![]() ![]() typical floor joists, rafters and built up beams take advantage of that. ![]() There is a 15% repetitive member bending strength increase for 3 or more members spaced 24" or closer together. I can help with simple things but I'll let you know if we are getting beyond my knowledge. I'm just a country carpenter, a little long in the tooth, which just means I've seen a little and learned a little along the way. It helps tailor answers or tells me to ask you questions. ? Tell us a little about yourself and what you're up to, I don't know if I'm talking to a school student or a bona fide NASA engineer. What are we up to here ponderosae, are you building something in particular, learning. The supplement of base design values is also on that tab listing major lumber species. In their you wil find the adjustment factors, for instance the repetitive member increase in base design bending Fb of 15%, duration of load adjustments, etc. You'll also find read only versions of the NDS there which is the reference for wood construction. under the publications tab on the awc website you can download DA6, standard beam equations. You can usually backcheck the output longhand to see where the problem is creeping in. If something is not making sense it is usually a GIGO problem. The equations behind beam calcs is generally the same for similar load conditions. I don't know the specifics of how their calcs are working, I've backchecked mine a time or two against the awc one and we seem to be on the same page. So it seems odd that four 2x12s spaced at zero inches apart can only span 16 feet for the ridge beam, according to the table for that (when the last calculator says they can span 46 feet when spaced one inch apart and handle the same load). It looks like the big difference with this last calculator was all about the spacing, because when I plug in a 20 foot span with 12 inch spacing it corresponds to the first calculator at 50psf live and 10psf dead loads, to recommend a 2x12 board as well (or larger when the load increases with that spacing). Lateral Support: at supports and point loads Table 17: 50 psf Ground Snow Load, 10 psf Dead Load the compression edge of the beam must be laterally supported at intervals of 24" or less. ![]() applicable to members used under dry-service contitions. All tables are based on uniformly distributed loads only. With a minimum bearing length of 0.71 in. So I'd expect multiple member roof ridges to allow for a greater span than floor joists, yet the opposite seems to be listed:ĪWC Maximum Span Calculator for Wood Joists and Rafters And looking at a calculator for just beams by themselves (not necessarily in a floor or a roof), when spacing them only one inch apart it vastly increases their load capacity. Looking at Southern pine, for example, it seems odd to me that spans for floor joists would be greater than spans for ridge beams, when the distance between boards is much greater in floor joists (given that multiple boards are used for ridge beams). ![]()
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