Concrete Pads

[Ryan Bazeley]

Hi,

 

I have 2 steel posts 152 x 152 x 37 that need to sit on concrete pads finished at DPC level. I am waiting for the structural engineers detail but I am assuming they each need to be cast on top of the footings but am unsure what size they would need to be 200 x 200??

 

Only posting as waiting for the detail from SE but it’s really holding me up so thinking about chancing it?

Edited May 28, 2021 by Ryan Bazeley

[George]

That's slightly unusual. Typically you'd take the steel down to the foundation level (and encase the steel with concrete) rather than to dpc. 

 

I'd wait because you may need to dowel the concrete upstand into the foundation below to take any horizontal forces, and/or cast in extra long holding down bolts. 

Edited May 29, 2021 by George

[saveasteading]

I like that detail, as it brings the steels up clear of dampness. It is not the norm but it is practical and what I have developed for big warehouse columns, just as for these here. 

Another advantage is you can build these plinths with great care to get the level and quality right.

BUT they are spreading quite a lot of load so they must be to the spec your Engineer decides.

That will give you a size (45 degree spread of load is a good rule of thumb). There may well be some steel in it. You will also need to tie the plinth very well into the footings with continuity bars, or drilled in and glued dowels.

You will then be drilling again to bolt the steels down, unless confident enough to cast them in exactly the right place (which will be better).

It is all more work, but better quality control.

 

For anyone interested, another reason for columns to be higher is that when you fit them you are not in a messy puddle and filth, with little room for a spanner, and the nuts can be inspected easily. Traditionally it is all 400mm down and then covered in concrete. I supervised a dismantling once and found that none of the bolts had nuts on them, and i suspect there are many more cases where a nut wouldn't go on, and concrete was quickly poured to hide it.

 

[George]

I don't like them because you're trying to get load down to the foundation. Horizontal forces especially are more difficult to transfer through a plinth. You can get round QA issues (although any decent groundworker should be able to manage HD bolts) with post fixed resin anchors. Steel encased concrete will take hundreds of years to corrode. 

 

Anyway - I'd be interested to see their detail OP, when it does arrive.

[kxi]

Our SE specified columns down to the pad foundations, and it's good to hear this is a standard approach, as I was apprehensive. Was spec'd initially with a fairly slimline concrete encasement, then liquid damp proof membrane painted over that to keep the dampness out:

 

 

(The new column is the left one, the right one is an old wall with foundation. Column encasement is isolated from the floor slab).

 

However, with my paranoia about steel corrosion and discussion with groundworkers we're now all agreed to instead have:

• A much thicker encasement - basically just filling up the holes above the pads with concrete
• A hydrophilic strip around the column base - to block any water potentially moving along the joint between pad concrete and fresher encasement concrete (pads were poured some months ago, encasement not done yet, and the columns now constantly sat in water)
• Floor slab cast right up to the steel - with isolation wrapped around the steel

I.e.

 

(Cat not to scale)

 

Now I look at this I see a potential dampness pathway to the steel along the underside of the slab.

 

Any comment from those in the know about the pros and cons of each option?

 

 

Column bolts were a mix of cast-in and post-fix resin, to which the steel frame erectors said cast-in was much preferable.

[saveasteading]

Agreed that the steel should not corrode when encased. My preference of a plinth is more for quality control in positioning and knowing it is bolted down tight. Ever tried to put a nut on a damaged thread, 400mm below you, in soggy leaves and cold?  I suspect that in a lot of cases, the nut doesn't go on properly if at all.

(Hint, leave the nut on top of the bolt to protect the start of the thread)

 

I was also once present when we put very big columns onto epoxy fixed bolts (by others) and the bolts pulled out while turning the spanner (the main contractor was of national standing, but is now no more and the world is a better place). I subsequently found the hardener capsules discarded adjacent.  Moral, make it easy for the groundworker to get it right, and easy to tighten the nut: I suspect a less interested  party might just have stopped tightening, and the concrete surround would disguise the problem...for a while

 

Have used the plinth detail, developed over 30 years and 300 projects ( and 6,000 plinths????). 

 

My point here is that the proposal of a plinth is perfectly good, and it is your Engineer's decision.

