Doc Holliday Posted February 23, 2015 Report Share Posted February 23, 2015 Ok, I'm back to the TV mounting here. I'm trying to find a formula for working out the tensile force (and to a lesser degree the shear force) of hanging this TV on the pull out bracket. I've had a little look on line but can't find anything readily available which doesn't require a science degree to understand it. To my mind, if an object weighs xx kg then this force is multiplied by X for every, say, 100mm it is away from its anchor point on the horizontal. Over to you eggheads... err! I mean you clever so and so's. Quote Link to comment Share on other sites More sharing options...
Karl_h Posted February 23, 2015 Report Share Posted February 23, 2015 Very basic is to multiply the weight of the 'thing' you are hanging by the furthest distance from the the wall fixing, however centre of gravity will also have an effect Quote Link to comment Share on other sites More sharing options...
Doc Holliday Posted February 23, 2015 Author Report Share Posted February 23, 2015 (edited) Ok, thanks Karl. So can you clarify that for me. The TV weighs 23.8kg. By What factor do we multiply it by? Millimetres, centimetres or metres? The furthest projection is 260mm off the wall, assuming I keep the TV parallel to the wall. I appreciate that any angle I place it at will have a bearing. I'm just trying to work out whether the fixings I've found will be any good. I spoke to the supplier today and his answer was 'It should be ok as everyone uses them' which is fine but it's not his TV he's hanging and I'd rather do the maths first than find out the hard way. Edited February 23, 2015 by Doc Holliday Quote Link to comment Share on other sites More sharing options...
Karl_h Posted February 23, 2015 Report Share Posted February 23, 2015 (edited) What fixings do you have? The type of wall you are bolting into will also effect the type of fixing you need. The layout of the fixing points will also play apart. The further from the wall the more 'pull' force will be placed on the top fixings, a good m8 bolt and suitable fixing combo in a solid wall will do, or some m8 coach type screws into a substantial noggin. Edited February 23, 2015 by Karl_h Quote Link to comment Share on other sites More sharing options...
figgy Posted February 23, 2015 Report Share Posted February 23, 2015 Doddle to work out use the suggested fixings and swing off it, if it holds your tv is going nowhere. Seriously a lot depends on hold of the fixing as the pulling out force would be greater than the shearing force as the bracket would act as a fulcrum lever on the fixings, like using a claw hammer to remove a nail. I used a flat tilt only wall bracket for my big tv as I didn't trust the swivel types, my parents put a mid size lad on one in their bedroom and when moved from the wall will not sit straight as the bloody thing has too much play in the hinges. Figgy Quote Link to comment Share on other sites More sharing options...
Doc Holliday Posted February 23, 2015 Author Report Share Posted February 23, 2015 It's a toss up between these - http://www.constructionfixings.com/productdatasheets/heavydutytoggleanchor.htm but are only 100mm long or these -http://www.toggler-uk.com/toggler-snaptoggle-bm5-long-207.html unfortunately the latter only do an M6 bolt at the length I would need. Ideally an M8 0or M10 would be ideal but the site says in concrete block work the tensile factor is 489kg. I like these as they anchor on to the rear of the blocks inside the cavity but M6 is chancing it a bit I would think. Even so, I would like to double check the figures so I know in my mind if they add up. The blocks I have are the old breeze block. Some may call them cinder blocks as they are made up of compressed cinder chippings so a good fixing is definitely an issue in them. They are 100mm blocks and there is a dot & dab plasterboard sheet on top of that so about 120mm all told is the thickness I have to go through. I'm aware that the more fixings there are the more the load is spread but those blocks tend to weaken if you have too many close together. The coach bolts or rawl bolt fixing you can get are ok to a point but once they slip in these blocks the fixing is useless and I just wouldn't trust the block work to hold the weight without anchoring on the back of them. So what is the formula for working out tensile stresses? Quote Link to comment Share on other sites More sharing options...
Karl_h Posted February 23, 2015 Report Share Posted February 23, 2015 Tensile strength is something like force x cross sectional area which you already have for the fixing, you need the pull force generated weight x distance so kgxmeters which for yours will be around 5.26 kg. So you have to be able to hold a verticle weight of 22ish kg plus a extra force of 5 ish kilos 'pulling' the bolt out of the wall, if that makes sense. I'm sure I may be corrected but that's a rough guide, the fixing will be fine the way it is fixed is a weak point, gorilla glue is a good thing to put on the thread as it will expand in the cavity and offer more support if the block is suspect. Quote Link to comment Share on other sites More sharing options...
WGD Posted February 23, 2015 Report Share Posted February 23, 2015 You can do all the calcs you like, but the weak point is the 20mm or so of "unfixed" fixing between the face of the blockwork and the face of the plasterboard. Better to cut a pocket and glue in a ply pad to fix through. Then use resin anchors. Belt and braces but you did ask for engineering input! Quote Link to comment Share on other sites More sharing options...
figgy Posted February 23, 2015 Report Share Posted February 23, 2015 If you get the right rawl bolts the more you pull on them the more they expand so will never pull out, use them for fixing gun cabinet, never pull it away from the wall without taking the wall with it. Quote Link to comment Share on other sites More sharing options...
