# 2x8 Hip Rafter Failure



## jar546 (Jul 8, 2013)

I am playing around with StruCalc to compare it against the prescriptive code for rafters and joists and finding that it is just a tad more liberal which makes sense because as I understand it, there is a bit of a safety margin built into the IRC tables. It appeared to be a bit more forgiving until I got to sizing a hip rafter. What I believe should have worked prescriptively, failed under StruCalc for moment. Here is what I did:16 x 16 building with a hip roof 6/12 pitch. 20psf live load and 10psf dead load. Simple enough, right? Wrong,.......or at least I think wrong.Looking at Table 802.5.1(2) I see that even a 2x4 would make this span but I would have chosen a 2x6. Does not matter because I chose a 2x8 hip rafter for this application. Up 2 sizes for a 2x4 and the next size up for a 2x6. If I were doing plan review I would not think twice about this if this was specified. Now I am curious as to why I have the following results from StruCalc. This is just comparative analysis for educational purposes so don't get uptight that I am using this program. Here is the report:

View attachment 1853


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## mjesse (Jul 8, 2013)

User error.

kidding.

I wouldn't know, since I've never used the software, but I have a hypothesis:

Using a 2x4 rafter and 2x8 hip on the assumed same plate height would require a really deep seat cut on the hip.

Does the software factor the portion of hip rafter not bearing on the plate and fail due to inadequate width?

The software report looks pretty cool, can you clip the "input" screen for curiosity sake?

mj


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## jeffc (Jul 8, 2013)

I think your work looks good. I have the exact same result. What do you do with the 691 lb (LL + DL) at the peak? That load would be for each hip so the total load to support all four hips would be 2764 lb (691 x 4).

View attachment 745


Strutcalc.pdf

Strutcalc.pdf


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## jar546 (Jul 8, 2013)

jeffc said:
			
		

> I think your work looks good. I have the exact same result. What do you do with the 691 lb (LL + DL) at the peak? That load would be for each hip so the total load to support all four hips would be 2764 lb (691 x 4).


I would think it becomes more of a thrust issue.


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## RJJ (Jul 8, 2013)

Call support!

Is there a central ridge? or do the hip rafters intersect at the peck?

What is the size of the common rafters?Just an old carpenter that has framed more then 100 hip in his day.

Thrust is not as significant factor in a square building of this size although the low slope of 6/12 a low pitch.


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## jar546 (Jul 8, 2013)

Unless you want to run rafter ties in both directions I can only see running them in one and having the opposite sided framed into them for the last 48" or so then using plywood as a diaphragm to control thrust.

How do you guys handle the thrust factor with hips prescriptively????


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## jar546 (Jul 9, 2013)

Wow, I ran this on beam check and it failed once again.  So in the example above, a 2x8 failed both StruCalc and BeamCheck when applied the same way.  Very interesting.


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## Dbronson (Jul 9, 2013)

jar:I think your tributary area is not set correctly.  The load is triangular and about 34" each side of hip.  (See the sketch)

View attachment 747


I'm not familiar with Strucalc, but inserting your parameters into Woodworks I get the section to work - about 60% of bending capacity is used.Regards,DB
	

		
			
		

		
	

View attachment 747


/monthly_2013_07/572953c8b9929_16Hip.jpg.f2488924d051ce4077c3b912163b8d3a.jpg


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## jar546 (Jul 9, 2013)

Dbronson said:
			
		

> jar:I think your tributary area is not set correctly.  The load is triangular and about 34" each side of hip.  (See the sketch)
> 
> 
> 
> ...


Dbronson, Welcome to the forum and I thank you for running this too.  If you look at the PDF from JeffC above you will see that we all have the same tributary, ours is shown in green.  My sheet is the report sheet that i attached in the first post, JeffC, who got the same answer showed the worksheet/design page.  I ran it with BeamCheck and still came up inadequate.

This is interesting. I am wondering if the the programs have the correct values for the lumber used.  Did you do yours by hand or with a computer program?


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## jar546 (Jul 9, 2013)

EDITED  Mistake, see next post below this one.

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## jar546 (Jul 9, 2013)

Now I messed it up again.  I did a 2x10 with beam check.  Let me try that again.Nope, failed Beamcheck for 2x8 in this application.

View attachment 749


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## Phil (Jul 9, 2013)

How about some guesses.

I am going to climb out on a limb because I know nothing about the IRC or the conventional wood design in the IBC (I mostly work with flat roofs, steel, conc, and masonry). But, I do not think the code requirements are based on the hip rafter acting as a beam. Rather, they are somewhat similar to ridge boards on a gable roof. I think a hip roof may need to be analyzed as a folded plate or maybe a space truss. Or, the requirements may be from some empirical study.


