Hold down straps - Insulated foundation

[SuperJohnG]

I couldn't really find any details with the search. 

 

I've a KORE insulated foundation, with a SIPS kit going on top. The concrete slab will be the FFL, hence cannot fit internal holding down straps, that are L shaped.  I have mailed Tanners, to check what the norm is. But can you just do the holding down outside with straight straps and screw into the EPS? seems like it's the only way, but not sure the EPS would be strong enough. 

 

I'm sure Tanners will tell me in the morning but keen to know what others done. 

 

Floor covering is LVT hence too thin to hide in grout or whatever.  

[Iceverge]

Can you fix straps to the rebar and pour the slab over them.

[SuperJohnG]

20 hours ago, Iceverge said:

Can you fix straps to the rebar and pour the slab over them.

I coukd put them in and attach to mesh. But here must be a better way. Its a PITA. 

 

Tanners came back to me today but said its uo to the superstructure engineer. 

 

@LA3222 any chance you can provide some input your the only other I know with an insulated foundation and SIPS. 

[LA3222]

21 minutes ago, SuperJohnG said:

I coukd put them in and attach to mesh. But here must be a better way. Its a PITA. 

 

Tanners came back to me today but said its uo to the superstructure engineer. 

 

@LA3222 any chance you can provide some input your the only other I know with an insulated foundation and SIPS. 

What is the purpose of the straps? There were no straps used for my build. The soleplate was fixed to the slab with concrete screws, the same for the baseplates on the two steels.

 

You got a drawing? May help people to see the issue.

[LA3222]

Oh, as for fixing into EPS I can't see that working. Its rubbish to stick stuff in it - the U shaped pins you get from Kore are crap and don't work great.

[jamieled]

I chipped grooves the same thickness of the l strap, fixed them with hammer fixings so they were flush with the surrounding concrete then put the flooring over the top.

[Gus Potter]

Hi all.

 

This a massive can of worms and much is to do with a lack of design coordination.

 

For all.. a rough overview.

 

For the last few decades once we started using lighter roofs on masonry cavity wall construction (trussed rafters as opposed to heavy cut timber roofs) we found that the roofs got sucked off. We then started to fix a metal straps to connect the roof to the heavy cavity wall to hold it down.

 

Moving on to timber frame / SIPS. At ground floor it was recognised that when you have a timber kit (include SIPS) that under more extreme wind uplift the wind could lift the TF and the roof so we had straps at the bottom. The straps are usually turned into the outer leaf of masonry and this held the whole lot down.

 

However, that is only part of the story. Now we have this desire to have large openings/ glazing. When the wind blows it wants to tip the building sideways so we need to prevent this. Previously we had a heavy masonry wall / smaller openings. For the building to move sideways the wind etc had to overturn the portions of walls, but the mass of the walls and the shear strength of the mortar resisted this.

 

Now we know that a SIPS panel can carry vertical load. If you stand a SIPS panel up and push it from the top end along it's length .. it's intuitive that it is not going to move much.. call this inplane stiffness.

 

When you have for example panels that are higher than their length, at ground floor the wind can generate significant uplift forces at the bottom corner of the panel. You fix a strap to the panel.. and you can calculate the number of nails etc that are required here to resist this force. But the strap needs to be anchored at the other (bottom) end.. and rawl plugs and a few screws won't often cut the mustard by a very long way... to the extent that you have an unsafe structure.

 

If you have SIPS (which are a fancy version of a traditional kit panel) then if you have a masonry outer leaf you are back to roughly traditional kit design. But if you don't have a masonry outer leaf to ballast (if you have a big set of bifolds this ballast is not there)  this you have to find another solution to address the overturning of the SIPS panels. One way is if you have a steel frame that is designed to account for uplift...

 

If you don't have a heavy outer leaf of masonry / other frame then you need to find something that is, or connect the straps to the foundation / your ICF raft for example. The other BIG problem is that you need to make it buildable, practical for a ground workers to set out, (difficult for concrete workers if you have steel projecting when they are concreting the slab.. if you want this you will pay a premium) if not they will often avoid your job like the plague or bung on a big sum.

 

The point of this post is to highlight the following: @SuperJohnG " Tanners came back to me today but said its uo to the superstructure engineer. "

 

To address this issue of the interface between the superstructure and the underbuilding requires a lot of thought and associated design cost, detailing and communication between the different designers. But the super structure Engineer can't do their job unless the foundation Engineer has made suitable provision for connection of the superstructure to the foundation.. it gets even more complex folks when you have basements with ICF, tanking, cold bridge mitigation etc

 

In summary while it seems to be attractive to shop about and try and get the lowest price for each part of the design package it's actually very often false economy.. and be unsafe. Why not find someone that can do the lot, coordinate all of it, while making it buildable. I think some members of BH have chosen this route.

 

I appreciate that ICF / SIPS and so on are a hot topics on BH but these developments require some complex detailing to make sure that the building is still safe while maintaining the insulation envelope. One could be cynical and suggest that it's not in the interests of the material supplies ect to point out the additional design coordination element (and associated  fee plus contractor cost element) required to achieve a coordinated, buildable in cost terms, safe design.

 

By way of encouragement. Structurally, sitting a house on thick insulation is not new. Cold store designers have been doing this for years, as have say the Canadians. But the devil is in the detailing to address cold bridging, tying a SIPS panel to the found / basement etc.

