Gina's Observatory Build - Abbreviated - Pier Construction

[Gina]

This abbreviated write up has been overdue for ages but at last I'm making a start I have split the pier construction from the ground preparations and main shed design and build.

[Gina]

Hole dug for pier base. Something like 30" (about 750mm) cube. Went down about 6" below the top of the clay level - it was very heavy going.

[Gina]

Here we have the preparations for the pier and base. Base and pier were to be poured all in one day - not giving enough time for the concrete to set between pourings from the mixer.

The bottom part of the pier base was held in by the soil but the top needed shuttering. Here I rummaged round for anything suitable. Mainly pieces of wood but I also used an old radiator that happened to fit one side just nicely. A couple of pieces of wood held it in place.

This next photo shows the side shuttering plus the rebar rods to reinforce the concrete. The four almost upright ones go right up the pier itself to reinforce that. The bars were hammered well into the sub-soil for extra strength and rigidity.

[Gina]

Because pier and base were being poured all in one go without letting the base set first, it was necessary to stop concrete flowing out from under the pier tube, so top shuttering was needed. I used an old piece of half-inch chipboard with a round hole cut in it for the pier. This was reinforced with a couple of pieces of wood and arranged to be screwed down to the side shuttering to hold it against the concrete pushing upwards.

Here is the top shuttering in place and also showing the pier rebar, held together at the top with a jubilee clip to keep them in place during concrete pouring. The string carried a plumb-bob to get the rebar in the middle of the pier all the way up.

[Gina]

To take the mount and at the same time, further reinforce the concrete, there are four 16mm threaded rods. I drilled a block of wood with four holes to space out the rods and a centre hole that took the top of the rebars. The lower ends of the rods were attached (temporarily) to the rebars with cable ties. These were just to hold the threaded rods in position while the concrete set.

The bend at the bottom of the rods are to stop any possibility of them pulling out or turning in the concrete.

[Gina]

Because it would be very difficult to get concrete past the wood, I replaced it with stiff galvanised wire - heavy duty fence wire - adding the wire while the wood held the positions, then removing the wood. The gaps between the pieces of wire and around the outside allowed concrete to be trowelled into the pier tube (more of which later) and in between the rods..

[Gina]

This next photo shows the container for moulding the concrete pier. I had no luck finding what I considered a good size pipe to form the pier (a couple of suitable pipes have since become available) so decided to use a 7" cardboard postage tube but with plant pots to widen the pier at the bottom where the stress was greatest. I managed to find two plastic pots which fitted nicely together and with the smaller one fitting the inside of the cardboard tube. This increased the diameter of the pier to 12" at the bottom. This photo shows the pier container parts fitted in a hole cut in the top shuttering of the base. I cut the bottoms out of the plant pots, of course

To hold the pier assembly in place and stop the plant pot dropping down, the lip at what is normally the top of the pot, was held up against the chipboard with a couple of pieces of wood, themselves held by wood screws down through the chipboard so that they could be unscrewed after the concrete was set, to allow the chipboard to be lifted off, leaving the pot in place.

[Gina]

There then followed what must have been about the most exhausting day of my life! Pouring the concrete for the pier and base. At least I had the use of a cement mixer to actually mix the concrete and tractor and link box to transport the ballast from a big bag by the gates and bags of concrete from the shed to the site. This next photo shows the setup.

I was able to back the tractor and link box down to the edge of the site and use a three-quarter piece of plywood to go across to the edge of the shuttering, supported on concrete blocks, to support the mixer. The raw materials were put in the link box and shovelled into a galvanised bucket to carry to the mixer. Water was put in a plastic dustbin with a garden hose and scooped out to put in the mixer.

[Gina]

The following set of photos show the filling of the base of the pier.

[Gina]

After this I screwed the top shuttering on, complete with plant pots attached. Then the pots section of the pier was filled with concrete using a garden trowel.

Here we see the plant pots filled with concrete. The concrete was poked into all the corners as it was poured - or at this stage trowelled into the pots.

[Gina]

Next stage was to place the cardboard tube on top of the plant pot, line it up and fill it with concrete with the trowel, tamping it down around the rods as it was filled.

Here are a couple of photos of the pier filled to the top. The concrete part of the pier was then complete. Phew!! Got it done just as it was getting dark. These photos were actually taken the following day.