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## jar546 (Jul 9, 2013)

Phil said:
			
		

> How about some guesses.I am going to climb out on a limb because I know nothing about the IRC or the conventional wood design in the IBC (I mostly work with flat roofs, steel, conc, and masonry). But, I do not think the code requirements are based on the hip rafter acting as a beam. Rather, they are somewhat similar to ridge boards on a gable roof. I think a hip roof may need to be analyzed as a folded plate or maybe a space truss. Or, the requirements may be from some empirical study.


Prescriptively a hip must be sized as a beam when the pitch is less than 3/12 which this is not.  We have 2 different programs calculating this hip at 6/12 so I think this is a straight forward calculation for the software.  Unlike ridge boards which are essentially only needed for nailing purposes, the hip does bear a lot of weight whereas a ridge board could be removed and the ends of the rafters tied together with a gusset plate and still meet the code prescriptively.  If you take a look at the sheets we provided with the results, you may find the problem if one exists.


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## RJJ (Jul 9, 2013)

So what is the length of the Hip?


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## globe trekker (Jul 9, 2013)

Jeff,

For clarity to some of us "slower ones", what grade of SPF did you use, and what

spacing of rafters did you use from Table R802.5.1(1)?   Thanks!

.


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## jeffc (Jul 9, 2013)

SPR Structural valuesHere are the values that I used. Good discussion everyone.

View attachment 750


SPF Values.pdf

SPF Values.pdf


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## jar546 (Jul 9, 2013)

I just had another person run the same test and the 2x8 failed.  So at this point 3 people with 2 different programs are finding failures and 1 person with a 3rd program got it to run using 60% bending capacity.

I was using SPF #2 values by the way as originally specified.


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## Dbronson (Jul 10, 2013)

jar:Oops - I forgot to load the second side.  2x8 doesn't work, in fact a 2x10 fails with our software - though only by 1%.  Curiously the moments reported are somewhat different between programs.  The rest of the values are pretty close.  I'm guessing that the programs integrate the loading in slightly different ways hence the variance. Also, our software is an older version, however the design values are exactly the same.  
	

		
			
		

		
	

View attachment 1766

	

		
			
		

		
	
Regards,DB

View attachment 752


Misc2.pdf

Misc2.pdf


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## MASSDRIVER (Jul 10, 2013)

RJJ said:
			
		

> So what is the length of the Hip?


12 feet to point lines.

Brent.


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## jar546 (Jul 10, 2013)

Dbronson said:
			
		

> jar:Oops - I forgot to load the second side.  2x8 doesn't work, in fact a 2x10 fails with our software - though only by 1%.  Curiously the moments reported are somewhat different between programs.  The rest of the values are pretty close.  I'm guessing that the programs integrate the loading in slightly different ways hence the variance. Also, our software is an older version, however the design values are exactly the same.
> 
> 
> 
> ...


Thank you.  Well, that about does it.  4 people and 3 different programs all failing a #2 SPF 2x8 as a hip rafter for a 16x16 building with a 6/12 pitch and a 20#LL and 10#DL.

Make sure you update your values for SYP due to all of the changes.


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## MASSDRIVER (Jul 10, 2013)

RJJ said:
			
		

> So what is the length of the Hip?


12 feet to point lines.

Brent.


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## MASSDRIVER (Jul 10, 2013)

jar546 said:
			
		

> Thank you.  Well, that about does it.  4 people and 3 different programs all failing a #2 SPF 2x8 as a hip rafter for a 16x16 building with a 6/12 pitch and a 20#LL and 10#DL.Make sure you update your values for SYP due to all of the changes.


 Can you say from the data what it is that fails? _s the lumber itself failing or is the design a fail, like spreading or seperation or connections failing?_

_Brent_


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## jar546 (Jul 10, 2013)

MASSDRIVER said:
			
		

> Can you say from the data what it is that fails? _s the lumber itself failing or is the design a fail, like spreading or seperation or connections failing?__Brent_


_In all cases, it looks like bending was beyond the design limits.  The bending moment failed which is kind of like the internal stress of the framing member.  It was not a deflection issue.  I ran the calcs for doug fir and it still failed._


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## Sifu (Jul 10, 2013)

I guess I am slow here but I don't see a hip rafter the same as a common rafter.  I know 802.3 says if the pitch is less than 3/12 the member must be "designed as beams" but I still fail to see how a hip and more importantly a valley can be sized by the same chart a common rafter.  The WFCM seems to support this.  First, by not mentioning hip and valley rafters; they use the term hip and valley beams, and second, by giving us a table for sizing these beams.  If mu quick read of the thread is accurate by WFCM t3.28 you would have a beam that fits in the 8x8 area or 11'4 horizontal span row which would yield 2-2x6 (depending on loads).  I am curious what the programs say about that.  I have questioned engineers about why no single member greater than a 2x6 is offered in this table and been told that the seat cut becomes too risky for them.

