I was going to buy some 4x4 timber for veranda posts. However I have some steel posts 65x65x2mm.
I wonder how would one calculate what size steel post can be substituted for the timber post?

  • So a 2-1/2 square what is the wall thickness?
    – Ed Beal
    Jun 18, 2020 at 23:05
  • 2mm is what I have, but I am more concerned with what does it need to be to match in strength a timber post.
    – anm767
    Jun 18, 2020 at 23:11

1 Answer 1


There are many variables with the steel supports 1) type of steel, 2) grade of steel, etc. and with the wood supports: 1) species, 2) grade, etc.

I’ll use A36 structural steel and SPF No.1 grade wood for comparison.

A 2 1/2” x 2 1/2” x 8’ high square structural tubing can support about 28 kips (28,000 lbs.) in axial (vertical) load.

A 4x4x8’ high wood post can support about 9 kips of axial load.

If you’re thinking of a different type of loading, please advise.

  • Is there a calculator or a chart for this? What is the vertical load for 2x2 for example. Can't be asking you for every size I might come across.
    – anm767
    Jun 19, 2020 at 4:10
  • I use the “AISC Manual for Steel Construction”. It’s expensive, but maybe there’s a free limited on line use. Just know that the steel column is going to be about 3 times stronger than wood. I’d guess that the connections are going to be more important than gaining an extra 500 lbs. of allowable load.
    – Lee Sam
    Jun 19, 2020 at 4:22
  • Could you tell me the vertical load for a 2x2" by 8' please? 2.5 mm steel.
    – anm767
    Jun 19, 2020 at 10:27
  • My books do not go that small. Probably because it’s tendency to buckle. Remember, this is for axial load only. If the connector put any torsion (twist) on the column, it will buckle and fail. If your columns bolt onto the side of something, that loading will cause buckling because it’s not axial loading...it creates twist.
    – Lee Sam
    Jun 19, 2020 at 16:01

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