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2x8 Hip Rafter Failure

jar546

jar546

CBO
Joined
Oct 16, 2009
Messages
12,724
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Not where I really want to be
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

View attachment 1853

/monthly_2013_07/572953eae71a3_ScreenShot2013-07-07at10.37.03PM.png.2bca97096e0e10268e261b63437ded1f.png
 
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
 
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).
Click to expand...
I would think it becomes more of a thrust issue.
 
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.
 
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????
 
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,DBView attachment 747

/monthly_2013_07/572953c8b9929_16Hip.jpg.f2488924d051ce4077c3b912163b8d3a.jpg
 
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) View attachment 1758 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
Click to expand...
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?
 
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.
 
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.
Click to expand...
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.
 
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!

.
 
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.
 
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 1766Regards,DB

View attachment 752

Misc2.pdf

Misc2.pdf
 
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. View attachment 1766

Regards,

DB
Click to expand...
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.
 
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.
Click to expand...
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
 
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
Click to expand...
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.
 
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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