Pad foundations for RSJ Goalpost

[Jamie McNaught]

This might be the wrong forum, but I think this is more specific to the foundations than the RSJ (but feel free to re-direct this to the RSJ forumn if needs be).

 

We're building a garden office, but given the small size of the garden we want to include a 4.8m bifolds to try and keep as large an open space as possible. Rest is standard marmox+100mm sips+ cedar cladding.

 

We've included a UC 152x152x30 goalpost to support the bifolds / avoid deflection and the rest of the SIP build. Calcs assume:

250kg doors

120kg oak rsk cladding

+ 50% of 908kg (SIP's, skylight, wood and snow - we're in Camb, but you never know) - the other 50% is assumed to be loaded on the rear wall (we've ignored loading on side walls).

 

This gives a total load of appox 820kg on the beam, causing a deflection in the middle of 3.75mm which we'll compentate for with a timber slightly thiner in the middle to hang the bifolds from (I think - it might be 3.75mm is fine).

 

So......

with 820kg load + 282kg goalposts can we put this onto a standard raft slab of 100mm conrete in centre and 300mm concrete at sides?

 

Do we need to thicken this up / re-enforce at the goalpost feet, or is the weight really too trivial?

 

Sketchup below - yes, I'm aware the feet are missing from the goalpost.

[Mr Punter]

I think you will need some reinforcing under the column bases.  It depends on the soil type.  If they were just pads they may need to be 1000 x 1000 x 300 with 2 layers of A393.  With your slab you could just place this locally as long as the thicker bit is fairly wide.  You need proper base plates on the columns and grout under.

[Jamie McNaught]

Thanks Mr Punter - afraid I don't know the soil type - I shall ask our builder who will lay the foundations. Is there an easy way to tell soil types? (Googling is not helping thus far).

 

If I understand you correctly (soil type dependant), if the slab was thickened to 300mm with 2 layers of A393 mesh for areas of 1000mm x 1000mm with the RSJ base plates in the centre, then this would be sufficient?

 

And yes, base plates will be added to the drawing (and the steelwork!).

 

[Mr Punter]

I am no engineer and I am just going by other jobs I have seen.  Your slab as proposed may be fine but half a tonne would be quite a point load.

 

Because this looks quite expensive it is probably worth getting some calcs from a structural engineer.  Sometimes they can save you money by simplifying.  Is this structure outside of the scope for Building Control?

[Jamie McNaught]

21 minutes ago, Mr Punter said:

I am no engineer and I am just going by other jobs I have seen.  Your slab as proposed may be fine but half a tonne would be quite a point load.

Yes, I appreciate all of this is "best guess".... yours will be a lot better than mine!

 

28 minutes ago, Mr Punter said:

Because this looks quite expensive it is probably worth getting some calcs from a structural engineer.  Sometimes they can save you money by simplifying.

We're almost at the point where we could hand this to a structural engineer (i.e. we know the building weight, loads etc), but budget is very tight now - I'm trying to get this in below £10k (which I know I am going to miss, but hopefully not by much) with me doing most of the graft once we're out the ground.

 

If anyone knows a SE who can provide rought calcs and isn't wanting to get super detailed I'd be grateful... I suspect (and I could be very wrong) that given the small weights and size, over specing the foundations etc is probably going to be cheaper than an SE doing super detailed calcs to try and save costs.

 

27 minutes ago, Mr Punter said:

Is this structure outside of the scope for Building Control?

It should be Permitted Development - height will be 2.4999m  (when the roof light is retracted) ?

 

 

[PeterW]

3 hours ago, Jamie McNaught said:

Thanks Mr Punter - afraid I don't know the soil type - I shall ask our builder who will lay the foundations. Is there an easy way to tell soil types? (Googling is not helping thus far).

 

If I understand you correctly (soil type dependant), if the slab was thickened to 300mm with 2 layers of A393 mesh for areas of 1000mm x 1000mm with the RSJ base plates in the centre, then this would be sufficient?

 

And yes, base plates will be added to the drawing (and the steelwork!).

 

Soil type ..?? Dig down about 18” and get a handful of the soil from the bottom and look at it .. then squeeze it in your hand, the more it stays together the more clay it has. Then drop

it onto a concrete slab from about 3ft up. If it deforms into a flat blob then chances are you are on clay that is shrinkable so you’ll need a decent edge foundation all round with good sized pads at the front. The more sandy it is, the better with depth and mass. 
 

