Keletica's Dialectic
Weighty Subject Print E-mail
Written by Gayle Knapp   
Friday, 05 June 2009 02:08

I'd been lucky until this year to be in outdoor shows that allowed staking into the ground. Now, I have a show this summer that requires 50 lbs of weight PER LEG on my canopy. So, I went on the web to see what others have done. I found pricey weights and bags at commercial sites that wouldn't easily get me to that 50 lbs/leg. I found "jury-rigged" ideas like cement blocks, jugs of water and sandbags. But, again the volume of 50 lbs of any of these would be unwieldy or downright too cumbersome. Then, I found the suggestion of using cement-filled tubes with eye-screws that secure to the tent legs using rope tied to the "rafters" and velcro around the lower legs. But, no details about how to do this were given--particularly to guarantee the weight (one of my potential venues has city inspectors signing off on the set-up). So that y'all don't have to rediscover my wheel, here's what the procedure is for getting your desired weight. My example detailed below will yield 25+-lb 4-inch PVC tubes. For the mathematically inclined I've included the formulas for easily redesigning this setup for different weights.

I grabbed my calculator and searched the web for essential information. Concrete has a density of 145 lb/cubic foot; the inside diameter of 4-inch (Schedule 40) PVC pipe is 4.026 inches or 0.34 feet; the quoted weight of the 4-inch (Sch.40) PVC tubing is 2.01 lb/ft (however, my empirical measurement gave 2.2 lb/ft).

So, put on your analytical thinking cap and let's DO THE MATH.

To achieve a desired weight tube, you need to calculate the volume you'll need using the density of concrete. You find the volume of cement required for the desired weight by solving the ratio:

145 lb/ft3 = 25 lb/V
or, rearranging and cancelling units
V = 25/145 ft3 = 0.172 ft3

The formula for the volume (V) of a cylinder is

V = pi*r2*h
where pi = 3.1416
r  = radius of the inside of the PVC pipe = 2.013 inches or 0.168 ft
h  = height of the pipe (unknown to be solved for here)

Filling in the knowns gives

0.172 = 3.1416*(0.168)2*h
h = 0.172/0.0884 = 1.95 ft

ERGO, a 2-ft length of 4-inch PVC pipe filled to the top with concrete should weigh 29 lbs (don't forget the pipe contributes significantly).

Now, put on your hard hat.

 

First, a shopping trip to Sprinkler Supply (4-inch PVC pipe) and Lowe's (everything else) or your personal favorite hardware store. Here's my shopping list for enough supplies to make 50 lb/leg:

16 feet (minimum) of 4-inch Schedule 40 PVC pipe (my store sold 20-foot lengths)
8 caps for 4-inch PVC pipes (**see note below)
1 can PVC primer
1 can PVC cement (and instruction sheet)
3 80-lb bags Quikrete concrete mix, ready to use
8 eye-bolts including nuts (5/16 inch diameter and at least 3.5 inches long)
60 feet (minimum) clothesline or rope
coarse sandpaper

**NOTE

  • a. Not all stores carry the caps that fit the Schedule 40 PVC pipes.  More often, they carry 4-inch PVC caps designed to fit over drain pipe. These caps do not fit over schedule 40 PVC pipes.  However, their outside diameter is exactly the same as the outside diameter of the pipes.  There is a MAJOR price difference between the drain-pipe caps and schedule 40 PVC-pipe caps. The drain-pipe cap is 10 times cheaper. The instructions below are for making the anchor using these cheaper drain-pipe caps.
  • Although 4-inch (O.D.) drain pipe is available, it is NOT PVC. It is HDPE which is a totally different polymer and the PVC cement would not chemically react to form adhesive bonding.
  • Also, the drain-pipe caps are flat whereas the PVC-pipe caps are domed. Using the drain-pipe caps provided another advantage--the capped pipes stand erect by themselves.


Let's get down and dirty. Making the canopy anchors requires the following steps:

1. Cut the pipe into 2-foot lengths. Using coarse sandpaper, sand one end flat and relatively smooth, removing all burs. Also, sand the lip of the drain pipe cap until it is flat--this will increase the surface area for bonding the cap to the pipe. weight_prep2_c

2. Verify that the flattened end of a drain-pipe cap matches the sanded end of a 2-foot length of PVC pipe without gaps. Rotate the two pieces to find the best fit, sanding again if necessary.  Follow the PVC primer/cement instructions to prime and then cement the flattened cap to the end of the 2-foot length of pipe. These two pieces do not need to seal perfectly, but they should form a solid bond.
weight_prep3_c
weight_prep1_c3. Put the nut about 1/4 to 1/2 inch from the bottom of the bolt. Using heavy duty pliers, pinch the threads below the nut so that the nut can no longer thread off. Including the nut on the bolt will make it less likely that eye-bolt might pull out of the concrete.

4. Prepare the concrete following the instructions on the bag and fill the tubes with enough concrete to achieve the final desired weight. I did mine to a final weight of 27.5 lbs and checked each tube as it was filled on my bathroom scale. IMPORTANT: You must tamp the concrete firmly several times during the filling process to insure that no air is trapped.

weight_prep5_cweight_prep7_c5. Once each tube is filled, center an eye-bolt in the top, pressing it in until only the eye is above concrete level.

6. Set the tubes in a dry place where they can cure undisturbed for at least 5 days.

7. If desired, you can trim the excess PVC pipe. My recommendation is to trim to just ABOVE the level of the eye-bolt.

weight_prep8_c8. At a local Dollar Store I found a set of eight plastic coasters with a diameter that perfectly matched the I.D. of the PVC tubes. A small slit in the middle allowed one to slip over the eye-bolt and camouflage the cement top.


9. Clothesline (8-foot lengths) tied to the eye-bolts finished the anchors.

The anchors will be tied to the canopy "rafters" above each leg using a taut-line hitch knot. This knot allows slack in the rope to be taken out but doesn't slip.