1 reply to this topic

[texcoco]

I am putting a Skyshed POD on an trailer for a mobile observatory. Right now I'm doing visual and planetary imaging, so I don't need a rock solid pier for long exposures, but still need to do this right to accommodate a 6" f/8 refractor weighing about 30 lbs with accessories.

 

I have a 36-inch, 108 lb pier made from a 6" H-beam on hand and would prefer to use it in this setup. It will need a base for that sits on top of the ground, no burying, no holes. The trailer will be jacked and leveled, and under some circumstances even anchored to protect it from high winds. It goes without saying, but I'll say it anyways, in use, the pier will be isolated from the trailer when in use and secured prior to transportation.

 

A platform of 2x4s and 3/4" plywood on top of the trailer is being considered to improve the mating surface for the POD, this would give a couple of extra inches for a pier base to retract for transportation. The trailer might be modified to increase axle clearance. This would also impact distance between the observatory floor (and the bottom of the pier) and the ground below.

 

The first destination of this setup will be a farmer's field with clumps of grass and dirt. No doubt it would be inadequate to simply secure the pier to a large block of concrete.^# Even with site prep, I'm sure it would rock around like crazy.^##

 

Now the big question, does anyone have experience using a "Y" shaped concrete base for their pier? I calculated the concrete below at about 520 lbs. Leveling would be accomplished by adjusting the nuts between the bottom flange of the pier and the concrete. I'm particularly worried about the 24 inch long "arms" flexing.

 

Your thoughts?

 

 

^#Similar to the 5-section pre-cast pier used successfully when buried, in this thread: https://www.cloudyni...-concrete-pier/.

^## In my experience, even burying and back filling should be avoided. See: https://www.cloudyni...pier/?p=8508875

[Phil Sherman]

I'd guess you can prevent flexing of the arms by using the same technique that is used in bridge construction. You will need to imbed prestressed cables between the center post and the ends of the arms. My concern with the arrangement you diagrammed is the arms settling into the ground for a few hours after being lowered. Another alternative might be to make the "arms" steel "U" beams that are welded together at the middle of the Y and the U filled with concrete. The concrete base would be cast around the Y junction. If the arms are angled down towards the ends of the arms, I'd also bend the ends to make a plane between the last 4-6" of all of the arms.