Green Larch Timber Frame

[kav]

Great stuff, been looking at building my bro a shed out of larch, inspired now:thumbup1:

[ucoulddoit]

Another progress photo as the frame is complete and the roof is on, and there will be no further work until the spring while the larch wall cladding is milled over the winter.

 

There have been a few comments/concerns on other threads about structural grading of self-milled timber, structural use of green softwoods, etc. so I thought there might be some interest to hear my views about structural use of green softwoods. This post is quite long……

 

I used BS 4978 'Visual strength grading of softwood - specification' as the basis for visually grading the larch, and used the wet graded category. I based the structural calculations on GS grade (general structural) rather than the higher strength SS (special structural) even though much of the milled timber complied with the higher grade requirements. When milling timber for a specific project, there is a bit of a chicken and egg situation with the calculations as the size of a beam or column is assessed by carrying out calculations which include an assumption of the strength grade, before the timber is milled. So if the assumed strength grade is high, there is a risk that a significant amount of the milled timber will be a lower grade and hence the wrong section size/too small for the proposed use. Each individual piece of timber needs to be strength graded. It is not possible to determine the strength of the pieces of timber that will be milled from a log by looking at the outside of the round log or by knowing the timber species or by testing a small sample. Individual timbers milled from the same log may have different structural grades. For instance, if a graded timber is cut lengthwise into two pieces, each piece should be re-graded. Having said that, it was pretty obvious from looking at the available larch logs that the best timber would be from the bottom few metres of the trees where there had been no branches for many years. So I selectively milled the logs which worked well.

 

For the strength properties of the larch I used BS 5268 part 2 which also includes strength properties for British grown douglas fir, pine and spruce. Each species is designated as either GS or SS, but the species are not interchangeable. Larch GS grade is stronger than the GS grade for the other three species for instance and if ‘dry’, is equivalent to C16 softwood.

 

To allow for the timber being 'green' and wet graded, I reduced the allowable stresses on the basis that it was service class 3. For bending stresses, the reduction factor is 0.8 (80%), so a fairly modest reduction. Compression strength is reduced by a factor of 0.6, so much more significant. Service class 3 applies to timbers with a moisture content above 20% at the time of installation, with no upper limit. This reduction factor is not specifically intended to be used for green timbers. It is used for external timbers and timbers with thickness exceeding 100mm which are difficult to dry. So this was a point at which I used judgement and made a pragmatic decision that for a small scale project for my own use which didn't need statutory approval, the reduction factor was acceptable. I wouldn't do this for a high value structure such as a frame in a house without further research about the wet grading, but to be honest, it looks fine.

 

In terms of construction details I used standard oak framing principles but increased the joint proportions slightly and this seems to be fine but I'll need to see how the joints withstand distortions and shrinkage as the frame timbers dry over the next few years.

 

I visually graded the larch timbers for this frame myself, but I have to confess that I didn't carry out a really detailed inspection of every timber. Only the central rafter and the corner columns needed to be sized based on the calculated stresses so I did inspect and visually grade these carefully. The sizes for all other elements were based on either aesthetics (e.g. all three rafters are the same size although the loading on the central rafter is much higher than on the eaves rafters), or the joint details to allow for the oak peg spacing, etc. I used my judgement to ensure that there were no significant defects at the points of highest stress in these timbers. As the three rafters were all milled the same size, I was able to select the best one for the most highly loaded central rafter and did the same for the columns. Also, I milled the timbers well over the length required (as it was free) so that I could exclude defects from critical points such as joints. I’m sure this overall approach has been done by timber framers for centuries……?

 

All that might seem a bit over the top for such a small outbuilding, but I've found it interesting to see that the final design is more or less what I might have built using only my judgement for the timber sizes and details. Also, reassuring that hopefully it will survive the winter storms. It has been very time consuming to fabricate the frame and if I hadn't been set on a traditional form of construction and the inherent aesthetics, it would have been much quicker to use lap joints with bolts. I wouldn’t hesitate to use green larch again for a project like this provided I could source slow grown old trees. It has just over 60 cu ft of milled green larch which I suppose would have cost between £1500 to £2000 in green oak including transport so with a bit of creative accounting, this one small project has more than covered the cost of my chainsaw and alaskan mill!

 

The only problem so far? My wife has decided it looks to be far too good for a timber drying shed and is keen for it to be completed as an 'artist's studio' with the larch frame left exposed on the inside........

 

 

Andrew

[Richard 1234]

Looks great well done👍

[pup]

Excellent post excellent project

Interesting to hear your thought processes

And I also agree it looks good 👍🏻

Alun

[agg221]

That's very interesting. I have never used softwood structurally but obviously I have encountered it in conventional buildings and your corner posts end up being much heavier section than I would have guessed. Looks good though.

 

Alec

[ucoulddoit]

Thanks for all the comments. The columns are 200 x 200 which is pretty chunky for a total roof area of about 26m2. The vertical loads are very low compared to the post capacity so it was the wind loads and the fact that the bracing mortices remove a lot of timber at the point of maximum bending which determined the column size. I could probably have reduced the size a bit, but decided not to rely on resisting any tension force in the diagonal braces and the oak pegs at the end connections. Although each wall has two diagonal braces, only the one that is in compression for a particular wind direction is assumed to be working. Probably a bit over engineered, but it does mean the frame is quite robust and should be suitable for a change of use in the future.

 

Andrew

[Woodworks]

That looks great Andrew

 

Love your threaded bar means to fix it down. Much better than having it buried in the ground and the subsequent rotting at ground level which is happening to one of our barns.

[nick1854]

but I've found it interesting to see that the final design is more or less what I might have built using only my judgement for the timber sizes and details

 

This is my experience of making small timber framed buildings too Andrew. I haven't looked into the engineering/structural grading aspects as much as you have, but found that by building "by judgement" can work well. For me "judgement" was built up by looking at other timber frames, reading how professionals frame and my own knowledge of working/using different types of timber (eg. you develop a sense of what a particular section of timber will span unsupported). Also as a default option for safety and peace of mind use a bigger timber!

 

You have made an excellent job of the bracing Andrew, nice tight joints.

 

How are you going to do the cladding?

[ucoulddoit]

Cheers Nick. I'll be using vertical larch boards a bit like 'Yorkshire Cladding' used on farm buildings, but with a second layer of boards on the inside which will hopefully encourage good airflow, but keep the rain out. I'm planning to mill some thick slabs from logs using the Alaskan mill, so that I can manhandle them out of the wood and have them milled locally on a band mill. I'm pleased at how the joints turned out, but it was time consuming scribing the joints, cutting them, trial fitting, further work on the joints, etc. and the timbers were so heavy to move about. I suppose with practice, you can get a good tight joint almost first time?

 

Andrew

[muttley9050]

Great job. Very impressed. There's a special feeling when you build something that you have taken from a tree to a finished product.