Repetitive Member Factor (1.15) applied to 3-Ply Wood Beam? 6

[Ron247]

Is it okay to apply the Repetitive Member Factor to a 3-ply wood beam? I have done it in the past based on my reading of the NDS but thought I would inquire here. I have an existing 3-Ply wood beam and since the NDS says 3 or more members in contact or spaced not more than 24", I assume I can use the Cr factor of 1.15. The beam is currently 9% over without the Cr.

 

[jayrod12]

I don't know about the NDS but here in Canada they call that the system factor, and it applies to regularly spaced members. So if you had 3 ply beams at 24" spacing then sure, but if it's a single beam then definitely not

 

[pmtrevisan1]

Jayrod.
I disagree as I read the code (both NDS and O86) and it states 3 or more members spaced at max 24" (600mm) on center. if you have a 3 ply beam spacing is less the code and therefore the repetitive stress (system factor) applies.

my understanding of this factor is to take into account components that are stronger than code values (#1/#2) grade.

 

[TehMightyEngineer]

I wouldn't, by my understanding that's not the intent of the Cr factor.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[jayrod12]

3 or more members regularly spaced. How can you apply the repetitive member factor to a beam. The wood design manual here has tables for built up beams. I'll have to check tomorrow, but I cant imagine they include an additional 15% bump in capacities just because you went from a 2 ply to a 3.

 

[JAE]

We use the Cr factor for 3 ply beams.

Wood tabulated stress values are set up for single members where the statistical variation in wood quality, even when visually rated, can result in lower strength due to random anomalies.

So if you have multiple members, either spaced or bundled, that statistical variation is less powerful and you can have the same assurance of strength with the Cr factor applied to the group as there is to one member.

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[pmtrevisan1]

agree with JAE
which my interpretation of the code and what I alluded to in my original post.

jae said it better.

 

[KootK]

My understanding of the repetitive member factor is identical to JAE's. Statistics.

I would argue that the bump is actually more applicable to a 3-ply whatever than it would be to spaced members. Better odds of better load sharing between 3-ply when you ditch the sheathing acting as a distribution member.

On the other-hand, Jayrod12 is our reigning tip-master / demigod of the week. So there's that.

 

[TehMightyEngineer]

Interesting thread from 2008 that discusses this:

https://www.eng-tips.com/viewthread.cfm?qid=213584

JAE you seem to have had your position changed. That thread might be why I don't use Cr for single members as that's about the time I started paying attention to engineering tips.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[StrucDesignPE]

The commentary of the 2005 NDS states:
"The criteria for use of the repetitive member increase are three or more members in contact or spaced not more than 24" and joined by transverse load distributing elements such that the group of members performs as a unit rather than as separate pieces. The members may be any piece of dimensions lumber loading in bending, including studs, rafters, truss chords, and decking, as well as joists."

"The repetitive member increase also applies to an assembly of three or more essentially parallel members of equal size and of the same orientation which are in direct contact with each other. In this case the transverse elements may be mechanical fasteners such as nails, nail gluing, tongue and groove joists, or bearing plates. The required condition is that the three or more members act together to resist the applied moment."

Based on this, I would agree that C_r may be used for a 3-ply beam fastened together.

 

[WinelandV]

I don't do much wood design anymore, but I did a fair amount of wood formwork design in a much previous position. Is your 3-ply member one your are having the contractor build up himself, or is it premanufactured? If the latter, I would be wary that it's already been accounted for in the tabulated stresses. (Note, I could be WAY off base, I'm just spit-balling here)

 

[Ron247]

It is an existing beam. The Owner is changing the usage from a flat roof to a deck. The live load goes from 20 psf to 40 psf. That is what is causing the increase. I have always used Cr for 3 ply or more beams but never really sought out guidance on whether I had interpreted it right. Th NDS does say "in contact or spaced not more than 24". It has been my understanding that it is related to statistical difference in relation to variable wood strength.

