MortarThePoint Posted October 30, 2023 Share Posted October 30, 2023 I've been thinking of scabbing timber onto the side of my stair stringers, but fear it could stress concentrate. The scabbed on timber wouldn't quite be full length of the stringer due to brackets at the bottom. It would be 145x33 mm timber each side of the middle two stringers and on the nonvisible side of the outside stringer. The forth stringer is bolted to the blockwork wall. It's essentially a beam thickened up for most but not all of its span. It will reduce deflection, but am I introducing a problem by creating a stress contraction when the reinforcement ends. As it stands the stringers are good and stiff already, but it's tempting to belt and braces. Link to comment Share on other sites More sharing options...
MortarThePoint Posted October 30, 2023 Author Share Posted October 30, 2023 @Gus Potter Is this a simple Structural Engineering question? Link to comment Share on other sites More sharing options...
joe90 Posted October 31, 2023 Share Posted October 31, 2023 Not sure what “scabbing” is, do you mean simply adding timber to stiffen the stringers? (Waiting for Gus to answer). Link to comment Share on other sites More sharing options...
SteamyTea Posted October 31, 2023 Share Posted October 31, 2023 4 minutes ago, joe90 said: Not sure what “scabbing” is I had to look it up, was some gruesome pictures. Then I found one that showed a plank overlapping a joint. 2 Link to comment Share on other sites More sharing options...
markc Posted October 31, 2023 Share Posted October 31, 2023 (edited) The additional mass of some doubling is almost irrelevant compared to anticipated loads on a stair. if anything, beef up the stringer connections top and bottom Edited October 31, 2023 by markc 1 Link to comment Share on other sites More sharing options...
MortarThePoint Posted October 31, 2023 Author Share Posted October 31, 2023 (edited) 7 hours ago, joe90 said: “scabbing” Sorry, I am pinching the term from my truss designer and it may be wrong. It's just adding timber along side to strengthen the beam/joist/truss/stringer. This is effectively the system I am considering as viewed from above with a load acting down onto it. Additional timber is screwed on either side of the main timber. (note: angled white gap is to avoid image being too long and isn't actually a gap) Edited October 31, 2023 by MortarThePoint Link to comment Share on other sites More sharing options...
joe90 Posted October 31, 2023 Share Posted October 31, 2023 https://homeinspectionsinohio.com/what-is-sistering/ This what I know it as. Regarding your stress concentration area I don’t think it will be a problem, you are looking at deflection (bounce) over a length not shear. (But @Gus Potter will put me right ( when he turns up 🤷♂️). Link to comment Share on other sites More sharing options...
SteamyTea Posted October 31, 2023 Share Posted October 31, 2023 I have always heard stress concentrators called stress raisers. I don't think this would qualify as a stress raiser as, apart from the extra force caused by the extra mass, there is no real difference. There may be a case where the dynamic loads are concentrated more towards the fixed ends, but in this case the lower overall deflection probably helps, not hinders, the situation. Link to comment Share on other sites More sharing options...
MortarThePoint Posted October 31, 2023 Author Share Posted October 31, 2023 Deflection would definitely be improved, but I wondered if it increased the risk of failure. If it were to fail it would be where the beam assembly narrows to being just the main timber width. Link to comment Share on other sites More sharing options...
joe90 Posted October 31, 2023 Share Posted October 31, 2023 2 minutes ago, MortarThePoint said: increased the risk of failure. I think not, you can over engineer but why, I think you are over thinking it. Link to comment Share on other sites More sharing options...
MortarThePoint Posted October 31, 2023 Author Share Posted October 31, 2023 There is no noticeable bounce at the moment, but whilst I have the structure open it is tempting to add more timber 🙂 I just want to be sure I'm not creating a problem by doing so. Link to comment Share on other sites More sharing options...
joe90 Posted October 31, 2023 Share Posted October 31, 2023 25 minutes ago, MortarThePoint said: I just want to be sure I'm not creating a problem by doing so. In my opinion, no. Link to comment Share on other sites More sharing options...
