Larson truss

[Russell griffiths]

Did I read on here recently about somebody building with a Larson truss type wall or jji joists

i think it was being designed by a specialist company and then pre cut by another company. 

 

Any body remember. 

[Jeremy Harris]

Our house is built with a Larsen truss system.  The inner studs take the structural loads, the outer ladder frames sit on the protruding under-slab insulation and provide a space for the cellulose fill to be injected, plus they add some bracing and lateral stiffness to the structure.

 

It's essentially exactly the same as the system that John Larsen came up with, with 300mm deep trusses, but prefabricated in a factory and with slight lower thermal bridging.  Mind you, John Larsen did come up with the idea back in 1981 - makes you wonder why it's taken us so many years to pick up on the idea..................

[IanR]

There's a couple of people using Cullen Timber Design to design JJI Joist frames with the possibility of having them pre-cut at the factory, before delivery. 

 

Edited to add: but remembering back I thought you were one of them so you know about CTD already. 

Edited December 3, 2017 by IanR

[Russell griffiths]

I have looked into this @IanR and would like to find someone who has actually done it, as I would like to go and have a look at theirs. 

I really can’t get my head around what build type I want so would like to tour some sites with differant build types. 

[IanR]

That makes a lot of sense. @bissoejosh was the other member that was using CTD if I remember correctly, although at early stages currently I believe.

 

Mine is a CTD designed frame, as part of a Touchwood package. My build is complete so the frame is not at all visible. I have lots of photos of the frame going up, if there's anything specific you need to get your head around then I may have it captured. 

 

I got quite involved technically with my frame design, swapping CAD models back and forth with CTD while we resolved issues around interfacing with the existing steel frame of the building we have converted, and they're are truly excellent at what they do.

 

I don't think there'd be any discernible difference between a completed house built with Larsen Truss Frame than that built with a I-Joist frame. If you are erecting it yourself then you may have to make up the trusses yourself as I'm not aware of anyone setup to produce these as supply only, for a stick build. (I may be wrong I've never searched for this option) Whereas I-Joists are a stock item that is just being cut to length either in the factory or on site.

Edited December 3, 2017 by IanR

[Russell griffiths]

I seem to be going around in circles, I originally wanted to stick build on site as I had done this previously, but then moved towards icf after reading a couple of blogs and doing a bit of research. 

Our new house design has some large roof overhangs 1.5m in places so I thought I would need a company like Cullen to design the roof structure, so started to do some more looking at projects that they do, and here I am looking at timber frame again. 

 

Im sure life’s not meant to be this hard. 

[bissoejosh]

We're going with an i-beam frame, all designed by CTD to be stick assembled on site from factory cut timber - hopefully much like a giant air-fix kit. I looked at Larsen but couldn't find a supply only provider. As mentioned CTD are refreshing to work with compared to many others and I feel they could tackle virtually any frame design issues.

 

 

Edited December 4, 2017 by bissoejosh

[Jeremy Harris]

Has anyone looked at drilling large holes though the I beam webs to reduce the thermal bridging and effectively turn them into Larsen trussses? 

 

If using blown cellulose, big holes like this would also allow an easier fill.  The webs in an I beam can tolerate loads of big holes with no reduction in stiffness or strength - they are only solid because it reduces manufacturing cost.

[IanR]

I don't know that the advantage of a Larsen Truss frame is thermal bridging within the wall structure. When you compare the two systems,  I-Joist may have a lower timber fraction than Twin-Stud.

 

The below is making assumptions on the timber sizes within a Larsen Truss based on images I have found, if someone can come up with better timber sizes I'm happy to update. For the centre nogging I've assumed 3 noggings within a 2.4m panel.

 

***Updated after JSHarris reply below

 

I'm surprised MBC appear to use 4x2 for the noggings, I would have thought they could get away with thin ply, but that's not what the pictures show that I could find. I've also always wondered why, if the outer stud is non-structural, why it's not a smaller section than the inner stud.

 

There maybe be opportunity to optimise the MBC truss.

 

I don't believe there is much scope for cutting holes within the webs of the I-Joist. I can't find the document at the moment, but I was supplied one that was fairly restrictive on the amount of web that could be removed.

 

However, the guys that did the pumped cellulose on my build commented that closed off channels were much preferred as it guaranteed even fill. They were critical of open panel systems as they didn't feel it would be possible to fully fill at even pressure.

 

In my view the advantage of a Larsen Truss over I-Joist is being able to sit the outer stud over an EPS upstand. This allows for a simpler perimeter condition on the slab to avoid a thermal bridge between wall and floor. I-Joist require a slightly more complicated perimeter to do the same.

