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Timber rafter ceiling joist connection
8

Timber rafter ceiling joist connection

Timber rafter ceiling joist connection

(OP)
I am using method of joints to calculate the tension force in ceiling joist but the result is 2 times the computer run or any roof framing book table, anyone has any suggestions, for example if uniform load is 54 lb/ft for LD=1 on the rafter(Horz) and the span is 17 ft with 1 ft overhang each side, the reaction at the wall = 459 lbs, the ceiling joist tension force=T=1377 lbs now if you use any framing book, they recommend 6-16d nail which capacity is 1/2 around 650 lbs which is close to computer run result, am I missing something here?

RE: Timber rafter ceiling joist connection

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.

RE: Timber rafter ceiling joist connection

(OP)
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,

RE: Timber rafter ceiling joist connection

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


RE: Timber rafter ceiling joist connection

(OP)
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/lumber_rafter_collar/structural_engineering_framing_rafter_collar_tie.html)

the result is 1/2 of joint method.

RE: Timber rafter ceiling joist connection

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.

RE: Timber rafter ceiling joist connection

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)

RE: Timber rafter ceiling joist connection

(OP)
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.

RE: Timber rafter ceiling joist connection

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

 

RE: Timber rafter ceiling joist connection

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

RE: Timber rafter ceiling joist connection

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.

RE: Timber rafter ceiling joist connection

(OP)
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.

RE: Timber rafter ceiling joist connection

(OP)
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?

RE: Timber rafter ceiling joist connection

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.

RE: Timber rafter ceiling joist connection

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.

RE: Timber rafter ceiling joist connection

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.

RE: Timber rafter ceiling joist connection

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.

RE: Timber rafter ceiling joist connection

"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.

RE: Timber rafter ceiling joist connection

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.

RE: Timber rafter ceiling joist connection

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.

RE: Timber rafter ceiling joist connection

Another way to understand this is to calculate the compression in the top chord or rafter as the vector equivalent of the reaction at support, then calculate the tension in bottom chord/ceiling joist as the vector equivalent of the top chord/rafter.

If you use the method of joints approach for a truss with few panel points or for this rafter/ridge board and tensioned ceiling joist example, you just aren't getting the true picture. Or: envision a truss that has its first panel point very close to the reaction. In that case, you will get closer to the correct numbers, as opposed to the truss with few panel points. In both trusses, the bottom chord tension should be equal, in reality.

RE: Timber rafter ceiling joist connection

TTK/SacreBlue
"apply the UDL directly to the rafter you will get the higher (double) tension force in the ceiling joist"
If this is the result you got there is a problem somewhere.
I analysed the pinned truss with both the UDL and 459 lb load at the apex and obtained identical tie tensions (the lower value). This result appears to be backed-up by the framingdesign.com results.
I agree the rafter results are different, the UDL case obviously has shear and moment, however the average of the varying axial compression is equal to the apex point load case (by my software results).

What you are telling me is that the tie tension force is identical for the same total load, whether applied as a UDL or single midspan point load. I'm convinced that can't be the case.

Cheers

RE: Timber rafter ceiling joist connection

(OP)
using method of section and take a moment about the apex, then ceiling joist tension force will be the lower one, which is the same as computer run, I think you should draw the FBD.

RE: Timber rafter ceiling joist connection

I'm coming in late to this one, as I've enjoyed the discussion so far!

For a symetrical rafter arrangement, supported at the ends only (and neglecting cantiler eaves overhangs), half of the total vertical load (due to rafter load only) is the magnitude of the end reactions.

Now the member orientation at the support is the ceiling joist is horizontal and the rafter is inclined. Assuming axial load only in a pinned truss, the ceiling joist can not contribute. The vertical component of load in the rafter is thus equal to the support reaction. Given the geometry as 4V:12H the horizontal component of force is 12/4 times the vertical force. The only member which is present to resist this force is the ceiling joist so it will take THREE TIMES the vertical reaction force.

By my way of thinking it does not matter if the rafter is loaded with a udl or point loads as the ceiling joist tension force is a function of geometry and support reaction only. The only way I can think of which would change this is if the ceiling joist is otherwise loaded or if the entire weight is applied as a point load at the ridge.

Now, who agrees with me?

