Scissor Truss Connection To Top Plate 3

[courtnvm]

Could someone please clarify what is the correct way to detail the connection between the scissor truss and stud wall top plate. I have read several threads on this website regarding this, but it doesn't seem like there has been any clarification.

Is it incorrect to provide connection at both ends of the scissor truss to prevent lateral thrust assuming that the wall would move with the truss. My thought is to install the connections after the dead load has been placed.

The project I have this on is a 40' x 80' church located in the middle tennesse area. 2/3 of the roof is scissor trusses and the other 1/3 is common roof trusses.

My huge dilemma is over the top plate connection and top plate design.

If anyone has encountered this problem, please help shed some light.

Thank you,
Val

 

[jike]

If the wind load on the wall needs to go into the roof diaphragm, then the truss to wall connection needs to bepositive (non-slip type). I normally require a minimum I for the truss to limit the wall movement to an acceptable amount.

 

[youngstructural]

I'm a little late, but I'm also with jike on this one... The amount of thrust you are going to get is directly proportional to the deflection of your truss (and hence to the spread or opening of the scissor). I would check the maximum possible opening, translate this into a load case on the wall (by solving for the force required to prevent this same opening force) and then apply this as a load case to the top of the wall...

Anyone else have any thoughts? You've got me curious here Val, please let me know how it's worked out...

Cheer,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[bjb]

in the past we have specified a value for horizontal deflection that in our judgement is acceptable. it's been a while since I encountered a scissors truss, and I can't remember what sort of limit we specified. I would be relatively conservative with this because although a flexible truss can have adequate strength, if the owner notices the walls being pushed out and distress to the wall finish materials he/she will wonder if the structure is going to fail, and they will also be upset at cracking finish materials. You will definitley hear from them about that.

 

[RARWOOD]

Installing the end connections of a scissor truss after dead load deflection is commonly done. This requires some careful thought about detailing, when heavy timber trusses are used.

Sometimes other elements of the structure will be installed after the trusses are set, but before thetruss end connections are installed. These other elements may not provide enough clearance to install the remaining items to complete the truss connection.

Both youngstructural & bjb have good advice. In practice when I design a scissor truss I try to limit the horizontal deflection to a limit that in my judgement is acceptable.

The important thing is to work with your truss supplier and draw on their experience.

 

[structurebeton]

I don't think the wall can be design to "take" the load from the displacement caused by the scissor truss. When the displacement is allowed, the load "disappears". The question really is, how much displacement is acceptable, and if the walls are temporarily braced at the top, during installation of the roof, then all the displacement to take into account would be the live load... Any advice on how much displacement is acceptable?

 

[twinnell]

Try this thread. thread1066-116989

 

[seattlemike]

I designed a scissor truss with a bunch of architectural stell plate connections, so I had the luxury of luxury to work with:

I looked into elastomeric bearing pads like the ones used in bridges, but didn't come up with too much: I determined that I would not actually have enough shear force to cause the pads in question to deform laterally. (although you'd think there would be a product out there somewhere that could do this.)

However, I did find bearing pads which could have a thin, specially-polished stainless-steel plate fabricated on one side. Landing the end of the beam with another polished steel plate, I could have a near-frictionless slip condition allowing the roof scissor trusses to spread under the 130 psf snow load.

However, these roof trusses land on ridiculous large beams: like 8"x16" or 12"x24", not walls, so the conditions may not apply. For wind & seismic, the roof diaphragm spans between neighboring wings of the house.

Pretty sick.

SeattleMike