SteamyTea Posted October 31, 2023 Share Posted October 31, 2023 (edited) It gets into Moments of Inertia, which I always thought was a strange use of the English language for a static beam. The Moment is just a point in space, and the Inertia is just the ability to resist force, force is, in part, an acceleration. https://www.engineeringtoolbox.com/area-moment-inertia-d_1328.html Making a beam wider does not add much bending resistance, making it deeper does. If you were to structurally glue a 0.15m bit of timber under the runners, then attach that to the wall, that would make it much stronger and stiffer. Edited October 31, 2023 by SteamyTea 1 Link to comment Share on other sites More sharing options...
MortarThePoint Posted October 31, 2023 Author Share Posted October 31, 2023 That would strengthen and stiffen it, but affect aesthetics so keen to avoid Link to comment Share on other sites More sharing options...
MortarThePoint Posted October 31, 2023 Author Share Posted October 31, 2023 2 hours ago, SteamyTea said: making it deeper does That would strengthen and stiffen it, but affect aesthetics so keen to avoid Link to comment Share on other sites More sharing options...
SteamyTea Posted October 31, 2023 Share Posted October 31, 2023 1 minute ago, MortarThePoint said: That would strengthen and stiffen it, but affect aesthetics so keen to avoid You don't have to double the depth, a small increase can have a big effect. Okay about with a beam calculator to get a feel of what will happen. Link to comment Share on other sites More sharing options...
joe90 Posted November 1, 2023 Share Posted November 1, 2023 Where is our @Gus Potter ???? We need you mate. Link to comment Share on other sites More sharing options...
SteamyTea Posted November 1, 2023 Share Posted November 1, 2023 26 minutes ago, joe90 said: Where is our @Gus Potter ???? We need you mate. Behind his slide rule maybe. Link to comment Share on other sites More sharing options...
Gus Potter Posted November 1, 2023 Share Posted November 1, 2023 (edited) On 30/10/2023 at 14:25, MortarThePoint said: I've been thinking of scabbing timber onto the side of my stair stringers, Hello all and @MortarThePoint hope all is good at your ends. There is plenty here to look at from Mortar's photo. My thoughts are this. Start with the Architectural appearance. Not sure if you are going to clad the stair like the Americans do in a high quality stable timber.. can look great and a I'm big fan of that when I get the chance... but you need a stable platform (not too shrinkable soft woods) to do this. The overall perpendicular distance from the nose of the tread to the underside of the stringer doesn't look too bulky.. do you really want to add extra material to the underside? How will that look compared with the rest of the hall we can't see? Structurally stairs are bit different as essentially they are two beams (the stringers with treads linking the tops) spanning a gap but are at an angle. The stair you have looks pretty close to the 42 deg maximum allowable pitch. Just say it is 45 degrees for ease of demonstration. Some of the basic checks we do when designing a stair is to make sure the treads (these are flat horizontal beams) are strong enough to span between the stringers, then we check the stringers and then the connections at the top and bottom of the flight. But as the stringer is at an angle we take the downwards load and convert that into it components which are a load acting perpendicular to the stringer and an axial load. The above shows the green load (you and your pals) on the treads converted into the component (vector) loads. Basically the bending force on the stinger is reduced by about 30% so that is good news as timber is very good at carrying axial loads along the grain. But what we are trying to do is get a stair that is not bouncy as well as not falling down. Beams (stringers in this case) bend in two ways.. in their plane and also by twisting.. as soon as they twist they deflect more and loose stiffness so bend even more... and then things often just get worse from there on. For BH folk! One thing to recognise here is that you can walk up a stair and feel movement.. highish frequency = "twangy" as opposed to low frequency which tends to make you nervous. My personal view.. it's actually a good feeling on a domestic stair as I think it gives it life. It "twangs a bit" but still feels safe and comfortable to walk up... that's my view for houses. You spend all that money on say other parts to bring the house to life.. why not let the stair resonate with you as you move about the house? Take a high end example say an oak frame.. it's living and moving.. why not make the stair feel "alive" My self build had a "lively stair" and it felt great.. but was still safe. I other words you can over design and make a stair dead both Architecturally and dead in terms of having a responsive house. Compare this with the stairs you go up in sports stadiums (I hope) they feel solid and dead.. but for good reason. What would you choose? Anyway back to strength and stiffness. If you take the stringers in isolation they will be