Timber rafter ceiling joist connection 8

• Aug 28, 2005

1
• #2

TTK

Structural

Sep 24, 2003

85

Based on your numbers and the information given, I am guessing the roof pitch is 8V:12H...is this correct?

If so, the tension force in the tie for the loading given is 689 lbs, half of what you are calculating.

When you use the method of joints the result that you get at that joint IS the force in the member, you do not need to add in the force from the joint at the other end of the member (I'm assuming that is what you did).

For your problem, just take the vertical reaction and multiply by the roof pitch to get your ceiling joist force:

Ceiling joist = Vert Wall Reaction * Roof Pitch

459 lbs * (12/8) = 688.5 lbs.

 

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• Aug 28, 2005

• Thread starter
• #3

Calc1

Structural

Feb 24, 2005

26

No the pitch is 4V:12H, and no I just took the joint by itself, as T=c cos(teta) and C=R/sin(teta), teta pitch angle=18.434,

 

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• Aug 28, 2005

• #4

TTK

Structural

Sep 24, 2003

85

Well then at 4V:12H the force would be what you calculated, and I would check your software if it is giving you half this value.

With this flat roof you will get significant connection forces that aren't really accounted for in the "standard nailing details".

Here's a link to a pretty good study that looked at traditional nailing patterns for various roof loads.

http://www.huduser.org/Publications/pdf/roof_connections.pdf

 

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• Aug 28, 2005

• Thread starter
• #5

Calc1

Structural

Feb 24, 2005

26

Thanks TTK,
But how come all the roof framing books have table that show 6-16d nail for that span is enough, which is not, and computer run at

http://www.framingdesign.com/structural_engineering_framing_rafter.html

for Lumber rafter with collar tie or
(

http://www.framingdesign.com/rafter...l_engineering_framing_rafter_collar_tie.html)

the result is 1/2 of joint method.

 

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• Aug 28, 2005

• #6

TTK

Structural

Sep 24, 2003

85

Do the roof framing books have the # nails required for each roof pitch or do they just list 6-16d for this "standard" connection?

I did look at the online program in the link you provided, but is too "black box" for my peace of mind. I sent the website manager an e-mail notifying them of a potential bust in their program...let's see if they respond.

Bottom line though, is you need to specify more nails in this connection than the "standard detail". You could use bolts if you are concerned with the density of the nailing pattern.

 

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• Aug 28, 2005

1
• #7

apsix

Structural

Sep 21, 2004

1,358

The correct tie tension force is;
(459/2) lbs * (12/4) = 688.5 lbs
Only half the reaction is used to calculate truss axial forces as the other half is the rafter reaction at the support (ie. only the rafter reaction at the apex is used).
(trust my explanation is understandable)

 

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• Aug 28, 2005

• Thread starter
• #8

Calc1

Structural

Feb 24, 2005

26

But how could the joint be in equilibrium, if using only 1/2 of the reaction?, if you look at any truss analysis, the reaction is used to calculate the bottom cord tension force.

 

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• Aug 29, 2005

• #9

TTK

Structural

Sep 24, 2003

85

apsix, please expand on your explanation....it seems to defy logic.

 

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• Aug 30, 2005

• #10

TTK

Structural

Sep 24, 2003

85

Calc1,

See PDF in link below from Trus Joist. They have nailing patterns for heel connections for various loadings and roof picthes. Note the high number of nails required for the 4V:12H roof pitch.

http://www.trusjoist.com/PDFFiles/2080.pdf

 

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• Aug 30, 2005

1
• #11

SacreBleu

Structural

Apr 7, 2005

427

Allowable load per 16d @ LDF=1.0 is about 112# (depending on wood species), so you are looking at about 12 nails. That seeems impractical...no wonder light metal-plate connected trusses are so common. That roof pitch of only 4:12 is not good, especially since I assume this is snow loading.

 

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• Aug 30, 2005

• Thread starter
• #12

Calc1

Structural

Feb 24, 2005

26

Thanks TTK, yes, I have seen that report, that is why it made me to question this idea of 1/2 reaction. Have you run any analysis program and model the rafter and ceiling joist system and compare the lateral force at the support Vs 1/2 reaction?
Scareblue no the loads are DL+LL=40 psf @16" with rafter to be 2x8.

 

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• Aug 30, 2005

• Thread starter
• #13

Calc1

Structural

Feb 24, 2005

26

I think apsix might have a point, if you draw freebody diagram, then only one rafter lateral force will be applied for ceiling joist tension force, this could be true only for rafters and ceiling joist and not for truss,I am just trying to find a logic for it, what do you think TTK?

 

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• Aug 30, 2005

• #14

apsix

Structural

Sep 21, 2004

1,358

The udl is applied at the joints as a point load, ie. 229.5 lbs at each end of each rafter. The point load at the outer end of the rafter acts at the support and contributes directly to one half of the reaction, and therefore will have no effect on the ceiling tie force.
It then follows that the "other half" of the reaction is used to calculate the tie tension force.
The same principle is true for more complex trusses, however the point load acting at the support will be less than half the reaction, depending on the number of joints along the truss.

 

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• Aug 31, 2005

• #15

SacreBleu

Structural

Apr 7, 2005

427

apsix,
I disagree - you have to calculate the tension in the ceiling joist (or bottom chord of a truss) as the vector equivalent of the compression force in the rafter (or top chord), to get sum of horizontal forces equal to zero.

 

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• Aug 31, 2005

• #16

TTK

Structural

Sep 24, 2003

85

I agree with SacreBleu, which is consistent with my previous statements.

You can't just automatically convert the UDL to two point loads at each end of the rafter and then say the bottom point does not cause a tension force in the ceiling joist because it is offset by the end vertical reaction at the top plate.

The UDL must go through the rafter as a compression force before it ever gets to the top plate. I stand by my earlier statements.

 

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• Aug 31, 2005

• #17

SacreBleu

Structural

Apr 7, 2005

427

Furthermore, if the FramingDesign.com website is offering online engineering to the general public, and their so-called software is calculating one-half the ceiling joist tension magnitude, they are doing engineering a great disservice.

 

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• Aug 31, 2005

• #18

apsix

Structural

Sep 21, 2004

1,358

"You can't just automatically convert the UDL to two point loads at each end of the rafter "
Unless I'm mistaken the method of joints requires point loads to be applied at panel points.

"you have to calculate the tension in the ceiling joist (or bottom chord of a truss) as the vector equivalent of the compression force in the rafter (or top chord), to get sum of horizontal forces equal to zero"
Agreed, however it is the value of the compression force which is being debated.

Try analysing the truss in your favourite frame analysis software, using pin joints, and UDL and equivalent point loads as 2 separate load cases.

 

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• Sep 1, 2005

• #19

SacreBleu

Structural

Apr 7, 2005

427

apsix,
The compression force in the rafter doesn't care how you analyze it. If you do a method of sections at the center of the span, you will see.

 

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• Sep 1, 2005

• #20

TTK

Structural

Sep 24, 2003

85

I think I understand the confusion now.

If you try to analyze this system as a true truss (i.e. members can only take axial force), then you must convert the UDL to point loads at each end of the rafter because the members cannot take direct loading in this model and you will indeed get half the tension force in ceiling joist.

But....that is not a correct model for how the system is behaving. It is not a true truss since the members can take shear and moment in addition to axial force.

If you model this in your favorite analysis software as frame elements but with pinned connections and then apply the UDL directly to the rafter you will get the higher (double) tension force in the ceiling joist....this is the correct model.

 

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