I would I be chucking a couple of sheets of mesh into the whole slab and make it all structural. 

[Jamie McNaught]

31 minutes ago, PeterW said:

Soil type ..?? Dig down about 18” and get a handful of the soil from the bottom and look at it .. then squeeze it in your hand, the more it stays together the more clay it has. Then drop

it onto a concrete slab from about 3ft up. If it deforms into a flat blob then chances are you are on clay that is shrinkable so you’ll need a decent edge foundation all round with good sized pads at the front. The more sandy it is, the better with depth and mass. 

 

Thanks, soil descriptions are helpful. I'm assuming if it hits the ground and crumbles (rather than remaining as a flat blob) then it's more sandy than clay?

 

By edge foundation do you mean strip foundation or do you mean thick edge on the raft? I thought raft / slabs were ideal when on clay and strip foundations were going to get prohibitively expensive? (Appologies, to this point I've ignored foundations and got on with the design of the stuff above ground).

 

31 minutes ago, PeterW said:

I would I be chucking a couple of sheets of mesh into the whole slab and make it all structural. 

 

Yes, looking at the cost of A393 this doesn't seem like a huge cost. Would including this below the RSJ's have the affect of reducing sheering issues with the load fron the RSJ's on the slab?

 

I'll try to knock up a more detailed slab drawing - builder wants to show this to an SE now.

 

[Gus Potter]

Hello Jamie.

 

You may be able to look at this in a different way.

 

A lot of Bifold / Sliding doors now days are bottom supported. That means the floor etc carries the weight. Think about a lift and slide set of sliding doors, you turn a big handle, it lifts the doors and this lets them slide. Most of the weight is carried by the bottom runner and the top runner is there to stop it falling out and moving in. There are other types that don't need a massive handle, but they are a bit more higher end.

 

Let's say you go for bottom supported doors and now you can follow a different design path. What you look to do is to hold up the roof over the opening rather than the roof and the associated weight of the doors and the added deflection the door weight brings. Now you only need to design for the roof load rather than the roof load + the weight of the doors.

 

Your starting point here is to go back to the roof loads. If you want a decent job then mostly roofs are designed for it's own self weight + an access load. They are also checked for wind loads and for snow drift loads. Mosty snow drift loads don't govern the design unless you live at higher altitude or say in Aberdeenshire etc.

 

To get you on the ball park on a small roof like this. If you fix it well at the corners the wind load will not be an issue, also, unless you have the "cave" right next to a larger building off which snow can blow this snow drift problem often "goes away"

 

A standard roof access load is about 60kg / sq metre before safety factors, roof self weight varies a bit but you can work this out depending on what you want to make the roof from.

 

Jamie.. post some more on how you are going to construct the walls & roof . If you have an inclination towards timber construction this drives the design, you could go down the composite insulated metal panel route, then over clad that...,and there are more options.

 

For something like this you often don't need a heavy found, pads and so on. In fact, with something like this you almost want it to "float about on the ground below" Folk may cringe at this but let the thing move about, just control where the movement takes place, design for the movement and save money. It sounds complex but it's not really, much is common sense.

 

Oh, and lastly if you can make some savings on the founds and goal post and opt for a higher end doors with thinner mullions then choose you sizes. You may want to knock it down later and use the doors to fit into that fab extension that is next on the bucket list!

 

If you are able to do some  of the work yourself you could get something pretty good for your budget.

 

 

 

 

 

 

 

 

 

 

 

 

 

[Jamie McNaught]

 

13 hours ago, Gus Potter said:

A lot of Bifold / Sliding doors now days are bottom supported. That means the floor etc carries the weight. Think about a lift and slide set of sliding doors, you turn a big handle, it lifts the doors and this lets them slide. Most of the weight is carried by the bottom runner and the top runner is there to stop it falling out and moving in. There are other types that don't need a massive handle, but they are a bit more higher end.

 

Let's say you go for bottom supported doors and now you can follow a different design path. What you look to do is to hold up the roof over the opening rather than the roof and the associated weight of the doors and the added deflection the door weight brings. Now you only need to design for the roof load rather than the roof load + the weight of the doors.

The reason for going with bifolds is to maximize the opening from office to garden - while lift and slide doors are great, we'd at best get a 66% opening v's bifolds at about 90%.

 

I'm pretty certain they are top hung, but waiting to hear back from manufacturer (they don't put their install instructions online so difficult to tell). However, if they are bottom hung, when the doors are open, you will have a huge amount of lateral force pulling forward out of the building? ie still a need for a strong support?

