(Fall Marching) The Towers

Duae Turres

(The Two Towers)

This show theme was not about the Lord of the Rings but more of a Yin and Yang concept. Below is the concept sketch I was given. 

Design Criteria:

1. The towers need to be 30 feet tall with the sides having asymmetrical blocks which have abstract art graphic panels placed in each block which would be attached with cord or wire ties. 

2. The towers will be transported in a 12 foot box truck.

The director thought PVC pipe would be the material of choice to build the towers. Well as it turned out that this project became an exercise in compromise.

Click on the images to enlarge.

Concept Sketch

This was my original layout drawing to determine the angle of the taper and pipe size which turned out to be 3 degrees and 2" diameter pipe. This is where I realized PVC would not work.  With a 3 degree taper there would be no way to put in the cross pieces with standard 90 degree PVC pipe fittings because they would be angled down and the cross piece would not fit without bowing the pipe. It also would be so heavy and have so many fittings that it would probably not stand up to the stress of lifting the prop upright without breaking.

Back to the drawing board! After a trip to Home Depot I discovered EMT Thinwall Conduit. It was cheap, very light and strong and seemed the perfect solution. Best of all smaller pipe sizes fit perfectly inside the next larger size to make telescoping structures. To make the structure as strong as possible I laid out a design with diagonal bracing after being inspired by the design of a cell phone tower I see driving to work.

Here is a closeup of the base drawing. The tower is 3 sided. There was no point in making a tower with 4 sides because the audience will only ever see two. Saves weight and material. The drawing is messy but I understood it and it gave me the dimensions I needed to to build the towers. 

I had twelve 2X12X8' lumber to use to build the bases. I located the 3 vertical poles 1.5" from the edge of the base so I can drill and run 4" lag bolts into the base and through the tubes to secure them to the base. The triangle shaped base is a double thickness of wood so the bases are 3" thick. and weighs about 150 pounds and they are bolted together with carriage bolts. Marching band props need HEAVY bases to keep the prop steady during moderate breezes outside! Alway keep this in mind when designing yours... heavy base...light prop.

I mounted the base in a wood fixture made from 2X4's that is level and plumb and pulled a perpendicular level line out from it to establish the horizontal plane for the rear tubes. The line is attached to the base on a line between the mount holes for the rear tubes of the tower. The rear tubes are perpendicular to the base. The front tube is pitched toward the rear tubes at 3 degrees. The rear tubes are pitched 3 degrees toward each other. It is hard to see in this picture but the length of each vertical tube is 29' 6".

The two additional wooden supports hold the ends of each 10 foot section. The bottom tower section uses 1" tube, the center is 3/4" and the top is 1/2" tube. The bottom tube passes through the base and is secured to the base with lag bolts. The center section tube is inserted 3" inside the 1st tube.  The top was the same as it was inserted 3" inside the 2nd section. I drilled the pipe and inserted a 1/4" bolt 3 " down from the top of the tube to act as a stop. Then drilled 1.5" down for another 1/4" bolt that actually secured the tubes together.

From the drawing I knew the spacing of the rear tubes at the top and marked the wood support that width on the centerline. Then it was as simple as placing the tubes in position and eyeballed them to make sure they are straight from the top to the base. They were spaced about 6.5' at the base and a little over 3' at the top. I secured the tubes in place by screwing a drywall screw on either side of the pipe to hold it to the wood supports.

The drawing gave me the dimension to the top tube at the top of the tower so measuring up from the bottom support I added another cross brace (the one closest to you in the picture). Then I pulled another line by pulling the line through the hole I drilled in the base for the front vertical support to the centerline of the wood brace at the top of the tower. Then I placed the cross braces for the two center sections to the line. Then placed the tubes for the front vertical and secured to the wood braces with drywall screws. 
Now it was time to install the tower triangle bracing.  I marked the locations for the triangle bracing on the vertical tubing 12" above and below the two middle sections, 6" down from the top and 24" up from the base.  The braces are 2 pieces. The brace attached to the front tube is one piece and extends from the front tube to the two rear tubes and a separate tube across the back of the rear tubes. Now is a good time to tell you that all of the bracing is attached using 3/16" steel pop rivets. 2 rivets at each attach point. The method we used to fabricate the  rear brace was to measure the width and cut the tube a little long and then flatten the ends of the tube using a hammer and anvil (old school style). Install it by drill and rivet it to the inside of the rear vertical tubes. To make the brace from the front to rear tubes we cut a length of tube longer than the length of the 2 sides and flatten the tube in the center. Then take a 6" C clamp and clamp it in the center of the flat. Now bend each side 45 degrees to make a 90 degree bend but at the apex you will have a flat that is easy to drill and rivet to the inside of the tube! Nice huh?  Then place the flat you just made against the inside of the front vertical and mark where to hammer the flats at the rear verticals. Just repeat the process top to bottom riveting a horizontal brace at the top, center and bottom of each section. FYI all of the bracing is 1/2" EMT top to bottom. The excess flat past the rivets was trimmed with a sawzall and hammered down around the vertical tubes so it was tight against them. Click on the pictures to enlarge them to see this detail.

