L Bolts for astronomy pier (mild steel or stainless)

[stevebb]

Hi,

I am after a bit of advice please.

Groundworks are about to begin for the observatory and I want to use L bolts in a template and set the bolts into wet concrete.

I have been looking at prices on Fleabay and wow, the stainless steel ones are quite expensive  (M16 x 300mm are £44 for 4).  Could I get away with zinc coated mild steel at half the price (M16 x 440mm) or am I setting myself up for trouble ?

The pier is inside the observatory though appreciate when observing on a cold evening there is a possibility of moisture/ice building up.

Also, is 300mm enough - I have been googling and although see lots of photos I have struggled to get exact measurements.

Thanks in advance,

Steve

ps the holes on the base of the pier are pretty big hence thinking about M16 - again, if this is overkill and M12 would be suitable please advise 

[symmetal]

The zinc coated ones I saw only had a 3 micron coating and were recommended for indoor use only. If the coating gets scratched or worn rust will set in quickly. Stainless steel is resistant through its whole thickness and not just the surface. A4 grade stainless is better than A2. I would go for the largest bolts that would fit the pier and mounting plate just to add more rigidity. Mine uses M20 A4 bolts using a 300mm diameter concrete pier. I believe they were about 300mm long. The extra price of the stainless over the zinc is not much compared to the cost of the equipment it will be used with. Once set in concrete they can't be replaced if they go rusty. Well not without a great deal of hassle. 

Alan

[pete_l]

Rusting requires free oxygen and there isn't much of that inside the concrete of your pier. However, it is quite possible for the part of the bolts exposed to air to rust. Tthough treatment/paint is pretty good at preventing that.

However, as symmetal points out, is £20 really going to make much of a difference in the overall scheme of things? If you didn't spend it on your pier, you'd probably only waste it on enjoying yourself!

[stevebb]

Alan and Pete thank you so much for your advice, stainless it is 

Steve

[Bizibilder]

The trouble with stainless is that it can be difficult to bend - I used zinc plated studding (allthread) for my pier in the observatory and it is still perfectly Ok after four years "outside".  I do take an occasional look under the false floor and the rods have a very slight covering of rust on the cut ends but seem otherwise OK.  Obviously I can't see what is below ground but as Pete says there shouldn't be much oxygen around down there so i suspect that any rusting is minimal.

I used 12mm studding if I recall correctly and had to get it to red heat and use a hefty hammer to bend it!

[symmetal]

39 minutes ago, Bizibilder said:

The trouble with stainless is that it can be difficult to bend

That's true, but I believe Steve was looking at the already bent bolts to take the hard work out of it. 

Alan

[iwols]

just a thought has anyone glued straight bolts in the concrete after the cenent has gone off?

[r3i]

I got my threaded bars from e-stainless (mine were M12) :  http://www.e-stainless.co.uk/stainless-steel-threaded-rod-studding-c22.html

I cut the rods to about 50cm in length, ground a slot at the bottom of each then fixed a small length of rebar (also from e-stainless) into the slot using JB Weld: 

[Davey-T]

1 hour ago, iwols said:

just a thought has anyone glued straight bolts in the concrete after the cenent has gone off?

I drilled my holes in the concrete after it had set and fixed stainless steel rods with 2 pack stuff, worked really well and easier to get the spacing right.

You have to buy a special gun to inject it so a bit pricey if you're only going to use it once, don't use the ready mixed stuff in glass phials is crap.

Dave

[Jkulin]

My Pier is 750mm3 and I deliberately chose not to insert rebar into it whilst wet as I wanted the ability to fine tune.

Once the concrete had set I left it for about a week, then placed my pier into perfect alignment and then drilled into the concrete and glued using the correct glue available from you local hardware/builders merchant. The glue was in a mastic tube and you only needed to use your standard mastic gun holder, insert the glue then push your raw bolts in and allow to set over night, the glue bonds with the concrete chemically and you cannot get the bolts out again no matter how hard you try.

I would defy anyone to move my pier unless you owned a jumbo jet :-) The bolts would go before the bonding glue.

Here's the thread about my pier: -

[symmetal]

Like Dave and John I drilled holes in my pier after it had set and fixed 3 lengths of 300mm M20 studding (A4 Stainless) using Anchorset Red 300 2 part resin. It fits a standard sealant gun with 2 bags in the tube which mix when they flow down the supplied helical cored applicator tube. It lets you fit them exactly where you want them without the risk of dropping them in wet cement. 

Alan