads, and the thing to look out for is a large horizontal deflection at the top of a short wall.  1/2" on the top of normal wall (theoretical deflection ) is not a big deal in my opinion.

RE: scissor truss horizontal movement at bearings?

I had a similar question and asked a truss manufacturer and he said: "TPI allows upto .75" movement under LL and 1.25" under TL for scissors trusses, but most contractors ignored it".

However, if you look in the Simpson Strong-Tie 2005 Catalog, on page 143, they have Truss Connectors that allow up to 1.25" of movement: TC24, 26,and 28.  I'm considering using these on a 70' span but, my question would be, do you put one on each end of the truss? I think one end would be the correct way.

RE: scissor truss horizontal movement at bearings?

You have to ask yourself what happens if you allow the roof/ceiling to move relative to the supporting wall.  What about the finishes?  And what braces the wall?  Deflection will happen, but it doesn't necessarily have to be by relative movement between the roof and wall.

RE: scissor truss horizontal movement at bearings?

(OP)
jheidt2543
A 70 foot scissor truss is a big one, or would be where I am.  Interested to know how the movement is accomodated and the horizontal loads picked up with a light gauge steel connector?  I've always had a bit of a problem with these light gauge steel connectors.  There much abused on site but look good in Simpsons catalogues though.
Is the movement the plate guys are talking about a result of wind, maybe unbalanced snow load?

RE: scissor truss horizontal movement at bearings?

I've been to a couple of the seminars that Simpson puts on and admittedly they are a promotion for their products, BUT they have the engineering behind them.  They are well worth the free admission.  Simpson is the first to admit that installation is the key to the success of the connector.  What I like about them is they are tested, not calculated load tables.  The seminar presents a lot of case studies and pictures showing what happens when the connectors are installed wrong. They also provide a lot of info and aids for field installation. I'm not connected (sorry for the pun) to Simpson, I just find them the easist to work with and specify.

The 70' span scissors truss is not an every day truss! In this instance, the top chord is 3.5:12 and the bottom chord is 1.5:12.  The supplier ran the truss calcs prior to final design of the building and the theoretical movement under full loading is 0.60" under LL and 0.90" under TL. The sidewalls are 2x8's at 24" o.c. 14' long on a 2' concrete stub wall.  The live load L/def = 280 for the wall, IF it moves that much, all on one side. In thinking this over, I'm not sure the "slip connectors" are really needed.

RE: scissor truss horizontal movement at bearings?

It should move about half of that amount on both sides unless there is some restraining force on one side like a building abutting to it.

RE: scissor truss horizontal movement at bearings?

Again geometry will tell the tale, but most likely the horizontal deflection is simply a result of your gravity load case, or maybe snow if you are in a high snow area.  Ask for an engineered drawing from your truss manufacturer.  Better yet go sit down with them.  They should be able to show you shear/moment, deflection, axial and loading diagrams for every load case.  Most don't know what they mean, but you do.  In fact, they can probably emal them to you.

Wind loads create more upward forces on the truss than they do downward.  So really, the wind isn't a factor.  Actually wind will create negative horizontal deflections even in the 90mph regions.  Remeber, the wind is trying to lift the truss like an airfoil.  If you have a pin/roller and the truss peak is lifted, the roller has to move toward the pin.

RE: scissor truss horizontal movement at bearings?

The 0.5" horizontal deflection is most likely from gravity load cases. It needs to be taken into account in the overall building design. As has already been mentioned, if you attempt to prevent the movement it will change the design stresses on the truss, but more importantly it may cause a horizontal thrust that could cause the walls to fail.

The diaphragm may decrease the horizontal deflection some, but the reason the note is there, is that the truss engineer feels that the diaphragm won't take out all the horizontal deflection.

The truss was probably designed with pinned-roller supports, if it was pinned-pinned you would have horizontal reactions, not a horizontal deflection. One end should be installed with a fixed connection and the other end with a connection that allows it to move the 0.5".  If both ends are installed with roller connections, the entire roof could move undesireably.

The truss manufacturer should be able to provide all you need for each load case. If they can't (or don't know how), the engineers at their plate suppliers will have the information.

RE: scissor truss horizontal movement at bearings?

(OP)
RJ2005
Scissor truss.
".. if you attempt to prevent movement it will change the design stresses on the truss, but more importantly it 'may' cause a horizontal thrust ... fail."
" .. the truss engineer feels the diaphragm won't .."
Pinned/Roller support is a modelling convienence, but you have to start somewhere, yes.  From a modelling perspective then I understand if you in the field construct a pinned/pinned condition this would then change the computer model and would change the forces in the truss as modeled pinned/roller and result in a horizontal force at one of the reactions? both of the reactions? (scissor truss).

I guess my original question was, given that the horizontal movement of the wall can be accomodated and then therefore does move horizontally as constructed pinned/pinned are we not then back to our original assumption of pinned/roller with the wall movement putting the truss back into the original assumptions made by the plate company pinned/roller and therfore no changes in the forces with in the truss members?  I'm sure there is a better way to put that.  But you get my 'drift' so to speak.

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