I need to create above ground, mobile "footings" for metal poles to hold sail cloth. I was thinking of a cardboard tubing, filled with concrete to hold a sleeve to insert the pole into that will hold a metal pole. the pole will hold a sail cloth. I am thinking of a 3-4' high, 16" wide concrete tube form. will this be sufficient to hold the poles for sail cloth?
It will be very hard to give guidance as to if a particular size of base will be adequate for your application. Here are some of the variables to consider:
- Length of the metal pipe that holds the sail cloth. The longer (and presumed higher) this pole the more lateral leverage it can apply to the base.
- Span of the sail cloth overhead. The longer span you have for the sail cloth the more tension it will place on the metal poles and in turn on the base.
- Placing the base into the ground. I know that you desire a mobile application but be aware that putting the base assembly into the ground will give it a lot more stability that just letting it sit at ground level. It would also look a lot better than having a huge block of concrete sitting there. I can easily imagine a buried base that would only project enough from the ground level so that the dirt and debris does not easily get into the slide in hole for the metal poles.
Do also consider that even at the size you asked about that these bases would not be so "mobile". Concrete weighs approximately 150 pounds per cubic foot. A 16" (i.e. 1.333 feet) diameter concrete pillar 4 feet high is going to be:
V = 3.1415 x 0.666 feet x 0.666 feet x 4 feet
V = 5.57 cubic feet
Weight = 5.57 x 150 pounds
Weight = 836 pounds
That does not seem particularly mobile to me.
If you're building something that is hard to tip over, it needs to be wide and heavy. Your poles unfortunately have a huge lever arm compared to the base. The weight of your base multiplied by its radius needs to be more than the tension in the sail cloth multiplied by the height of the pole. If you have a 6' pole and a 16" wide base, the leverage works against you by a factor of 9 (because 6 * 12 / 8 = 9). I.e. you'd need at least 9 times more weight than tension!
Instead I think you'd be better with a simple pole and guy-ropes tied to weights. If the guy-ropes were really long, and the weights had a friction co-efficient of 1, the weights would need to be at least as heavy as the tension to avoid sliding. As the guys get shorter and steeper, the weights need to be heavier. At 45 degrees, about 150% of the tension. At 60 degrees, 200%.
Then your next problem is estimating how much tension the wind will cause. This is nearly impossible as the wind varies so much. I suggest estimating the force required to rip a corner off the sail cloth, as this is the most force that the sail can exert on the pole.