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caducus
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# Posted: 30 Mar 2018 08:39pm - Edited by: caducus
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I'm trying to find load tables for beams/girders, but all I can find are tables on decks.
Trying to figure out if triple 2x12s can span 6' for a cabin with a loft, no snow load.
Can anyone point me to a table?
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ICC
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# Posted: 30 Mar 2018 09:44pm - Edited by: ICC
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The IRC has tables that can be used. Link here...follow the link forwards; go to chapter 6, walls. Scroll down to Table 602.7.1. (Lots of good info before you get to table 602.7.1.) Table 602.7.1 is for exterior bearing walls... the values are close enough for under the floor girders. Err on the side of being too large a beam rather than cutting corners. There are column sections for various snow loads, 30 psf ground is the general minimum though 20 psf minimum is customary in no snow southern areas. That covers winds and heavy rains then. There are also columns for various building widths - wider the building the greater the load on the side walls. Those width values may be extrapolated for non listed widths. Just be careful with doing the math correctly.
There are also sub-sections of the table for different loading. Err on the conservative side and for a cabin with loft, 2 beams, one down each side, use the section for roof, ceiling and two clear span floors. Roof, ceiling and one clear span floor could be used if you are certain the loft loads will be nothing more than a mattress and simple bed frame, minimal other furniture. When trying to do your own estimates in place of hiring an engineer I figure I'm still money ahead to go for the extra materials when choosing a heavier load scenario than being cheap.
The IRC has a wealth of info but it is not a how to do it book. It's just the facts, methods that have been proven over the years. Interpreting the info can be intimidating to some. Also READ the footnotes!! Exceptions are often listed and may require upsizing of structural members in some circumstances.
The beginning of that chapter has great info on nail sizes and quantity per joint. That info is quite specific too on nail shank diameter. If you use a nail gun most of those nails are = to box nails, not common nails.
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caducus
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# Posted: 31 Mar 2018 02:32pm
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That's super helpful, thanks.
Any idea how is psf calculated in these tables? It's easy for me to use the table, knowing my snow load, or lack thereof, but I'm just curious how they arrive at a number like 30psf when a small cabin can easily weigh 60 psf.
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ICC
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# Posted: 31 Mar 2018 03:28pm
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Quoting: caducus I'm just curious how they arrive at a number like 30psf when a small cabin can easily weigh 60 psf.
Not sure exactly what you mean --- the 30 psf is snow which comes and goes --- that doesn't have anything to do with the weight of the cabin or whatever.
The columns headered with Ground Snow Loads (PSF) of 30, 50, 70 are the typical maximum snow loads found on the ground at any one time at the geographic location in question. They are derived from historical data. Here in the west the ground snow load can vary hugely by just being of the other side of a ridge only a few miles away.
Structure weight is not mentioned directly, but will be derived from the norms of 40 psf for the LL for living spaces with a reduced 30 psf LL used for sleeping only areas, plus 10-to 15 psf for structure materials (DL).
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DaveBell
Moderator
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# Posted: 31 Mar 2018 06:22pm
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ICC, is there a section that covers Ridge Beams?
I've looked at Ch 8&9.
I was planning on 3 ea. 2x12's spanning 12'.
Snow load 30.
Thanks,
Dave
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ICC
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# Posted: 31 Mar 2018 08:33pm
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The IRC leaves ridge beams to the engineers. As well as the strength and stiffness of the ridge beam the supports have to be designed. Height affects the dimensions of the columns; more height means the column has to have a larger cross-section or other structural members to resist bowing when under the vertical loads. Plus a shallow pitch, less than 3/12 I think needs to provide sufficient uplift resistance as winds can make a shallow pitch act like an airfoil. Not to mention the foundation which needs to have the capability to support the ridge beam column loads. I think there are too many variables that might need consideration to get into prescriptive tables for a ridge beam.
You could try using a beam calculator like the one here. Use the simple beam, uniform load calculator. You need to separately reference the species of wood to obtain some values to plug into the various fields, plus estimate loads. Then adjust things to get a pass in each column; bending, deflection and shear. Then use one of the column calculators to size the supports and carry that down to how big a footing is needed for the type of soil. Many variables.
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caducus
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# Posted: 1 Apr 2018 05:24pm
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"Structure weight is not mentioned directly, but will be derived from the norms of 40 psf for the LL for living spaces with a reduced 30 psf LL used for sleeping only areas, plus 10-to 15 psf for structure materials (DL)."
That's what I'm talking about. I'm curious where 30-40 psf comes from when a building can weigh twice that. Just curious how those numbers are derived.
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ICC
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# Posted: 1 Apr 2018 06:26pm
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Ah! I mispoke, misunderstood. The 40 PSF or 30 PSF is "Live Load"; LL; and that is just what you put into rooms plus the people. The tables, chairs, stuff in the drawers and so on. That is not going to cover special things like metal turning lathes, big gun safes, etc. When considering the floor joist spans all you are interested in is the LL plus the DL (dead load being the weight of the floor joists, subfloor, carpet, whatever).
Usually, the only time anyone thinks about the total structure weight is if they have to tear it down and haul it to the dump where they charge by the ton, or a house mover or foundation contractor who may have to lift the house for a new foundation, or maybe the odd curious person. 
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