steel column fixings to concrete pad fondation. help!

22 Jan 2013
Reaction score
United Kingdom

I’m just about to fit some steels on my self build extension extension and I have the following questions-

Do I need to use any kind of dry pack mortar or can I bolt straight down onto the pad foundation I have laid. It’s pretty flat as I levelled and smoothed it of after pouring.

Secondly I wasn’t away that the structural engineer had specified 12mm chemfix (H.D) bolts and I haven’t laid these into the concrete prior to this.

Is there an alternative way so I don’t have to smash out this 1.2m3 pad foundation? I know there are some resin type methods with threaded rod but it’s all a bit new to me.

Please see photos attached and thanks for your help.


  • chemfixy.jpg
    52.7 KB · Views: 889
Sponsored Links
Thanks Newboy, do you know what size hole you'd drill. If the rods are 12mm how much bigger do you allow the hole?
14 mm hole for M12

ABSOLUTELY CRITICAL - the hole must be cleaned out properly, all traces of dust removed etc. & MI's for the resin followed.

JUST AS CRITICAL - pads checked / referenced to each other for position & level, holes for the studs spot on, resin fixing allows no room for errors in position
Sponsored Links
It's worth bearing in mind that you haven't indicated the loads you're planing to impose on the studs
Sorry what does that mean exactly and how do I find that out?
This was all drawn out by a structural engineer.

There will be minimal load on the bolts. The baseplate should be grouted once lined and levelled. Usually shimmed to suit then a neat cement slurry is poured to solidly fill the void between baseplate and foundation.

Do you have to shim and slurry the steel?
If level can it just be fixed Down directly to the pad foundation?
What type of slurry do you use? Thanks
You can just fix it to the foundation as the loads a quite low in this instance. However, Usually the engineer would detail some tolerance into the design to be taken up with shims & grout. This also helps get uniform load spread from baseplate into the foundation concrete.
Thanks dom_k,

Where can I buy this grout from in the East Midlands? Are there any nationwide builders merchants that sell it?

The column will eventually be supporting a second story extension.

When packing up the base plate to the column do you just put shims under the plate in between the predrilled holes or is each rod packed up below on washers and then the hole thing back filled with the non shrink cement grout?


looks like chemical or resin anchors then! They are great when used properly.

Mark the bolt positions very accurately and predrill. Use an airline to blow the dust out, or a small tube duct taped onto the end of the wife's vacuum cleaner to suck it out before any resin anchor bolts are used. Ideally the top of the concrete base would be set lower than the theoretical level. On most general steel structures we allow 25mm grout thickness and 100mm bolt projection to provide a generous tolerance as most ground workers mess up the levels. Anything is better than nothing though. Normally mild steel shims of about 100mm x 50mm are placed at the centre of the base and accurately levelled, then finally the column goes on top and gets bolted down. It depends on the circumstances whether you drill and fix the bolts before or after erecting the steel. I recently built a steel framed workshop at home where I drilled and fixed after erection as there was nothing that needed the bolts to provide stability during erection, everything was tied together by other members or temporary bracings.

It is a potentially dangerous time once the column is bolted to the base if it starts getting blown over or gets hit by anything as there are the tremendous forces trying to pull the bolts out of the ground. Stay clear of unsupported columns! Think of the mechanical leverage a long column has over your little bolt group at the bottom. So, get the column tops tied to something very quickly and be very careful until they are. I have seen columns pull out holding-down bolts and it isn't pretty. Structural engineers don't always consider these temporary issues when designing the hd bolt groups, they are usually more inclined to consider the working loads. In some circumstances expanding bolts give you that instant fix that chemical anchors don't.

Once everything is aligned, plumb and level, make some simple timber frames around the base plates and pour in a runny sand/cement grout so that it fills the voids. It's purpose is to spread the load from the flat column baseplate onto what is a very uneven surface of concrete. That is why a void under the column is highly preferable to a close fit.

As an aside, talking about resin anchors reminds me of an incident I remember from many years ago. I was working on a job where we had a steel erector who got upset at a former employer who owed him money. He allegedly 'borrowed' a load of tubes of the two-part chemical resin we were using to fix stainless steel bolts into tunnel liners and paid the guy a nocturnal visit where he apparently injected it around his car doors. Not really sure how that saga ended but I'll bet if he really did it, they would have had to grind the doors off.

Thanks colinc

What is the best material for the studs to be made of?

I've seen stainless steel, bzp, carbide steel etc etc and different grades.

I have some m12 x 160 anchor studs but by the time I've left them sticking out the resin they may only have 100mm bearings in the concrete? The structural engineer specified 150mm bearings with the h.d bolts.

You can purchase the RAWL m12 x 190 anchor studs but I think these are bzp or something like that.

Or I could purchase stainless steel m12 threaded rod cut to 200mm / 250mm?


The best answer is whatever your engineer specifies. He is the one who gets sued if it goes wrong, so give him the opporunity to be more specific. All the advice here is really just informing you about how we have built stuff. All I can say is that stainless steel is not necessarily better and every holding down bolt I have encountered was unprotected steel, usually grade 4.6.

Have a chat with the guy.



DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links