Built-up Wood Header Calc 1

[kar108]

Should the Size Factor C_F for a built-up header of (3) 2x10s be 1.1 or 1.2 when using the 2005NDS Supplement Table 4A Adjustment? The notes do not specify if the factor applies to the component 2x10s or the built-up assembly. It seems that using the larger factor of 1.2 might be double dipping if I am already using the repetitive member factor C_r=1.15.

So which is correct
a) F'_b=F_b*C_F*C_r= F_b*1.1*1.15 or
b) F'_b= F_b*1.2*1.15
???

 

[haynewp]

It talks about load sharing distribution. The header needs all members to take the load, how can there be redistribution? I think it should be called redundancy factor instead of repetitive use. Again if this is the case, can I apply Cr to an isolated glulam beam? Doesn't it meet the same criteria you quote? What if I think of one big solid sawn beam as made up of a lot of smaller 2-4" sawn beams connected by nature. Can I use the repetitive use factor then? The whole thing doesn't make engineering sense no matter what it says.

 

[apsix]

"The whole thing doesn't make engineering sense no matter what it says."
Of course it makes sense, I'm not sure the same can be said about your example.
If it was purely due to load sharing with other members because it is deemed (somehow) that they are not fully loaded, then the principle should be valid for all construction materials.
The repetitive member factor is used in timber design because it is statistically very unlikely that all the members in a connected group of 3 or more will have the characteristic design strength. Due to the high variablity of timber properties it is likely that most or all of the members will have a strength significantly higher than the design strength.
This is not a comment specifically on NDS (about which I know nothing), but rather, on the basis for British and Australian Standards.
Also refer <

http://www.timber.org.au/NTEP/menu.asp?id=187>.

 

[hokie66]

Like apsix, I am unfamiliar with NDS, but I agree with the logic of applying a repetitive member factor to members which together share a load. The Australian reference he gives does a good job of explaining the concept, and our Standard allows this increase. Manufactured members, e.g. glulam, LVL, already have the stress increase built in because of their consistency.

 

[haynewp]

hokie, you are right again.

 

[JJMT]

Drinking isn't just limited to a p-delta proplem it also has nonlinear materal properties.

CJSchwartz - I agree that the references suggest that a stress increase for typical headers is appropriate. I feel this would be fine to incorprate into the IRC header tables where there are prescriptive construction requirments. I'm leary of its application with regard to the NDS, where I'm able to design a large range of built-up header and beam configurations.

GT1878 - I'm not sure the commentary really clarifies anything.

To recap I feel the discussion is centered on two concepts:
1) Is the intent of repetative member factor to account for the likelyhood that some of the adjacent members will be stronger than the expected design value.

2) Or, is its purpose to account for the redistirbution of load between framing members and partial composite action.

I feel the majority of the evidence leans toward the second.
The NDS seems to have specifcally not included built up headers or beams in its list of members and I think we can agree that they are not similar to joists, truss chords, rafters, studs, planks, or decking. In addition the commentary is clear that that you can not apply a repetitive member factor to an assembly composed of adjacent 2x10, 2x8, 2x6 and 2x4, or to an assembly of only two 2x10s. Both of these assemblis have the possiblity of adjacent members being stronger than expected.

Now there is the possibilty that the initial intent of this factor has mutated into a more narrowed definition of just load sharing. Specifically, the reptitive member factor was first added to the 1968 NDS edition based on a recomendation in ASTM D245. This would suggest the factor has some expected allowable stress component. Unfortantly I don't have a ASTM handy to further investigate.

My final comment on this matter is that because of the complex nature of allowable wood stresses I can not be sure that the obvious benefit of using multiple 2x members over a large solid sawn member is not accounted for with other adjustment factors or within the base allowable stress value. If I was designing a header and came out overstressed by 15% I would just note the top plates, rim joist, etc and move on, but then engineering judgement is a relative thing, I've hung small block chevy engine from a single grage truss with some 10 penny nails with great succes.

 

[jechols]

I bet you could apply the increase to all members and never have a problem. If you occupy a structure made of any material type that was built during WWII (in the US and probably other countries as well) you already are in a structure designed for higher stresses. The allowable stress was increased to save materials. That seems so strange to me, a "war factor".

 

[JAE]

Here's the section on C_r in the 2005 NDS commentary. It appears to include 3 member headers (see last paragraph):

 

[GT1878]

Which was my point. I had the same idea as JAE on the Cr but the commentary clearly shows that a 3ply header qualifies.

I personally believe apsix has the correct logic on why.

JJMT I believe your questions are answered in the commentary that I referenced and JAE posted. The answer is both.

 

[JAE]

But GT1878,

I still think I won't use it for the reasons I posted above. Despite the commentary, which seems pretty clear, I just haven't used Cr for headers in the past and with the quality of wood we get these days, the fact that headers are usually fully loaded at some point in their lives, and the fact that I'm an old stick-in-the-mud, I'm using 1.0 on headers and 1.15 on joists/rafters, etc.

 

[hokie66]

And that is what engineering is all about. Codes are our guidelines, but each engineer must decide for himself what provisions are appropriate to use.

 

[DonPhillips]

I disagree. The codes are the minimums. Guidelines imply the option to ignore.



Don Phillips

http://worthingtonengineering.com

 

[hokie66]

I suppose. But in this case, JAE is just exercising his option to ignore a less conservative provision.

 

[GT1878]

JAE, I agree. The logic you used earlier is exactly how I felt about Cr... Its how we learned and that shapes our thinking and intuition. If we had learned from someone who did use it for 3ply headers, we'd probably slap that 15% in there at the blink of an eye. Bottom line is that NDS says its ok....
I'm with the post that said to make it 3 2x12's and quit fighting it if its that close.

 

[JAE]

Sort of a commercial...

But this thread is typical of what is found here at Eng-Tips.

A "nuanced" question arises from our work, we discuss it from the different perspectives of our education and experience, we research a bit, we discuss the ramifications of that research, and then we conclude a bit.

This thread then exists for future searches by others who run into the same issue. We all benefit. Thanks for the great discussion all, and thank you kar108 for the initial post.