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?

 

[jerseyshore]

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

 

[Greenalleycat]

Well this is minorly worrying as I have never heard that and never thought about it in my own designs
Do you have a reference for that?

 

[ChorasDen]

Do you have a reference for that?

This seems to come up on these forums every couple months. NDS has a reference that says not to load below the neutral axis, see image below.

American Institute of Timber Construction has a document that discusses wood connection detailing. APA has as similar document that also discusses connection detailing.

 

[driftLimiter]

The construction of the LVL seems it would be less susceptible to issues with this arrangement. The footnote doesn't mention this for structural composite lumber.

 

[dik]

Bad code advice... it should be changed.

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

-Dik

 

[ChorasDen]

The footnote doesn't mention this for structural composite lumber.

This is because AWC NDS chapter 8 tells us to refer back to the manufacturers code report, then the manufacturer will say to go back and design per the NDS.

The construction of the LVL seems it would be less susceptible to issues with this arrangement.

Can you elaborate on this? The manufacturing method of laminated veneer lumber would make it less conductive to resisting tension perp failure. LVL is made by peeling a log and adhering the wood veneer together with an exterior rated adhesive/resin. This allows the wood grain to be in alignment, which if anything, would increase the likelihood of tension perp failure when loaded below the neutral axis, because the lack of cross laminations and/or slope of grain.

 

[ChorasDen]

Bad code advice... it should be changed.

dik, can you please elaborate on how you have determined that this is bad advice and should be discarded? Tension perp is a funny thing, it has very low (not reliably quantifiable) design values, and it's an energetic failure method, not progressive like bending failure would expected to be.

 

[dik]

For timber it is less of a problem, but for structures in general, loading below the shear centre acts counter to most stability problems. This is most noticeable for steel construction, in particular long cantilevers. I've occasionally addressed cantilever crane beam issues using the Eurocode approach, which I think is the best approach out there.

I cannot think of any condition where loading below the shear centre is detrimental, even for wood. As long as you can make a safe connection. My CAN$0.03 (2 cents US). Can you provide me an example where loading above the shear centre is better?

To reiterate... bad code.

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

-Dik

 

[driftLimiter]

My understanding was that not all of the laminations are oriented the same for LVL. So I took at look at an ESR report for LVL and turns out you are correct the veneers in LVL are oriented in the same direction. I did not see anything in the ESR report regarding connections below the neutral axis.

You are also correct that the ESR report says to design connections per NDS. But if NDS doesn't say structural composite, I don't see that this applies following the letter of the law.

Time to reach out to Weyerhauser and ask them, surely someone there has some guidance on this.

 

[dik]

I'm pretty sure composite doesn't make any difference... maybe for uplift, in a rare condition.

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

-Dik

 

[ChorasDen]

drift, that's convenient you'd say that, I've been a bit coy in the past on these forums about my profession, but my day job is an engineering representative of the largest wood product manufacturer in North America. Believe me when I say that my opinion above represents the wood products industry as a whole.

 

[ChorasDen]

I cannot think of any condition where loading below the shear centre is detrimental, even for wood. As long as you can make a safe connection.

Over a couple drinks I could tell you some stories of testing at AITC labs that I expect would cause you to change your mind on this topic, or at least reconsider your stance. Regardless, tension perp is a very real and very catastrophic failure mode in wood design. Particularly in that it is sudden with little to no warning, unlike bending, buckling, or even some forms of shear failure. You'll experience no additional deflection or displacement of the member until sudden, complete failure of the wood member. Thus, giving occupants of the building no visual warning of deficiencies in member capacity or design until it is too late.

 

[dik]

A bad connection will fail, even if it's above the shear centre...

Other than splitting with heavy timber construction, it's not likely a problem with LVL, PSL or similar ilk. There is no need to have that restriction. I've been playing engineer for over half a century (and still active) and, IMHO (not very humble, sometimes) there is no reason for this.

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

-Dik

 

[driftLimiter]

@ChorasDen

If its such a bad problem why doesn't the industry publish anything on this, why is the code unclear?
Telling us horror stories about some work in a lab is one thing, but if this is really true why is it not published clearly in the text or some other technical document?

Its not clear to me how cross grain tension in the LVL causes the same issue as Sawn lumber or GLBs.

 

[ChorasDen]

Interesting questions, Is the code unclear? NDS calls it out quite clearly in chapter 3.

Chapter 8 of NDS limits tension side notches at bearing to 1/10 of member depth on structural composite lumber. In addition to shear checks, this limit is also to reduce potential for cross grain tension failure at notch corners, the wood handbook from forest products laboratory teases this idea out as well.

LVL is manufactured in such a way as to basically ensure that all the grain is oriented in the same direction, and that grain is also oriented in the axial direction of the member. We do not have the advantage of slope of grain that causes an angular adjustment to the tension perp vector that you might expect to see with sawn lumber and glulam.

 

[driftLimiter]

I think the table is unclear, and most of NDS discussion about perpendicular tension is rather unclear as well.

What is a heavy load? medium load?

Does placing the bolt above the beam cl really preclude perpendicular grain tension?

The ESR report from the manufacturer fails to mention anything about this.

So we are left with the table provided in Ch 12, and the paragraph (on tension members) in chapter 3. Which calls out sawn lumber and GLB specifically.

So what is the answer to the OP question, is 90# hanger load below the centerline of the LVL a heavy load or medium load? If its neither of these then there shouldn't be any issue according to the code. Except the nebulous text in 3.8.2 that says avoid wherever possible and reinforce if needed. Is that for heavy loads & medium loads, or all loads?

We have hashed this out before regarding simpson hurricane ties. I feel like there isn't a good common consensus.

 

[driftLimiter]

@ChorasDen Idk why I seem feisty about this in general I try to avoid the condition. I've just always felt that code guidance on this topic is very lacking and it is a commonly occurring condition.

I found this research paper that backs up the cross gain tension concern in LVL and also gives some practical guidance for reinforcement of it.

[URL unfurl="true"]https://ir.canterbury.ac.nz/bitstream/handle/10092/7674/Thesis_fulltext.pdf?isAllowed=y&sequence=2[/url]

 

[dik]

and therein lies the crux... best to know the mechanism of failure.

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

-Dik

 

[jerseyshore]

I'm confused, the NDS specifically leaves out LVL's and PSL's from that footnote, but you're saying those are actually worse than the sawn & glulam for this condition? Seems odd that the code leaves SCL out in a table like that.

This is just a rando guess, but is it possible that the code and some of those diagrams say avoid loading wood below the neutral axis is only because 99% of beams are braced with joists top bearing/ top flush thus leaving the bottom of the beam a little more free and more prone to cross grain tension issues?