MortarThePoint

The outer leaf for most of the house's cavity walls are going to be flemish bond. We'll cut bricks in half to make 'header' part of the bond as you'd expect. What I was wondering is whether we should widen the cavity slightly to make the laying easier. Half a standard brick is 215 / 2 = 107.5mm whereas the standard brick width is 102.5mm. The blade of the grinder will take a bit off, but it's enough work to cut bricks in half let alone cut a slither off. The cavity is to be 100mm mineral wall. Should we tweak the cavity width to say 105mm to make life easier or could that create it's own issues? I discussed it with a pair of brickies, one said no as he likes a 'tight cavity' whereas the other said yes and the compromise was to go with 102.5mm, but I wanted to see what others thought.

I've realised the figures for the deflection were wrong as I should have used: d_max = W*L^3 / 3*Y*I []NB: I used Y for Young's modulus but table uses E] as set out on the second line of the BEAM BENDING table. I didn't read the table properly as I assumed M=W*L applied to the deflection as well, but it doesn't ( M = W*(L - x) ). That changes the deflections figures to: 4x2 : d_max = W*L^3 / 3*Y*I = (150*9.81) * 600^3 / (3 * 7200 * 4E+6) = 3.7mm 5x2 : d_max = W*L^3 / 3*Y*I = (150*9.81) * 600^3 / (3 * 7200 * 7.6E+6) = 2.0mm

I think I understand what you are suggesting, effectively making a timber I-beam. From the calculations I am content that it will be stiff enough without that. There will be a fair amount of cold bridging, but the plan is to fill the gaps between the 4x2 noggins with 100mm Celotex then with 18mm CDX ply on top and 9mm CDX ply beneath. If I make it too warm then a log burner would turn it in to an oven. Probably already the case actually as not many shepherd huts will have a 100mm PIR insulated envelope. That's better insulated than we are planning for the house.

ProDave, you're right I'm sure it would make a big difference and lower the maximum stress considerably. I did mention "their weight wouldn't be carried solely by one beam", but that perhaps understates it. I'm happy that C24 4x2 would be sufficiently strong, but wanted to crunch some worst case numbers to get some confidence. I may double up a couple of the beams at the doorway though.

HEALTH WARNING: AMATEUR AT WORK!! Right so I am going to have a go at playing Structural Engineer. I've found a copy of BS5268 - Structural use of timber. Table 8 of this has "Grade stresses and moduli of elasticity for various strength classes" and lists: C16 - bending parallel to grain 5.3N/mm2 and minimum Y=5800 N/mm2 C24 - bending parallel to grain 7.5N/mm2 and minimum Y=7200 N/mm2 My cantilever length is L ~ 600mm My maximum moment with 150kg at ~600mm is M = 150kg*9.81N/kg*600mm = 9E+5 Nmm A beam's second moment of inertia is I = b*h^3 / 12 (where b with its cross section width and h its cross section height) The resulting maximum deflection is d_max = M*L^2 / 2*Y*I The bending stress is sigma = M*y / I where y is the distance to the neutral axis so h/2, so sigma = M*h / 2*I For C24, Y = 7200 N/mm2 4x2 h = 100mm, b = 47mm Second moment of inertia: I = b*h^3 / 12 = 47*100^3 / 12 = 4E+6 mm4 The maximum deflection: d_max = M*L^2 / 2*Y*I = 9E+5 * 600^2 / (2 * 7200 * 4E+6) = 5.6mm. Bending stress: sigma= 9E+5 * 100 / 2 * 4E+6 = 11.25N/mm2 > 7.5N/mm2 SNAP! 5x2 h = 125mm, b = 47mm Second moment of inertia: I = 7.6E+6 mm4 The maximum deflection: d_max = 3.0mm Bending stress: sigma = 7.4N/mm2 < 7.5N/mm2 OK However, that is based on permanent loading, for short term loading you can multiply the permissible stress by a loading duration factor, K_3 = 1.5 (from Table 17 of BS5268). That raises the limit to 7.5 * 1.5 = 11.25 N/mm2 which makes the 4x2 OK (just). There are other factors (like the depth factor) which I could make fractionally greater than 1. My scenario is very pessimistic and in reality the 100kg person wouldn't be out at 600mm and their weight wouldn't be carried solely by one beam. I need to think carefully about how I secure the beams to the chassis so as not to weaken them. I'm thinking like this, but possibly only 2 screws holding the blocks on so that the screws are closer to the neutral axis. Do these calculations look correct? HEALTH WARNING: AMATEUR AT WORK!! http://www.learneasy.info/MDME/MEMmods/MEM09155A-CAE/020-Compare/Compare_to_formulas.html

Good thought, it will be pretty top heavy

Last chance for me to change my mind on the timbers I'm looking to use for the base. The trailer has two rails that go the full length of the trailer and are 1270mm apart. The width of the hut is 2200mm or 2400mm, not decided yet. If I presume 2400mm, that means the hut overhangs the sides of the rails by 565mm on each side. Do you think 4x2 timber will be strong enough for that cantilever? They are on ~400mm centres and the panels and weight of the hut itself is likely to be equivalent to less than 50kg bearing down on the end of the 4x2. Adding that someone could stand against the wall and that could add as much as another 100kg. Does 150kg on the end of a 4x2 cantilever 600mm long sound OK? There is lots of safety margin in that as the person's weight wouldn't be solely supported by the one beam and the walls etc will be much less than 50kg (the panels way <20kg per 400mm).