 

Re the recent drawings by kxi above. I would always paint the steel in bitumen up to slab level including the underside of the base plate, where the welds may be the weakest part, and some gaps under the plate. Why not? 10 minutes work.

 

The column either has to be on the absolutely exact level of footing or else design to allow 25mm shimming then dry-packing (that also facilitates adjusting the column to vertical)

Waterproofing around the column, you just do the best you can by cutting  lapping and sticking dpm. If the concrete is dense enough there won't be any significant water absorption anyway, or problem. I can't see that you need to get complex with hydrophilic strips.

 

kxi, is the cat drawing for scale and now a requirement for building reg's?

And is that the cat that traditionally leaves footprints in all new concrete slabs?

[George]

How do you manage shear forces and significant uplift?

[saveasteading]

16 minutes ago, George said:

How do you manage shear forces and significant uplift?

 

 

Tie bars from plinth to mass base. The bigger the load the bigger and deeper the bars.

 

Moderate accuracy required to get them in the zone of a shutter, but much less accuracy and difficulty than for putting the bolts in perfectly with a mass pour.

We  took to building the shutter with permanent blockwork.

 

For a big base, these are L bars down to near bottom. 

 

All Civil Engineers have to experience both site and office, but tend not to hands on help with concrete pours in nasty weather. I introduced this method when I had to reset bolts in such circumstances.....and thought there must be a better way.

 

On paper the plinths cost more. In practice their perfection has greater value in my opinion.

[kxi]

On 30/05/2021 at 10:52, saveasteading said:

Re the recent drawings by kxi above. I would always paint the steel in bitumen up to slab level including the underside of the base plate, where the welds may be the weakest part, and some gaps under the plate. Why not? 10 minutes work.

 

The column either has to be on the absolutely exact level of footing or else design to allow 25mm shimming then dry-packing (that also facilitates adjusting the column to vertical)

Waterproofing around the column, you just do the best you can by cutting  lapping and sticking dpm. If the concrete is dense enough there won't be any significant water absorption anyway, or problem. I can't see that you need to get complex with hydrophilic strips.

 

Thanks. 

Current state is columns shimmed & grouted (and sat in water for days every time it rains) so missed the chance to paint under the baseplate:

 

 

 

 

So ditching the hydrophilic strips and sticking with just bitumen, what are your thoughts on A vs B?

 

A - Bitumen paint the column to top of slab (as per your usual), fill the pit with concrete, then lay slab right up to the column - with isolation joint taped to steel:

 

 

 

B - Encase base of column up to top of slab in concrete, then bitumen paint the outside of encasement, and isolation joint on the outside where it meets the slab:

 

 

I was assuming A is less work since you don't need the formwork for each column encasement, but which is likely to have more longevity? B keeps dampness further away from the column?

 

Some discussion of this in https://www.structuremag.org/wp-content/uploads/2014/09/C-SD-ABetterBasep13-151.pdf which illustrates an A-like one as a solution, but doesn't discuss a B-like one.

 

(The cat is more likely to be a rat these days as the pack of 21 cats long since extinct due to inbreeding).

 

[saveasteading]

The trouble with the first one is that concrete goes into the recess of the H shape. If you can isolate it neatly and get a float in then it should be ok.

It may need thicker isolation to allow shrinkage without causing cracking.

[George]

On 30/05/2021 at 22:40, saveasteading said:

 

Tie bars from plinth to mass base. The bigger the load the bigger and deeper the bars.

 

Moderate accuracy required to get them in the zone of a shutter, but much less accuracy and difficulty than for putting the bolts in perfectly with a mass pour.

We  took to building the shutter with permanent blockwork.

 

For a big base, these are L bars down to near bottom. 

 

All Civil Engineers have to experience both site and office, but tend not to hands on help with concrete pours in nasty weather. I introduced this method when I had to reset bolts in such circumstances.....and thought there must be a better way.

 

On paper the plinths cost more. In practice their perfection has greater value in my opinion.

 

I'm still not convinced and I wouldn't like to rely on vertical tie bars to transfer shear. I have specified plinths when other factors drive it, but by and large I prefer to get a direct load path to the footing. I've pulled 120 year old steels out of footings and, although heavily corroded at ground level, they're absolutely spotless where encased in concrete - and this was agricultural sheds so exposed to the outside. The combination of no oxygen, alkaline environment and being within the building envelope should mean risk of corrosion is minimal. 