[Gina]

After letting the concrete set and harden for a week it was time for the "unveiling" - removing the shuttering etc.

Firstly, unscrew and lift off the top shuttering.

Then the side shuttering...

[Gina]

The unveiling of the pier :-

Firstly the cardboard tube, having served it's purpose of containing the concrete while it set and hardened.

Later on I decided that the plant pots should go too. They didn't look right and had served their main purpose of retaining the concrete while it set and hardened.

Firstly the top pot...

Then the bottom one.

Finally for the pier, levelling the pier adapter ready to take the mount. The construction of the pier adapter is in a thread thread of it's own. (To follow) In fact, since making this and taking the photo I have learnt that levelling the mount isn't necessary - the full adjustments can be achieved with the mount polar alignment.

[kirkster501]

Very good - jealous of people who have room and horizon to build an observatory !!!

Also, liking the black cat poking out at the bottom left of one of the pics I am a real cat lover !!!

[Gina]

We are too In fact I have a lovely black and white lying on my arm ATM, purring away happily

[Astrokev]

Hi Gina

Can you remind me how you get access between the plates to secure the EQ6 retaining bolt?

cheers

Kevin

[astrosathya]

I would hazard a guess that Gina would probably have to remove the top plate entirely, mount it to the EQ6 and then mount the EQ6+top plate assembly onto the bottom plate?

Would it not be better to replace the washers with springs Gina? This way you can level the mount the same way as collimating a newt.

Regards,

Sathya

[Gina]

Hi Gina

Can you remind me how you get access between the plates to secure the EQ6 retaining bolt?

cheers

Kevin

Sorry for the late reply. There's enough room to get a spanner in. Another way is the undo the 4 big nuts and hoist the mount plus top plate etc. as a unit, move sideways and lower to the floor. A "sky hook" on the roll off roof plus a block and tackle provide this facility.

[Gina]

I would hazard a guess that Gina would probably have to remove the top plate entirely, mount it to the EQ6 and then mount the EQ6+top plate assembly onto the bottom plate?

That's is indeed an easier way to do it as I've just posted above.

Would it not be better to replace the washers with springs Gina? This way you can level the mount the same way as collimating a newt.

I learned later that leveling the mount is quite unnecessary - it can all be accommodated in the polar alignment. If I knew then what I know now I could have simplified the whole pier head mounting

[laser_jock99]

That's is indeed an easier way to do it as I've just posted above.

I learned later that leveling the mount is quite unnecessary - it can all be accommodated in the polar alignment. If I knew then what I know now I could have simplified the whole pier head mounting

Good point. Mount leveling adjustment is not necessary on piers, the polar axis adjustment on the mount is all that's needed (assuming it's reasonably level to start with!). Many pier designs are serriously weakened by having un-needed 'leveling plates' built in. The main consideration should really be some means of accessing the mount retaining nut.

[andyboy1970]

When I built my pier I covered the threads sticking out the top of the pier with cling film to stop the concrete going all over the threads.

I was also amazed by the amount of air released from the concrete after I poured it and tapped the side of the tube/shuttering, the concrete level dropped by about an inch.

[Gina]

When I built my pier I covered the threads sticking out the top of the pier with cling film to stop the concrete going all over the threads.

Good idea - didn't think of that

I was also amazed by the amount of air released from the concrete after I poured it and tapped the side of the tube/shuttering, the concrete level dropped by about an inch.

Yes, I noticed that too. Needed topping up.

[Tinker1947]

There are machines to assist with removing air bubbles ect.......never used one myself but maybe for a Pier they might add strength.......

http://www.alleneng.com/view/concrete-vibrators

[Gina]

I think my pier is plenty strong enough - well over-engineered in fact

[andyboy1970]

There are machines to assist with removing air bubbles ect.......never used one myself but maybe for a Pier they might add strength.......

http://www.alleneng....crete-vibrators

I believe it can also cause problems if you use it for too long, well so i've been told.

When I lightly banged the pier after pouring it was like a bubbling hot spring, you know the volcanic ones.

Another thing was the heat generated by the setting concrete, I covered the pier and wet the fabric covering twice a day for the first several days. (It was the end of May).