LRFD Wood Design

[Perception]

I am a faculty member that teaches wood design as part of an advanced structural design course.

In the past I've primarily taught ASD with a short introduction to LRFD wood design. My understanding has been LRFD wood design is not commonly used in practice given the extra factors and with the reference design values being in terms of stress already. I'm curious, is this still the case? For those that do regular wood design, how often are you utilizing LRFD vs ASD?

 

[dik]

I think foundation design was the last to go SI...

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

-Dik

 

[Perception]

This is valid for Allowable Stress Design vs. the newer Allowable Strength Design used in steel design...but it's fundamentally incorrect for ASD (either stress or strength) compared to LRFD. LRFD is a completely different statistical model for comparing demand and capacity.

I agree LRFD is fundamentally different with the statistical modeling, but I feel like they could have made that change and still kept everything in terms of stress. I wonder what the motivation was at the time to move to everything in terms of strength.

 

[phamENG]

I wonder what the motivation was at the time to move to everything in terms of strength.

Because we got over our fear of partial plastification? As an industry, we came to accept that M[sub]y[/sub] is not the point at which a laterally stable, compact section fails. It doesn't fail until we've formed a failure mechanism through a plastic hinge or hinges. This turns the calculated stress into something of a moving target because of the simplifying assumption that strain about F[sub]y[/sub] becomes infinite. So by defining a bending moment capacity, we define a point beyond initial yielding that we're okay with the beam getting to at ultimate loading. And as I mentioned above, the stress at that point varies based on the shape factor.

The result is a more materially efficient design. To compare allowable stress vs allowable strength, let's look at a W21x93 in bending. (F[sub]y[/sub]=50ksi, S[sub]x[/sub]=192in[sup]3[/sup], Z[sub]x[/sub]=221in[sup]3[/sup] - yes, to anyone not in the US, our S's and Z's are backwards)

Stress: 0.66F[sub]y[/sub]=33ksi => M[sub]allowable[/sub]=33*192=6336k-in.
Strength: M[sub]n[/sub]/Omega=50*221/1.67=6617k-in

That's a 4% increase in bending strength for that beam. Not a lot for one beam, but it can add up on larger buildings. If the Empire State Building benefited from that, it would save about 2,400 tons of steel (about $1.6M in material alone at today's hot rolled prices).

 

[XR250]

If the Empire State Building benefited from that, it would save about 2,400 tons of steel (about $1.6M in material alone at today's hot rolled prices).

Possibly - if strength was controlling and not serviceability. Also, you could argue the WTC may have collapsed quicker if it was designed using LRFD (maybe it was - IDK)

 

[driftLimiter]

What about approach examples similar to AISC. They solve all examples in ASD and LRFD in tandem. I think if teachers start implementing this approach for wood design the LRFD wont be so scary to future engineers. If we can get this part of the industry to switch over to LRFD then we can finally be done with ASD for good.

Other than the manufactured components publishing LRFD design values, LRFD design for wood is as easy as adding one more adjustment factor.

I wonder what the motivation was at the time to move to everything in terms of strength.

Strength Limits become quite handy for interaction checks. It is also a more effective means to include instability into the member design and unity check. I also appreciate that is based off a statistical analysis that includes all relevant parameters and their variability.

Another thing to consider is that it takes away the subjective 'Allowable' strength question. It is based off failure(ultimate strength) which is simple to understand for any member, then the statistics go running from that point.

 

[phamENG]

I think another reason ASD has held on so strongly is the wide variability in material strength. Our current allowable stresses are based on 5 percentile sample strengths and then get a safety factor applied to them. (One reason that wood structures that "should fail" stand for 150 years - 95% of the lumber out there is stronger than we say it is.) And for wood, since there is no yielding and we need to stay firmly planted in the elastic region, keeping it all stress based is a sensible way to go.

we can finally be done with ASD for good

What about foundations? Around here, geotechs only provide service level allowable bearing pressures.

 

[driftLimiter]

I think another reason ASD has held on so strongly is the wide variability in material strength

Good point.

What about foundations?

Ya I guess the jury is still out on this one.

Still though it would be nice to have only one set of unified load combinations.