Many thanks to those who have answered my questions during my journey to this pier. The pier design set out to follow the percepts of Avandonk, a large diameter pipe with four tapered gussets.
Dimensions base plate : 25 mm thick 500 mm diameter
pipe :1200 mm long 220 mm external diameter 10 mm wall thickness
gussets : 10 mm thick 900 mm high tapering from 135 mm to 10 mm
to plate : 25 mm thick 250 mm diameter
adaptor plate: 30 mm thick 185 mm diameter
concrete : Notionally 1000 mm cube, the original hole grew between digging
and the pour resulting in 1.3 cubic metres of concrete. The reinforcement is an open topped 900 mm cube box of arcmesh and the cage of bolts used to attach the pier.
Assembly:- The pipe has both ends trued square by machining in a lathe. The top and bottom plates have central holes and a machined groove to receive the pipe. A threaded 16 mm rod was passed though these holes and the pipe and the pieces clamped by tightening nuts on the rod. All welds joining the pipe to the plates and gussets are continuous.
In order to minimise movement at the pier to concrete join the bolts are as snug a fit as practical in the base plate. This requires accurate assembly of the bolt cage. When the base plate bolt holes were drilled a plywood template of the hole pattern was taken. This template was used to make a cage of eight vertical 460 mm long 16 mm bolts joined by two rings of 12.5 mm rod. One ring to be 100 mm below the top of the concrete and the other at the bottom of the bolts. The length of the bolts is approximate, the different lengths show in the amount of bolt showing through the base plate.
When the concrete was poured the top was leveled as accurately as possible and the cage was pressed in so that the webs would be close to north/south east/west. Two wooden boards were located with nuts top and bottom on two paires of bolts. This allowed the setting of the depth of the cage when these boards touched the concrete. The four lower nuts remain in the concrete. As Avandonk points out the base plate needs to be held down at the outer edger of the gussets, so the surface of the concrete within the ring of bolts was dished out to about 1 mm at the centre . This also accommodates the inevitable slight deformation of the bottom plate due to the welding on its top surface.
The top plate has six tapped holes making a hexagon, three contain blanking screes and three take set screws attaching the adaptor plate for the HEQ5-PRO mount.
The whole assembly was lowered onto the bolts and four nuts progressively screwed down until the base plate made contact with the concrete then all eight nuts were tightened. A south line was scribed on the adaptor plate and a 8.5 mm square tool steel azimuth post located by a central set screw.
Our luck was good, the top plate is spirit level reversible east/west and just off that north/south. On Saturday night I used it for the first time. Easy star and polar alignment, what a joy to use afer the alfoil legged Skywatcher tripod.
