# Aluminum square tubing capacity question

I have an awning that I am attaching to the corner of the house. I needed to extend it so I got a piece of 6063 square aluminum tubing. It's 1 inch wide with .125 wall thickness. It has about 2 feet sticking out. I'm wondering if this will be strong enough. I originally had some Home Depot .0625 thickness made out of God knows what as far as alloy. It broke after about a week or two when it got windy. I'm wondering about how many lbs stress this new material will handle. It's 6063 but not sure what temper it is but I believe it is tempered.its a high quality tube from a specialty shop that seems to specialize in aluminum tubing

• Guaranteed the wind will show you the next weakest point. Could be the fixings or the facade of the wall... just wait. Oct 6 '20 at 16:29

If we scale the picture the screw attaching the rod to the facia is at 1/10 the length of the rod. So it imparts 10 times the force of the rod to the siding.

By eyeballing the fabric it is 8-10 square. Meaning a garden variety gust wind can generate roughly $$15 \cdot 64= 1000/4 =250\text{ lbf}$$ uplift at each corner.

$$250 \cdot 10= 2500\text{ lbf}$$ on the screw going through the wall siding. The force is going to tear off the siding.

We need to use a stronger member and more substantial connections and reduce the moment arm on the rod.

• The metal is attached with two 3 inch deck screws into solid wood and there is zero movement in that area, I'm not concerned about that part failing at all. When it failed before the metal broke in two. The new piece is much stronger, like more than twice as strong. It feels about three times heavier but I didn't weigh it. Are you saying the rod will have 250 lbs of stress on it? The fabric is very light and cheap if it gets that much stress the fabric is guaranteed to fail the first time that happens. Oct 7 '20 at 0:35
• @Jack M. I don't know what wind zone you are but in major parts of US gusts of 70 miles per hour is not unexpected. Yes i think the weak link is the deck screws. Oct 7 '20 at 2:13
• @kamran if you get some time, could you please elaborate where did 15 and 64 came from? I suspect it has to do with the wind loading, and the area but I can't be sure. I also could not figure out why 15*64=1000/4. I'm sure you have a perfectly good reasoning, so this is me just trying to learn something. Oct 7 '20 at 11:48
• @NMech, wind load as per ASCE 10, and many similar codes have many options of design depending on the geometry and height, and location of the buildings. A frequently used code is ASCE 7-10 Sections 27.1.5 & 28.4.4: Wind Loads for MWFRS in an enclosed or partially enclosed building shall not be less than: § 16 psf (ultimate or ~10 psf ASD) for walls § 8 psf (ultimate or ~5 psf ASD) for roofs. But after seeing what happend to some awnings I designed I go with 15-20 psf to take into account the wiplash. Yes I assumed 64 sqf are and 15lbf wind uplift. Oct 7 '20 at 17:50
• so its 15*64=960 lbf, which you approximate to 1000lbf and then divide by 4 for each corner of the fabric? Oct 7 '20 at 18:09

I am just going to go with the procedure here, and the formulas. Then I'm going to give you a numeric example.

The maximum Force that a cantilever beam can support without yielding is given by ( I could go through the derivation but I don't think its relevant):

$$F= \frac{2 \sigma_{allow} I}{b \cdot L}$$

where:

• $$\sigma_{allow}$$ : allowable stress
• $$I$$: Moment of inertia of Square beam $$I=\frac{1}{12}\left(w^4- (w-2t)^4\right)$$
• $$b$$: breadth (and height) of square beam
• $$t$$: Thickness of square beam
• $$L$$: the distance of the application of the force to the support of the beam.

Given:

• $$\sigma_{allow}=10[ksi]$$ (a conservative estimate for aluminium)
• $$b=1 [in]$$
• $$t = 0.125 [in]$$
• $$L = 2[ft]$$:

You should find that:

• $$I = 0.057[in^4]$$
• $$F_{max} = 47.5[lbf] (=211[N])$$

I'm more comfortable with N, so if in doubt check the conversion.

My suggestions are the following:

• Go for something beefier. One inch will not be adequate. Just the weight of the tent might be straining it too much. (But I can't be sure from the picture)
• Select steel preferably, if it gets windy where you are. This (whimsy) structure will oscillate, and it will fatigue the aluminium. The aluminium has notoriously poor fatigue properties. You can check by looking at the failure surface in your previous beam. If the failure surface is shinny, then it was fatigue.
• I have no idea what those letters and equal signs combined with numbers mean. it might as well be written by a martian I would understand that just as well. I'm just wondering approximately how many pounds of stress the new piece will handle. Oct 7 '20 at 0:38
• Stress isn't measured in pounds... But he says that 47.5lbf of force could be applied right there in the answer. Oct 7 '20 at 7:21

The question is "how much wind force is developed here". Depends on size and design of awning and wind conditions. An awning is similar to an aircraft wing which develops a high force. It would be stronger if the bar were attached with a brace over it rather than a couple screws through it ; the holes made stress concentrators/weak points. The next iteration ,if needed, could be a side rail from a ladder. Or; look at that ladder to see how something is made relatively strong with aluminum

I'm not an engineer by any stretch of anyone's imagination, but I have 30 years in aluminum extrusion. In the conditions you have, I would agree with going to steel. You probably have a 6063 T6 tube. With the amount of force generated by wind, you'll have enough cold-work in the aluminum to make it brittle, then it'll simply snap. If you're dead-set on aluminum, try to find some 6061 alloy tempered to T6 (you'll get 10 ksi more yield strength) with a wall around 0.200 or so. If you could do a drawn round tube in T8 or T9, it would have double-duty as an elephant beater. But also, yeah find a better way to connect it to the structure. I never realized how much power was in the wind, until I tried my hand at kite surfing, holy cow, it's like tying yourself to a semi-truck.