quite flexible and bounce about. But the key thing is how the treads are fixed to the top of the stringers as it is mainly the treads that prevent the stringer form twisting... and thus misbehaving. The weak spot is.. and I think you have clocked this is that when the stringers dry and treads shrink they will split the stringer along the line of the bottom of the stepped notches. Now you lose all the stiffening etc that the stair relies upon. I think the split will start on the outside of the stringer away from the wall due to the treads shrinking and the way they bend.. they cause inward rotation so the split should start on the outide face of the stringer. If it was my stair I would get some 18mm marine ply and template that to the photo of the stringer away from the wall. Then glue and screw that to the stringer with structural glue say Cascamite. The screws only act as clamps while the glue sets. That will do two things. Main thing is it stops splitting of the notched bit away from the main body of the stringer.. so the tread still stops the stringer from twisting.. and the ply will also act to resist the some of the bending forces not least as you have effectively thickened the stringer. Lastly though you need to check the supports at the top and bottom of the flight. @markc As a ball park figure a domestic stair is generally desigend to carry about 150kg/ square metre on plan plus a safety factor of 1.5 to 1.6. Say 260 kg on plan to allow for a bit of self weight of the stair. Some of that will get transfered to the inner stringer fixings.. but just check your supports at the top and bottom are good enough. Hope this helps you make an informed descision. Main thing I want to know is how you are going to finish the stair, hand rails, glass? Just to add a caveat.. make sure you don't compromise fire protection etc! You may need to line the bottom of the stair? Edited November 1, 2023 by Gus Potter 1 1 Link to comment Share on other sites More sharing options...
MortarThePoint Posted November 2, 2023 Author Share Posted November 2, 2023 (edited) 9 hours ago, Gus Potter said: Start with the Architectural appearance. Not sure if you are going to clad the stair like the Americans do in a high quality stable timber.. can look great and a I'm big fan of that when I get the chance... but you need a stable platform (not too shrinkable soft woods) to do this. The overall perpendicular distance from the nose of the tread to the underside of the stringer doesn't look too bulky.. do you really want to add extra material to the underside? Yes, I'm building it as the Americans do so will be clad with nice timber or have better painting surface on the side. There will then be plasterboard under the sloped section. I like the proportions as they are so am not keen to make the stringer bulkier by adding timber underneath. I know it's less effective at adding stiffness, but alongside is hidden so doesn't affect aesthetics. There are 4 stringers in the structural skeleton each made out of 2by material (45mm) and the distance between notch and edge leaves 148mm of continuous timber which felt like enough. 9 hours ago, Gus Potter said: My personal view.. it's actually a good feeling on a domestic stair as I think it gives it life. That makes sense. The stairs as they are feel very stiff and I can't notice any life in then so perhaps I have missed a trick there. Being less experienced I always lean towards adding strength and I guess that's the downside. The mountings of the three stringers not against the wall: Bottom: Bird's mouth plus a framing anchor each side. I have upgraded the framing anchors since the photo below to more substantial ones (BPC) which had stated load data which looked plenty strong enough. Top: 150mm x 90mm x 10mm steel Angle Section bolted to the UC 203x203x52 with 4no. M10 bolts. There are then 3no 6.3mm structural timber screws (TimberFix) into each stringer, plus a 6.0 x 100 woodscrew. The natural action of the forces serve to tighten this area as the Angle Section is on top of the bottom flange of the UC beam. The stringer against the wall is bolted to the blockwork (M10 Masonry) plus 50x50 washer at every tread in the position shown by the red dots below, so outside just outside the 148mm continuous section and where the timber widens to its thickest. Here are some more views of the overall look, minus spindles and handrails which will be added. Two spindles per tread. There will also be a newel post at the top and bottom of the stair. The treads will be varnished sapele and the riser, stringer trim, newels and spindles will be painted. There will then be a carpet stair runner up the middle of the stairs and carpet covering the half landing. 7 Edited November 2, 2023 by MortarThePoint Link to comment Share on other sites More sharing options...
joe90 Posted November 2, 2023 Share Posted November 2, 2023 55 minutes ago, MortarThePoint said: The stairs as they are feel very stiff and I can't notice any life in then After @Gus Potter comments above I think that says it all, you could park a car on that 😱 1 Link to comment Share on other sites More sharing options...
