Steel Column corner detail

[Coyne]

Hi, 

I am halfway through construction of a small coastal chalet - I am building it myself and it’s the first time I’ve done this so I’m hoping this forum can help with gaps in my knowledge as I move from the “design on a page” to the reality of building it… 

 

The construction is timber frame with a steel skeleton frame. The steel columns in the corners are fixed to the concrete slab foundation; the timber frame inbetween the steels is built on 2x concrete blocks to keep DPC >150mm to ground level. The problem I have is where the steel base plate's wider than the steel column, and the concrete blocks are outside the base plate - leaving a void at the bottom of the I-beam under the timber sole plate in all of the corners - which is currently letting in the rain, and I fear in future will be a route in for rodents. 

 

How should I fill this void? Should the timber frame OSB sheathing and external wall insulation extend down to the slab and I fill the void from the inside with insulating foam? That solution doesn’t seem rain proof… Or should I create a concrete post base around the steels, up to the height of the concrete blocks? This seems a much better way to weather proof the corners and prevent driving rain coming in but seems tricky in reality to do without getting in the way of the external wall insulation reaching down to the slab. 

 

And a second question: On some of my walls, the OSB sheathing doesn’t sit up tight to the steel column - there is, on some walls, a gap of ~10mm because the timber frame stud work is wider than the steel I-beam. Outside the OSB will be external wall insulation boards, breathable membrane, cladding batons then timber cladding. Do I need to fill in these gaps on the inside of the OSB sheathing with insulating foam? The external wall insulation is to keep the steel within the warm side of the building - so I fear that quite large air gaps next to the steel will prevent the EWI from doing its job? 

 

Thanks so much for any help!

[saveasteading]

Good drawing to explain the situation.

A few questions.

Is this already built to the stage you describe?

If it is, then photos would be good. If not, there is perhaps more you could improve.

Is the steel outside an insulated inner box?

[Coyne]

Hi, 

 

Thanks for fast reply. Yes, I have built as far as OSB sheathing on the frame. The wall build up from here will be external wall insulation boards, membrane, batons and cladding. (I will take photos and add later today)

 

So the steel frame sits inside the insulation layer. It is packed with insulation (as is the timber frame) and there is another layer wrapping the entire frame and OSB externally.

 

I did miss adding a load-baring thermal break between the steels and the foundation slab (the steel company I used hadn't heard of them and it's only since the frame went up that I found them online - so I'm aware of the cold bridge I've created with the foundation slab), but otherwise the steels should be fully enclosed and I'd like to prevent there being a thermal bridge on the upper face of the base plate. I assumed spray foam was the the better option for filling the void for this reason? 

 

[saveasteading]

OK. Steel plates direct on concrete is normal. Anything strong enough to make a thermal break won't be making much difference.

I've prob had 2,000 columns put direct onto concrete so a product is new to me ...can you tell me its name?

I will await photos before further comment. 

What is the distance from outside of steel to slab edge, and will your cladding be outside that?

[Coyne]

That's great news you've had so much experience with steel! Thanks so much for your advice on this. 

 

I've attached a photo - the steels at the front of the building (that will receive the serious coastal wind) have widened ground pads in the concrete slab of 750 squared, so the distance from the steel to the slab edge is much wider here (up to 350mm) shown in this photo. The steels at the back of the building are almost flush with the edge of the slab. 

 

Ideally I'd like to fill the inside of the void with spray foam to insulate the steel - but I'm not sure how to weatherproof the outside. My finished external wall will be another 80mm wider than this photo once external wall insulation and cladding are fixed. 

 

The thermal break base plates I saw online (but didn't use) are these: https://farrat.com/structural-thermal-break-connections

I'm relieved to here they aren't common practice! 

 

Thanks again for your help

[George]

On 14/03/2023 at 10:17, saveasteading said:

OK. Steel plates direct on concrete is normal. Anything strong enough to make a thermal break won't be making much difference.

I've prob had 2,000 columns put direct onto concrete so a product is new to me ...can you tell me its name?

I will await photos before further comment. 

What is the distance from outside of steel to slab edge, and will your cladding be outside that?

 

My go to for thermal breaks is Farrat - although I've only ever used them for beam-column connections they do a base plate break:

 

https://farrat.com/wp-content/uploads/2022/09/DS-STB-Farrat-TBF-22a.pdf

 

Alternatively, you could also design the baseplate to be supported only by the bolts and make them M24s+. Then leave a gap on shims and surround the baseplate with insulation.

 

But overall I doubt the thermal losses are significant enough to worry about. 

 

 

Edited March 16, 2023 by George

[saveasteading]

On 16/03/2023 at 11:46, George said:

supported only by the bolts and make them M24s+

NO.

These are holding down bolts, not holding up. It isn't a bad idea to build on the bolts as a temporary measure, but it has to be grouted up. Even 24mm bolts are liable to bend under vertical and rotational loading.

And the column isn't just sitting on bolts, but on the threads of 4 nuts.

 

It could work but needs detailed calculation.

[George]

Oh yeah it'd need an engineer to design it. That's why I put the '+'