 

I agree without the bifolds there is less load on the opening... but this is still a 4.8m opening and investigations thus far have highlighted that this can't really be done in wood (we looked into Oak, but no manufacturer wanted to talk about deflection on 4.8m). Catnics are max 4.8m, so we could perhaps switch to that by reducing the span (https://catnic.com/products/lintels/bhd100), but that is still £400 - the goalpost RSJ is £650.

14 hours ago, Gus Potter said:

Jamie.. post some more on how you are going to construct the walls & roof . If you have an inclination towards timber construction this drives the design, you could go down the composite insulated metal panel route, then over clad that...,and there are more options.

 

It's a SIP build - 100mm walls, 150mm roof, celotex floor on slab. Walls are build on 100x65 Marmox Thermoblock, 2x4 sole plate, 2x3 sip inserts. Roof is secured to sips all the way round with bolts as 400mm and to i-beam. See below for more details. 

Walls End Elevation

100mm SIPs, sip splines, 2x3 ends.

 

Walls - Plan View.

100mm sips, SIP splines and 2x3 Timber ends to surround.

 

Roof - End Elevation - Sectional View through middle of Skylight

 

Roof - Plan view

Timber is 2x5, except where three beams where is is 2x5, 2x6, 2x5. All gaps are filled with 150mm SIP's exept skylight in centre.

 

14 hours ago, Gus Potter said:

For something like this you often don't need a heavy found, pads and so on. In fact, with something like this you almost want it to "float about on the ground below" Folk may cringe at this but let the thing move about, just control where the movement takes place, design for the movement and save money. It sounds complex but it's not really, much is common sense.

 

Oh, and lastly if you can make some savings on the founds and goal post and opt for a higher end doors with thinner mullions then choose you sizes. You may want to knock it down later and use the doors to fit into that fab extension that is next on the bucket list!

 

If you are able to do some  of the work yourself you could get something pretty good for your budget.

 

The hope was that we could float it, however, the long term plan is it will tie into an extension (there is now plans to "knock it down") so possibly making sure it doesn't float but actually stays put is the way forward. 

 

[Jamie McNaught]

17 hours ago, PeterW said:

I would I be chucking a couple of sheets of mesh into the whole slab and make it all structural. 

 

23 hours ago, Mr Punter said:

I think you will need some reinforcing under the column bases.  It depends on the soil type.  If they were just pads they may need to be 1000 x 1000 x 300 with 2 layers of A393.  With your slab you could just place this locally as long as the thicker bit is fairly wide.  You need proper base plates on the columns and grout under.

 

Something like this?

100mm slab (inc 1x A393) 300mm at edge

1000x1000x300mm pads to support RSJ (inc 2x A393)

all cast in a single pour hopefully?

[Mr Punter]

Would it be easier to do the pads with a concrete mixer, chock up and bolt down the base plates so your goal post is nice and level, then pour the rest with readymix?  Don't want to be messing with heavy bits of steel and finishing concrete on the same day.

[Jamie McNaught]

15 minutes ago, Mr Punter said:

Would it be easier to do the pads with a concrete mixer, chock up and bolt down the base plates so your goal post is nice and level, then pour the rest with readymix?  Don't want to be messing with heavy bits of steel and finishing concrete on the same day.

I might be adding confusion as the feet are not acutally in that diagram - they're not 'under concrete'. See below.

 

I'm imagining shuttering it, including the 100m slab and pouring in one to ensure it forms a single slab with no weak joins. Hopefully, the lower pads wont overflow due to weight of concrete pushing down from the 100mm slab (like this

 

Could the bolts for RSJ's be put in while concrete is wet using some form of template to ensure they all line up for when actually attaching the pillars when contrete is dry? Or would you drill and stick them in with resin afterwards?

[Russell griffiths]

Any reason your posts are uc I would use a 90x90 box section as I believe it will be easier to insulate around and clad over. 

[Jamie McNaught]

deleted

 

Edited October 20, 2020 by Jamie McNaught
Messed up, didn't reply to other post so deleting.

[Jamie McNaught]

31 minutes ago, Russell griffiths said:

Any reason your posts are uc I would use a 90x90 box section as I believe it will be easier to insulate around and clad over. 

