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
???

 

[NKT]

The size factor should be based on the size of the individual pieces of the built-up assembly.

The repetitive member factor should not be used for built-up members: it is intended to account for redistribution of loads to adjacent members, as in floor loads transferred to adjacent joists through beam behavior in the sheathing. This factor represents a level of redundancy that is not present in a single (built-up) beam.

 

[JAE]

I would not use the Cr factor - this is a single beam, not a system of floor joists.

The size factor, C[sub]F[/sub], is used based on the single piece (2" = 1.1 for a 2x10). This factor is used to adjust for the fact that as you get larger and larger members out of a tree, there is a higher probability of flaws, knots, etc. in the wood and thus a lower safe stress.

 

[DaveAtkins]

I agree with the comments above--but I do use the repetitive member factor for headers. The NDS specifically talks about members which are in contact when it discusses the repetitive member factor. After all, if one of the three plies of a header is a little weak, it can transfer its load to the other two plies.

DaveAtkins

 

[Structures33]

This factor is applied to the actual size - not the built up size. So you would use 1.1 as your size factor.
As an aside, the repetitive member factor has nothing to do with the fact that there are three 2x10's. It has to do with the spacing of the members. If the members are, for example, joists spaced at 16" o.c. you can use it. If the member is a single (3)2x10 (like a single beam), this factor does not apply. So in your case, if you have (3)2x10's @ 12" or 16" or 24" on center, it applies.

 

[DaveAtkins]

But the NDS does say that the members can be in contact--wouldn't that mean in a header?

DaveAtkins

 

[KBVT]

I agree with Dave, although the repetitive member is often utilized in only joist scenarios, it can be applied to any 3 bending members with a spacing of less than 16". A center to center spacing of 1.5" applies here. You can use the Cr and the Cf, they are two completely different adjustment factors.

 

[JAE]

I don't agree that the rep. factor applies to headers.

It says, [red]"when members are used as joists, truss chords, rafters, studs, planks, decking, or similar members which are in contact or spaced not more than 24" o.c., are not less than 3 in number and are joined by floor, roof, or other load distributing elements adequate to support the design load."[/red]

Here's my reasons from the quote above:

1. It doesn't use the term "headers" in the list above.
2. The mention of "in contact" is referring to the decking.
3. The load coming into a header is not via a "distributing element".

and finally - the main reason:
4. A header is designed for the TOTAL load coming into it. The intent of the Cr factor is to suggest that if an overload is applied to one joist or rafter, then the distributing elements can transfer that load (when the overloaded member deflects away from the load) to other members which are not loaded and therefore create a safe system. In the case of a header, there are no adjacent non-loaded members. Each of the 3 2x10's are designed for 1/3 of the total load. So load transfer to an adjacent, lightly loaded member to take up the slack can't happen.

Therefore the Cr factor doesn't apply.

 

[CJJS]

JAE,
I am not clear on your interpretation of "in contact". I read it as meaning the individual members can be in contact or can be spaced at 24" o.c. or less.

Based on ANSI/TPI1-2002 Page 29, "Repetitive member factors have long been utilized in wood esign to account for the load-sharing effects of assemblies that strengthen them beyond the strength assumed in designing the single members within them."
Page 30, "These increases apply to chord members where three or more trusses are positioned side by side, are in contact, or are spaced no more than 24 inches on center and are jointed by roof sheathing, flooring, gypsum, or other load distributing elements attached directly to the chords"

I realize this specification is for wood trusses, but the principles should be the same for beams.

 

[JAE]

CJSchwartz - To me, the principle is all about load sharing for adjacent members that aren't fully loaded when the "design" member is overloaded.

If you have three trusses, and one gets overloaded, the adjacent ones can help. This is because the decking, or bridging, etc. can distribute the load to the other members.

When you have "one" member that is comprised of three pieces (be it 2x10's or multiple trusses in a girder truss) - all three pieces are designed for exactly 1/3 of the total load. If you apply a Cr factor of 1.15 to them, then you essentially are saying that piece 1 gets help from pieces 2 and 3, piece 2 gets help from pieces 1 and 3 and piece 3 gets help from 1 and 2.

But they can't help each other because they are all taking their full share of load - you can't overload one of the 2x10's and not the other two.