I have taken 802.3 in a different light.  I beilieve that all structural ridges, hips and valleys are beams but that if the pitch is less than 3/12 802.3 requires them to be "designed" (engineered), otherwise the WFCM could be used.

Just one more illustration of the weakness of this section....IMHO!


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## globe trekker (Jul 10, 2013)

Jeff,

Here are the current span tables for rafters, from the American Wood Council, 2012 Edition

Span Tables for Joists and Rafters.

http://www.awc.org/pdf/STJR_2012.pdf

.


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## jar546 (Jul 10, 2013)

Sifu said:
			
		

> I guess I am slow here but I don't see a hip rafter the same as a common rafter.  I know 802.3 says if the pitch is less than 3/12 the member must be "designed as beams" but I still fail to see how a hip and more importantly a valley can be sized by the same chart a common rafter.  The WFCM seems to support this.  First, by not mentioning hip and valley rafters; they use the term hip and valley beams, and second, by giving us a table for sizing these beams.  If mu quick read of the thread is accurate by WFCM t3.28 you would have a beam that fits in the 8x8 area or 11'4 horizontal span row which would yield 2-2x6 (depending on loads).  I am curious what the programs say about that.  I have questioned engineers about why no single member greater than a 2x6 is offered in this table and been told that the seat cut becomes too risky for them.I have taken 802.3 in a different light.  I beilieve that all structural ridges, hips and valleys are beams but that if the pitch is less than 3/12 802.3 requires them to be "designed" (engineered), otherwise the WFCM could be used.
> 
> Just one more illustration of the weakness of this section....IMHO!


Yep, weak at best.  There is no chart specifically for hip and valley rafters for a pitches 3/12 and greater.  In the IRC there is no chart for hip and valley rafters whatsoever.  There is language specific to the nominal thickness and correlation to the nailing face being equal to the cut end of the jack rafters.

In the example that I used of a 16x16 building with a hip roof, 10# deal and 20# live load, with a simple 8' span, even a #2 SPF 2x4 would make the longest span for a jack rafter (9'4" max @ 16" o.c.) yet a 2x8 does work when you run the numbers, yet it appears to work prescriptively when using 2x4 jack rafters.


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## jar546 (Jul 10, 2013)

So, after a long conversation with the AWC and then with the ICC's head residential guy, it has been determined by my simple mind that:

1) There is no prescriptive way to size a hip or valley using the IRC unless the pitch is below 3/12 because you can then use Table 3.28 from the WFCM for sizing Hip & Valley Beams.

2) If you want to size your 6/12 pitch hip or valley per WFCM 3.28, it can be prescriptive but possibly overkill.

3) There is no table for a hip or valley in the IRC

4) I don't think there is such a thing as a hip rafter or valley rafter.  Only hip beam and valley beam.  They both support the weight of other framing members.

5) Just because there is a requirement for a hip to be wide enough to accept the cut end of a jack rafter does not mean that it is properly sized or compliant.  That is just one of the minimum requirements.

So the onus is on the applicant to prove that the hip or valley structure will support the load imposed as required.


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## RJJ (Jul 11, 2013)

YA! The one size larger rule has been there for years. Also, we had this discussion on the ICC web site a number of years ago. You need a DP for the plan. Never has been a table for hip or valley! You must calculated the assumed dead and snow load for the area in order to size the HIP! A hip is a beam in all basic cases and works differently then a ridge.


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## RJJ (Jul 11, 2013)

Oh ya just for the record last night when I had some time I laid out the roof for a 16 X16 building.

With a 1' overhang the Hip's  are 13' 7 23/32 long

4 common rafters are 10' 3 29/32 long

Jacks are 8'10 1/2"

7' 45/8 "

5' 10/4"

4' 4 27 /32"

the roof angle is 26.5651 degrees

the roof area is 362.24 sf.


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## jar546 (Jul 11, 2013)

RJJ said:
			
		

> Oh ya just for the record last night when I had some time I laid out the roof for a 16 X16 building. With a 1' overhang the Hip's  are 13' 7 23/32 long
> 
> 4 common rafters are 10' 3 29/32 long
> 
> ...