Rupert Posted February 23, 2015 Report Share Posted February 23, 2015 12mm threaded bar,cut to length you need. Drill 25mm holes in block and fix with resin anchors' Put nut,a pair of repair washers and another nut on and fix bracket between washers, Quote Link to comment Share on other sites More sharing options...
Marki Posted February 23, 2015 Report Share Posted February 23, 2015 (edited) It's probably your terminology that is causing the confusion, either that or I'm confused on what you are asking for ;-). That wouldn't be unusual. Tensile force would be stretching the material to a point of breaking while shear force is more apposing forces. I'm not sure these are appropriate means for choosing the fixing since even a 4mm standard steel bolt would be more than capable. What may be more appropriate is a torque calculation which should give you the amount of force that is trying to lever out the top fixings. You need the distance between the wall mounting bracket holes, the center point of the cantilever in relation to the same mounting holes and the length of the arm. This should allow you to calculate the pivot point of force and then the force on the top mounting. Unless the cantilever is very long I'd be surprised if it added more than 5-6kg load so roughly 30kg. Then again, I like surprises. Try searching for prying calculations or prying actions. My apologies if this is miles away from what you wanted ;-). Personally, I think you are not focusing on the real problem, None of the maths can cope with how crappy cinder blocks are, I know, I have them too. EDIT: Typing that took too long. Karl_h said it a lot simpler than I did. Edited February 23, 2015 by Marki Quote Link to comment Share on other sites More sharing options...
pegleg31 Posted February 23, 2015 Report Share Posted February 23, 2015 You can do all the calcs you like, but the weak point is the 20mm or so of "unfixed" fixing between the face of the blockwork and the face of the plasterboard. Better to cut a pocket and glue in a ply pad to fix through. Then use resin anchors. Belt and braces but you did ask for engineering input! That's what I'd do Quote Link to comment Share on other sites More sharing options...
Alex C Posted February 23, 2015 Report Share Posted February 23, 2015 It sounds like my walls are made from the same stuff as yours. I had a nightmare trying to fix my cabinets to it. The best solution by far is drill the deepest hole possible and use the resin anchor stuff with a lenght of thread bar. The problem with any mechanical fixing is with the blocks being soft you just end up compressing it with the fixing. It may seem tight to begin with but it could eventually work itself loose. Hope you get it sorted either way Quote Link to comment Share on other sites More sharing options...
lambhat Posted February 23, 2015 Report Share Posted February 23, 2015 There are some unknowns, especially how far the top holes on your bracket are above the lower edge of the bracket (I guessed 10cm) and how many of those holes there are (I guessed 2). Some other things could be material but probably don't have a big influence unless you've got an weirdly shaped bracket. Anyway, based on those guesses the tensile load for an individual one of the top fixings is going to be around 30kg, so you're well within the theoretical capability of the fixings. Who knows what they'd provide in practice in a non-ideal wall though. Quote Link to comment Share on other sites More sharing options...
Harnser Posted February 23, 2015 Report Share Posted February 23, 2015 I think I would just stand the TV on a table . Harnser Quote Link to comment Share on other sites More sharing options...
grrclark Posted February 24, 2015 Report Share Posted February 24, 2015 It is very simple really, the tensile strength is a measure of the mass x force, but to all intents that is quite simply just the weight of the TV. So if the TV is 30kg and it was hung vertically from a single fixing directly above the TV then the tensile strength of the fixing has to be greater then 30kg. If there is 2 fixings then they need to be greater than 15kg each, etc. As the fixings are horizontal then in theory it would be a shear value that would be important, but effectively the TV is rotating around it's centre of gravity, so there is a moment of torsional rotation. Way too complicated a formula, so to simplify if it was a 45' degree pull then you would need more than 15kg of tensile strength and 15Kg of shear strength for your 30kg TV if suspended on one fixing. The TV would effectively be pulling in two directions evenly so only half the force in each direction. If you have 4 fixings then divide by 4 and so on. The above assumes you are fitting relatively flush to the wall. Assuming 100% vertical force, If your fixing goes through the 6" of the cinder block and is fixed along it's length then you effectively spread the load evenly along that length too, so if using a single fixing the 30Kg TV would exert 5Kg of load every inch. If you have 4 fittings then the 30kg is suspended evenly over 24 inches of block, so 1.25Kg of load per inch. As mentioned earlier there is a torsional load in practice, so there is a slight horizontal load as well as the vertical, so in practice there will be less than 1kg of load per inch on your cinder blocks. If you use more fittings the load becomes even less. At the risk of being a wee bit flippant, your cinder blocks are holding the whole house up, a TV suspended over a few fittings that are well installed wont make the slightest difference. Quote Link to comment Share on other sites More sharing options...