I've been using this, so choices are a bit limited. If you can point to another tool or resource that'd be good - I know little about OC / UB / box sections.

https://shop.mrbeam.co.uk/

 

work in progress pillar cladding plan below - pine 2x6 spacer to bifold frame + oak cladding. There will probably be another pine strip to hold the inner oak in place (which will be longer). The outer oak pannel will be secured to the battons for the cedar cladding (with Tyvek house wrap behind it. The rest of the ibeam volume would be filled with PIR/celotex. Pretty much same detail for cross beam.

Edited October 20, 2020 by Jamie McNaught

[Gus Potter]

Hi Jamie.

 

I would go back to basics before you open the wallet/ purse / gourd or raid the money tree in the garden.

 

You have three walls and a roof with no openings. You do have quite a "stiff box" to work with. Most walls and roofs are pretty stiff. Think about them as deep beams, the roof, if it is detailed well it acts like a deep beam and thus able to transfer load down to the foundations quite adequately via the walls.

 

If you have bottom supported bifolds, then yes they are heavy but when you work out the horizontal loads that appear at the head of the doors (when they are open) you may often find that they are no more onerours than the loads that occur when the doors are shut and the wind is sucking them outwards.

 

It's just my view but bottom supported bifolds are more common now.

 

Logically it's ok to float one part of a structure and connect it to another. You just need to design for the differential movement. Ask this. Say you have an 18 th centuary house, you add an extension, both will move differentially relative to the seasons, temperature and so on. Both the old house and the new extension often still float on the ground (unless maybe on similar rock), just they move up and down differently. All materials are elastic and some can also expand/ contract differently if the are sensetive to moisture.

 

Before you go down the "steel route" and the associated foundations, deal with the cold bridging etc then it may be worth while going back to basics and avoiding all the extra interfaces and assosciated labour cost. This way you may be able to bring it all in budget and still have a competant structure / safe design that will still allow you to connect the house to it later.

 

In summary you may get this within budget if you go for bottom hung doors, do away with all the steel, base plates and thickened founds and approach the design in a different way.

 

 

 

 

 

 

[Jamie McNaught]

16 hours ago, Gus Potter said:

In summary you may get this within budget if you go for bottom hung doors, do away with all the steel, base plates and thickened founds and approach the design in a different way.

Thanks for the reply Gus, I really appreciate it - please bear with me on this.... I'm really keen for your input on an alternative to RSJ's.

 

Contrary to my earlier comment, I do agree with you that the door frame, attached to the roof will not pull/deflect outwards at the top - the roof when assembled is strong enough absorb this.

 

The issue I think we currently have is that we somehow need to span the roof either left to right (using the side sip walls) or front to back (using the rear SIP wall and RSJ or an alternative) (the original plan). I'm aware the side walls (in a front to back span) will transfer roof weight to the ground - calcs I've done for RSJ don't take this into account as that's complex to do and I was aiming for a 'worst case' scenario.

 

Options:
span left to right - the span is 5.2 -> 5.4 (building width) to support the roof. SuperSIPs (http://www.supersips.uk.com/index_htm_files/span table.jpg) insist on 2x5 (x1), 2x6 (x2) & 2x5 (x1) for 4.8m, without a roof light. With a roof light + 5.4m span we don't have details - trying to get this. We could use glulam as a replacement for (or addition to) the 2x6, but as the standard height for glulam is 225mm v's 150mm SIP thickeness means we lose the flat ceiling. We'd need a minimum of 3 spans of this with 1.2m sips inbetween. 3 Gluman 90x225x6m is £465 (https://www.bucklandtimber.co.uk/glulam-beam-cost-calculator/). This seems expensive and results in a ceiling with two joists - we are tight on ceiling height. And I've no idea if it works!

 

span front to back - this needs something for the beams&sips to rest on at the front. Original plan was the RSJ. I'm not sure what the alternative is. Possibly Glulam again, but we're back into worries about downwards deflection affecting the operation of the bifolds - I know we can get a gluman that can handle this but no idea of specs - I'm working on calcs. Additionally, the glulam will need support at each side - the SIPS are not strong enough for this point load at the front - so probably kiln dried 2x6's or some other post. This then gives us the goalpost config we had before with the roof load transfering down in the same way as the RSJ, albeit the glulam is lighter than RSJ (half?). At which point we're back to looking at pads to transfer the weight - albeit not as much.

 

Steel goalpost RSJ is £570 and I know it will work / not deflect. Yes, it technically needs larger foundations, but the load at the feet would not be much more than Glumam. If we try to span left to right, we have a huge span that I don't know how to handle.

