Fastening to LVL Below the Neutral Axis 7

[XR250]

It is my understanding that you were not supposed to attach a vertical load to an LVL below the neutral axis. I have a situation where I have an LVL (4) 1 3/4x18 supporting framing on top and 2 ft. of brick on the face with a lintel. If I use a an L5x3 1/2x1/4 lintel fastened at 16 inches on center, the shear load is 90 lbs. per connection. The fasteners would be located 3 3/4" from the bottom of the beam. Is this enough to worry about or should I weld some tabs to the lintel to get the bolts above the neutral axis?

 

[XR250]

Thanks y'all for your insights.
Still on the fence about which way to go unfortunately!

 

[dik]

The hottest places in hell are reserved for those... engineers that sit on a fence. (with apologies to Dantes)

It could be that engineered lumber, unlike dimensioned lumber and heavy timber are not prone to splitting and the connection that caused tension perpendicular to the grain is not applicable. This is something the engineered lumber companies should look into and address...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Eng16080]

I don't know the best answer to this question, but as another data point:

From "Design of Wood Structures" - Breyer - 6th Ed. (p. 14.14-14.16), it's recommended to avoid suspending multiple loads below the neutral axis. "Tension stresses perpendicular to the grain tend to concentrate at the fastenings. Wood is weak in cross-grain tension, and tension perpendicular to grain can interact with horizontal shear to cause splitting." Concerning an isolated load: "An isolated light load may be suspended below the NA of a beam. However, the load should be limited to one that can be transferred with small fasteners." Furthermore, the book recommends a minimum distance from connection to bottom of beam, in this case, of 6 inches. By "light" load they seem to be implying something very light, like electrical conduit.

 

[dik]

That's why it's necessary to know the mechanism of failure, and how this is applicable to engineered lumber.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[jerseyshore]

Side note, how great it is that the NDS uses terms like "heavy or medium" and this book says "light load...transferred with small fasteners". They just have to leave things ambiguous for us.

 

[XR250]

How are you bracing the beam to prevent torsion if the framing sits on top?

I'm not - just like everybody else!
Not a failure mode I have ever witnessed.
How do you do it?

 

[jerseyshore]

Well it's a little different if you're attaching a load to the bottom of the beam, but still close to it's center line; not a lot of eccentricity there.

But attaching an angle or any eccentric load is going to create torsion for sure. I've spoken to AWC directly and they said wood has effectively 0 capacity in torsion and must be avoided.

Typically I try to make everything work with loose lintels in your situation. Or avoid any chance of torsion on a beam. If I absolutely can't and have no other options I will have the joists/ blocking/ braces frame into the side of the header/beam near the load to prevent it from rotating.

 

[jayrod12]

Almost every wood guardrail I've ever seen puts some form of torsion on something. Only since Simpson Strong-Tie really started pushing their guardrail and deck details have I seen more robust detailing that eliminates the torsion via blocking. For years I was looked at like a crazy person for my guardrail details, I feel vindicated now.

Therefore, I would be considering the detailing similar to guardrail details in terms of resolving the torsion into supplementary framing.

 

[dik]

I'm the odd man out... I don't know why they would make that statement, in particular engineered wood. I posted this in another thread and surely these beams have a least a little torsion; they are curved in both axis...

https://www.constructioncanada.net/...timber-arena/?qnewsletter=20230607&en_click=1

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[XR250]

The header is 4 ply - so pretty chunky. Never have seen anyone worry about this in a garage door header with brick veneer and only a 2 ply LVL.
Does not seem to be a failure that is occurring even though technically it ain't right.

I have another area on this house with a 20 ft. span and 10 ft of brick above it with 15 ft. tall walls.
In that case, I am using a 14x6x3/8" tube steel header with a lintel plate and HSS6x4x1/4 jambs to resist the torsion.
Believe it or not, my local steel supplier stocks that header.

 

[jerseyshore]

I'll have to find the exact quote from them, but I asked about this exact situation (2x or LVL beam supporting brick) and they said something like because of the tension perp to grain wood has no capacity.

jayrod, are you talking about regular wood guardrails with 2x's bolted to the side of wood posts? Because eccentrically loaded posts are okay and there is a calc for them in the NDS.

