Predicting Panel Mass SS - Lookover

[John Greenlaw]

Hello again everyone!

Can someone confirm that I am doing this calculation correctly? I have used lexan as a half template and found a piece of lexan "in the square" to calculate the area.

Density = Mass of lexan square / LWH of lexan Square
Density = 3.156kg / 86.2cm x 52cm x 0.59cm = .00119kg/cm3

Area = Mass of template / density x H of template
Area = 0.523kg / .00119 x 0.591cm = 743.6

Panel Volume = Target Thickness of Panel (#2 Honduran Mahogany) x Area
Panel Volume = .312cm x 743.6 =232cm3

Panel Mass = wood density x panel volume
Panel Mass = .00119 x 232cm3 = .276kg or 276grm

[Jim watts]

John, you lost me on your area calculation. Pause.... .00119 is in Kg/cm correct? Now that I've considered the area calculation, that's an interesting way to do it.

[Trevor Gore]

Your panel mass is about double what I'd expect. I think you may have omitted a factor of 2 (remember you're using a half template).

[John Greenlaw]

Hi Trevor!

I think I have the predicted panel mass formulas worked out. The predicted masses in my table are half of what is in Table 4.5-3 but maybe that is because those masses are for a full panel and my calculations are for a half panel? Also, I noticed the density is slightly off in my table compared to Table 4.5-3; you were mentioning that to someone else before that the values will be slightly different due to excel?

[Trevor Gore]

The answers can be slightly different due to Excel, but it's most likely that slight differences are due to rounding errors. If you hand transfer data from an elastic constants spreadsheet to a panel mass spreadsheet it is unlikely that you will be transferring all 15 significant figures that Excel works with, so some rounding errors will ensue. Likewise, if you transfer my spreadsheet data from the book back into a spreadsheet, the book only has a few significant figures, certainly not 15, so there will be some rounding error. However, 2* 80 is 160grm, whereas I had 155grm. I would normally expect to be closer than that, e.g I could put 156 vs 155 down to rounding, but 5 in 155 is over 3%, which is stretching things.

Try using the top area that I used. See footnote 33, p 4-62, Design.