question on truss supports

[Lion06]

This issue came up in the truss forum (not one of my posts, but I did chime in), but it's been dead for a bit with no real resolution, so I'd like to get some opinions here.
In the case of a truss (scissor or otherwise, but especially scissor trusses), if the truss is designed as simple-simple, the bottom chord is in tension and one end (or both) would move horizontally even if just a bit.
If you know, for example, that the horizontal movement of one end of the truss is going to be 1/2" per the fabricator, I think you need to actually allow for the movement and NOT try to eliminate it without having the truss fabricator redesign the truss.
Here is my reasoning. If the truss is designed to allow horizontal movement of one end of the truss, the bottom chord will be in tension only. As you start to restrict that movement by introducing a beam at the top of the wall (or whatever other method you choose) the bottom chord will end up going into compression as it tries to displace horizontally and is restricted. Now a truss chord that was designed for tension is taking compressive loads and this is definitely not a good thing.
I would think that you either:
1. Need to tell the fabricator up front that horizontal displacement of either end of the truss will not be permitted to allow him to design it properly.
2. Allow for the displacement the fabricator tells you to expect.
I just think that being told to expect 1/2" horizontal movement and then trying to restrict it is a bad idea.
Any opinions.

 

[haynewp]

I also first thought the walls would move out slightly to accomodate the thrust, only if it was a very stiff perimeter beam the truss was sitting on it could restrain it enough to compress the bottom chord.

If they are designed for uplift, then the bottom chord is likely ok as hokie said. What I am initially concerned with is that even if the truss was designed for significant uplift, that if it was sitting on very stiff perimeter beams under a dead+live case that there would still be a greater stress reversal in some web member due to the downward gravity plus the restraint at the ends. Also, is the max web member force in any member always greater for gravity with a roller at one end than gravity with restrained at both ends?

 

[hokie66]

If trusses are to supported on rigid walls, concrete beams, or the like, it seems to me that is another load case that should be allowed for in the design. Require the truss to be designed for both pin-roller and pin-pin condition.

 

[jechols]

connect2

the slotted connector only allows movement until the nails are fully seated. it can easily be done after MOST of the dead load is applied. obviously the drywall won't be in place yet. I know the contractor can do it because they always wait to put on the huricane clips until right before drywall goes (around here anyway).

j

 

[bdruehl]

well, this issue just came up for me on a "precriptive" or conventional wood construction... so, the trusses were done with no regards to engineering ofcourse..especially interior shear walls....

so i went out to inspect and noticed that the interior shear walls of the second floor were not attached to the trusses. So... the house is only 30 feet wide, so i told the contrator to attach the wall to the truss - with a35 or h2.5 - whatever he wanted... (trusses perpendicular to wall).. movement wasnt an issue in my mind...

because, the interior shear wall required by the conventional construction method certainly needs to be attached to the roof in my mind or else its just a nice plywooded wall sitting alone in space doing nothing,(by the way, i called truss manufacturer and they said they cannot design an interior lateral pathway without also having the wall be a bearing pathway!!)

so, in comes the inspector who says absolutely that you cannot attach the truss to the wall because the issues in this thread. I say horsepucky, better than not attaching..even if the truss wasnt designed for the transfer...

so, i called out slotted clips (alternating sides becauuse you only get one direction of "F1" shear (simpson clip HTC), but with a 1/4" gap required, you only get a whopping 155# per clip, whooopie dee doo...

still better than nothing i think...

seems there should be something in the code addressing truss attachemtn for conventional framing.