Marki Posted February 24, 2015 Report Share Posted February 24, 2015 I think I would just stand the TV on a table . Harnser Quote Link to comment Share on other sites More sharing options...
joe1978 Posted February 24, 2015 Report Share Posted February 24, 2015 I've recently done this but with a solid stone wall instead of cinderblock. During my qualms it seemed the real problem isn't the holding strength if the relative fixings it was the problem of bridging the gap between plasterboard and fixing point to avoid compression damage and poor attachment. I used these for my install.... http://buyrigifixonline.co.uk Typical plastic rawl plug which then has a metal sleeve to bridge the gap then the required bolt go's in, M8 or M6. My TV weighs 23kg I have an articulating arm mount extended to approx 60cm(!!) to clear a chimney breast attached to wall by 2 M8's and 2 M6's as I was limited to the hole size in the actual mount. I have swung off it and I weigh 12 stone so the TV is going nowhere. For completion this is my mount I used http://www.theplasmacentre.com/wall-brackets/tv-wall-brackets/ultimate-mounts-um301l.html?bp=99&menu=Filtered Joe Quote Link to comment Share on other sites More sharing options...
Dick Posted February 24, 2015 Report Share Posted February 24, 2015 If it's going on an internal wall and there is access to the other side you could drill right through and use studding, nuts and big washers. Recessed on the non tv side and filled and finished. In my experience any expansion fitting in cinder blocks usually works loose as the cinder material just keeps compressing away from the expansion fitting. Quote Link to comment Share on other sites More sharing options...
deershooter Posted February 24, 2015 Report Share Posted February 24, 2015 (edited) You lot just need to get out more ! my gun cabinet is held to the wall with 6 off 8 mm wall rawl bolts I weigh 25 stone and can sit on the top including the weight of 9 guns inside The thought of working out the tensile strength has never entered my head Common sence has worked for me for the last 48 years even if it's a little over engineered Work things out properly and you might just use what's strong enough common sence will make shure it won't fall off the wall .The size of the holes in the bracket and how many should give you an idea Edited February 24, 2015 by deershooter Quote Link to comment Share on other sites More sharing options...
Marki Posted February 24, 2015 Report Share Posted February 24, 2015 my gun cabinet is held to the wall with 6 off 8 mm wall rawl bolts I weigh 25 stone and can sit on the top including the weight of 9 guns inside Photo or it never happened! Quote Link to comment Share on other sites More sharing options...
Doc Holliday Posted February 24, 2015 Author Report Share Posted February 24, 2015 You lot just need to get out more ! my gun cabinet is held to the wall with 6 off 8 mm wall rawl bolts I weigh 25 stone and can sit on the top including the weight of 9 guns inside The thought of working out the tensile strength has never entered my head Common sence has worked for me for the last 48 years even if it's a little over engineered Work things out properly and you might just use what's strong enough common sence will make shure it won't fall off the wall .The size of the holes in the bracket and how many should give you an idea I don't see how this would have a bearing on anything. The material you are fixing in to is every bit as important as the fixings used. You can use M10 rawl bolts 125mm long but if the material is prone to crumbling, as these blocks are, then it would be as much use as a chocolate fire guard. Thanks Karl_h and Grrclark. Most helpful. I appreciate what you are saying re the blocks holding the house up but there is nothing 'hanging off' the block work. I just know what an absolute pain they are to get a good fixing in. Quote Link to comment Share on other sites More sharing options...
Yellow Bear Posted February 24, 2015 Report Share Posted February 24, 2015 If you can drill through to the cavity you could use snaptoggles - this effectively bolts to a "bar" behind the block. The ones I looked at quote 120kg so more than adequate. may be the answer. Quote Link to comment Share on other sites More sharing options...
Harnser Posted February 24, 2015 Report Share Posted February 24, 2015 What you really need is a 6 foot by 6 foot square half inch think steel plate ,bolted right through the wall to the outside . I would use 47 one inch diameter bolts . You will then be able to weld or rivet the TV to the steel plate . The ends of the bolts showing through to the outside can then be used for hanging your hose pipe and other garden implements . Job done . Harnser Quote Link to comment Share on other sites More sharing options...
grrclark Posted February 24, 2015 Report Share Posted February 24, 2015 Thanks Karl_h and Grrclark. Most helpful. I appreciate what you are saying re the blocks holding the house up but there is nothing 'hanging off' the block work. I just know what an absolute pain they are to get a good fixing in. I used to have a no fines house and that was a nightmare for fixings too. Honestly though, there is such a little amount of load on each fixing that it won't take much to anchor it. As you tighten the fittings into wall you will place much more tension on the fitting than the weight of the TV will exert. If you were to take the back panel off the TV you will find the support beams for the fitting to mount to will be attached to the TV chassis with nothing bigger than M4 machine screws. I would say resin bonded fittings would be best and if your worried put 8 of them in the wall, that way less than 4kg total load each fitting. Quote Link to comment Share on other sites More sharing options...
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