Load Duration Factors for Engineered Lumber (LVL)

[TPStructural]

Can the NDS load duration factors be used when designing engineered lumber (LVLs)? For instance 1.15 increase in allowable moment capacity with snow loads etc.?

 

[phamENG]

The short answer is yes. If you're using NDS, look at NDS 2015 chapter 8. That tells you how to adjust the reference design values for LVLs and other Structural Composite Lumber materials.

This is for use with the reference design stress values provided by the manufacturer - NOT the load tables. The load tables are usually pre-adjusted based on a set of reasonable assumptions.

 

[SteelPE]

This should be an easy question but it isn't. I just run BC Calc and let it figure all of that out.

According to this design literature the answer is yes:

https://p.widencdn.net/jrrpdq

Under the design values... note 2 reads:

2. This value is based upon a load duration of 100% and may be adjusted for other load durations.

 

[phamENG]

Hmm...I'm not a fan of the way they present their load allowable stress tables. The "Allowable Stress Values" table header is misleading. It should be "Reference Design Values" so the engineer can us NDS procedures to determine either the Allowable Stress (aka Adjusted Reference Design Value) or the LRFD capacity.

 

[SteelPE]

phamENG.

I can't say that I have ever actually used these tables and charts. As I said, I use their program to determine wood requirements (it's free and quite effective). I have always thought the load span tables were more for the contractor to figure out beam sizes when an engineer is not involved.

Also, is LRFD design in wood really a thing? I can't say that I have ever witnessed any calculations being performed in wood using LRFD criteria.

 

[jayrod12]

Maybe not LRFD, but up here in the frozen north all materials are designed using Limit States Design.

 

[phamENG]

SteelPE - I don't pay much attention to the capacity tables or the load tables, but the stress tables are important. I tend to use Weyerhauser's ForteWEB (Microlam LVLs) (I'm pretty sure I spelled all of those wrong) when I need it. Though I usually pick one or two beams and run hand calcs to verify. The printouts always have that pesky "results should be verified by a registered design professional" notice on them - so I usually verify at least one or two.

And yes, LRFD is a thing. I have not used it, nor do I know anyone who has. The biggest reason for me is that I haven't taken the time to understand what it is and how it's different. The ASD stuff makes simple sense, so I'll stick with it until I either have to change or I have enough time to figure out why (or when) LRFD is better.

 

[XR250]

@pham,

My feeling is that if I want to make less money, I will switch to limit based design (especially in wood)