# Forum Home Renovation Metalwork & Welding  Steel strength

## slepax

Hello, 
I'm not sure this is purely metalwork related but this is a question about steel  :Smilie:  
I plan on putting up a sail-shade and after reading about recommended post dimensions figured out 100x100x4 is the way to go. However, to make the end job nicer, using 75x75 posts would fit much better by replacing existing pool fence posts. Now obviously 100x100 should be stronger than 75x75 so I was thinking to up the gauge, but would it have the same durability? 
The thickets 75x75 I could find is 6mm gauge. Trying to compare them on OneSteel website I've notice the mass for 100/4 is 11.6kg/m and the 75/6 is 12kg/m. Does that mean the 75 would be stronger? Denser? I'm worried about the post flexing due to high winds or weight of water when it rains. 
Many thanks!

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## David.Elliott

Shade Sail? Should not water go through?

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## ringtail

Shade sail posts should be installed on a 5 ish degree angle too. But the original question of strength. No, not necessarily. Wall thickness is one thing but a column is a column. They are meant to be used in compression. That's not to say they can't be used in other applications and that's where the engineers come in. Maybe heavy gauge 75 nb tube might suit better. Depends on wind load, sail size, footing depth etc....

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## CraigandKate

Gee your really thinking about this one! 
Simplistically, just looking at the bending moment (assuming that is the highest load in this application) you lose strength by reducing the cross sectional area (100mm down to 75mm) you can basically compare the two using half the beam width divided by the second moment of area (just looking at one axis) which gives 0.021156 for the 100x100x4 and 0.028323 for the 75x75x6. In this case the smaller number is better, so effectively the 100x100x4 is roughly 30% stronger. 
However I assume that in this case the load is going to be perpendicular to the fence? How about some 75x125x4mm that will give you a better second moment of area for the loaded axis but still fit in your fence positions.

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## Moondog55

OR
Use the 75mm SHS and weld on a length of 50 * 6 flat bar in the direction of pull??
It's how they make strong trailer drawbars using less material I believe

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## Marc

What you can not hide, make it a feature. Sail post should be at an angle so make it something unrelated to the fence, use round post at an angle and away from the fence?

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## Moondog55

Related to the strength of the pole is the depth of the hole and big posts need deep holes

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## PlatypusGardens

Round posts seem to be most common for sails...

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## paddyjoy

> Gee your really thinking about this one! 
> Simplistically, just looking at the bending moment (assuming that is the highest load in this application) you lose strength by reducing the cross sectional area (100mm down to 75mm) you can basically compare the two using half the beam width divided by the second moment of area (just looking at one axis) which gives 0.021156 for the 100x100x4 and 0.028323 for the 75x75x6. In this case the smaller number is better, so effectively the 100x100x4 is roughly 30% stronger. 
> However I assume that in this case the load is going to be perpendicular to the fence? How about some 75x125x4mm that will give you a better second moment of area for the loaded axis but still fit in your fence positions.

  Comparing the moment of inertia between a 75x75x6(*1.16 million mm**4)* and a 100x100x4(*2.23 million mm**4)*, wouldn't the 100x100x4 be 92% stronger?  75 x 75 x 6 Steel Data  100 x 100 x 4 Steel Data

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## Marc

Not really, here is a deflection calculator for beams supported at both ends Stress and Deflections in Beams

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## Moondog55

Isn't a post a vertical cantilever tho??
Craig&Kate?

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## Marc

cantilever beam calculator ...  Cantilever Beams

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## paddyjoy

> cantilever beam calculator ...  Cantilever Beams

  Based on that the 75x75x6 will deflect twice as much as the 100x100x4

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