# Calculating correct size joist to support a column

Im working on some plans which will be checked by an engineer but would like to do a decent job to begin with and would like to educate myself more.

I have a loft with an LVL beam resting on a (4) 2x6 column and would like to figure out the size of the supporting joist to add under that. I've played around with calculation for section modulus, support reaction etc but missing the part on how to translate that to which size timber I would want to use and looking for resources/help on this.

As seen in diagram the loft has also a mid support beam so my column will support something like 59"x56" at lets say 40psf live load going into column yes? I think max deflection on IRC2018 for floor is L/360. My floor joists are 2x10 24"o.c. w center blocking with a clear span of 126".

How would I take let's say a 2 ply LVL 1.75x9.25 (specs) and see if that worked?

I'd love to learn the calculations within reason (within the scope of simple wood framing like this with a single point load). I've looked for videos etc but not finding the right thing - either too simple (continuous load) or too complicated and not covering how to apply to timber selection. Willing to buy a book/course if there is something good.

Thank you!

• This looks like a work assignment or a homework problem, not a DIY problem.
– JACK
Commented Apr 22, 2023 at 22:36
• The support you'd add under that would be another beam, even if there were joists parallel to it. Commented Apr 23, 2023 at 2:51
• Beam, joist.... yes - it will be a multi ply or fat LVL/PSL. @JACK - no its as much DIY as you can be - DIY engineering :). This is my house plans but while i have hired an engineer (actually 3... lets not go into that) the current one is shit, am out of time and this is the final piece. He claims I need a 5 -ply 2x10 which I find had to believe for this small section of loft- after he made a dozen mistakes I called him out on I dont trust it. Ultimately this is checked by my code examiner but want to do my homework. Commented Apr 23, 2023 at 4:32
• Can you pour concrete at that point and put in a short column that will take the load directly to the ground?
– Mark
Commented Apr 23, 2023 at 15:12