MortarThePoint Posted November 2, 2023 Author Share Posted November 2, 2023 16 minutes ago, joe90 said: After @Gus Potter comments above I think that says it all, you could park a car on that 😱 Yes, seems like sistering the additional timber is over the top. 1 Link to comment Share on other sites More sharing options...
Gus Potter Posted November 4, 2023 Share Posted November 4, 2023 On 30/10/2023 at 14:25, MortarThePoint said: I've been thinking of scabbing timber onto the side of my stair stringers, but fear it could stress concentrate. The scabbed on timber wouldn't quite be full length of the stringer due to brackets at the bottom. It would be 145x33 mm timber each side of the middle two stringers and on the nonvisible side of the outside stringer. The forth stringer is bolted to the blockwork wall. It's essentially a beam thickened up for most but not all of its span. It will reduce deflection, but am I introducing a problem by creating a stress contraction when the reinforcement ends. As it stands the stringers are good and stiff already, but it's tempting to belt and braces. What a tidy job.. well done. Those birds mouths at the bases of the stringers look great! The weak spot is at the top as you are fixing into the end grain where the notches for the treads can split off. Follow the load path which is.. each tread is on a non continuous piece of timber to some extent... the notched part. The notched part transfers the load to the main continuous part of the stringer. At the top you have three fixings into the end grain, probably only two of these are really doing the work, hard to tell. Assume that at some point the timber will split to some extent. If you plate the outside stringer with ply and glue any potential problem should be mitigated. The ply will also give you a good stable background to glue / fix the facing timbers.. in other words you kill two birds with.. On 02/11/2023 at 09:05, MortarThePoint said: Yes, I'm building it as the Americans do so will be clad with nice timber or have better painting surface on the side. Fantastic! Can you post more photos as you go to let folk see the results. It's a great way of doing it if executed well. You have spent the time making the underlying structure stable at low cost.. now you get to play with the high end finishes.. which should be cheaper comparitively as they are non structural. Link to comment Share on other sites More sharing options...
MortarThePoint Posted February 3 Author Share Posted February 3 (edited) On 04/11/2023 at 21:50, Gus Potter said: If you plate the outside stringer with ply and glue any potential problem should be mitigated. I'm avoiding plywood and assume OSB wouldn't be a good alternative. Could I glue a flat nail plate in the area of the screws instead? I'm thinking of the Simpson style that is just like a drilled sheet of 1.5mm steel. Perhaps NP15/140/180. I'd use CT1 which would squeeze through the nail plate holes and leave it well stuck. I'd want to apply anything only to the inboard surface of the outside string as my dimensions there are critical Edited February 3 by MortarThePoint Link to comment Share on other sites More sharing options...
Gus Potter Posted February 4 Share Posted February 4 On 03/02/2024 at 18:27, MortarThePoint said: I'm avoiding plywood and assume OSB wouldn't be a good alternative. Could I glue a flat nail plate in the area of the screws instead? I'm thinking of the Simpson style that is just like a drilled sheet of 1.5mm steel. Perhaps NP15/140/180. I'd use CT1 which would squeeze through the nail plate holes and leave it well stuck. I'd want to apply anything only to the inboard surface of the outside string as my dimensions there are critical Hiya.. The glue thing is ringing alarm bells here! I'm not saying it can't be done.. just before you do something like this you need to really understand how the materials are behaving. Also imagine you came back to me and said.. Gus can you prove that is ok by calculation? Now that would cost you a lot and it may not be possible without physical testing. Try and seek out an alternative if you can.. do one of your great 3D models as can't quite picture what you intend at your end. Simpson et al have a huge range of bracketry so keep looking and you may find one that fits the bill.. with some load data tables. Link to comment Share on other sites More sharing options...
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