Edited December 4, 2017 by IanR

[Jeremy Harris]

There's only three noggins per storey, though, with a twin stud.  and the timber is standard stud work timber, so 89 x 38, inside and out.  The noggins are also 89 x 38, as are the top and bottom plates.

 

The fill point is really more about speed, as every bay has an individual fill hole at the top with a twin stud wall, just as with an I beam.  The difference is that the cellulose partially fills the adjacent bays through the big gaps, so once the first bay has been filled the rest along a run are a bit quicker.  The high pressure ensures the fill is even, either way.

 

FWIW, our roof is cellulose filled I beams, with an added bit of studding spaced 10mm off the bottom of each I beam to increase the depth to 400mm.

Edited December 4, 2017 by JSHarris

[IanR]

26 minutes ago, JSHarris said:

There's only three noggins per storey, though, with a twin stud.  and the timber is standard stud work timber, so 89 x 38, inside and out.  The noggins are also 89 x 38, as are the top and bottom plates.

 

Thanks for the timber sizing, the original drawing has been updated and that brings the timber fraction more similar. The difference is now not decisive on whether to use one option or another.

 

I still see an opportunity to improve the twin-stud over what I have drawn, reducing the thermal bridge further.

Edited December 4, 2017 by IanR

[jack]

1 hour ago, IanR said:

I'm surprised MBC appear to use 4x2 for the noggings, I would have thought they could get away with thin ply, but that's not what the pictures show that I could find. I've also always wondered why, if the outer stud is non-structural, why it's not a smaller section than the inner stud.

 

I'm sure it's a combination of simplifying the machinery/jigs they use for cutting and assembly, and minimising the number of material types they need to keep in stock.

 

Also, the outer stud does have panels attached to it, plus may need to hold potentially heavy cladding. I think it would be a challenge to greatly reduce the cross section of the outer stud while still providing a good fixing for, eg, battens. 

[IanR]

The I-Joists outer flange at 47x45 does the same functions (+structural support), so the section could reduce to this. And the web could likely be ply as other manufacturers of Larsen Truss use.

 

But we're into the arena of small gains so it is likely, as you say, that the choice is one of simplified stock rather than achieving ever gain possible.

Edited December 4, 2017 by IanR

[Jeremy Harris]

In essence, you can make stiff beams lots of different ways, but all use the same core principle of having the primary load bearing elements as far away from the centre as possible in order to increase the second moment of area (or moment of inertia).  The objective is to have as high a second moment of area as possible with the least amount of material.

 

The shear web (which is really noggins in the Larsen truss design) only takes shear loads and keeps the structural member separated so as to maximise the second moment of area.  It's very common to have large holes in sheet shear webs, to lighten the beam and reduce cost.  The stress in the centre of the shear web is zero, which is why it can have big holes cut in it.  The most common example of this we see is with Posijoists, where the shear web is just a load of shaped nail plates keeping the two structural members apart.

 

There are some timber I beams around with punched out holes in the shear web, really just to make it easier to run services, but these could just as easily be used to reduce thermal bridging in a wall or roof structure.  The problem is cost, I think.  Basic I beams are really very cheaply made when you look at them, and the OSB shear web is the cheapest way of doing the job.  OSB is OK for a lot of things, but if you cut lots of holes in it fairly close together it's shear strength isn't great.  Ply webs would be ideal for this, but the cost would be a fair bit higher, I'm sure.

 

The reason for twin stud walls using noggins and not ply spacers, as John Larsen originally used, is down to ease of manufacture.  It's a great deal quicker to just position noggins and press on nail plates than it is to faff around nailing on ply webs.

 

 

 

 

[ProDave]

I have just finished wiring a house all built of I beams. All I know is where they cut large holes in the web for soil pipes etc, they reinforces on both sides of the web with gusset plates. That tends to suggest a large hole would otherwise loose too much strength.

[Jeremy Harris]

1 minute ago, ProDave said:

I have just finished wiring a house all built of I beams. All I know is where they cut large holes in the web for soil pipes etc, they reinforces on both sides of the web with gusset plates. That tends to suggest a large hole would otherwise loose too much strength.

 

Beam theory says it has virtually zero effect of stiffness or ultimate strength, and I beam joists are designed to meet maximum allowable centre deflection criteria, so are massively under stressed, anyway.  Take a look at the main spar on an aeroplane - that's a simple beam that's massively loaded in comparison to anything in a house (the margin may be around 1.5 for an aircraft, versus around 5 for buildings, where there is a greater probability of unknown loadings being applied) where the web will often be full of whopping great holes.

 

The problem with OSB web I beams is that the OSB used is not great, plus OSB relies on having lots of bonded wood chips over a large area to gain its shear strength.  Anyone who has cut a bit up will know that small bits of OSBare really weak in shear compared to large sheets.