Flat Use Factor ...

Good stuff.

When we lay the 4 x 6 flat, our Cfu goes from 1.0 to 1.05. (You can see that in NDS Table 4A.)

What that means is that our unit strength, or design value, increases ... our `psi' value. (The strength of the material increases by 5%.)

BUT, the strength of the member, however, decreases.

Let's check ...

The reference resistance or reference capacity, for bending, is M = Fb S ... (it comes from the equation f = M/S and setting f = Fb).So, (and for the time being NOT showing the other adjustment factors, which are equal in the two orientations) ...

... for the 4 x 6 on edge ...

S = bh^2/6 = 3.5 (5.5)^2/6 = 17.65 in.^3M = 2540 psi x 17.65 in.^3 = 44,820 lb-in. = 3735 lb-ft.

... for the 4 x 6 on the flat ...

S = 5.5 in. (3.5 in.)^2/6 = 11.23 in.^3M = 2540 psi (1.05) x 11.23 in.^3 = 2667 psi x 11.23 in.^3 = 29,950 lb-in. = 2496 lb-ft.

So,... by laying the thing flat, the unit strength of the material went up (2667 psi vs. 2540 psi), but the capacity of the member went (way) down (29,950 lb-in. vs 44,820 lb-in.).

BABY!

NDS Commentary on Adjustment Factors ...

NDS Commentary on Adjustment Factors ... here.

Note: the form factor, Cf, is no longer used.

Sink your teeth into this one ...

Wood Column Design Using the 2005 NDS LRFD Provisions ...

http://www.associatedcontent.com/article/2031526/wood_column_design_using_the_2005_nds.html?cat=15

While the equations for design check and Column Stability Factor for axial loading of a wood column are the same for both ASD and LRFD in the 2005 Dual Format NDS, the `numbers’ certainly are not.

Dual ASD and LRFD Beam Stability Calc using 2005 Dual Format NDS

Here is a calc of the Beam Stability Factor in both ASD and LRFD for a wood beam using the 2005 Dual Format NDS. Interestingly, the formula for CL is the same in both formats, but the `ingredients' are different, and so thus the end numbers.

Enjoy: (and note - this is a draft - comments welcome): http://www.associatedcontent.com/article/2021852/beam_stability_factor_using_2005_dual.html?cat=15

Another one ...

Owner: "Does the fact that it is standing presently have any merit?

Consultant: "The structure is empty and it's summer. Put a couple layers of sandbags on the roof - to represent the design snow load plus overload, and a couple of layers of sandbags on the main floor system - to represent occupancy plus overload, and then we can talk. But I don't want to be inside while you're doing it."

Quote of the week ...

"Jeff, on your plans it specs a 5-1/8 x 16.5 glulam. We put in a 4 x 12. Is that OK?"

ESR 1539 / NER 272

Here is a link to ESR 1539 ... replaces NER 272.

This document is awesome. It expands on fastener sizes, requirements, etc., diaphragm and shear wall information ... and, what is really cool, is that it gives illustrations of, for example, the fasteners / connections spec'd in IBC Table 2304.9.1.

Enjoy: http://http//icc-es.org/reports/pdf_files/ICC-ES/ESR-1539.pdf

So, when a Contractor asks about using one fastener over another (that I have spec'd) ... I simply say, " ... well, what does NER 272 (now ESR 1539) say?" "Oh, you don't have a copy? ... my goodness! ... what's your email address? Here."