RE: Timber rafter ceiling joist connection

Calc1,
Finally you got it right and you will agree with apsix.
I guess apsix try to explain the FBD at the support.
Imagine a king post truss - two truss elements at each roof slope (three node points).
Then, ceiling tension = 3/4 x Reaction x 12/4.

RE: Timber rafter ceiling joist connection

Seeing this long discussion, I decided to model this up and, to my surprise, the tension in the ceiling joist is indeed 688.5 lbs.  Seems that the rafter carries some of the 459 lb reaction in shear.  If you put the total force (918#)at the ridge line, the shear in the rafter is zero and the tension in the ceiling joist is 1377 lbs.

RE: Timber rafter ceiling joist connection

jmiec,
That may be true due to the way your software handles, this, but the unescapable truth is that the tension in the ceiling joist is 1377 lbs no matter what. The problem with the way statics is taught in college is: You aren't informed of the "real-life" considerations when doing those textbook examples, nor is the professor savvy of the pros/cons of different methods.

RE: Timber rafter ceiling joist connection

I agree with PBA - your explanation is very clear. I drew this thing out - including the 4:12 slope triangle. It is really obvious that in order to solve this problem, first turn off the computer, get a pencil, calculator, and common sense.
Reaction = 54x17/2=459 lbs
Tension in ceiling joist = 459x3=1377  lbs (by proportional triangles).
I am amazed that more than one computer program computes anything so basic incorrectly!

RE: Timber rafter ceiling joist connection

I have read all the above comments and at first I thought that it was just a misunderstanding of the facts.   It seemed to be a simple static’s problem and I think that   Reaction = 54x17/2=459 lbs
Tension in ceiling joist = 459x3=1377  lbs (by proportional triangles).

So I too ran it thru STAAD and got half the value.
Now I am utterly confused.  I believe it is 1377 but I don’t understand how STAAD gets half that value.

I guess that is why I distrust computer programs.

RE: Timber rafter ceiling joist connection

It would be interesting to contemplate that if there ever was a failure, the plantiff's engineer would really seize upon this fact.

RE: Timber rafter ceiling joist connection

It is amazing! that all forgot the FBD the first thing we learned in analsis class 101.
Case 1: one truss element at each slope UL(54plf) applied at top chord; convert it to nodal CL(229.5#), Reaction =459#.
tension at bottom chord = (459 - 229.5) x 12/4 = 688.5#
Case 2: two truss element at each slope UL(54plf) applied at top chord; convert it to nodal CL(114.75#), Reaction =459#.
tension at bottom chord = (459 - 114.75) x 12/4 = 1032.75#
Case 3: one truss element at each slope TL(54plfx17'=918#) applied at the ridge; no nodal CL, Reaction =459#.
tension at bottom chord = 459  x 12/4 = 1377#
Case 4: two truss element at each slope TL(54plfx17'=918#) applied at the ridge; no nodal CL, Reaction =459#.
tension at bottom chord = 459  x 12/4 = 1377#
Calc1, run each case with your favorite program and verify; how the puter program do more than manual calc and be amazed by it's capacity!!
yep, time to time we need to brush our brain . . .

RE: Timber rafter ceiling joist connection

oops forgot spell check! posting again
It is amazing! that all forgot the FBD the first thing we learned in analysis class 101.
Case 1: one truss element at each slope UL(54plf) applied at top chord; convert it to nodal CL(229.5#), Reaction =459#.
tension at bottom chord = (459 - 229.5) x 12/4 = 688.5#
Case 2: two truss element at each slope UL(54plf) applied at top chord; convert it to nodal CL(114.75#), Reaction =459#.
tension at bottom chord = (459 - 114.75) x 12/4 = 1032.75#
Case 3: one truss element at each slope TL(54plfx17'=918#) applied at the ridge; no nodal CL, Reaction =459#.
tension at bottom chord = 459  x 12/4 = 1377#
Case 4: two truss element at each slope TL(54plfx17'=918#) applied at the ridge; no nodal CL, Reaction =459#.
tension at bottom chord = 459  x 12/4 = 1377#
Calc1, run each case with your favorite program and verify; how the puter program do more than manual calc and be amazed by it's capacity!!
yep, time to time we need to brush our brain . . .