Showoff.

I bet you were framing the whole thing with 2x4's.  LOL


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## RJJ (Jul 11, 2013)

NO! As stated I have framed well over a hundred large hips in the day. Cut more rafter then I wanted to count!

The hip is a strange animal especially when it is a bastard hip. Before all the code improvement we always went on size larger and DF SS#1 for the hip to carry the load. With LVL and such most DP will spec them for the length and because of the added load carrying ability.


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## Sifu (Jul 12, 2013)

jar546 said:
			
		

> So, after a long conversation with the AWC and then with the ICC's head residential guy, it has been determined by my simple mind that:1) There is no prescriptive way to size a hip or valley using the IRC unless the pitch is below 3/12 because you can then use Table 3.28 from the WFCM for sizing Hip & Valley Beams.
> 
> 2) If you want to size your 6/12 pitch hip or valley per WFCM 3.28, it can be prescriptive but possibly overkill.
> 
> ...


Couldn't agree more.


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## ewenme (Jul 12, 2013)

Jar: Try-out the hip/valley sizing in StruCalc. It's fun to what-if there.


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## DRP (Jul 14, 2013)

Hopefully attached another graphic showing the tributary area just to show the plane it's on maybe clearer.

View attachment 759


I'm coming up within spitting distance longhand with max moment of 1478 ft-lbs compared to your 1509, min req'd section modulus @ 1208 psi is 14.68"3 a 2x8 has a section modulus of 13.14"3, still blowing out by ~11%





> Originally Posted by MASSDRIVER  Can you say from the data what it is that fails? _s the lumber itself failing or is the design a fail, like spreading or seperation or connections failing?Brent._





> _In all cases, it looks like bending was beyond the design limits. The bending moment failed which is kind of like the internal stress of the framing member. It was not a deflection issue. I ran the calcs for doug fir and it still failed. ._


_The failure is of extreme fiberstress in bending. The outermost strap of wood in tension on the bottom of the hip (extreme fiber is the eng term) in #2 SPF is allowed to resist, with its' calculated margin of safety, 1208 psi. By my numbers in a 2x8 we are generating 1350 psi. Bend a popsicle stick and watch the bottom surface, notice the fibers begin to tear on the convex side as it fails, there's a fiber failure in bending.Usin JAR's 1509 ft-lb max moment x 12=18,108 in-lbs max momentdivide 18108 in/lbs by 13.14"__3__ section modulus for a 2x8= a required Fb' of >1378. #2DF-L is good for 1428psi... checks in my book. But if you use either DF-L north or south the base design value drops by 50 psi, enough to blow out... the default in your calc must be the weaker more conservative value, which is good. Cutting it too fine anyway. If you step up to #1 Fb" is 1587 psi and you're out of the weeds._
View attachment 759


/monthly_2013_07/hiptrib.jpg.b3cf4128cb3ecd346ff8d77ac1fe4183.jpg


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## jar546 (Jul 14, 2013)

2x8 Hip Rafter Failure

Thank you for the very informative response.  This truly shows the reason why hips and valleys can't be prescriptive.  It looks like we have 4 different calcs and the all failed the 2x8 in this simple yet common design

Sent from my iPhone using Tapatalk


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## RJJ (Jul 14, 2013)

DRP: I believe if you use a 2x10 DF #1 ss  you will find a pass. I have tossed out the long hand calculations I did when running the rafter lengths, but if need be I can do it again.


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## jar546 (Jul 14, 2013)

2x10 does work.


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## RJJ (Jul 14, 2013)

Thanks! I was going to have to turn the trash can upside down!


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## DRP (Jul 14, 2013)

RJJ said:
			
		

> DRP: I believe if you use a 2x10 DF #1 ss  you will find a pass. I have tossed out the long hand calculations I did when running the rafter lengths, but if need be I can do it again.


First a point of grading order;

#2 is the standard grade, then #1 is stronger, then Select Structural (SS)

For in the field quick grading, SS is very clean, almost clear, very small well spaced defects. Then remember quarter, third, half. The cross section through the piece for a #1 can contain a defect of 1/4 the section for a #1, 1/3 for a #2, 1/2 for a #3.

a SS or better Dougfir-Larch 2x8 north, south or center of range passes a #1 straight(senter of growth range) DF-L passes as well


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## RJJ (Jul 14, 2013)

I should have typed it #1 or better SS. I do understand the grading. Many years ago I work in a saw mill. Some of the trees were cut for the lumber industry. The main function was boat keels for sail boats. The rest of the work was busy work while clear the woods. Grading was a daily function and the better the grade produced the more money. That adventure died with the fiberglass boats. I sure loved that job regardless of the danger.