 

I should hope no-one is attempting concrete pours in poor weather! Especially when HD bolts are being set. 

 

 

@kxi B - is the more normal detail.

[kxi]

17 minutes ago, George said:

 

@kxi B - is the more normal detail.

 

Thanks. Given you mention corrosion above ground, is it worth bringing the encasement in B up a bit proud of the slab, say 100mm (where practical) to bring it out of a zone where a) damp dirt might collect against the column or b) occasional floor washing might splash on it?

 

In terms of the thickness of the encasement on B - presumably it should be at least ....50mm...thicker than the baseplate width? So end up even thicker round the column.

Edited June 1, 2021 by kxi

[George]

Before I answer that, a few clarifying questions...

 

• Is this all within the building envelope?
• Is anything going on top like a screed?
• Is the steel galvanised? 
• Are you boxing in the columns?

 

I would still paint the steel with bitumen if you have access.

 

Usual detail is to encase with min. 100mm concrete surround and ensure it is well compacted between the flanges. 

[kxi]

3 minutes ago, George said:

• Is this all within the building envelope? Mostly yes - apart from two external columns that will be fully concrete encased top to bottom (with reinforcement)
• Is anything going on top like a screed? Mostly no, but some has a 'normal' floor on top with DPM, insulation, finished floor
• Is the steel galvanised? Yes, though some small knocks
• Are you boxing in the columns? A few, but mostly not

 

Reason for variation is the building has multiple parts with differing usage. Though none with an aggressive environment.

So you would paint the steel with bitumen even in B? AND the encasement in bitumen as well or just the steel?

Reason for caution is I'm paranoid about any steel corrosion and building aiming for 150+ year lifespan. 

 

 

[George]

Anywhere with a floor going on top I'd finish the encasement flush. 

 

If this is going to form the FFL, then you could slope it slightly to allow water to run off. 

 

Paint bitumen on the steel and run it up to DPM level. It is smelly so don't go too far up. 

 

I'm not sure you need to paint the concrete - @saveasteading - what do you think?

 

Galvanising is great protection. If it has damage then galvafroid can help repair it. But it is quite resistant to damage anyway as the zinc provides protection if it's within the same electrolyte.

 

150 years is ambitious... no building will last that long (no because things corrode, but because nature takes over) without maintenance so it's out of your hands I'm afraid. Build it as well as you can so it'll see you out. That's all we can do!

 

 

Edited June 1, 2021 by George

[saveasteading]

So much to comment on. A short essay.

 

150 years? more I would say.

When modern buildings are said to be designed for 50 years. This is more a matter of the design codes for weather events. 50 year occurrence of storm/snow etc. I think anybody buying a new house would hope it lasts much longer, but some may well not from what a lot of us have seen.

The usual reason for end of life of a building is that it is in the way of something else....new development.

Next is that it is simply out of date stylistically or functionally and nobody wants it. Thus factories are demolished after 60/70 years.

I would guess that after these, the average life of a house is 150 or more.

 

My career was in commercial and education buildings. They all look fine after up to 40 years except for the 2 that are replaced by housing, or due to superficial abuse by the operators.

All ok for the next 200 years if looked after.

 

My house is 90 years old, wooden and with no founds.

prev was 100, and 300 and moved seasonally and in the wind. Lots of maintenance but is good for the same again.

 

Shear in rods to plinths. This must not be done lightly. In my case the buildings were big and light and there was uplift, turning and shear. Although I am qualified to design these, I always had a Structural Engineer do the calcs for certainty. Lucky 6,000 times perhaps. So, yes they are safe and can lead to higher quality, but not to everyone's taste.

You are right that this may not be cost-effective for 2 small bases, even though the loads are probably all downwards.

 

Concrete encasement protects steel, and there is not much oxygen to support rusting, . There is always a chance of gaps under the baseplate so that is the biggest risk. Not essential.but the extra cost of bitumen paint is tiny.

 

The photo of the column in the water demonstrates my argument about bringing the base plate higher for control. That is a comfortable space for working in too, and it is not always so.