As per the drawing the column in one direction has alength 7 m and other direction 3.5 m by assuming it is pinned in bot end. If there is no beam in the middle it shall be 7 m in both direction. Considering if the mid beam is weak and can not prevent the lateral buckling so you must neglect it...
For every structure elemen by knowing the structure element end joint constrain and the shape of load (distribute, concentrate...) it is in blast resistance design book. Depends on Bigs formula there are a tables to provide the stifness, and the mass can be calculated then by knowing mass and...
In general spiral stirrups Vs normal circular stirrus. The column with spiral carry more load depends on the spiral pitch. I can give you in EcP which is similar in BS but away from CSA.
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the pile cap shall be rigid as it transfere the axial force and also convert the momenet to adding axial and tension to the pile. The puchinh check is essential and also the stresses. But 6" is very thin it can not consider a rigid to transfere load so it needs to be address.
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As you mention it is a cantilever type wall in this case choose the horizontal bar as the minimum based on your design code and standard
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It is important to do the hand calculation to feel about your structure. for tanks it is very easy i do it be excel sheet in case of trial for sections. With PCA it is easy but you need to handle your structure system as transfere the slab load on the walls then design the walls in two direction...
i hope to return to dnv for lifting specific for padeyes design or with examples in https://www.elsevier.com/books/marine-structural-design-calculations/el-reedy/978-0-08-099987-6
If it is near to the sea so sure the ground water will be salt and its concentration like the sea/ocean salt concentration...but as mention it is flactuated it is called a splash zone which is 2 ft up and down the MSL. So you shall take a lot of precaution for this foundation to maintain its...
As per ASCE to be near to the shore; but if there are a high rise buildings separate between your builing and shore even it is near to the shore in this case you can relax the wind load by shielding effect or use category C
Really, for normal building with beam and slab system the beam and column connection; and column and foundation it is partially fixed. But in design we consider it pinn connection. So after applying the load the straining action will be redistributed and the code accept a cracks between column...