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After considerable discussion on CN and endless thought on alternatives, over several years, I am now leaning more and more towards a post and plywood clad pier 12' high. No, more complex, arrangement would offer the almost guaranteed stiffness and structural support for my massive 200lb mounting plus OTA(s) so far above the ground. Nor could anything else be made for so little cost in simple materials and work. Concrete foundations and cast or block built piers are not without potential problems due to the very high mass placed so far above the ground. The compound pendulum effect is not unknown in amateur circles.The thermal storage of so heavy a block and pier warmed by sunshine should not be forgotten if so exposed.
A round, conical pier might be more cosmetically attractive, if it could ever be seen below the covering platform. With plans to box in the platform sides for storage or shelter the pier is very unlikely to be visible. Nor would it suffer from wind loads once so enclosed. The small amount of extra space provide around a conical pier would not be worth the considerable extra work in making such a tall, round and tapering object. Not to mention the careful cutting of the the essential, flexible plywood to clad it. Nor would a circular footprint be so large and stable as the much simpler, square, truncated pyramid. BTW 'Truncated' simply means it has its top chopped off.
Four 4"x4" corner posts, resting on buried, adjustable height, concrete carport anchors, would rise from a 4' square base to a 2' square top. Standard [metric] 4'x8' exterior plywood sheets would reach to, or slightly above, the planned, 8' high, observing platform surface. Four sheets would be needed as a minimum covering. With one sheet applied vertically to each face. This requires only a pair of slanting cuts to each long edge of each sheet before screwing it to the corner posts and horizontal bulkheads.
Each line can be scribed on the back of the sheet to exactly match the sloping posts or previously applied plywood face. The portable circular saw and a guide batten will make short work of the two straight cuts. There is probably not an hour's work in the main pier cladding. The pier could be clad while vertical or even lying on the ground.
Given the weight of four 3/4" sheets and the necessary weight of the heavy corner posts, bulkheads and cross braces a vertical cladding of the already mounted framework makes most sense. Te plywood cladding sheets need only be propped up on suitable blocks or bricks off the ground to the required height.
Each line can be scribed on the back of the sheet to exactly match the sloping posts or previously applied plywood face. The portable circular saw and a guide batten will make short work of the two straight cuts. There is probably not an hour's work in the main pier cladding. The pier could be clad while vertical or even lying on the ground.
Given the weight of four 3/4" sheets and the necessary weight of the heavy corner posts, bulkheads and cross braces a vertical cladding of the already mounted framework makes most sense. Te plywood cladding sheets need only be propped up on suitable blocks or bricks off the ground to the required height.
Extra plywood would, of course, be required for the top section of the pier. More would be needed to make perforated bulkheads at vertical intervals internally to further resist twist (torque) effects. Timber cross-braces, also placed at intervals, would support these heavy plywood bulkheads.
The concrete anchors could rest on a thin layer of sand on top of large and thick paving slabs to avoid sinking into the rammed, or plate-vibrated sand or gravel base. The space beneath the entire platform would then be back-filled with bulk sand and gravel around the anchors. The nearest sand and gravel merchant sometimes uses his huge bucket loader for local deliveries in confined spaces.
Vertical tilt adjustment of the pier to vertical would be managed by the threaded, galvanized steelwork provided by the concrete anchors. A plumb line from the central top of the pier could be matched to cross strings stretched across the base. The plumbline could even remain in place for an instant check of perpendicularity of the tower to monitor any changes. Central cut-outs in the bulkheads would allow clearance for the plumbline and electric cables. Or even provide bearing surfaces for a central, rise and fall, subsidiary pier.
Vertical tilt adjustment of the pier to vertical would be managed by the threaded, galvanized steelwork provided by the concrete anchors. A plumb line from the central top of the pier could be matched to cross strings stretched across the base. The plumbline could even remain in place for an instant check of perpendicularity of the tower to monitor any changes. Central cut-outs in the bulkheads would allow clearance for the plumbline and electric cables. Or even provide bearing surfaces for a central, rise and fall, subsidiary pier.
The main pier would pierce the observing platform at 8'. With the usual isolation gap provided, to avoid footfalls being directly conducted to the mounting and telescope. The concrete anchor supports for the platform would be arranged to provide the maximum distance between themselves and the central pier supports.
The top of the pier would probably employ multiple, horizontal layers of thick plywood to provide a firm resting place for the heavy mounting's, 3/4" [20mm] aluminium base plate. Vertical, galvanized studs would anchor and provide fine tilt adjustment to the mounting base plate. Fine adjustment in azimuth [horizontal rotation] to point the mounting squarely at the Pole Star will have to be provided at the mounting's own base plate level.
If the pier should prove, against all expectation, to be sensitive to tipping forces then a reinforced plywood bulkhead near ground level could be loaded with paving slabs. Though, unfortunately, this ploy would greatly increase the ground pressure on the pier's concrete anchors. It might be necessary to increase the load bearing area to reduce possible sinking problems. Just allowing the loaded plywood plate to rest on sand would probably suffice.Though this might well shortcut the adjustability of the ground anchors. Or even short circuit the deliberate gaps placed between the pier and platform anchors buried in the sand. Sand can carry vibrations.
There is the possibility of providing a rise and fall subsidiary [top] pier to allow for different mounting heights to suit different OTAs. A car screw jack could manage the lift-and-lower effort over a rather limited range of about a foot.
The adjustable height central pier could be formed from a square plywood 'pipe.' With heavy timber, internal reinforcement it could use upper, main pier bulkheads as sliding bearing surfaces. Perhaps with PTFE/Teflon pads to keep friction low without binding. External clamping around the pier 'pipe' to lock it securely in place is possible, once the required height has been reached. Vibration from flexibility must not be allowed to undo the design benefits of the massively proportioned pier!
Alternative lifting and lowering devices are possible for a much taller sub-pier carried inside the bulkheads. An externally mounted chain hoist, for example, could lift and lower the smaller central pier via the hook and chain carried down inside the main pier to the inside bottom of the sub pier via suitable holes made for the purpose. Or a boat trailer winch might manage the task. A straight, central pull would seem optimum to avoid binding causing friction and wear.
The sheer size of the lower, main pier offers a very useful volume for potential storage. Provided, of course, that any cutouts for access doors, do not weaken the plywood cladding. Its "stressed skin" qualities must not be compromised by very large cut-outs nearing the edge of the cladding sheets. Though this will not be remotely necessary given the near 4' width down at the base. Waterproofing of the main pier could be easily achieved with an outer layer of pond liner. 'Flashing' with the same material could also be arranged at the pier/platform gap.
Soil has a limited ability to resist pressure. Its so-called soil bearing capacity. I found a figure of 2.5kg/cm^2 online for small buildings with point foundations much like my planned pier. Let's assume that each cast concrete anchor has a base of 20cm squared. That's 400 square cm. Or 400 cm^2 each. Multiply by four and you have the total surface area which will support the entire pier and everything later mounted upon it. 4 x 400 = 1600 cm^2. Divide 1600/2.5 and you have the total load bearing capacity = 640kg. Now we need the weight of the pier itself.
Soil has a limited ability to resist pressure. Its so-called soil bearing capacity. I found a figure of 2.5kg/cm^2 online for small buildings with point foundations much like my planned pier. Let's assume that each cast concrete anchor has a base of 20cm squared. That's 400 square cm. Or 400 cm^2 each. Multiply by four and you have the total surface area which will support the entire pier and everything later mounted upon it. 4 x 400 = 1600 cm^2. Divide 1600/2.5 and you have the total load bearing capacity = 640kg. Now we need the weight of the pier itself.
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