Bolting down steel plates

[Osprey]

I will be bolting down a large number of 8mm thick Galvanised base plates onto a reinforced concrete raft. The plates are 300 x 300 and each fixed by 4 M16 anchor bolts:-

plates while pretty darn good, are not perfectly flat ... the welding on the upper faces (300 high T section in 8mm steel) Plus in places the concrete may be a couple of mm out.

We are only talking about slight rocking of the base plates, and the torquing each of the bolts to 100 lb ft will probably pull bases flat.

Just wondered if I should 'paste' or put anything on the under side of the Galv plates to help bed them onto the concrete ?

These are being used externally and will be subject to wet & full UK temp range. (hence the galv)

[Derek Geldard]

Two or three thicknesses of DPC wouldn't hurt.

DG

[George]

Won't the raft sink?

[Andrew Mawson]

I would be tempted to bed them on a thin layer of external mastic to keep out moisture. The pull down bolts will probably as you say actually conform them to the concrete. Alternatively you could bed them on a cement rich thin grout as you would with the base of a large machine tool.

AWEM

AWEM

[somebody]

In message , Osprey writes

Hi,

Yes it is generally good practice to use a cementitious grout between steel and concrete....

provides a good bed between the uneven surface of the concrete and the typical tolerances in steel fabrication.

You can buy it as an admix for cement slurry (usually 1:1 cement:water + admix) or premixed as above.

Hth Someone

[Owain]

No, not if he ties enough empty oildrums round the sides.

Did your mummy send you to brownies instead of cubs?

Owain

[Phil L]

A structural engineer I know insists on plates like this being taken down to

2mm of the concrete, then neat cement and water poured into the gap, then a day or two later the nuts tightened to required torque - it just gives the underside more surface area to bear down onto

[somebody]

I would seriously question your structural engineer. Neat cement and water can crack like crazy paving and has questionable structural integrity. One pour may be strong, the other may fall apart, ie - it cannot be relied upon.

Now if he had said "neat cement and water plus an appropriate admix" I'd agree with you. Cement and water alone is just a "cement slurry", it has unknown properties and unknown stability.

A fully specified cement slurry is properly called a cementitious grout. It has exact, known behaviour. From my *very* limited experience it is specified at 10mm to 75mm between the plate and the concrete and most importantly, that known behaviour includes its structural (bearing) capability.

It is possible your structural engineer just assumes that the contractor/fitter/diy'er will know to include the admix - it's happened to me :-) That 2mm thingy though sounds a little off the wall IMHO.

Hth Someone

[Osprey]

Read the replies - I have previously seen cement grout used when bedding down uprights on a large steel building .. this is when there were cast in fixing bolts. In this case plates will be flush to floor ... how do you envisage the grout getting under the plates ...?

Should it be mixed thick and a layer spread on the plates, or some form of mini shuttering made and slurry poured in after plates bolted down ? .... the only issue I see is that by then plates will probably have distorted to match the concrete, and grout will not flow under the plates?

An idea (maybe) .. I drill the holes and start fixing plates ... just taking up tension on all the bolts, then put a frame around the plate - maybe 3mm wider all round ? .... pour in grout, encourage it under the plate by vibration ... then torque up plates while wet ... and allow to set.

Welcome any better ideas ... what is the sequence for machine bases ?

Good ideas ... just need some further ones on implementing this.

[Osprey]

In my case gap is from 0 to 3mm ? .... does that mean it's of no use ?

[Osprey]

After suggestions - Looked at the Sealocrete web site .. the products they recc for anchoring all require a sizable min 5mm ... so looks like I shouldn't bolt direct to raft, but should 'force' a gap and and pour. Then torque bolts down 72 Hrs later ? Anybody know if I'm understanding this correctly ... does putting serious compression load on grout after it has set cause no problems ?

[somebody]

I think there is a PDF on the Sealocrete site giving instructions, but yes, shim the plate to be bolted down. Form a shutter around the plate, pour the grout, wait specified time (72 hrs?) and torque it down.

The grout has a specified n/mm cured strength (probably in the 80 n/mm range) which is far higher than the strength of the probably C30 concrete you are bolting onto - absolutely no problem torquing down onto the grout.

Hth Someone

[Rick Hughes]

I did see the Tech PDF ... but it only mentions torquing down onto studs (assumes you are use cast in studs) ... the slab is actually a bit stronger than C30 ;-) ... but it's good to know you can torque down onto the grout (once it's set)

The sequence I will follow then would be ..

Drill holes Shim plates with 10mm steel spacers Fine adjust to get plates plumb Torque to 75% on finished setting Create formwork, giving aprox 20mm all round base plate. Pour cementitious grout Let it set for specified time (24 hrs = 30 N/mm2) Torque down bolts to final 100 lb ft

Does the above all sounds logical ?

The only issue I can think of .... the grout will be poured and will surround anchors & bolts threads .. when I then go to give final torque ... will threads turn with no problem ?

Do I need to consider this ? .....pre-treat threads in any way ? or even perhaps look at full torquing down (onto shims) before grout is poured.

Thanks for any advice on this.

[somebody]

In message , Rick Hughes writes

As you've correctly surmised, it assumes precast studs (in fact not pre-cast but a hole drilled and a stud epoxy resin'd into the hole).

I don't mean this to be a cop-out but has an engineer specified these anchor bolts (I assume a rawl bolt type fixing)? Your 100 ft/lbs (135n/m) through the expansion fixing would exert a fairly high expansile stress on the concrete. The main quality of concrete is it's compressive strength, not shear or tension.

I think I'd be tempted to ask the original engineer for some pointers, and/or ask Sealocrete.

You must use the grout though for a few simple reasons....

- You have concrete specified at "x" n/mm strength

- Your plates are sized to exert less than "x" n/mm load

- Look at the concrete carefully, it isn't flat. It has peaks and troughs, both visually and microscopically. If you could calculate the actual bearing area of those peaks it might easily be "x" n/mm times three of four or more.

- The grout is designed to ensure an evenly spread bearing.

Sorry, doesn't really help does it :-(

I'm away from 'puter for the next few days, but let us know how you get on.

Cheers Someone

[Osprey]

Yep this method of fixing was specified by a structural engineer ... and full calcs etc. Sleeve anchors are being used as they 'spread' the load better so it is stated.

Raft is C30 mix over a whole shit load of steel ... again purpose designed, not just 'stuck some steel in' ... its 550 mm thick at location of the plates,

All fixings are at least 150mm away from each other and from raft edge to comply with design spec ...

I'll go with the 'fully torque down' onto 10mm packers, then formwork & pour.

Couldn't get the Sealocerte product - none of the merchants had it in stock, but found a FEB (now BASF) product that has same performance. In fact the wording on Tech sheets is a direct cu & paste ... don't know who was first though ?