 

So, am I missing some alternative solution? Am I too worried about the span left to right?

 

(will get back with options for spanning 5.4m in SIPs as soon as manufacturer gets back to me).

Edited October 22, 2020 by Jamie McNaught
Fixed sentence.

[Jamie McNaught]

SIP manufacturer has suggested a flitch beam - this should provide much of the strength of the RSJ without the cold bridging / cost and can be hung from the sips. Any thoughts?

[Mr Punter]

I would try to span the roof between your panels rather than on a beam.  Could you then get rid of the goalposts and extra footings?

 

Flitch beam is a good plan to span the top of the opening.

[Gus Potter]

Hello Jamie.

 

A few random thoughts, based on your budget of say 10k and doing a lot of the work yourself.

 

Have you thought about going for a different look. What about a composite panel roof with steel purlins. Your wind and water tight pretty much right away. Also, you could spend a bit on the flashings to get a contemporary / but less industrial look that could compliment the existing house and the shading of the doors. This would /could then lead to either a couple of hot rolled steel portals at the gables with wind bracing along the back wall. Or you could explore cold formed steel portals as well.

 

You could overclad parts of the composite with larch say or some other material. There is loads of stuff you can play with. You could over clad at you leisure as you look ahead to maybe linking in the house at a later date.

 

I think you may be pushing your luck with flitch beams this size as deflection / bolting etc may trip you up when you come to the detailed analysis. Also, you would need a heavy flitch (thick too!) plate, may be hard to get one that long anyway that is straight and true.

 

You could have a look at glulam. Here you could have a deeper one, still sit the bottom roughly flush with the ceiling but let the top side sit above the roof, thus creating a bit of a parapet. This may look ok as the vertical proportion of the beam over the door may fit well with the vertical height of the doors. You could maybe do something here to incorporate some subtle external lighting. I'm not sure but the likes of Carrara Hill has made some posts on how you can externally light a structure so folk can't see in at night. Also, if you have a bit of a parapet and there is some snow on the roof it may stop more of the water dripping as it thaws?

 

Have a look about for Glulams that have been miss measured, it happens, you may get lucky and pick them up for a song.

 

You could go back to the steel portal idea. Here, to keep the beam depth down you can weld a top / and or bottom plate onto a beam. One effect of this is to reduce the deflection and can provide a thinner accessible flange to drill and fix things too. Also, you can get more insulation in too. But also you could creat a good ledge to rest the sips panels on as you need some end distance for the fixings into the sips panels.

 

If you transfer load to the beam this way it is not central and the beam will start to twist i.e. torsion. Have a word with the sips panel designer and see if they can put a good heavy end on the sips panel. This may allow you to bolt through the web  / up through a bottom plate of the beam along with some brackets to transfer the torsion back into the roof. I'm not sure how far you will get with this as there will be a design for the sips manufacturer, and it's complex. It may be too small a job to make it worth while.

 

Have fun!

 

 

 

 

 

[Jamie McNaught]

Hi @Gus Potter, thanks for the details reply and ideas. Sorry about the slow response, I've been afk for the last week.

 

On 25/10/2020 at 21:56, Gus Potter said:

Have you thought about going for a different look.

What would be the reason for the different look - is this to bring it in on budget (we are almost there with SIPS) or some other reason? I don't really want to start investigating alternatives, worring about insulation issues etc when we know that SIPs are perfectly capable (and quick to put up!).

 

On 25/10/2020 at 21:56, Gus Potter said:

This would /could then lead to either a couple of hot rolled steel portals at the gables with wind bracing along the back wall. Or you could explore cold formed steel portals as well.

This is a very different approach - what are the benefits of this versus SIP's? Have you got any links to off the shelf pricing to try and compare the costs?

 

On 25/10/2020 at 21:56, Gus Potter said:

I think you may be pushing your luck with flitch beams this size as deflection / bolting etc may trip you up when you come to the detailed analysis. Also, you would need a heavy flitch (thick too!) plate, may be hard to get one that long anyway that is straight and true.

With the flitch beam being bolted to the roof SIP, would this not reduce the danger of this? I've no idea I have to say, just going on the SIP companies recomendation (although *still* waiting on an actual calc + dimensions).

 

On 25/10/2020 at 21:56, Gus Potter said:

You could have a look at glulam.  Have a look about for Glulams that have been miss measured, it happens, you may get lucky and pick them up for a song.

