Loggia Ledger

[Gus Potter]

On 19/11/2022 at 11:11, PeterW said:

We are aware there are many professionals on here and generously give their time and advice on the forum. To that end there is a clear statement in the terms and conditions 

Peter.

 

To you and all the mods. Big thanks from me and thank you for all the time you put into BH to make it work.

 

I'm not a legal eagle, I know bit but probably enough to be dangerous! I think you have our back on Build Hub but even if you don't then.. to make a forum like this work you need all sorts. I chip in as it's a friendly site, have got to meet some folk in person and made new friends. Not bad eh?

 

I learn loads of stuff here not just from the pro's but also from the new members that are coming on the site with innovative ideas, asking questions.. challenging the norm. Without new members and you old sorts (not me as I'm only 58)  doing your tenth self build the site would not have the richness that it has. If you doubt then have a look at the Farming Forum.. yes it is commercial.. not my cup of tea but they have the depth and breadth of contributers that makes it work. BH is the same without the commercial and that "edge" you get on commercial sites.

 

In terms of liability. When I was training and cutting my teath one of my mentors said (runs a top notch SE Consultancy) .. Gus you are worried about getting sued too much.. Clients don't want your caveats.. they want answers and good commercial ones too. Study hard (we are giving you the tools to educate yourself @SteamyTea) and you will be able to give the right answers / solutions.. ! If you do then you won't be sued.. well you might by some dafty but folk are folk, that is the price you pay for having an exiting job. Enjoy your job, do what you are good at and have fun.

 

So in that context lets look at

On 21/11/2022 at 19:00, MortarThePoint said:

Are you saying that the rafters themselves could do that by using a bracket on the rafter. Interesting!

Ok lets go back to my earlier post.

 

On 17/11/2022 at 00:27, Gus Potter said:

2.39 * 1.375 / 2 = 1.64 kN/m. In other words the "DESIGN" load on the ledger is 1.64 kN per metre run of the ledger.

Say the rafters are spaced at 600mm centres. Thus the shear load at the top end of each rafter is 1.64 * 0.6 (spacing) = 0.98 kN.

 

Below is an extract from the Simpson brochure for the 7090 brackets.

 

I have not copied it all but the bracket capacity changes depending the grade of timber and nails you use so please check this yourself. The main thing to take away from this is how you read the table. This table is based on Eurocode design and the value we are interested in is the characteristic value. In the previous post I showed how we worked out the DESIGN loads.. we worked out the loads and then applied safety factors. BUT we also need to apply safety factors to the Characteristic loads for resistance as we are using limit state codes.. which in this case is the brackets. Generally for simple timber design we apply a material factor of safety of 1.3.

 

Thus you can see for the 7090 bracket the characteristic shear load with 35mm nails is 7.6 kN. We divide 7.6 / 1.3 = 5.84 kN which is well above the 0.98 kN. Caution.. read the fine print as the values are based on two brackets.. one each side!

 

Now that seems overkill in terms of holding the roof up.. but

On 14/11/2022 at 14:00, MortarThePoint said:

I'm trying to finalise my thoughts around this loggia

 

What you do here is pile up logs and inevitably they will end up pushing the vertical columns (timber posts) outwards. The 7090 bracket also work in tension that have a declared value. The brackets can take tension where as joist hangers commonly can't.

 

Earlier we looked at the resin fixings.. these to can take a bit of tension..

 

For all if you are trying to get you head round this sort of thing try and work out what forces you have and follow where they go.

On 21/11/2022 at 19:09, MortarThePoint said:

Would it be possible to use hidden woodscrews like below?

Will have a go at this but not tonight as screws into the end grain of timber are not.. "that good".. I would try and avoid any end grain screw that is subject to a tensile force.

 

 

 

 

 

 

 

 

 

[MortarThePoint]

9 hours ago, Gus Potter said:

On 17/11/2022 at 00:27, Gus Potter said:

2.39 * 1.375 / 2 = 1.64 kN/m. In other words the "DESIGN" load on the ledger is 1.64 kN per metre run of the ledger.

Say the rafters are spaced at 600mm centres. Thus the shear load at the top end of each rafter is 1.64 * 0.6 (spacing) = 0.98 kN.

 

Below is an extract from the Simpson brochure for the 7090 brackets.

 

I have not copied it all but the bracket capacity changes depending the grade of timber and nails you use so please check this yourself. The main thing to take away from this is how you read the table. This table is based on Eurocode design and the value we are interested in is the characteristic value. In the previous post I showed how we worked out the DESIGN loads.. we worked out the loads and then applied safety factors. BUT we also need to apply safety factors to the Characteristic loads for resistance as we are using limit state codes.. which in this case is the brackets. Generally for simple timber design we apply a material factor of safety of 1.3.

