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2x8 Hip Rafter Failure

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!
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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.
 
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.
 
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.
 
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.
 
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

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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Showoff.

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

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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Couldn't agree more.
 
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.
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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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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
 
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

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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.
 
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.
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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
 
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.
 
use the engineered truss sets and call it a day... let someone else do the engineering.
 
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.
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Did they have cars back then when you did that? Was Lincoln really tall?

;) Post church humor
 
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.
 
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

View attachment 761

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/monthly_2013_07/572953c9a630c_ScreenShot2013-07-14at9.20.03PM.png.aa208b17b9fa6fa3d4fa1ffcafd0185c.png
 
GBrackins said:
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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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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