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?
RE: scissor truss horizontal movement at bearings?
RE: scissor truss horizontal movement at bearings?
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?
RE: scissor truss horizontal movement at bearings?
RE: scissor truss horizontal movement at bearings?
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?
RE: scissor truss horizontal movement at bearings?
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?
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?
RE: scissor truss horizontal movement at bearings?
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 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?
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.