 

[jayrod12]

I'm talking the reverse where the posts cantilever off of the deck framing. And if there's a calc in the NDS, then they are obviously accepting some form of torsion is a thing.

 

[Eng16080]

Some more information per NDS 2018, Commentary Section C3.8.2 - Tension Perpendicular to Grain:

"Average strength values for tension perpendicular to grain that are available in reference documents (181, 183) apply to small, clear specimens that are free of shakes, checks and other seasoning defects. Such information indicates that tension design values perpendicular to grain of clear, check- and shake-free wood may be considered to be about one-third the shear design value parallel to grain of comparable quality material of the same species (9). However, because or undetectable ring shakes, checking and splitting that can occur as a result of drying in service, very low strength values for the property can be encountered in commercial grades of lumber. For this reason, no sawn lumber tension design values perpendicular to grain have been published in the Specification..."

So, if I were to run some real numbers on this and not rely on vague language concerning what light, medium, and heavy loads are, I would perhaps take one third the shear parallel to grain value published by the LVL supplier and use that as a tension perp. to grain value. I would be hesitant to use this value for sawn (non-engineered) lumber considering the above language and the much higher likelihood of undetectable defects versus an engineered product. In terms of throwing around rough numbers (don't hold me accountable), if the shear parallel to grain value of the LVL is 285 psi, then tension perp. to grain would be 95 psi. Considering a 4-ply LVL (7" wide), then for every foot of beam length, the beam can support about an 8,000 lbs force (7in x 12in x 95 psi) applied perp. to grain. I don't know if this is the full story, though. Considering that there is also horizontal shear forces acting in addition to the perp. tension, it seems that the 8,000 plf may be too high. Still, if we took only 1 percent of this value, it seems that that alone would be adequate to support the lintel in this case.

Curious what others think.

 

[dik]

It doesn't make sense (not for your comment Eng16080, but for torsion stresses)... I would have thought torsion stresses of approx 100psi minimum, might be in order. Saying 0psi in not my idea of engineering.

Your take, Eng appears to make sense... I'm not sure what the effect of the neutral axis has on this.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[jerseyshore]

jayrod, I guess based off what Eng16080 posted, having an eccentrically loaded post parallel to the grain vs perp to grain is the difference there.

And for the deck posts like you mention, this is probably why Simpson shows tension ties at all of those connections to reduce any torsion.

 

[jayrod12]

I agree, that's how I've always detailed guardrail posts. With tension ties and blocking to resolve the torsion. I used to get looked at like a crazy person. Now I just send Simpson's technical bulletin when there are questions.

I'm failing to see how a cantilever guard post puts load parallel to grain, the only load I'm seeing is cross grain bending (essentially pure torsion) when the guardrail posts are bolted to the rimboard/edge joist.

 

[JAE]

I'll chime in late here.

The "below the axis" issue is intended to prevent tension perpendicular to the grain of sawn lumber and Glue Lam members comprised of sawn lumber.

In both cases the hanging load, below the axis, create direct tension across the grain.

For LVL, or SCL, there really isn't any "grain".
The wood has been jumbled up, so to speak, and the idea of tension perpendicular to a plain of grain doesn't really exist. There's no longer any real grain there.
This is why, in my opinion, the NDS doesn't include SCL in that provision.

However, having said that, XR250, as a prudent engineer trying to always avoid possible abrupt failures, etc. and with concern over a bolt hole in wood with tension in two directions (bolt downward and longitudinal due to bending) I'd be strongly tempted to use the straps as you suggested in your original post.

 

[Craig_H]

I have a personal interset in this subject, since I am in the midst of desiging a complicated roof framing system and was considering connecting some dormer ridges to a main ridge below the neutral axis. A quick google search turned up this article:

https://www.researchgate.net/profil...eer-Lumber-LVL-and-Cross-Banded-LVL-LVL-C.pdf

Snip of the conclusion below:

 

[dik]

Thanks, JAE... I'm not so sure that glulam beams are an issue. I can see it for dimensioned lumber and timbers...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JAE]

dik,
Well the NDS included GLB in that provision. If you think of 2x4's used in a GLB there is a grain direction longitudinally in each lam of the beam still.