We have, but this is cost prohibitive and it's size + weight might become an issue. My understanding was it would be as heavy as a flitch plate but bigger. In all honesty I've not investigated much as the SIP manufacturer was not keen and price seemed high. Given time is an issue, I'd prefer not to hold out for a mis measured one.

 

On 25/10/2020 at 21:56, Gus Potter said:

You could go back to the steel portal idea. Here, to keep the beam depth down you can weld a top / and or bottom plate onto a beam.

Ok, I'm confused - I suspect I am missing the obvious here. In what way is welding a top & bottom plate onto a steel portal not just making an RSJ goalpost? And would this not have all the problems with cold bridging that the RSJ would?

 

On 25/10/2020 at 21:56, Gus Potter said:

One effect of this is to reduce the deflection and can provide a thinner accessible flange to drill and fix things too. Also, you can get more insulation in too. But also you could creat a good ledge to rest the sips panels on as you need some end distance for the fixings into the sips panels.

This was the original plan for the RSJ - it would form a ledge to rest the SIPS on - albeit we'd not get torsion as it's stupidly overspec'd. Drilling would be an issue, but it comes pre-drilled at 400mm centres, so a piece of structural timber would be bolted on.

 

On 25/10/2020 at 21:56, Gus Potter said:

If you transfer load to the beam this way it is not central and the beam will start to twist i.e. torsion. Have a word with the sips panel designer and see if they can put a good heavy end on the sips panel. This may allow you to bolt through the web  / up through a bottom plate of the beam along with some brackets to transfer the torsion back into the roof. I'm not sure how far you will get with this as there will be a design for the sips manufacturer, and it's complex. It may be too small a job to make it worth while.

This is why we were originally going with an off the shelf RSJ - too overspected to twist. There are already holes in the web - a 2x6 would be bolted to this and then the SIP bolted to that.

 

[Gus Potter]

Hi Jamie.

 

I'll have a stab at at some answers! Good points you make, there is always more than one solution!

 

1/ Different look.. yes my thoughts were that to get this within the budget of 10k it may be worth while conducting an appraisal of the concept design so far. Sometimes this can really help, although it seems at times you are going backwards. But that is part of the design process.

 

2/ Portalised structure - I spent time working with a few contractors a couple of years ago who were developing the "higher end man cave / garden room concepts. These were coming in at about 20k and the world was you oyster after that, well insulated with adequate founds. We were looking at both hot / cold rolled steel and also diaphragm structures - similar to SIPS.  With these the labour cost initially was round about 1/2 to 1/3 of the overall cost, broadly extensions and so on tend to be closer to 1/2 the cost. The aim was to drive down the labour towards the 25% mark. I think the SIPS are another option, matter of preferance really, SIPS have less cold bridges perhaps, portalising things provides more of a "mechno" type frame that folk may more comfortable with? If you want a ball park figure for a cold formed steel shed go online and ask for just the frame only, you this may give you and idea.  As a commercial venture the savings lay in deriving an acceptable ground / foundation solution that could be rolled out to the mass market, the cost of the superstructure had less of an influence in comparison.

 

3/ I would think carefully before you rely on the SIPS panel to straighten a plate, especially once its bolted to the timbers which themselves may not to true, these too will warp and move about.

 

4/ Yes a glulam would be quite chunky, heavy too as you say, but you can fix stuff to them easily, especially if you are more DIY orientated.

 

5/ Welding a top and bottom plate on? I think you can detail this so you get most of the insulation on the outer side of the web, thus more of the steel is on the warm side? .. apart from the edge of top and bottom plate.. food for thought? Yes, you are making a goal post but if you want to keep the top of the goal post as shallow as you can this is a way of doing it. The force distibution and connection design can be / is different around the goal post but the explanation is very lengthy so I'll leave it out as folk may loose the will to live.

 

6/ Origional plan for RSJ. You can site drill a 12mm dia hole fairly easily with a cobalt tipped drill bit on site provided the steel is on the ground before you need to resort to a mag drill. I use a 110v Bosch drill I've had for years. Don't be put off buy having to site drill a few holes, just a few mind, say ten or twenty in an accessible part of the beam or plate.

 

In summary it looks like you are well on your way with the SIPS route and happy with the approximate costs. In this case,  if it was me, I would cart on, get it built and start to enjoy it! You seem to have put a good amount of effort into finding something that suits you, that is important too!

 

All the best.