Wood Roof - Thrust 9

[Buleeek]

Hi everyone,

I'm analyzing the TFEC Bulletin No. 2018-12 "Behavior of statically determinate and indeterminate rafters". If I use a collar tie (or strut) system (with no horizontal reaction at the bottom of the rafter)can I assume there will actually be NO horizontal force at the wall top plate? I know that collar ties should not be used to carry the thrust, but struts yes (located higher than top plate level in this case).

I am not sure what situation occurs in a real life. I can't assume that the rafter has no thrust at the bottom, since it is resting on the wall plate and is always notched/nailed/connected.

What do you usually do in such situation? That assumption determines a wall design and how much thrust it will carry.

Thanks,

 

[1503-44]

If you still need something to do, running all with the same stiffness would be interesting, maybe 2x10's ?, now that I finally got past the sign convention. That sign "unconvention", least from what I was used to, was a big surprise to me. But like I say, in the end I'm glad that was a sign convention problem and that the final result didn't contradict my intuition... at least not too much.
Thanks again for doing all that analysis work. Really nice of you.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[BAretired]

The structure is statically determinate if supports are pin/roller. Reactions and member forces are not dependent on member stiffness. You don't need a computer to determine the tie force or the reactions. Not sure how many times I have to say this.

My sign convention for member force has always been that tension is '+' and compression is '-', but as stated by others, sign conventions are variable. The program really should state the convention being used to avoid confusion.

BA

 

[r13]

BA,

Isn't that nice when your hand calculated predictions are confirmed by the computer analysis? On the other hand, when there is deviation, albeit small, don't you interested in knowing the cause/reason (other than rounding obviously)? I don't think learning is one way street, practicing is another matter though.

 

[Buleeek]

One question about about the static from RISA. Shouldn't the tie be modeled as "pinned" (only tension/compression)?
Thanks,

 

[r13]

Yes, you can use "axial force only" (truss) member, as usual is the case, for which will give you the exact solutions as hand calculation.

 

[r13]

I hope this sign convention makes sense.

 

[r13]

If you still need something to do, running all with the same stiffness would be interesting, maybe 2x10's ?

Wish granted. But a general shape rather than 2x10s. Note the roof slope has changed to 1V:2H (6':12') to be more realistic. There is still self weigh in the models that I don't know how to get ride of it at this moment. I'll try later.

 

[Buleeek]

retired13,

When specify BLC (Basic Load Combinations) remove any "gravity Y" load. That will eliminate the self weight load.

 

[r13]

Buleeek,

Appreciate! It works

 

[1503-44]

Does this spreadsheet make sense?

Enter span, pitch and load on sheet 1

sheet 1 rafter tie at variable height roller supports
sheet 2 rafter tie at variable height fixed supports
sheet 3 frame without rafter tie and two fixed supports
sheet 4 truss on roller supports

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[BAretired]

File won't download.

BA

 

[1503-44]

2nd upload here

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[1503-44]

I tired both and neither work. I reported the problem.

Link to a copy in DropBox. Sorry but you will need a DropBox account to download it.

https://www.dropbox.com/s/92ep4h5bh9ku6e3/RAFTER TIE FRAMES & TRUSS.xlsx?dl=0

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[BAretired]

For those who do not have a DropBox account, see below:

BA

 

[BAretired]

Something is wrong with that sketch. With span = 20 and W = 100, R can't be 1000, so which is correct?

Edit: Okay, I see: should be w = 100, so W = 2000, then R = 1000 would be correct.
Then M@B = 1000*4.5 - 100(4.5)^2 = 3487.5 (ok)
Tie Force = WL/8h = 2000*20/8(3.333-1.5) = 2728 (ok)

BA

 

[1503-44]

That's just the first worksheet. There are 4 worksheets in the downloads I provided, each with various support conditons.

But there seems to be a problem with balance of horizontal forces at B.
AB = 0
BCh = 870 compression * 12/12.65 = 875 left
BDh = 2727 tension to right
2727-875 = unbalance of 1902
What's going on with that?

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[BAretired]

On the sketch, Rafter Tie is 2727 tension, which I believe is correct, but then shows rafter tie of 825. What is the reason for this?

BA

 

[BAretired]

That's just the first worksheet. There are 4 worksheets in the downloads I provided, each with various support conditons.

Okay, I see that now, but it is only the first sheet that pertains to this thread because the top of stud wall can't act like a pin. So on the first sheet, how do we interpret the two values shown for rafter tie force, namely 2727 and 825?

BA

 

[r13]

Note, if both ends pinned, you will have horizontal reaction at supports.

 

[1503-44]

I was looking for the tie force by sum horizontal forces at B
If BC is determined from sum vertical loads at C, gives BC axial load of 275/4*12.65 = 870 compression
Horizontal component is 870/12.65*12 = 825
[highlight #FCE94F]Then sum of horiz forces at B yields tie force of 825 tension
So you can't have the 2727 you get from sum of moments at C[/highlight]

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"