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.

 

[jike]

Don't restrict it let it move! If it is setting on stud walls, each wall will move out 1/4".

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.

 

[ltlspf]

jike,
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?

 

[slickdeals]

The only way the bottom chord might go into compression is if the restraint offered exceeds the tension in the bottom chord. Does that make sense?

 

[Lion06]

Maybe towards the center of the span. But near the ends where the tension isn't high, a small restraint could easily result in a large compressive force.
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.

 

[swearingen]

I think the an answer is hidden in jike's comment. It really depends on the stiffness of the support. In a model with a pin/pin connection, the pin is infinitely stiff, which we all know is never the case. What are the ends of the truss sitting on? It may be perfectly fine to cinch them down if the support can breathe a little (1/4" at each end).



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[Lion06]

I would agree with that, however, the trusses closest to the end walls (walls parallel to the trusses) would have a very stiff support since the supporting wall will experience very small deflections so close to its ends. In my opinion, that warrants using a pin-pin condition as it more closely represents the actual condition.

 

[jike]

The bearing walls will move outward to accomodate the lengthening of each truss. Near the end wall there may be additional restraint but generally it is ignored.

 

[jike]

ltlsfp:

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?

 

[Lion06]

jike-
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.

 

[jike]

We are only talking here about 1/4" movement! The 1st truss is generally not at the end wall, but 24" away. Can the wall top plates deflect 1/4" 24" away from the support?

 

[Lion06]

I agree the numbers in this example may not be that much, but it is the concept I am after. What if it were an 80' span and required 1" of movement?
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.

 

[jike]

This is exactly why I require the scissor truss manufacturer to limit the horizontal movement to a number that I am OK with. Additionally, I have the truss manufacturer do a preliminary design so that we can make sure we are not asking for something that will be unreasonably expensive.

 

[jechols]

In the past I have used the slotted Simpson truss connector on alternating ends and a regular connection at other end. When the initial connection is made I require the nail not be fully seated until the dead load has been applied (or at least most of it).

J

 

[Gumpmaster]

StructuralEIT,

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."

 

[tngolfer]

Our office follows Jechols train of thought and slotted Simpson anchor (TC24 or 26). You can always ask the truss designer to use a stiffer truss design to limit your thrust force and deflection.

Here is a pretty good document on subject:

 

[youngstructural]

I absolutely adore Wooten's paper, and give a copy to every coop or new graduate engineer I work with. Of late I keep getting questions about "what is ASD", but that's because New Zealand has gone fully limit states, even for geotechnical work.

It's a fantastic paper, and you can find a copy here:

http://www.aisc.org/Content/ContentGroups/Documents/MSC_Docs/wooten.pdf

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...

 

[jike]

I would expect that even with slotted connections at alternating ends, all the trusses would move together because they are tied together by the diaphragm.

 

[hokie66]

I agree with jike's reasoning completely. Even if restraint does introduce compression into the bottom chord, there should be no problem. The bottom chord should be designed and braced to resist compression due to wind uplift, and that force will be greater than the force due to restraint.

The numbers in StructuralEIT's example seem reasonable, and the chord should easily handle that condition if it is a robust design.

 

[connect2]

good ref. tngolfer.
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.