Owner Building a Strawbale House
in the Bega Valley,
NSW,
Australia.



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Structural Steel Framing

We are using a ladder-truss style structural steel framing in our house. The base is a monolithic raft slab, chem-set allthread rod bolting to plates at the bottom of the ladder-truss steel columns, ladder-truss spanning between the steel columns (skillion roof), topped by steel C-purlins and colourbond corrugated iron sheeting.

Frame to Slab

You can either have the all-thread rod inserted in the concrete at the time of the pour (cheap and easy, but you have to get it in the right place, otherwise you need to cut it off and drill holes for the rod), or you can put the rod in afterwards. I chose the later as I haven't finished the exact location of the columns, so this gives me freedom to change things around.

The base of the steel columns has a 10mm steel plate welded to it, and through this are two M16 bolts (really pieces of all-thread rod) which are "glued" into the slab. I drill 18mm holes in the slab at the required locations, add a measured amount of "chem-set" goop, hammer in a length of M16 all-thread rod (follow the instructions on the pack), and after some curing time it is ready to go. The "chem-set" goop is a two-part chemical from ramset, and outgases, smells noxious (etc.) but is strong and can be used close to the edge of the slab. I probably would have inserted the rod into the concrete during the pour if I was better prepared.

All thread rod can be purchased pretty cheaply at a Nut & Bolt store in 3 metre lengths.

The plates are left 20mm above the top of the slab (using nuts or small chunks of steel to keep the plate off the slab), firm up the all-thread rod nuts and adjust so that the column is in its final position (best left until all the frame is up, plumbed and squared) - then pack the gap between the plate and the slab with dry pack grout - ask for it by name, accept nothing else, mix it as dry as you can (and still be able to ram it in under the plate). Getting it in under the plate is not easy, if you can get it in as a very dry mix you can remove the packer immediately and the grout will support the weight - but this is difficult. We ended up making a wetter mixture and had to leave the packers in place until the grout had set (note: leave the packers in a way that makes them easy to lever out after the grout has dried !). Do not mix too much grout at once (you only have 30 minutes, and it takes a while to push it in under the plate, working only from one side, avoiding air inclusions).

Steel Columns

These were specified by the engineer, just follow the plans. In my case the columsn are made from two 65x65x4mm Square Hollow Section tubes joined by 25x25x2mm SHS in a zig-zag fashion. The 65x65 SHS is about 170mm apart (300mm deep all up), and they go some 2600 to 3600mm high. At the base is a 10mm steel plate with holes for the all-thread rod. The top is welded to the truss (next section). It is all fully-welded (no stitch welding). All steel (except the base plate) is galvanised, all welds (and the base plate) get a zinc-rich paint applied. Weld on a level surface, we placed some spare 25x25x2mm sHS on the concrete slab and levelled it (packed under it in places) to read less than 1mm deviation (using a dumpy).

Steel Trusses

As above, except these are 400mm deep all up, and the longest span is 10,000mm. The trusses extend beyond the columns about 1,000mm to provide a big eave for the house. The eave extension is not as deep as the rest of the truss so that it doesn't look as bulky under the eaves.

Along the top of the trusses some purlin brackets (8mm galvanised steel plates with two or four holes in them) are welded to the truss and provide a solid location to bolt the purlins onto.

Steel Purlins

These can either be C-section or Z-section. We chose C section because it is slightly cheaper (Z-section can be single-span double lapped, ie: add 15% to your overall purlin purchase; C-section can be double-span with no lap, ie: heavier & longer lengths, but no need to overlap the joints). Next you should specify exactly where your purlins bolt onto your trusses, so that the factory can pre-punch holes in the purlins (no extra cost, saves you a lot of work). The purlins in our case ar 990mm apart, and the longest single purlin is about 12,000mm. We used 200mm C-section purlins.

The Roof

On top of the purlins goes "roof safety mesh" (sortof like chicken mesh with bigger holes) followed by foil-backed insulation (called a blanket) - in our case, fibreglass due to cost, but could be wool or polyester; followed by the roofing iron.

The roofing iron is trimmed (or flashed) with a bit more colourbond, custom bent to suit our roof (custom bent flashing is cheaper than two or three bits of standard flashing, and fits your roof requirements).

Assembly

The columns were moved and erected by hand - a two person job, but manageable. Either you have enough space to run your concrete drill through the plate in place, or you need to place the column, spray-paint through the holes, remove the column, and drill where the paint is. Loosly bolt in place (hand-tight in my case).

We used a crane to lift the trusses up onto the columns. The trusses would be over 200Kg each, so other options for erecting them exist, but the crane was fast and easy (not to mention cool to see a new crane !). The trusses were clamped onto bits of scrap metal tack-welded to the columns (later on removed by angle-grinding and touched up with zinc rich paint). Welding of the truss to the columns was done in place.

Bolt in extra bits (struts, diagonal bracing), pack under the column plates, and true everything up. Stuff in the non-shrink dry-pack grout. Wack on your purlin, though over the roof safety mesh, add insulation and roofing steel, and you are mostly done (just trim, gutters, walls and runnign services to go ...)