It is nice and I wondered about that. Also considering the US style of vertical battens on sheet:

In the short term I'll lave the insulated panels exposed as they have an external skin of painted metal. In the long run I'll probably go with feather edged board.

I thought I would canvas opinions. I was planning to use studding on the end walls to stiffen them up and help against wind loads etc. Whilst the insulated panels are stiff, I'd rather have something a bit extra to help against shear. That's what the 4 sheets of 18mm plywood are to help with. I was wondering if people thought 18mm was overkill? The other 18mm plywood is for the floor.

A viewing panel for the sewage tank would make it easier to tell when it's almost full.

We may end up moving it around a bit, otherwise it would work well

Good idea with the skirt. It should be really handy for all sorts of things. I'll try to keep this thread topped up with progress.

It should be fun and provide a useful bit of space.

Thought I'd add a picture of the chassis. It's pretty high for a shepherd's hut, but use what you've got. Lots of sanding and red oxide paint in the coming days.

I'll work out where to put the windows more from the shepherds hut perspective. It may be a while until it gets used as a site hut unfortunately

Covid-19 has stopped my self build in its tracks just prior to dig and pour of foundations. I have massively got the twitch to make something and have some materials on hand and time to do it. Thinking what to make I have decided to make a hybrid site hut come shepherd's hut. I have a couple of old (but sound) 5.2m x 2m trailer chassis which I'll use one of for this. I stripped the 100mm coldstore panels off a building that got demolished and have 100mm Celotex that came out of the slab. I've done some basic CAD and thought I'd share what I am up to and see if anyone has any thoughts. Haven't thought about windows or internal stuff, also likely either clad with feather edge or plywood and vertical battens (US style). Base is 5.2m long and 2.2m wide so overhangs the side of the trailer by 100mm on each side. Doorway is currently 1800mm wide. Shown as 2000mm high but the panels could be cut to raise the ceiling a fair bit. Front View: End View: I haven't spent the time filling in the part of the panels that would extend up to to meet the curved roof. Section View: I've allowed for the roof / ceiling to be made out of curved corrugated sheet steel with 150mm of loft insulation and 5mm sheet plywood curved to make the ceiling. You can see the uncovered frame of the endwall. Notice the timber cross braces that extend into the 'living space' but rigidity. Skeleton: All made of 4x2 (95x47) timber. The insulated panels are quite strong themselves but need to be restrained. The plan it to use scrws top and bottom into the timber to secure the insulated panels. End wall: All timber 4x2. Propose to use 18mm plywood screwed to interior surface to provide shear stiffening. Base: All 4x2 timber with 18mm Exterior grade plywood sheets screwed on top or bottom (if bottom then add timber flooring to inside of hut. I think it needs: Skeleton: 53m of 4x2 untreated + 4 sheets 18mm plywood Base: 57m of 4x2 treated + 5 sheets exterior 18mm plywood Roof: 13m2 curved corrugated steel, 13m2 150mm loft roll, 5 sheets 5mm plywood When finished as a site office I can line the insulated panels with 5mm plywood and paint etc adding guestroom like furniture. What do people think? Will it stand up?

Well that was a close call! I had a feeling that this evening's announcement was going to be what it was and that has stopped work. Thankfully the dig and pour was due to be tomorrow and not today, otherwise they said they would have had to dig out the pour and start again (at their cost but still). I'll be doing a trickle of small jobs on site, but that's it for now. No materials and no foundations in so not much I can do. I'd been measuring their mark out and it was spot on, so I was getting excited about the foundations going in. Could be a few months now, but at least it's not half way through. Weird emotions relief and disappointment at the same time.

I'm sure there would be, not quite sure what but I have a vision in my head of the mesh hanging from timbers laid across the trench.

No keen to have a polystyrene separator, feels like an expansion joint. Probably made sense for your setup but I'd like the foundations to behave as one.

Hi I'm probably being a bit vague here. The clay is stiff/firm but above it there is sloppy wet cay that is I guess the water table. So I guess that means we are below the water table.

The subsoil is very sloppy and he foundation quite long with a number of corners. They are suggesting doing the blue shaded bit on the first day and the orange shade bit on the second day:

Due to the nature of the soil the ground work team don't think they can dig the whole foundation and pour in a day so they are proposing to pour in stages anyway. They should be able to confirm that cement is deliverable first thing and then dig and pour same day. Any shutdown that might be announced is unlikely affect the day of announcement so at least that day's work can be concluded. There is the risk that the second day gets called off which wouldn't be good as the concrete is more likely to 'bond' the two halves together if it is fresher I imagine. Does anyone know about any downsides of having the foundations done in two pours like this? The Structural Engineer has recommended "to dowel foundations at the construction joints, use 6no. H16 dowels, 600mm long encased 300mm deep in concrete on each side of joint; install dowels in three rows evenly spaced and providing minimum 150mm edge distance." I don't know how many would be on each row and will ask. The ground works team were talking about starter bars. What do people think?

The Structural Engineer has specified a maximum of 750mm of blockwork below external ground level and maximum 450mm below internal ground level (i.e. under void). Looking at these two pictures highlights that I should make sure it all works as the Structural Engineer has finished floor level way above external ground level whereas it will be 150mm above external ground level. The floor beams and screed are 375+75=450mm thick and the void 300mm so that makes internal ground level 450+300-150=600mm below external finished ground level. Based on this I'd expect the pour would come up to level with the ground under the void. Ground level outside has been reduced during demolition and needs to be built back up by 600mm.

Good thought about the insurance, they may have specific conditions. I'd take precautions anyway.