question on truss supports
question on truss supports
(OP)
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.
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.






RE: question on truss supports
Incidentally, don't use a slotted connection at the truss bearing and the top of the wall because wind load still would have to transfer up into the truss and roof diaphragm.
RE: question on truss supports
So how would you allow it to move and still keep the lateral load transfer with the diaphragm if you don't use a slotted connection at bearing?
RE: question on truss supports
RE: question on truss supports
We did this a while back for a building that had a truss failure. We modeled the truss to try to determine if it was adequately designed to begin with. When modeling it, the question came up about whether it should be modeled at pin-roller or pin-pin. The pin-pin model had VERY high compressive forces in the bottom chord near the supports.
I just opened the model again. Near the ends, 7k of tension turned into 1.4k of compression and near midspan 4.25k of tension turned into 2.5k of compression. All of this is at the bottom chord and just by changing the one support from a roller to a pin. That seems pretty significant to me.
RE: question on truss supports
If you "heard" it on the internet, it's guilty until proven innocent. - DCS
RE: question on truss supports
RE: question on truss supports
RE: question on truss supports
Without the slotted connection, everything is stable. The walls move out slightly (1/4") to accomodate the lengthening of the truss. Wind load is transfer from the wall to the truss and roof diaphragm. Everything is stable!
If you used a slotted connection, how would you transfer wind load from the wall to the diaphragm without the structure moving the length of the slot?
RE: question on truss supports
I understand it is generally ignored, but how does one justify that? The wall will provide restraint. This isn't like a concrete beam where you just say let it crack and behave simple. If the bottom chord fails in compression, that is a serious problem.
RE: question on truss supports
RE: question on truss supports
I initially was curious about how others approached it. Now I am curious as to why.
It just seems to me that having 1.5k of compression where you were designing for 3 kips of tension isn't a good idea.
RE: question on truss supports
RE: question on truss supports
J
RE: question on truss supports
It's largely a matter of scale. 1/4" of movement is relatively small. In a wood framed building, feel happy if anything is within 1/4" of where you want it to be, that's why they invented flashing and sheetrock mud.
If I were looking at 1" of movement, I might start trying to design to accomodate it, or resist it.
This issue of scale comes up often. Think of deflection on a wind column. L/240 is about ~.6 inch on a typical building column. Go to an 80 foot tall warehouse column and L/240 is 4 inches! One is insignificant, the other is pretty noticeable.
If you can find a copy of it, read the article "Wooten's Third Law and Steel Column Design" by Jim Wooten in the second quarter 1971 Modern Steel Construction. A quote from it "we must confess that with all our uncommon knowledge, we cannot compute the actual stress at any point in a member. Having conceeded that, we can see it never was important anyway."
RE: question on truss supports
Here is a pretty good document on subject:
RE: question on truss supports
It's a fantastic paper, and you can find a copy here:
h
Not sure if you need to be an AISC member as I just keep myself logged in while at work...
Regards,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: question on truss supports
RE: question on truss supports
The numbers in StructuralEIT's example seem reasonable, and the chord should easily handle that condition if it is a robust design.
RE: question on truss supports
trying to transfer your lateral load into your diaphragm with a connector that allows movement doesn't work. trying to connect your slotted connector after all the dead load is applied, roof and cielings, is very difficult (constructability), for those of us where the snow load is much greater than the dead load doesn't work anyways, trying to allow the connection to move after the roof diaphgram is in place and finished doesn't work either.
Right spec a tolerable movement, lock it up and let the wall or frame move. Thank-you truss plate industry. There's more written on your disclaimer sheets than there is on you engineering data output sheets. CYA.
RE: question on truss supports
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?
RE: question on truss supports
RE: question on truss supports
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
RE: question on truss supports
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.