 

Thus you can see for the 7090 bracket the characteristic shear load with 35mm nails is 7.6 kN. We divide 7.6 / 1.3 = 5.84 kN which is well above the 0.98 kN.

 

That was the loading on the top (ridge) ledger wasn't it so not near the ceiling ties. In the US Simpson do a nice Face-Fix Rafter Hanger (LRUZ) but it isn't available in the UK unfortunately. They have an SPR product, but that's for 6x2 and pretty expensive.

Frame anchors are pretty discrete. I'm not sure which directions Fx,k , Fy,k and Fz,k are but if you pick the lowest one it's a characteristic capacity of 2.16kN for C24. Dividing byt the 1.3 safety factor that gives 1.66kN which is greater than the 0.98kN requirement so should work (?). They also specify use in pairs.

 

I like how these look if using Simpson Strongtie CSA screws which have an axial pull out strength of 1.28kN for the smallest type (CSA4.0x30). I expect they would improve the numbers of the table above. Following the load and comparing this to the tensional Design Capacity of the lower ledger (ceiling level) which was around 1.14kN per fixing I think, so these frame anchors would be stronger than the ledger fixing if the ceiling ties were on the same c/c as the ceiling ledger fixings. If the ceiling ties were 1200mm c/c then there would be two ledger fixings per ceiling tie and their strengths would become comparable (2*1.14kN vs 1.66kN) though the ledger fixings would win (based on the FA's table).

 

Is there any way the tensional load on the ceiling tie can exceed the shear load on the top ledger (which we know to be 1.64kN/m Design Load)? I can only think wind uplift might do that but it would need to overcome the weight of the tiles by that point and I suspect the roof would be in tatters at that point in a nuclear blast. Would it be the load of things resting on the pillars that are of concern?

Edited November 23, 2022 by MortarThePoint

[Gus Potter]

On 23/11/2022 at 09:37, MortarThePoint said:

Is there any way the tensional load on the ceiling tie can exceed the shear load on the top ledger

Hi Mortar.

 

Have not checked you figures as kind of off duty here.

 

These brackets look like a Simpson AE35. You have to wrap the top tab over.

 

On 23/11/2022 at 09:37, MortarThePoint said:

Is there any way the tensional load on the ceiling tie can exceed the shear load on the top ledger

Your train of thought is good. You mention wind.. probably won't govern unless it gets close to a mansard type roof which you don't have.

 

It's the practical side of things.. on a loggia folk will pile stuff up and use the posts to hold it all still.. that is where the risky horizontal loading comes from.

 

I would stick to the 7090 brackets.

 

Other way it to use ceiling ties.. but that compromises the vaulting effect to some extent.

 

As an aside for all on BH.

 

If you have an add on to the house with one side open and the rest clad you can encounter quite a lot of horizontal wind forces. This is what we call an open sided building.. has a dominant opening. You find this with hay sheds. In the spring you have the columns, it is just empty (hopefully) and the wind blows though. But say mid winter.. the bales are left at the sides and taken from the middle.. the wind blows in and can't get out the sides so it lifts the roof off for example.

 

 

 

 

 

 

 

[MortarThePoint]

18 hours ago, Gus Potter said:

I would stick to the 7090 brackets.

 

Other way it to use ceiling ties.. but that compromises the vaulting effect to some extent.

Ah, I think I may have misunderstood you. Are you saying that just having 7090 brackets at the top is sufficient and then there wouldn't need to be any ceiling ties at all? So like this:

[Gus Potter]

No you also need something at the bottom to stop rafter uplift. The birds mouth needs to be held in close contact with the wall head / lintel.

 

You could maybe use a truss clip but on the outside face of the lintels. Will be nicely hidden here, but you need to fix them before the sarking / tiles.

 

If you want to go for a "sturdy" job use the 7090's all over.. but some of the nails will be to close to the joint between the double lintels.. nail edge distance.

 

You best bet is to assemble is and do a trial fit with the 7090 brackets.. use common sense.. do they look right if not try a truss clip.

 

Generally a truss clip has 6 nails into the lintel / head binder and 6 into the rafters all with at least 19 mm edge distance for the nail in the loaded direction of the force if using a 3.75mm diameter nail... 5* 3.75 = 18.75mm ish..

 

Now that is quite hard to get right on site when you have angled timbers resting on a say a head binder. Have copied a table from BS 5268 for reference. But if you fit say truss clips on the outside the problem often "goes away"