 

In Super Johns case, if there is no heavy outer leaf of masonry there may be a solution where you can use the floating floor slab to provide the ballast, although this requires detailed analysis and maybe a change in thickness of the reinforcing and the depth of it within the slab. Another may be to design some thicker straps and connect them to the foundations with expanding anchors perhaps. If you encounter this type of problem then it may only require to be done locally. Much will depend on the panel layout, how the stability is achieved and the local panel uplift forces that occur due to the overturning effect.

 

 

 

 

 

 

 

Edited January 19, 2021 by Gus Potter
Additional info

[SuperJohnG]

Drama semi over - I think. The SIPS supplier has put hold down straps on the ground floor shown on his drawing. see here:

 

 

 

However on inspection of the Structural engineer drgs he hasn't shown any straps on the ground floor. only the upper floors to strap the roof down. So I think it's a non issue! 

 

@LA3222 they tie the kit down on some designs, see above (HDGS item). @jamieled thanks for coming back to me, I thought that was what I would have to do with must have be a PITA. 

 

@Gus Potter helpful and insightful as always thanks. My own choice to use two different was more one of needs must and to ensure I got the best people for the job, the SIPS deisgner found their superstructure engineer, who are very good and I appointed Tanners as they are experts for insulated foundation design. Fortunately I discusssed with the superstructure engineers if they would do the SER also, which they did and signed off of Tanners design  and also asked them to change a few items. So on the whole I am hoping it's merely a SIPS designer draughting issue and that's all. 

 

I will find out tomorrow and update the thread. 

 

For reference  - I am SIPS without masonry outer leaf, but I do have  steel portal frame on my big glass gable. 

[LA3222]

7 hours ago, SuperJohnG said:

My own choice to use two different was more one of needs must and to ensure I got the best people for the job, the SIPS deisgner found their superstructure engineer, who are very good and I appointed Tanners as they are experts for insulated foundation design. Fortunately I discusssed with the superstructure engineers if they would do the SER also, which they did and signed off of Tanners design  and also asked them to change a few items. So on the whole I am hoping it's merely a SIPS designer draughting issue and that's all. 

@Gus Potter, you mention getting one guy to do it all. I get that, but you tend to find that the Timber Frame companies have their own in house or go to designers for the structure. Like John, the company I used worked like this and like John I used Tanners for the foundations and the SIP company worked to that design.

 

I got the point loads etc for the house (the SIP part) and provided them to Tanners along with the ground investigation report. They then designed the foundations in the knowledge that the house was SIPs so had to work for the SIP soleplate etc and they also took into account the blockwork outer skin, designing the foundations accordingly. Once I had that design, I went back to the SIP company and got them to look at it and make sure there were no issues from their perspective with how the frame integrates with the foundations.

 

The slab design is sloped on the perimeter, this meant the SIP company had to check and adjust their fixing plan for the sole plate and steels in order to ensure there was sufficient edge distance for the fixings. 

 

The general point I'm making is that although ideally one person will carry out all the work, it tends to not work out that way. That is not an issue, it just needs a bit of back and forth chasing to make sure each side of the coin is working together via yourself.

 

In Johns case, it seems the SIP frame designer has incorporated a standard detail but without looking at the foundation design and considering the how! Everyone is human, these things happen, the key thing is that hopefully after a bit of discussion it can be resolved in a way which makes it practical for John to implement yet still achieve the desired effect.

[Russell griffiths]

I didn’t comment earlier as I’ve never built in sips. 

I have however built two timberframe houses, one of which was in a cyclonic area where wind uplift was a major consideration. 

Basically you had your corner profiles set out that would have the internal dimension of the walls marked on it. 

You poured your concrete slab and whilst waiting for this to dry you put a string line up to mark the wall position. 

You then pushed and wiggle your hold down straps into the concrete at the design locations.

As the concrete drys you hand finish around the strap and let it set.

The sole plate goes down and the strap should sit up against the inner face of your frame.

 

The last one I did the spec was that the straps extended all the way up to wall plate level to transfer loads. 

However we don’t get many cyclones in England.  

[Pete]

When I questioned my B/insp over this issue he just said if wind can move this building we are all in trouble!!

[SuperJohnG]

As a bit of an update. 

 

The superstructure engineer hadn't included them in the drawing....albeit is stating they ae definitely required to provide lateral restraint against wind loading. I had a few words about why were they not shown and seems to have been a missed detail that shoud have really went on engineers drawings. 

 

Anyway they are in their calcs and are required to help with wind loading and help against lateral loading (As a mech engineer it's what I'd call and overturning moment. Imagine you are pushing on the thin end of the SIPS panel not the inner or outer face, load getting applied from another wall that's being hit with wind. 

 

So the gist of it is, sort of found a solution where it removes my need to chip out concrete or plan positions of straps prior to pouring. we will add an l shaped strap underneath the sole plate, so it is hidden. Every 1200mm the sole plate will either have a rebate in the underside or just cut a bit out thick enough to take strap, straps will be Hilt nailed to slab and then SIPS rail that provides SIP guidance into the panel rebate will go on top. SIP panel is placed then hold down strap attached. Seems to work and importantly someone else will have to do the work not me. A nice image to make it easy to understand, once agreed and implemented I will update this thread for others who come along.