Yet another fuckup

The nice thing about swerving out of one’s lane once in a while is the resultant fuckups.  Sitting in a ditch waiting for a tow truck sucks on one hand, on the other hand it does break up the monotonous daily drudgery.

In the last couple posts I was going on breathlessly about calculating critical column loads.  In other words how much load can you put on a column before it fucks off on you and your porch falls down.  What I forgot to take into account is that sizes, pipe sizes in this case, listed in standards have tolerances.  Wall thickness for A53 pipe can vary by as much as -12.4%.  That means that wall thickness of a pipe can be more than 12% below nominal size.  That’s over 11.9%!  The practical implication is that there’s less steel to hold up the load.  This is something that one does need to take into account.  Now to be fair, that sort of thing does get lost in the factor of safety, but none the less.  This sort of thing is important.  The devil as they say is in your pants.  Or details.  I can’t remember.  Whatever.

So here’s the table of my calculated critical loads on nominal pipe parameters, critical loads on pipe parameters taking into consideration the maximum allowed variance,   As a Saturday night treat there’s also a column of maximum allowed loads as specified in HSS LRFD Column Load Tables by the Steel Tube Institute of North America.  Everything from that document has been reproduced here without explicit permission.  Let’s break down some of the acronyms

• HSS – Hollow Structural Sections – translation – metal with a hole in the middle
• LRFD – Load and Resistance Factor Design – how much stuff you can put onto the end of a metal with hole in the middle.

We’re doing a bit of hand waving here.  The pipe we’re using is not structural tube, but in the aforementioned guide they cover round tube (i.e. pipe) that’s the exact dimensions of NPS pipe.  The main difference is their material yields start at 45ks+ where as I’m ASSUMING that ASTM A53 pipe is uses standard ASTM A36 steel which is 25ksi.  In the end their maximum permitted loads are less than my calculations up to 3″ pipe.  So I’m erring on the safe side and using the smaller of the two numbers,  It’s all good.  I’m doing this to my own house.

Columns below:

• Critical load, lb, C1 – my calculations using nominal standard sizes
• Critical load, lb, C2 – my calculations taking into account tolerances
• STI – Steel Tube Institute allowed loads

There is significant difference between C1 (nominal parameters), C2 (minimum parameter values based on maximum allowed deviations), and STI.  Overall though I’m relatively happy with my numbers.  They are definitely in the “ball park”.  I will obviously use the loads from the STI document for my project(s).  The Internet is not sized to handle the kinds of hubris that would ensue if I tried to claim the superiority of my numbers over those produced by a collection of fine folks doing this sort of thing for a living.