scissor truss 3

[jwrassoc]

I need to check the capacity of an existing scissor truss roof. It is home made with plywood gussets. I have the measurements of all members, span (35 ft) spacing, pitches, etc. Does anyone know of a text which will give me a graphical or mathmatical method of analysis. NOT using a computer program
Thanks loads

J

 

[DaveAtkins]

Any Statics textbook will cover the Method of Joints. You can't check deflection, however, using this method.

DaveAtkins

 

[COEngineeer]

I just want to add something on Dave Atkins's reply. Since you wont be able to figure out deflection, you wont be able to figure out the horizontal thrust on both supports.

 

[RARWOOD]

If you had horizontal thrust a both supports your truss would be indeterminate. The truss deflection could be estimated by the use of the virtual work method or graphically. Any basic Structural Analysis test book should cover the above two topics.

 

[COEngineeer]

RARSWC, I havent design trusses before but how do you do virtual work if the top chord is loaded uniformly? (meaning the top chord is not only in compression and tension, but also in bending). I still remember doing virtual work on trusses but it was point load on nodes. Maybe just replace distributed load with equivalent point loads?

 

[MiketheEngineer]

CDEngineer -

In the old days (for wood trusses) that is exactly what we did. To simplify - we used to:

Convert distributed loads on truss to point loads at panel points.

Solve for axial forces.

Apply distributed loads to each member and using NDS check each member using Interaction Equation.

Deflection of truss as a whole can be checked with Virtual Work. Not too hard - but time consuming.

And the answers (which took quite awhile) were not that much different than today's answers using high powered computers.

But with more exacting analysis - they now wring out every inch of span available which saves money.

 

[COEngineeer]

So when you design scissor trusses, you limit the horizontal deflection right? What is the horizontal deflection limit? OR do you assume it is pinned on both side and figure out the thrusts? How do you make sure the wall can take it?

 

[MiketheEngineer]

COEngineer -

Very good question and you might want to pose that on the "Truss Engineering Design" forum. Things have changed over the years. They may have a different solution.

My thoughts - you cannot resist the horizontal reaction with a plain wood stud wall - just no way. There was a posting a couple of weeks ago where someone wanted to make some sort of frame (maybe even outlining the plate line) to try to contain these forces. And these forces are large even for small trusses.

Therefore - we used to use an anchorage clip that allows horizontal movement - Simpson and others have them. And let the trusses slide around a bit.

Years ago, I was an expert witness where an architect had modified a truss (making it a collar tied truss) and had not taken into account this scenario. The walls moved like 2'' and ruined the room. He lost. A collar tied truss is not exactly the same - but close enough.

 

[unclesyd]

You might want to rethink the software approach.
Under PC Software checkout the program WinWood. You can download a Demo or buy the program for $40.00.

http://www.archoneng.com/

 

[CJJS]

The limit on horizontal deflection is typically 1.25"