Steel equivalent to 4"x4"?

[Eric DeSilva]

I need to install some poles off my back porch to enable me to string up a sun sail. The sail manufacturer recommends using 4x4s, but that seems thick and ugly to me, so I was thinking about steel square stock or steel tubing. I had hoped to find some kind of equivalency table on the web, but no such luck. I'd rather go with something of a smaller OD and thicker wall than something thin walled with a thick OD.

Anyone care to venture a guess as to what might be comparable in strength to a 4x4? Square or round better?

[Don Alexander]

i have no idea on the answer to your question

however seems like a funny question to ask on a woodworking forum

then again maybe its just my sense of humor

[Matt Meiser]

I don't know either, but can you buy steel that warped and twisted that will shoot out water when you drill into it?

I assume you contacted the manufacturer and they can't/won't give a steel recommendation?

[Charlie Reals]

Uhh can't you paint or stain the 2x4's so they aint UGLY? They do make metal studs but they aint purty either.

[Cliff Rohrabacher]

I used 40' 19-kips 8" wide-flange steel I beam when I built my shop/garage.
Never regretted it. It's so much stronger than wood.
But for out of doors you would have to accept the fact of rust. Even indoors there's rust always present.

Ya might be better off with the wood. Besides it's easier to attach things to wood. With steel ya gotta drill and bolt.

[Jim Rimmer]

Check this link. It's the Steel Stud Mfrs website. Lots of technical info; I didn't read it all but maybe something can help you out. http://www.ssma.com/default.aspx

[Ed Hazel]

I would think 1 inch or 1 1/4 square tubing 1/8 thick would work.

[Matt Ellis]

depends on the load that it has to support, and the height of the column. you have to realize that a long, slender column like you're proposing has much, much less capacity than a slighter larger section that has a larger slenderness ratio.

it also depends on how the ends of the column will be restrained. need more info.

[Eric DeSilva]

@Don--yeah, that is why I posted it in Off-Topic. I know there are some folks here who do structural stuff, which is why I thought they might know offhand.

@MattM--I did end up calling. Mfr said "oh, don't use 4x4s, you need 5x5s, and if you are going with steel, use 4" diameter 8 gauge. Gulp. That seems... Overkill. I'm now going with the assumption they are simply trying to engage in overengineering to avoid liability. I don't live in an hurricane region, and I think I'll simply take it down off season or if weather is really bad.

@Charlie, Jim--I've seen data on steel studs v. 2x4s, but this isn't really the same thing. The steel studs have no structural integrity when they aren't attached to drywall, so those won't work. I was really looking at using something like 2x2 14 ga square steel tubing, which is a vastly different creature.

@MattE--I'm not sure I understand your "slenderness ratio" discussion--you mean large OD, thinner wall is better than thin OD, thick wall? That was part of what I was hoping to see, some kind of deflection table comparing those. I see your point about needing more info, and I'm gradually concluding the mfr's recommendations are BS based on exactly that. The mfr doesn't say "use a 3x3 up for up to an 8' pole, 4x4 for up to 10' and 5x5 for 12'--it is just "4x4" The basic idea is that one end is planted in the ground or bolted to a support, and the other end has an eye into which is hooked--more or less--the corner of a tarp pulled taut. It needs to be able to resist bending/breaking if the wind kicks up and blows the tarp up, but I can't for the life of me figure out how to calculate what that means.

The more I think about this, the more I'm thinking I'll just see if I can find a cheap source of the type of galvanized pole they use for basketball installs... Either that or use 1/8" wall 2x2 square steel tubing, which strikes me as serious overkill.

[Matt Ellis]

link to the manufacturer's website? if i can see what your talking about, i may be able to help you out.

if you're primarily worried about uplift loading, which would put the steel section in tension, i'd think nearly any section would work. standard, structural grade steel has a minimum yield point of 36,000 psi. that means you could put 36,000 lbs of tension on a 1 inx1 in bar before you got any permanent yielding/deformation. usually, you'd design around 24,000 psi as a safety factor.

-matt

[Don Alexander]

its always amazing to me how much info on widely varied subjects is available on this site , but it never surprises me when my sense of humor isn't shared by everyone/anyone else

on the topic though, don't know if this matters in your situation, but it might be worth noting that wood beams will hold your building up longer in a fire than steel ones, takes longer to burn through the wood enough to collapse than it does to get the steel hot enough to bend

[Eric DeSilva]

link to the manufacturer's website? if i can see what your talking about, i may be able to help you out.

if you're primarily worried about uplift loading, which would put the steel section in tension, i'd think nearly any section would work. standard, structural grade steel has a minimum yield point of 36,000 psi. that means you could put 36,000 lbs of tension on a 1 inx1 in bar before you got any permanent yielding/deformation. usually, you'd design around 24,000 psi as a safety factor.

-matt

That sounds like you are thinking of something more complicated. This sun sail is really just a big triangle. Think 3 more or less vertical poles sunk in concrete. At the top, you have some eye you clip (and tension) a corner of the sun sail--the sail is 12' x 12' x 12'. In order to shed water, in all likelihood two of the three poles are going to be slightly lower so there is a down tilt and rain will run off. That said, it is a sail in some sense, so in a wind, seems to me it is going to want to pull the various poles towards the centerpoint of the three. I'm thinking it is a simple lever, with the fulcum at the point the steel exits the concrete...

The problem with doing real math here is that I have no idea what kind of forces you would expect the wind to generate--maybe a couple hundred at most? Unfortunately, that is a couple hundred applied to one end of an 8' long lever.

[Matt Ellis]

if you're in arlington, the basic wind speed that is designed around for your area is about 90 mph. without going through all the math, a 90 mph wind is going to give you a loading of about 15-20 psf on the frontal area of a non- building structure. uplift forces would be similar.

hope that helps.

[Lee Koepke]

I am surprised the sail manufacturer doesnt have a recommendation for steel columns for you to use. Actually I think that tube steel supports would be a better support structure anyway. (not an engineer or designer). The forces acting on each of those supports will be very eccentric loads, and I would venture to say, no conventional load tables would provide you with accurate results. Load tables are generally horizontal or vertical point load type calculations, with more complicated moment calcs.

I worked on a much larger version of your sail for a commercial application and remember the reaction loads we had to compensate for in the footings, but we are talking very large spans. Maybe look up a similar manufacturer and see what THEY recommend.

[Chris Kennedy]

If I had to guess, your sail is a Coolaroo. I installed a square (actually, hyperbolic) a couple of years back, and I remember the instructions well. My family is Australian, and the instructions are really meant for an Aussie. When they talk about steel posts, they are basically assuming you can go to the local industrial park (for my family -- out Gordon Road in Mandurah, Western Australia) and you will find someone who has and can install such a thing. It is assumed that you can get such a thing, and honestly in the States, I don't think you can. This isn't a BORG product.

Good luck (and when you get it in, you'll love it).

Cheers,

Chris