MortarThePoint Posted October 31, 2020 Share Posted October 31, 2020 I'm using HiTherm+ lintels which have reduced cold bridging above windows but are mostly made to order, so not quick to get. The Structural Engineer made a note of the drawings which I didn't understand, but now do, which says avoid multi-truss bearings above lintels. This means avoid things like hip trusses and velux trusses/rafters bearing above the lintel. I've asked the lintel manufacturer to check the situation, but they are being a bit slow. In parallel I want to consider my options to add strength. The spans are 1800mm. Lintel specified is HT/HD100 with capacity of 35kN. First floor, 525mm below top of wallplate, 4 brick courses on outside. I'm not a Structural Engineer, but some logic suggests the following options could help: Upgrade to an XHD lintel (50kN capacity), but there are difficult lead times and circumstances there Add a Box lintel either screwed to the rear flange of the cavity lintel or more easily with a course of coursing blocks between (or just a mortar bed between?). A standard box lintel has capacity of 30kN so should add something useful (i.e. somewhere between 0kN and 30kN :-). I am sure if screwed on it would add its full capacity. With a course of coursing blocks I'd hope it would add at least half its capacity giving 50kN total?? Add a precast concrete (PCC) lintel. R15 2100mm has a load bearing capacity of 22.82kN/m so 1.8m should give a total of 41kN. This could be bedded straight onto the flange of the cavity lintel so I'd expect it should add a fair amount of strength. PCC may have a different allowable deflection at capacity so may be complex working out combined strength. Construct a broken lintel using a box lintel (2100mm standard 30kN, HD 50kN) and a single leaf lintel (e.g. IG L10 2100mm has 10kN). Brickwork only weighs 1kN. Options 2 and 3 are the easiest, but in most scenarios involve laying the lintel of a full lenght bed of mortar. Does that worry anyone? Does anyone have any thoughts as to how the lintel strengths may add together? It's crazy times at the moment and particularly for lintels it seems. Link to comment Share on other sites More sharing options...
Oz07 Posted October 31, 2020 Share Posted October 31, 2020 3 seems easiest 1 Link to comment Share on other sites More sharing options...
MortarThePoint Posted October 31, 2020 Author Share Posted October 31, 2020 The load/span table has lower figure than the CE cert so looks like 10.14kN/m --> 18kN. Not sure why the figure is lower. https://www.supremeconcrete.co.uk/media/2620/prestressed-lintels.pdf Link to comment Share on other sites More sharing options...
Mr Punter Posted October 31, 2020 Share Posted October 31, 2020 I would go with the load table and option 3. I could not view the pdf. 1 Link to comment Share on other sites More sharing options...
MortarThePoint Posted October 31, 2020 Author Share Posted October 31, 2020 Digging through the Structural Engineer's calculations I see he calculated a requirement of 21.5kN, and specified an IG L1/HD110 lintel with a capacity of 22kN. That was when he was using the wrong cavity size and the equivalent lintel is an IG L1/HD100 with 35kN capacity. ? I've gone with a HT/HD100 with capacity 35kN. 35kN is 159% of 22kN and I calculated that if all the load on the lintel is concentrated on the central 50% (so between 0.45 and 1.35 out of 1.80 span) the moment is 150% higher than if UDL across full span. The actual positions are better than mid span bearing, but this provides a useful worst case presuming ~45degrees of load spreading under the truss bearing. Quite relaxed now. I'll await confirmation from the lintel expert and keep the PCC option up my sleeve if needed. Link to comment Share on other sites More sharing options...
Mr Punter Posted October 31, 2020 Share Posted October 31, 2020 Did you choose HiTherm for the SAP friendly psi values? What insulation are you using? I am considering using some for a 150mm cavity with eps beads. Only a few in brickwork. The rest is rendered and I will use 2 x concrete ones. Link to comment Share on other sites More sharing options...
MortarThePoint Posted October 31, 2020 Author Share Posted October 31, 2020 Just now, Mr Punter said: Did you choose HiTherm for the SAP friendly psi values? What insulation are you using? I am considering using some for a 150mm cavity with eps beads. 100mm of EPS beads so similar arrangement just narrower cavity. 1 minute ago, Mr Punter said: Only a few in brickwork. The rest is rendered and I will use 2 x concrete ones. You could use a concrete lintel for the inner leaf and a single leaf lintel for the outerleaf brickwork. Is there a reason you prefer the concrete lintels? They do look a lot cheaper (Condell seem very cheap). There must be a reason Structural Engineers go for the cavity lintels though. Link to comment Share on other sites More sharing options...
PeterW Posted October 31, 2020 Share Posted October 31, 2020 Cavity lintels are lighter to handle and also allow various things like simplified trays and insulation. They are also cheap. @MortarThePoint what is your full wall build up now with 100mm EPS bead ..? 50mm internal PIR ..? Link to comment Share on other sites More sharing options...
MortarThePoint Posted October 31, 2020 Author Share Posted October 31, 2020 1 minute ago, PeterW said: Cavity lintels are lighter to handle and also allow various things like simplified trays and insulation. They are also cheap. @MortarThePoint what is your full wall build up now with 100mm EPS bead ..? 50mm internal PIR ..? No internal PIR, 300mm walls with U value of something like 0.26. I did a cost benefit analysis based on 100mm fibre batts vs 125mm or 150mm fibre batts and for me it didn't make sense. I then changed to blown beads very late in the game, otherwise I would have upped the cavity size since the wider cavity economics with blown beads are more compelling. With blown beads there is a fixed cost of labour and a variable cost of beads and so having a 50% larger cavity probably only adds 30% to the cost (guestimate). Link to comment Share on other sites More sharing options...
Oz07 Posted October 31, 2020 Share Posted October 31, 2020 What is the cost difference between 150 and 100. I've been using 150 since 2015 it's not been too taxing on costs. Trench blocks seem to be the worst part of it, lintels, ties and insulation didn't appear to break the bank by upping to 150mm Link to comment Share on other sites More sharing options...
PeterW Posted October 31, 2020 Share Posted October 31, 2020 1 hour ago, MortarThePoint said: 1 hour ago, PeterW said: 100mm EPS bead ..? 50mm internal PIR ..? No internal PIR, 300mm walls with U value of something like 0.26. How will you get that through DER/TER SAP etc..?? Have you massively increased the insulation elsewhere ..?? That is well above 0.18 which you would need to get for Building Regs. Link to comment Share on other sites More sharing options...
MortarThePoint Posted October 31, 2020 Author Share Posted October 31, 2020 Maybe I've misremembered the number Link to comment Share on other sites More sharing options...
ADLIan Posted October 31, 2020 Share Posted October 31, 2020 There is a table in L1A that mentions 0.18 in walls but this figure is not cast in stone. Possible to still get compliance with a wall at 0.25(ish). SAP gives a lot of design flexibility. Link to comment Share on other sites More sharing options...
Dave Jones Posted November 3, 2020 Share Posted November 3, 2020 did same above a gable window lintel, struct eng specified a HD catnic, the brickie wasnt happy with the defelection in it and added a pre-cast conc little to the inner skin at the back of it to stiffen it up. Cheap way to belt and braces it to stop any movement. 1 Link to comment Share on other sites More sharing options...
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