RE: Timber rafter ceiling joist connection

I have to keep repeating this over and over (it's pathetic, I need a life)...When you are doing your cases , you are getting an incorrect tension in bottom chord, due to the conversion of UDL to nodal loads. Merely a schoolboy exercise.

RE: Timber rafter ceiling joist connection

3
Don't throw out your software yet! Tension in the bottom chord really is 688.5 lb. Rather than try to explain in words here, have put a short, zipped PowerPoint presentation on my website at the link below. The file, named "Truss Analysis" is near the bottom of my home page. After unzipping, the presentation is in manual mode (keep hitting the enter key) so that you can go thru it step-by-step. Several of my continuing education PE review students did not believe it either, but it is true. Will discuss as requested.

www.SlideRuleEra.net

RE: Timber rafter ceiling joist connection

Leaving the error prone software out of this, take a free body to one side of the ridge line, then sum moments about the ridge line.  Then 459lb*8.5ft-54plf*8.5ft*8.5ft/2-8.5 ft*(4/12)*T=0, and T=688.5 lb.

RE: Timber rafter ceiling joist connection

I neglected to say that PowerPoint offered is merely a graphic REPRESENTATION of a hand solution using ONLY the method of joints. - it is NOT a software solution.

www.SlideRuleEra.net

RE: Timber rafter ceiling joist connection

SlideRuleEra,

End reaction of 459 lb is agreed. Why divide by two in the analysis? That is where I would diverge from your logic/

RE: Timber rafter ceiling joist connection

I am ashamed for myself and my colleagues.

As was pointed out in prior posts if we take a free body diagram and take the moments around the ridge  we have the load acting down and the reaction acting up  and the  joist (bottom chord) counteracting the results of these two moments and that is how we get the 688#

 I am ashamed because I have been working for over 40 years and have gotten away from the fundamentals of engineering and statics.

What also upsets me is that so many others apparently have fallen into the same trap.

RE: Timber rafter ceiling joist connection

Well I'm also not too big to admit when I am wrong...

In my initial haste to answer the post quickly, I solved the problem assuming the rafter was carrying axial load only (truss approach) but then later condemned those who tried to use a pure truss analysis method.....my apologies.

As the simple FBD noted above shows, the rafter takes a significant portion of the gravity loads in shear since it is a frame member and not a truss member.  The rafter axial load is greatly reduced, which also reduces the tension force in the ceiling joist.

My lesson learned....slow down and analyze the problem instead of trying to post a "quick" reply!



RE: Timber rafter ceiling joist connection

pba - The 459 Lb is not divided by two, the vertical forces of +459 Lb and - 229.5 Lb are summed. This is done at each  joint. The example that we are working on has the disadvantage of being symmetrically loaded, making it difficult to really follow the steps used in the method of joints.

Take a look at this slight difference in loading (54 lb/ft UDL & 300 lb point load)


I have placed another zipped graphic PowerPoint method of joints solution on my website. It is  titled "Truss Analysis With Point Load". The results are somewhat surpising.

www.SlideRuleEra.net

RE: Timber rafter ceiling joist connection

(OP)
Thanks everyone for participating, I am glad that such a fundamental case is resolved, I know that many in our field did not have a answer for this case, even those who I have opportunity to work with in this field, but again, I appreciate everyone inputs and sharing their knowledge. I have had many conflicting answers, from Residential Structural Design Guide: 2000 Edition (HUD2000 edition), design example 7.3 (Heel Joint) that used joint method and designed for higher tension load to structural exam (SE) sample problem that tension force was the lesser value based on FBD and take the moments around the ridge.

RE: Timber rafter ceiling joist connection

SlideRuleEra,

Thanks for the new slides. Yes you are correct! To my horror I must confirm that I've designed a great number of cut timber roofs with twice the necessary ceiling tie tension load. I am at least grateful that I've designed for twice and not half...

RE: Timber rafter ceiling joist connection

SlideRuleEra...
Thanks- I have seen the light. It is with some amount of amazement that I thought the world was flat...for a while.

RE: Timber rafter ceiling joist connection

Three cheers for apsix for holding his/her ground in the face of formidable opposition, and to Calc1 for bringing up the issue in the first place.

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