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Raised chord truss design
2

Raised chord truss design

Raised chord truss design

(OP)
Hi
Could somebody put me on the right line i'm designing a truss as above and this is my methodology.

Max moment = vertical reaction x horz. dist. to tie.
comp in rafter = vertical reaction/ sin pitch angle
tension in tie = ??????????
thrust = vertical reaction x cos pitch angle
load = 1/2 @ point loads at apex and 1/4 @ each wall plate?

Confusion is the only thing I'm sure of at the moment.

Thanks
Andy

RE: Raised chord truss design

Do you have a picture - not sure what you mean about a raised chord truss.  There are many differing definitions in the industry.

Sounds like you might be on the right track.  Just go back to your basic statics....

RE: Raised chord truss design

What is your technical background?  Are you an Engineer?

Mike McCann
MMC Engineering

 

RE: Raised chord truss design

(OP)
Hi thanks for the response I am an engineer that relies on software too much. The frame I am thinking about is just two rafters with a tie between them about a third way up the rafters. Thanks

RE: Raised chord truss design

ok.  

You are talking about a "collar tie" condition.  

There have been several discussions regarding this in the structural forum here.  Have you tried the search engine yet?

Mike McCann
MMC Engineering

 

RE: Raised chord truss design

FYI though, the collar tie tension force is just the maximum moment in the truss divided by the vertical distance between the centerline of the collar tie and the centerline of the top chord members at the ridge.  

Be careful and use the Hankinson formula correctly when figuring out the connection of the collar tie to the top chord members.  Sometimes helps to double up the collar tie - one on each side of the top chord - to double the allowable shear loads to the connectors.

Mike McCann
MMC Engineering

 

RE: Raised chord truss design

Now I know what you are doing and what everyone says is true.  A couple of things you will find out -
   the moment might be very high at the collar joint - the higher the collar - the higher the moment.
   as noted - that connection might be a bit of a bitch.
   you need to check for deflection or thrust from the chords to the walls.  Might/might not be a problem.

My house has the same roof type.  One day, rather bored - I sat down and quickly analyzed it.  Didn't really make it but it has been up 40 years with no apparent problems and I sure as hell am not going to change anything - even if it were possible.

RE: Raised chord truss design

(OP)
Hi thanks for all the help.
So what I am saying on the first page is right?
Would anyone be willing to send me a sample or point me in the diraection of one.
I really hate relying on computor outputs for my calcs.

RE: Raised chord truss design

The maximum moment may or may not be at the collar tie connection. You need to also consider the plf loading along the rafter from the dead, live, wind, seismic, etc. in the required load cases per the building code you are using.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Raised chord truss design

(OP)
Does the plf loading not get resolved into a series of point loads at the nodes?
Back to confusion
 

RE: Raised chord truss design

Yes they do - but you must check combined axial forces and moments coming from the PLF loading to verify integrity.

Max moment may come from the joint.  Check all cases.
 

RE: Raised chord truss design

amwc - "Does the plf loading not get resolved into a series of point loads at the nodes?"

If you are using nodes at 24" o.c. maximum, with a minimum of five between member to member connection points.

But if you are using nodes only at member to member connection points than the calculation will give the tension and compression forces in the members. You still have to account for the bending forces from the plf loads.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

RE: Raised chord truss design

If you want to model this on the computer, just lump the vertical forces at the three nodes, and show the supports as pin and roller.  

You will have to control the lateral spreading with the stiffness of the rafters.

This is a simple problem, really.

Mike McCann
MMC Engineering

 

RE: Raised chord truss design

(OP)
Thanks every body
It may be a simple problem and once I am confident on how to design one (or check a computer output on paper) I'm sure it will be simple. But at the moment I have no confidence in my calcs. When I go through the checks the compression is high with very little moment. So I'm guessing this is ignored in the rafter/ tie design and just used in the connection design?
Thanks again
A

RE: Raised chord truss design

Seems a bit dubious - might be correct.  Since we have no dimensions, loads, rafter material, etc -- it is impossible to render an opinion.

That said, you still need to do a combined forces test - in your case Moment % + compression %.  Also, the rafters will need some type of lateral bracing - say plywood roof or girts.  But it needs to be checked also.

RE: Raised chord truss design

(OP)
Well once again thanks for your time basically its a 18.5ft span I am designing for 1.5ft c/c frames. The loading is normal loading i.e. conc. tiles, snow etc. Its a 12/12 pitch. Using scott pine. or similar. I have got 2 x 6 for the rafters and 2 x 8 for the tie with one 2 inch toothed bolt each connection. But its proving this works is the problem.

A

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