Greetings from a practical f-wit

[Gus Potter]

Hi Mike.

 

9 minutes ago, MikeSharp01 said:

Other than thinking that a properly constructed Larsen truss is practical and therefore structurally OK I cannot comment further on them.

 

On 01/05/2022 at 07:53, MikeSharp01 said:

Filling in with solid insulation between Larsen Truss or I-Beams is a pain

Depends what side of the fence you sit on if so inclined. It could be properly designed SE wise but no use whatsoever to folk on BH in terms of buildability / labour cost. Your bang on with it being difficult to fit solid insulation between I sections for example.

 

Now your 50mm nail spacing on your racking walls? That I have not seen before but every day is a school day. It's interesting for me to see such close nail spacing. For example the racking forces are probably quite large.. so this will result in a fair bit of panel overturning effect, lots of uplift forces = plenty tying down straps and associated fixings. Also I'm curious as to how you SE has been interpreting the codes and taking account of what we call second order effects.. just nosey.

 

 

 

[Gus Potter]

6 minutes ago, joe90 said:

As I like dealing with wood rather than concrete etc and I found @Gus Potters post above fascinating

Hi Joe.

 

Thanks for the compliment, much appreciated. Pretty much every time I browse BH I learn something new. Some folk like numbers, some less but I try where I can, like a lot of other posters, to weave a lay persons "rough guide" and tell a storey when it comes to SE stuff. BH is supposed to be fun and friendly. I too struggle with some of the numbers especially when it comes to the insulation side of things but keen to learn at my end.

14 minutes ago, joe90 said:

But the cladding will be pushing against the edge of the OSB due to rotational  forces and fixed to the 50 x50 so It won’t bend (well it might bend outwards if negative air pressure is great 🤔).  But realistically anyone is going to nail at 150mm centres and may even glue as well so I think the Larsen truss is a great way of building a wall that will allow a lot of insulation (maybe I might do another build 🤔) (maybe not).

 

also if the 50x50 is a bit bent (as a lot is) it can be straightened to meet the straight edge of the OSB prior to nailing.

Joe I think I can see where you are coming from in that you are looking at each individual cladding board? But even though each board will be counter acting the effect of the one above or below they still have the net effect of loading the truss at it's outside edge. Think of the truss as a column. If you load a column about it's centre all the load goes straight down so you don't have a load set off from the centre of gravity of the column thus no initial bending effects. The same applies if you have a joist bearing onto a brick wall. If the point of bearing is not dead centre over the wall it will cause a bending effect in the wall as well as a compressive load.

 

That's a good point you make about nails at 150mm centres and that they seem close at first glance. However when we design timber frames and check for lateral resistance, to check the house won't move sideways, the starting point in say BS 5268 is to begin with a 150mm nail spacing around the edges of the boarding. Then we modify the nail spacing if need be. 9.0mm OSB is pretty thin and if the nails are too far apart then the OSB buckles not least between the nails, a bit like the thin backing on a kitchen cabinet. The backing on the cabinet keeps it square, same principle applies to a timber frame, just a TF is often more expensive?

 

I have copied, some info below. You can see in coulmn 2 the starting point for nail spacing internally and around the perimeter of the boards.

 

 

 

 

 

[MikeSharp01]

7 hours ago, Gus Potter said:

Also I'm curious as to how you SE has been interpreting the codes and taking account of what we call second order effects.. just nosey.

Cannot say really - I just made it as exactly as I could to his (it was a him) specification. He did explain that the racking strength was fine longitudinally (Basically North-South) but limited cross wise so he wanted the cross walls to be really stiff and nailed as stated. We do have a reasonable amount of hold down straps into the passive slab to the upright I-Joists, the sole plate is also held down separately while each I-joist is also fixed to the sole plate with Strong-Tie angle brackets and screws. I will have a look at his calcs and see what the uplift forces might be - not much I suspect given the adjacent dwellings. 

 

We do have the 150 spacing on the longitudinal and roof boards. All boards are 15mm OSB 3.

[joe90]

8 hours ago, Gus Potter said:

Joe I think I can see where you are coming from

If I had my time again I would have liked to study being an SE, as a practical type of chap I can usually work out what will stand up and what will break but would like to understand the science behind it. 👍

[SteamyTea]

49 minutes ago, joe90 said:

If I had my time again I would have liked to study being an SE, as a practical type of chap I can usually work out what will stand up and what will break but would like to understand the science behind it. 👍

Just a case of changing mass to force.

Then remembering that force at a distance is torque.

And a moment is a place, not a spot in time.

And materials fail differently.

Holes can reduce forces.

CT1 puts it all right.

[Simplysimon]

21 hours ago, Gus Potter said:

Your bang on with it being difficult to fit solid insulation between I sections for example.

perhaps, however, if you fit a stud at each end to your head and sole plate, then fit insulation prior to fitting a stud and work you way along it will be a lot easier.

 

 

21 hours ago, Gus Potter said:

I have copied, some info below. You can see in coulmn 2 the starting point for nail spacing internally and around the perimeter of the boards.

 

the nailing centres, does this take into account d4 glueing sheets to both sides of the stud and therefore the increased racking strength?

[joe90]

Re insulation, if you used rockwall , blown cellulose or poured insulation beads you can fill all voids.

[markc]

@Gus Potter only just seen this as it’s been a mentally busy week building a new design crawler chassis for a Mewp. But back to the post, I reckon you have nailed it (no pun intended). People rarely  look at shear strength against load in nailed connections, I’ve seen trussed rafters doubled and trebled with literally hundreds of nails when a a few in each member would have been more than enough.

[MikeSharp01]

12 hours ago, markc said:

@Gus Potter only just seen this as it’s been a mentally busy week building a new design crawler chassis for a Mewp. But back to the post, I reckon you have nailed it (no pun intended). People rarely  look at shear strength against load in nailed connections, I’ve seen trussed rafters doubled and trebled with literally hundreds of nails when a a few in each member would have been more than enough.

Triangles are everything I was always told.

[SteamyTea]

54 minutes ago, MikeSharp01 said:

Triangles are everything I was always told.

Obtuse

 

I like it when the angles of a triangle don't add up to 180°C.  Makes it more real.