Interpolate and Extrapolate

[wwhitney]

You won't find a general statement because it's not there.

OK.

Not all tables in the ICC codes allow interpolation. Where interpolation is allowed, the code says so at that table. If nothing in a table or in the code section pertaining to that table says interpolation is allowed -- then interpolation is not allowed.

This seems to contradict the earlier quote--you say there's nothing in the IRC that says "interpolation is only permitted when explicitly allowed," then you go on to say "interpolation is only permitted when explicitly allowed." So what is the basis for the latter statement?

Cheers, Wayne

 

[steveray]

No one going to touch that the interpolation only applies to the soil loads? Because that is where the footnote lives.....

 

[socaldano]

We know that in the IRC tables, we could interpolate to find a fitting value, but we cannot extrapolate because that is projecting. The AWC DCA-6 Manual, one of my favorites, specifically tells you that you can interpolate but you cannot extrapolate their tables and charts.

Does the IRC also make that clear?

I studied this, and for a plain english you can INCREASE the size or reduce the distance. I can use a 2x8 instead of a 2x6, or

We know that in the IRC tables, we could interpolate to find a fitting value, but we cannot extrapolate because that is projecting. The AWC DCA-6 Manual, one of my favorites, specifically tells you that you can interpolate but you cannot extrapolate their tables and charts.

Does the IRC also make that clear?

I believe they are saying that if your values are in between two values on the table, you can calculate between those two values. but if your values are outside the table, you cannot logically extend the values. You would need to get a professional engineer to make the calculations. For instance, if the snow load is 80, you cannot extrapolate from 50 and 70. But you can interpolate the value of 40 or 45 snow load from 30 and 50. But you cannot do that < 30 or >70. So you would have to use snow load of 30 for <30 or get an engineer to calculate the loads.