Todmorden pier, how high can it go?

[nightvision]

How high can a Todmorden block pier go?  I need to get to about 2 metres but not sure if it would be safe to go that tall.  I want to build something that will be removable.  It's a flat roof at 2.7 metres with a large hatch opening to 1500x1500mm.  The floor is concrete.  Any thoughts? 

[JamesF]

The blocks I have for mine are 435mm long, suggesting you'd need four and a half blocks or thereabouts to get 2m.  I imagine it would be fairly stable -- you'd have a pier that weighed well over 100kg so I don't think it would "flap about in the breeze", but I'd be concerned about the loads being placed on the walls of the bottom block if it were bumped.  Concrete is nowhere near as strong in tension as it is in compression.

That said, you could perhaps adapt the design slightly.  Given a suitably long drill bit (SDS bits certainly go long enough) you could drill all the way through each block a couple of times and run threaded rods through from top to bottom and into a resin anchor in the floor.  I reckon two 3m lengths of M16 rod would only cost about £35.  Or perhaps bolt the bottom block to a frame wider than the base of the block, the frame then being anchored to the floor with a larger number of fixings.

If you don't want to mess about with the floor at all, perhaps adding a set of box section steel stabilising legs bolted to the pier (or bolted to a steel collar that is bolted to the pier) near the top and braced against the walls or something might work?

An alternative might be to buy a couple of 8" x 4" oak railway sleepers of an appropriate length and bolt them together.  Stand them on end and add some wooden or steel legs for bracing.

James

[nightvision]

Thanks James for such a comprehensive reply; really got me thinking about the challenges.

I was considering the tension limitations of blocks and thought about some form of bracing, I like the idea of the steel rods running through the whole structure.  

Just realised there will be an access platform at about 1 metre so I'll have an opportunity to brace the structure at that point, not bothered about the platform touching the column as I will be mainly imaging remotely from the study.  The first 3 blocks would be braced by the platform leaving about 755mm to play with. 

[Dr_Ju_ju]

Put some form of shuttering around the column, & fill with concrete\postcrete etc ??  

[Peter Drew]

For a concrete flat roof installation I would use 4 x 600x600x50 paving slabs set as a square with a 5th one centrally bolted on to them through its corners, this would tie the whole construction together forming a stable free standing pier. I use a similar one to mount a 5" F5 APO as shown in the image. The cladding is 8mm plywood, added to give a more traditional look but it does add to the stability.

[Gina]

Very nice trig point ?

[nightvision]

I like that Peter, looks very stable, looks like 3 blocks tall.

[JamesF]

I think Peter posted some info in my build thread.  I'll see if I can find it.

James

[JamesF]

Perhaps there's a typo in one of Peter's posts, but in my thread he posted:

Quote

I have a 3 1/2 block pier for a 5" F15 APO the base has 5 600x600x50 concrete slabs, 4 in a square with the 5th bolted to the others through its corners but it is still freestanding.

I think it's probably the same one.

18" square 2" concrete slabs are not light by any stretch of the imagination, so I think they'd act as quite an effective stabilising mass for the pier.

The idea of a 2m tall 100kg+ pier falling over for some reason makes me apprehensive, so if I were to use that design I might be tempted to make a close-fitting ply box to encase the blocks and then brace them with ply/steel/timber.  If you have a nice solid platform half the height of the pier however, perhaps it all becomes much simpler.  Tie the pier in to the platform at that point and it all becomes far more stable.  You may not need to worry about bracing at all after that.

James