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Which is stronger, a round sign post or a square-cross-section sign post? I will be using one or the other to hold the top corner of a shade sail. I tried an aluminum pipe but a strong wind on the sail bent the pipe. I am about to buy a 10-foot-long sign post because they look stronger than the pipe. I found online a 2 by 2 inch square-cross-section post with a lot of holes in it. The steel is 14 gauge (0.0781 in) thick. I also found a 2 inch diameter hollow cylindrical post with no holes in it. The steel is 16 gauge (.065 in) thick. Which one should I buy?

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If you install the square post in a way that its face is perpendicular to the cord it has to support it will be stronger. It has a larger 2nd moment of the area (I) that way which makes it stiffer.

I would also try to figure out the prevailing wind and position the sail in a way it is not going to get a lift out of that angle and also the cord is tight to keep the sail flat. Make sure you minimize the sail's flutter by pulling it tight.

The sail, like an airfoil, gets a lift when it is laying parallel or near parallel to the wind. I would position it leaning into the wind so the wind will push it down as opposed to lifting it up.

Edit

I correct my error after a comment by aleph zero. the second moment of area, I, of a square section is constant regardless of the angle of the square.

$$I_u=\frac{I_x+I_y}{2} + \frac{I_x-I_y}{2} \cos{2\phi} -I_{xy} \sin{2\phi} \\ I_v = \frac{I_x+I_y}{2} - \frac{I_x-I_y}{2} \cos{2\phi} +I_{xy} \sin{2\varphi} \\ I_{uv} = \frac{I_x-I_y}{2} \sin{2\phi} +I_{xy} \cos{2\varphi} $$

In this cas the above simplifies to $$I_u=\frac{I_x+I_y}{2}=I_{xx}=I_{yy}$$ So the parallel orientation is equally as suitable as any, except it's easier to install and easier to find fasteners for. The rest of my answer is still valid.

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  • $\begingroup$ (1) "Stronger" and "stiffer" are measuring two different things. (2) A square post (like a circular one) actually has equal 2nd moments of area about any axis, and since the corners are further from the center than the middle of each side, it is stronger (but not stiffer) if it is oriented with the rope pulling across the diagonal of the section. $\endgroup$ – alephzero Nov 29 '20 at 21:35
  • $\begingroup$ @alephzero, I was getting worried about you, long time no comments on my posts, considering the Covid and all. My apologies you are right the (I ) is equal for the orientation. but still, the square is better than the pipe. As to 'stiff', I use the terminology I deem the most effective within the context. I understand the difference between the 2. I still recommend the parallel configuration for a variety of reasons: ease of finding fasteners for the cord, ease of setting the post, with 45 degrees requiring extra work. I'm glad you checked in tho. $\endgroup$ – kamran Nov 30 '20 at 3:10
  • $\begingroup$ Thanks, both of you. Please clarify one point. Did you consider the factor that the square post I am considering has holes along the entire length of all four surfaces? The holes appear to be about .33 inches in diameter and about .33 inches apart (judging by my measurements of a photograph). $\endgroup$ – David Dec 2 '20 at 22:14
  • $\begingroup$ Yes I did. the holes are designed not to decrease the strength of the post. $\endgroup$ – kamran Dec 2 '20 at 23:04
  • $\begingroup$ Thanks again. Purchased and installed. The project looks like it will stand up to a gale force wind. Your advice was very helpful. $\endgroup$ – David Dec 14 '20 at 23:31

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