So its sort of like depending on three drunks all leaning on each other for support.

That's the way I see it personnally.

However, I note the following publication is apparently a statistical study to prove me otherwise:

http://www.nahbrc.com/bookstore/er0603w.aspx

And check out table B4 and the paragraphs above it here:

http://www.huduser.org/Publications/pdf/panelize_wall_app_B.pdf

So from these two publications, my argument seems weak, I'd agree....but I don't read the NDS that way and for me, I'm sticking to my guns until the NDS is modified to explicitly state that headers are applicable for Cr factors.

 

[DonPhillips]

I think "in contact" allows for the doubling of alternate joists to get a a differnent spacing - like adding 1 joist every third to get your 16" centers to 12" centers.

Don Phillips

http://worthingtonengineering.com

 

[ctcray]

First off...I use the Cr on beams and headers but I'll admit, I hadn't noticed the NDS doesn't explicitly list beams and headers in its description.

JAE - I don't see how "similar members which are in contact or spaced not more than 24" o.c" is referring to contact with decking. Either the members are in contact {with each other} or they are spaced less than 24" oc. Requiring contact with decking seems just as extrapolated as assuming "beams and headers" should be included.

Also, "are joined by ... other load distributing elements adequate to support the design load" implies that if they are properly nailed together (nails being the load distributing elements), they would meet the requirements of this statement.



Since the allowable loads for lumber are based on the 5th percentile of actually tested specimen values, then further reduced by another factor of safety, I don't think giving yourself 15% back for a 3-ply beam is really pushing the envelope.

But hey, rather than waste time on engineering, bump it to (3) 2x12s or go to LVL.

 

[JJMT]

I had a quick discussion with the office and we are in support of JAE's conclusion. While we think you could 'fool' the building code offical into accepting the actual wording of the NDS, it is not the intent of the repetitive member factor. We feel the Cr factor is more related to load redistribution than an increase in stress.
My though experment goes as follows. If I test three 2x10s fastened together as a header i get a specific breaking strength. If I had tested those three 2x10s sperately and added together their breaking strengths I would get the same value. (the whole is equal to the sum of the parts). Now, if I had placed those three same 2x10s in a flooring system and tested each one seperatly. I would find that each 2x10 would carry a large load, due to the fact that the decking (load distributing member)applied some of the load to the adjacent members. (the whole is greater than the sum of the parts)

I feel the distinction comes in the design process. I design the header members to each carry a third of the load. I design the floor members for their tributary load.

While the argument could be made that the repetitive member factor accounts for the propablility that some of the adjacent members will have a greather allowable strength than expected. (There is also the proprability that they are weaker, when i build up headers i always but the the worst one in the middle of me header). This consideration is accounted for with the size factor, CF. Three 2x10s are stronger than one '5x10'. As support of this I provide the observation that there are size factors for Fb, Ft, and Fc, not Fb alone as is applicalbe for the repetiative member factor.

 

[CJJS]

I think JAE's references indicate that the stress increase is okay. Whether or not that is the intent of the NDS is another question. Clearly, based on testing, a "load sharing" increase in allowable stress is acceptable for multi-ply beams.

 

[DaveAtkins]

JAE,

Aren't three drunks leaning on each other considered structurally stable?

DaveAtkins

 

[haynewp]

Applying repetitive use to a built-up header would be similar to me to applying it to a single glulam beam.

Physicists use space time thought experiments, we use stability of drunk guys.

 

[connect2]

one must also consider how the load is applied, top loaded, side loaded ...etc., as well.

 

[JAE]

well, it depends on whether they were drinking beer or scotch.

Scotch has waaay more second order effects.

 

[haynewp]

The more you drink the more p-delta. It's a nonlinear situation!

 

[GT1878]

I think it would be a good idea for some of us to read the commentary for Cr. NDS 05 C4.3.9.

"It (referring to the 15% increase) is based on the increase in load-carrying capacity and stiffness obtained when multiple framing members are fastened together...."
"It reflects two interactions: load sharing or redistribution of load among framing members and partial composite action of the framing member..."
"The repetitive member increase also applies to an assembly of three or more essentiall parallel members of equal size and of the same orientation which are in direct contact with each other"
"The required condition is that the three or more members act together to resist the applied moment"