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## peach (Jul 14, 2013)

use the engineered truss sets and call it a day... let someone else do the engineering.


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## jar546 (Jul 14, 2013)

RJJ said:
			
		

> I should have typed it #1 or better SS. I do understand the grading. Many years ago I work in a saw mill. Some of the trees were cut for the lumber industry. The main function was boat keels for sail boats. The rest of the work was busy work while clear the woods. Grading was a daily function and the better the grade produced the more money. That adventure died with the fiberglass boats. I sure loved that job regardless of the danger.


Did they have cars back then when you did that?  Was Lincoln really tall?

   Post church humor


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## DRP (Jul 14, 2013)

Done the truss, darn uncomfortable, repaired and went back to hand framing. For many situations I'd probably let an LVL supplier do the math, say for a cathedral valley. If you're bored at some point JAR run a few larger scenarios on strucalc with the hip a size above and SS and see where it plays out. Interesting on cars from back when, they had ash frames. They were quite concerned at one point that the automobile would wipe out the supply of ash trees so they investigated alternative woods. Same as with baseball bats, they chose maple as a second choice. I'd have to call it a distant second.


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## jar546 (Jul 15, 2013)

Here are the results with a 2x10 as a hip "beam" since all hips and valleys carry weight.  I ran it on BeamCheck and StruCalc and both passed, unlike the 2x8 that failed.  As a reminder, the hip in question is for a:16'x16' building 6/12 pitch 20LL 10DL using #2SPF.  Even though jack rafters can easily be 2x4, a 2x8 is inadequate and a 2x10 is needed.  We have had 4 separate ways this was calculated and 4 different people came up with the same failure conclusion for a 2x8 contrary to popular belief.  Here are the results for the 2x10.  If you see any data discrepancies, please let me know.  I did it quickly but they seem similar.

View attachment 760


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## GBrackins (Jul 15, 2013)

now you see why Table 3-28 of the Wood Frame Construction Manual from a previous post "seemed" overkill ..... it wasn't


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## jar546 (Jul 15, 2013)

GBrackins said:
			
		

> now you see why Table 3-28 of the Wood Frame Construction Manual from a previous post "seemed" overkill ..... it wasn't


The interesting part is that the IRC refers to the WFCM for hip rafter for pitches <3/12 YET the WFCM makes no mention of pitch for using Table 3.28


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## RJJ (Jul 15, 2013)

Yes we had cars and I had three that would all be classic today.


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## RJJ (Jul 15, 2013)

YA to the pitch issue


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## DRP (Jul 16, 2013)

This was a table I saved from an article in JLC some time ago. There's really no reason there couldn't be a prescriptive table.

Hip/Valley Chart2


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## jar546 (Jul 16, 2013)

I have seen that chart.  I agree they could probably break it down into a chart but there must be a reason they still have not other than the WFCM 3.28 prescriptively.


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## DRP (Jul 16, 2013)

I suspect simply because no one has proposed it


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## RJJ (Jul 16, 2013)

That chart has been around for a while. Why the need for a chart? If they submit a plan with calculations and the sizing of the hip in relation to the jack and common rafters is within load requirements either pass it or request a DP's seal.


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## DRP (Jul 16, 2013)

As much as I hate more pages and tables their purpose is to avoid having to engineer things over and over that have the same solution. It makes common solutions prescriptive. I would rewrite the code language that all hips and valleys need to be designed as supported myself and then reference a table or allow engineering, the current language is pretty murky.


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## RJJ (Jul 16, 2013)

Yes I agree! It always has been. The problem is that unlike common rafters Hips & valleys take on a whole different realm of problems.

example being a Hip with a drop hip just a few feet below it would blow and table. Thus the need for design. Snow loads and drifting can also increase the ability for the hips to fail. The rule of thumb of using one size larger then the rafter size has always had its limits.

As referenced by Jeff the code could allow 2x4 to frame some of the members. Well that IMHO is and would be a mistake.


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## rleibowitz (Jul 16, 2013)

Unless I'm mistaken here's how you figure a hip rafter load for a 16x16 building with a total load of 30ft2....8x8x30lbs=1920ibs divided by 2 = 960 pounds of combined live and lead load 12 foot span which is the diagonal of the 8x8 load area...looks to me like a 2x10 does the trick if it complies with 2009 IRC section R802.3 Hip and Valley rafters shall be not less in depth than the cut end of the rafter. Hope this helps


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