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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 corner posts 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 by 2.5 and you have the
total load bearing capacity = 640kg. Now we need the weight of the pier
itself.
Checking online shows 3/4" plywood weighs about 70lbs per 4'x8' sheet. I'll convert the imperial figures to metric later. Let's ignore the slanting off-cuts for the pier's taper and add it as cladding material to the top of the pier. Our four sheets of 3/4 plywood = 280 lbs alone.
Our four corner posts are to be 4" x 4" at 3 lbs per running foot. Lets say our corner posts alone = 4 x 12' = 48' x 3 lbs = 144 lbs.
This is just for the bare geometric structure without any cross bracing or any stiffening plywood bulkheads. Cross braces have to be multiplied by 12 to = lbs per foot over four sides. Let's keep it very simple and have 4', 3' and 2' cross braces per side and round it up to 10' per side x 12 for the total weight in pounds for four sides That's another 120 lbs.
Our simple frame and cladding is already up to 120 + 144 + 280 = 544 lbs. I think we can all agree that a single person cannot possibly lift such a heavy structure up to the vertical and then place it precisely in the footing's metal shoes. So the pier will have to be built in pieces off the footings.
The framework could be safely built down on the ground. The cross bracing cut, fitted and drilled then disassembled again. Two framework "sides" could be built on the ground then set up to lean against each other just prior to being joined by cross bracing at the top. Only once all the bracing is added can the plywood cladding go on.
Each bare framework "side" will weigh [2 x 12] 24' + 10' x 3 = 103 lbs each minimum plus fixing bolts/screws. Probably just manageable for a fit person to handle alone. But rather risky of flopping over during erection or badly disturbing the anchors. The actual method of construction needs rather more thought!
BTW: 544 lbs is very roughly 250kg so the load on the soil below the concrete footings is well within tolerance for the bare pier. Until, of course, we add the weight of the massive mounting, counterweights and OTA. The top of the pier will also need to be heavily reinforced to accept the mounting. Add at least another 20lbs.
A conservative 200 + 100 + 50 + 20 = 370 lbs + 544lbs = 920lbs = ~450 kg.
The safety margin is shrinking rapidly! I did mention using heavy paving slabs under the carport anchors. These would provide a firmer footing than plain anchor blocks resting on the soil, gravel or sand.The anchor blocks themselves have considerable weight and should be included in the ground pressure calculations.
To reduce the risk of sinking, the sides of the pier could be supported on four more concrete anchors at the base side mid points. With a sturdy cross brace and 3/4" ply cladding these footings could take a decent proportion of the overall load for a greater margin of stability. Moreover, they will lie at the edge of the footprint where they will do most good. Internal plywood cladding could reinforce the edges of the pier considerably.
A more central support carrying much of the total ground load could easily cause unwanted rocking. We don't want a fraction of a millimeter movement up at the mounting height. A millimeter movement at the base would mean 3mm movement at the top of the pier. Slightly more if the mounting has greater height above the pier top plate. This demands total solidity without any flexure in the pier. The slightest sinking, vibration or tilt would throw the mounting's polar alignment and levels completely out. Making Goto commands a complete lottery.
You can't measure precision wormwheel movements with a length of string. So the pier base must be as solid as if resting on solid bed rock or a concrete slab. Large concrete slabs on packed sand may be the answer but only slabs of the paving variety. The soil under the pier is garden soil lying over soft clay. Load spreading will be vital to success. It's a shame I can't lay some of those thick steel plates they use on building sites to support mobile cranes and heavy vehicles. In their absence I shall just have to spread the ground loads as best I can with readily available and affordable materials.
60 x 60cm [2'x2'] paving slabs offer 3600 cm^2 each of surface area. That's 9x the surface area of the anchor footing x four. [Or eight if I double the number of anchors supporting the pier] Naturally these slabs would rest on well compacted sand. With more sand as a thin sandwich to prevent local loads from the anchors cracking the deliberately thick paving slabs. Loads applied to soil spread out at an angle. So the deeper soil and clay "enjoy" much lower local loads thanks to the successively greater area expanding with increasing depth.
I could further increase the number of paving labs by using four under each anchor footing. With an intermediate, large slab, load spreading to the lower slabs via compliant packing at their centers. The upper slab ideally wants to be rather larger to ensure its corners lie well over the centers of the four lower slabs. This will avoid tilting forces due to off-center loading by the compliant packing between the two layers of slabs. Compliance here helps to avoid cracking which would immediately undo the desired [even] load spreading.
A single, cast concrete slab in the ground would have to be quite considerable in area to support a 4'x4' pier. It is arguable that pouring four large, concrete footings would be ideal but I have no desire to get involved in hand mixing concrete in freezing mid winter. Though I have done so in the past for other projects where the ground was suitable for digging holes and the weather much warmer.
