New Building Technologies

[Michael Walsh]

I was at a trade show at the week-end and saw a couple of new building methods and was wondering if anybody on the list have used them and if so what were their experiences.

The first was an engineered flooring joist it was made from 75mm square timber frame with steel diagonal ties and occasional 75mm square reenforcing. It came lengths up to 5M. It seemed like a good idea as it allowed easy running of plumbing and electricals through the joists without drilling as they were hollow and it is supposed to have better flex and load carrying characteristics

Joist Side on, all horizontal and verticals 75mm timber, diagonals steel ties.

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The second item was a building block made from extruded clay it was a sandwich of a 125mm inner block, 100mm polystyrene insulation and 75mm outer block. They are used alot in Finland apparently. They are fixed using an adhesive rather than traditional sand/cement mortar. They were very light but seemed flimsy to me.

[Tony Bryer]

Catnic tried an idea like this about 20 years ago. The diagonals were V shaped galvanised pressed steel, with the ends and point being stamped out to form nails like the plates used to assemble trussed rafters and pressed into the top and bottom members in a similar way. They didn't catch on.

I would wonder about the resistance to rain penetration. In some respects a climate where it stays below freezing for months on end is less demanding that our freeze/thaw driving rain winters.

[Mary Fisher]

What were the reasons for it not catching on? Just too new, people too suspicious of something unfamiliar? Presumably it had been tested and proved to be suitable.

Mary

[Set Square]

So what was the overall size of cross-section? Do you have a feel for size of ordinary wood joist it would replace - in terms of equivalent strength and bending siffness?

[Tony Bryer]

At a guess (a) cost: though the builders probably weren't factoring in the time saved in drilling holes for servicing, sorting out twisted joists etc; (b) need to be ordered to length probably several weeks ahead, whilst for 8x2 you can phone a timber yard and they'll be delivered tomorrow; (c) they probably needed somewhat more careful handling which cannot be relied on on many UK sites; (d) they might (though might not) be damaged by overloading - as might occur when the bricklayers have loads of bricks or blocks stacked on boards across the joists. Ordinary solid timber joists might deflect a but they'd spring back.

(b) and (c) apply to trussed rafters too. With these though the time saving is more obvious. In my BCO days we did have trouble sometimes with badly stored trusses being damaged.

[Mary Fisher]

So the builders need to adapt their techniques ... that happens in all trades with all new products.

In my experience :-)

Mary

[The Natural Philosopher]

Basically its actually more expensive than just whacking down 6x3 softwood.

To make the most out of all this stuff you need a slightly different approach to design.

If you mass produced a complete celing/floor using them, so the blokes on site just laid it en bloc, it might be worthwhile.

All this clever stuff means you have to change teh way things are done. Pre-fabbing is vheaper, but you have to think about plumbing and wiring eairler, and have things like cranes on sie to lift in prefab chunks.

[Christian McArdle]

Wow, what was the name? I'd really like more information on this one.

I'm currently considering steel to get a 4m span under 100mm. A wood based product would probably be easier to install than steel, provided it can be approved for 30 mins fire resistance with only plasterboard below.

Christian.

[Tony Bryer]

It wouldn't help you on height. IIRC the Catnic ones I referred to had top and bottom chords of something like 75x38 flat (one of selling points was lots of nailing room for floor deck and ceiling) and the overall depth was about 300mm. So it had the stiffness of a 9x2 but weighed half as much.

With timber joists deflection is nearly always the limiting factor, and stiffness is proportional to the cube of the depth. Keeping the width and joist spacing the same, if you reduce the size from 6" to 5" the deflection goes up by (6/5)^3 = 1.75 and from 5" to 4" by (5/4)^3 = 1.95 - i.e. 2 x 50x100 = 1 x 50x125