Now fab and install the diagonal cross X braces. On each section just make an X from the inside of the top brace to the inside of the bottom brace for each section. Make sure the intersection of the X falls on the center triangle brace. Flatten the tubes at the intersection and the ends and rivet to the inside of the vertical tubes. Do NOT flatten the tube of the triangle brace at the cross of the X brace. Remember to ALWAYS attach flattened tubes to a round tube for strength. And make the flats as short as possible. Note: The triangle braces hold the SHAPE of the tower. The X bracing gives the tower rigidity! For maximum rigidity the ends of the X bracing MUST be attached to the vertical tubes. Do NOT attach to the triangle bracing except at the intersection of the X. It is more work to attach to the vertical tubes but it is critical to the strength of the tower. Click on the pictures here and below to enlarge them to see this detail. The closeup picture of the 2 bases bolted together (below) shows clearly how the bracing is oriented.
So how strong is this design??? I laid the top section (which is all 1/2" tube) on the ground  and placed a 2X4 under the top and bottom triangle brace (8 feet apart) and a 250 lb man was able to sit on the section in the center of the front vertical tube and the tower section did not sag at all! I wish I had taken a picture of that!

There is my guard girl posing by the nearly completed tower. But the main reason for showing you this is to look at the sag in the vertical tubes of the top section (closest to you). I am pretty sure this happened just from the weight of the triangle braces and I did not notice it before we installed the X bracing. The diagonal X brace locked the sag in place. I fixed it later by standing the top section upright and drilling out the rivets on the X brace until the verticals released and straightened out and then re drilling and riveting the diagonals. The center and bottom sections did not have this problem since they were larger diameter tubes.

Here is the band director admiring our work!

Here is the guard girl again marking out a craft paper template to send to the company that will make the vinyl panel that will be inside the towers.

Here are the bases bolted together. Since the base of each tower has only 2 wheels,  bolting them together makes the bases easier to transport. 4 wheels make it stable to move 300 pounds on and off the prop truck. Also note the third wheel 2/3's the way up the tower. It is attached to a 8" wide piece of 1/8" thick piece of aluminum which was made to quickly detach from the tower with clevis pins but we found we did not need to detach the wheel when lifting the tower and the audience could not see it.

A close up of the bases bolted together. See the shiny heads of the 12" long bolts that hold the 2 bases together? The wheels are attached to a 2X6 which is secured to the base with six carriage bolts and braced with diagonal 2X4's.  This put the fulcrum above the base to make lifting the tower easier. You will also notice the 2X12 blocks bolted to the bottom of each base at the corners. This keeps the ends of the bolts sticking through the bottom of the base off the ground. The mechanism at the bottom corners was something I came up with to level the tower if we had a field that was not level. As it turned out we did not use them so a 12X12 block at each corner would have sufficed but we set these towers at mid field where it is level.

Here is the graphic vinyl panel. It is 10' x 40 feet. It was designed to mimic fire.

This was the first full dress performance at a home football game. We did not have time to install the panel inside the tower so we just wrapped it!!! I show this as a demonstration that you can have a graphic panel printed many ways and  you can wrap this tower design and make it look any way you want.

Wrapping the inside of the tower was pretty simple. We first cut 9 pieces of 1/2" pipe and trimmed it so it ran the length of the tower from the bottom to the top horizontal with the butt ends aligned with the joints of the 3 sections. The pipe does not reach the horizontal pipe and that is on purpose. we want the panel to look like it is suspended inside the structure. Note the pipe location at the back corner sticking up. Next we had to attach the front center pipe to the panel before installing it. I snapped a center line down the length of the panel and positioned the center pipes under the panel on the chalk line and riveted the pipe in place with 1/8" pop rivets spaced 8" apart. We then screwed the pipes in place at the front horizontal taking care not to force the pipe into position causing wrinkles in the panel. For reference this is the top of the tower with the center vertical at the bottom left.

The next step was to lay the pipe on the panel and pull it taught to the outside corner and screw the pipe down to any diagonal or horizontal brace it touched. Again it comes up 2" short of touching the vertical tube. After the tubing is secured the vinyl panel was pop riveted to the tube. then the process was repeated for the other side.

It was great working in the shade under the newly completed stadium bleachers!

Here it is just wrapped. The wrinkle at the bottom is there because I have not trimmed the panel yet. We also had to mark and cut the panel so the sections could be separated. If possible always try to install panel inserts as one piece and then divide it. If we had cut the panel first and installed them separately there would definitely be misalignment and gaps. We did not paint the tower structure. The zinc coating was the color the director wanted.

Here are both 33 foot towers and bases packed in a 12 foot box truck! Note the 3rd wheel at the back of the truck. The tower sections nested nicely inside each other! Where did I get 33 feet in height? The top section had 6" of pipe extending above the panel. To get the look of the sketch we attached lengths of 3/4" pipe to the stabs. 2 foot lengths at the rear and a 3 foot length on the front vertical to give that sort of a crown look to the top of the tower.

Here is a video of the Marching show Duae Turres. If you watch to the end you will see the prop crew dropping the towers and rolling them off the field.

In Retrospect

Ok if I had to do this again I would have bought a welder and built it from EMT. I would have built it to the design concept which would be possible by welding the horizontal cross braces. Welded squares are not nearly as strong as diagonal braces but it  would have been strong enough. I would have known that 1/2" conduit would sag and added an additional wood brace at the mid point of the top section to eliminate the sag during construction. I would still have diagonal bracing in the rear where it would not have been seen. The towers would have had a totally different look. Welding gives you much more flexibility in design.
We did not have any mechanical issues with the towers through the season. They were assembled and broken down around 20 times and by the end they were still mechanically sound.