Trestle table leg design - strength calculation?

[Wakahisa Shinta]

I recently made a table top out of doug fir 8/4 x 6" x 9-12' boards. Final dimensions for each board is 1.75" x 5.75" x 7'. Table top final dimension is roughly 32" x 80". Picture below:
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The left over pieces are 1.75" x 5.75" x 36" - 48". I can use them to make the legs for the table, but not sure about the strength needed for such a top. I like simple, Shaker style, so minimalism comes to mind. This is what I have in mind. The feet and top cleats: 28" length, 5" at the highest point, tapering to 3" at the ends. The vertical risers are 1.75" x 5.75" x whatever length to make final table height of ~29". M/T joinery.

Of course there is a stretcher using approximately same dimension lumber.

I estimate the top to weigh about 90 - 100 lbs.

Are the leg dimensions sufficient in strength to hold up this top? Should the vertical risers be wider (i.e. glueing two 1.75" x 5.75" pieces together)? How do you calculate?

[Brian Tymchak]

Interesting question - so I googled around and found this link on Capacity of Wood Columns (https://courses.cit.cornell.edu/arch264/calculators/example7.1/index.html)

It seems that a dimensional 2x4 (Doug Fir) can handle up to 636 lbs in compression, if I interpret that web page correctly. Since you are likely to have at least 2 supports, I think you are going to be more than fine with your leftovers, unless of course you plan to park a car on your table... It will boil down to aesthetics and what you desire.

[Mike Nolan]

It is better than that. The capacity depends on length. 636 lb is for a 8.5 foot 2 x 4. For a 3.5 foot 2 x 4 the limit is 3400 lb. Go ahead and park your car on the table.
Actually the failure mode is someone sitting on the side of the table. I think a 2 x 4 is strong enough but you must make your joints strong enough.

[Jerry Miner]

The weight-bearing capacity of the legs will not be the weak point in this table (as shown above)--- what will matter more, IMHO, is the joinery design and execution. You will need to overcome racking forces in long dimension (center stretcher with shouldered, wedged through-tenons would be the classic solution.) and short dimension.

Doug Fir is a softwood (pretty hard as softwoods go, but...). A 5.75 inch wide trestle column on a doug fir foot may not remain rigid over time due to compression of the wood fibers. If it were me, I would double-up the trestle columns to 11" or so:

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