The NDS also outlines example members but fails to say "beams". I read an article once that stated it applied to beams but cannot recall what the article was. The plys are mechanically connected as required.

 

[XR250]

I was in a 70 year old house yesterday that had a 4x8 beam spanning 10 ft holding up 9' trib of floor and 12' trib of roof. It was sagging 2". Am I going to worry about a 15% overstress in a beam - not really.

 

[jayrod12]

Canadian wood design manual
2x10 SPF single member capacity 4.1 kNm
2x10 SPF Case 2 (repetitve member) capacity 5.72 kNm

SPF 2 ply 2x10 beam capacity 8.99 kNm (so 1.1 * Single member * 2)
3 ply 13.5 kNm (again 10% increase)

this 10% increase continues to 4 and 5 plys.

In my mind, this tells me that they are not taking repetitive member into account since the same 10% increase applies to a 2 ply beam.

 

[JAE]

TehMightyEngineer,
Ha! You caught me!
Yes, I recall that post and after reading through it realized that a built-up beam is definitely "repetitive".

Old dogs can and do learn new tricks I guess.

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[TehMightyEngineer]

Nice find StrucDesignEIT! That seems to completely address this. I looked for this last night but apparently didn't read the commentary.

Well, guess I'll also start using Cr in my multi-ply beams now.

JAE: Well, that was over 10 years ago now.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[Ron247]

I think this is the article I read a few years back.

http://myconstructionfinds.blogspot.com/2011/05/repetitive-member-factor-introduction.html

 

[BridgeSmith]

It appears the Canadian code takes a much different approach, applying a 1.4 repetitive member factor, but only a 1.1 factor for multiple ply beams. Hmm...perhaps because repetitive members separated by some space have greater ability to develop load capacity at different deflections? I guess I can see a scenario where some joists are less stiff than others, so require greater deflection to develop their full capacity. With laminated plies, this would not be able to occur. That's the only reason I can think of for the differences.

 

[Ron247]

Jayrod,

I am not familiar with the Canadian Wood Design but what is the 10% you are citing?

Also, the Case 2 you are referring to shows about 40% increase over your single member. Is that the repetitive factor for the Canadian code?

 

[jayrod12]

Ron,

Yes, so the single member is used for a straight up single 2x member that violates the requirements for repetitive member. But up here we call repetitive member "System Cases" where our increase is 40% over a stand-alone single joists however this is subject to a minimum of 3/8" board sheathing or 5/8" decking to qualify(i.e. System case 2).

That being said, we also have System Case 1 members in the joist design area that indicates "Case 1 systems are composed of three or more essentially parallel members spaced no more than 24" apart and are arranged so they mutually support the load. However, Case 1 systems lack the sheathing and fastening requirements to qualify as Case 2 systems; or they may include a more complex structural component such as a wood truss." Which when you go on to read it says if you qualify for case 1 but not case 2, you multiple the single member values by 1.10. so there may be some validity to applying to beams.

What I was saying is when comparing the capacity of 2, 3, 4, and 5 ply beams to the single member capacity there is a 10% increase over just taking the single member capacity and multiplying by 2, 3, 4 or 5.

In the Canadian code however, Mr is a function of bending stress, section modulus, and two factors Kz (Size factor) and KL (Lateral stability factor). Digging deeper, it appears they apply the 1.1 factor to the bending stress for beams prior to publishing the number. So we never see a "repetitive member factor" in our beam design formulae if you just pick the beam bending stress from the tables.

Long winded story cut short (TL;DR), In Canada even 2 ply beams have a repetitive member factor included, however it is only a 10% increase for all built up beams. The repetitive member factor for joists with floor sheathing is a 40% increase.

On the other-hand, Jayrod12 is our reigning tip-master / demigod of the week. So there's that.

Twice in my tenure, and I'm pretty sure I just got lucky.

edited for clarity