The main problem is that the ground below the platform and pier forms a lower terrace and is already 2' lower than the surrounding area. So pouring concrete would require the entire area was brought up to a suitable height first and then the footings dug in whatever bulking material was chosen. Hence my interest in using 2' tall, tapered, concrete, carport anchors. The anchors and their supporting slabs, would be buried in 2' of well compacted sand only once the pier and platform are safely in place.
There is no access for a concrete mixer lorry nearer than a hundred yards away. Running to-and-fro over such distances with heavy barrows of liquid concrete is strictly a young man's sport!
The framework could be safely built down on the ground. The cross bracing cut, fitted and drilled then disassembled again. Two framework "sides" could be built on the ground then set up to lean against each other just prior to being joined by cross bracing at the top. Only once all the bracing is added can the plywood cladding go on.
Each bare framework "side" will weigh [2 x 12] 24' + 10' x 3 = 103 lbs each minimum plus fixing bolts/screws. Probably just manageable for a fit person to handle alone. But rather risky of flopping over during erection or badly disturbing the anchors. The actual method of construction needs rather more thought!
BTW: 544 lbs is very roughly 250kg so the load on the soil below the concrete footings is well within tolerance for the bare pier. Until, of course, we add the weight of the massive mounting, counterweights and OTA. The top of the pier will also need to be heavily reinforced to accept the mounting. Add at least another 20lbs.
A conservative 200 + 100 + 50 + 20 = 370 lbs + 544lbs = 920lbs = ~450 kg.
The safety margin is shrinking rapidly! I did mention using heavy paving slabs under the carport anchors. These would provide a firmer footing than plain anchor blocks resting on the soil, gravel or sand.The anchor blocks themselves have considerable weight and should be included in the ground pressure calculations.
To reduce the risk of sinking, the sides of the pier could be supported on four more concrete anchors at the base side mid points. With a sturdy cross brace and 3/4" ply cladding these footings could take a decent proportion of the overall load for a greater margin of stability. Moreover, they will lie at the edge of the footprint where they will do most good. Internal plywood cladding could reinforce the edges of the pier considerably.
A more central support carrying much of the total ground load could easily cause unwanted rocking. We don't want a fraction of a millimeter movement up at the mounting height. A millimeter movement at the base would mean 3mm movement at the top of the pier. Slightly more if the mounting has greater height above the pier top plate. This demands total solidity without any flexure in the pier. The slightest sinking, vibration or tilt would throw the mounting's polar alignment and levels completely out. Making Goto commands a complete lottery.
You can't measure precision wormwheel movements with a length of string. So the pier base must be as solid as if resting on solid bed rock or a concrete slab. Large concrete slabs on packed sand may be the answer but only slabs of the paving variety. The soil under the pier is garden soil lying over soft clay. Load spreading will be vital to success. It's a shame I can't lay some of those thick steel plates they use on building sites to support mobile cranes and heavy vehicles. In their absence I shall just have to spread the ground loads as best I can with readily available and affordable materials.60 x 60cm [2'x2'] paving slabs offer 3600 cm^2 each of surface area. That's 9x the surface area of the anchor footing x four. [Or eight if I double the number of anchors supporting the pier] Naturally these slabs would rest on well compacted sand. With more sand as a thin sandwich to prevent local loads from the anchors cracking the deliberately thick paving slabs. Loads applied to soil spread out at an angle. So the deeper soil and clay "enjoy" much lower local loads thanks to the successively greater area expanding with increasing depth.
I could further increase the number of paving labs by using four under each anchor footing. With an intermediate, large slab, load spreading to the lower slabs via compliant packing at their centers. The upper slab ideally wants to be rather larger to ensure its corners lie well over the centers of the four lower slabs. This will avoid tilting forces due to off-center loading by the compliant packing between the two layers of slabs. Compliance here helps to avoid cracking which would immediately undo the desired [even] load spreading.
A single, cast concrete slab in the ground would have to be quite considerable in area to support a 4'x4' pier. It is arguable that pouring four large, concrete footings would be ideal but I have no desire to get involved in hand mixing concrete in freezing mid winter. Though I have done so in the past for other projects where the ground was suitable for digging holes and the weather much warmer.
The main problem is that the ground below the platform and pier forms a lower terrace and is already 2' lower than the surrounding area. So pouring concrete would require the entire area was brought up to a suitable height first and then the footings dug in whatever bulking material was chosen. Hence my interest in using 2' tall, tapered, concrete, carport anchors. The anchors and their supporting slabs, would be buried in 2' of well compacted sand only once the pier and platform are safely in place.
There is no access for a concrete mixer lorry nearer than a hundred yards away. Running to-and-fro over such distances with heavy barrows of liquid concrete is strictly a young man's sport!
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