Thorfun Posted March 9, 2020 Share Posted March 9, 2020 Hi all, we're in the process of getting quotes for timber frames and one company is offering I-beams rather than Posi Joists. The say that they prefer to use the Steico engineered I-beams as they have less deflection than Posi Joists. now, if I knew what that meant I might understand it! But as I don't I was wondering if anyone can explain what that means and whether there's any concerns or if it's just simply not true or something that doesn't need to be worried about. cheers. Link to comment Share on other sites More sharing options...
Gone West Posted March 9, 2020 Share Posted March 9, 2020 22 minutes ago, Thorfun said: The say that they prefer to use the Steico engineered I-beams as they have less deflection than Posi Joists. It just means how much they bend in the centre for a given weight on them. It's not that straightforward though as it depends on whether there are strongbacks fitted to the metal web joists and whether the cord/flange sizes are the same. A little light reading on the subject. https://www.steico.com/fileadmin/steico/content/pdf/Marketing/UK/Tec_Guide_Construction/STEICO_tec_guide_construction_en_i.pdf https://www.wolfsystem.co.uk/media/78349/easi-joist_tech_guide_4th_edition_nov_2010.pdf Link to comment Share on other sites More sharing options...
Thorfun Posted March 9, 2020 Author Share Posted March 9, 2020 just a little light reading, hey? ? I'll crack on with that. I know people on here have used I-beams and others have used Posi Joists. I like the idea of Posi Joists for the ease of first fixing but if there are potential issues using them then why do people? would this simply be a companies preference or (with my cynical hat on) a product they make a higher margin on? 1 Link to comment Share on other sites More sharing options...
Moonshine Posted March 9, 2020 Share Posted March 9, 2020 Also bear in mind that a 12mm deflection limit is the maximum they usually design to, and some people (me included) are looking at specifying a deflection of 8mm. Link to comment Share on other sites More sharing options...
Gone West Posted March 9, 2020 Share Posted March 9, 2020 4 minutes ago, Thorfun said: I like the idea of Posi Joists for the ease of first fixing but if there are potential issues using them then why do people? We didn't have any problems using metal web joists and as you say they are much easier for first fix. 1 Link to comment Share on other sites More sharing options...
Taff Posted March 9, 2020 Share Posted March 9, 2020 45 minutes ago, Thorfun said: just a little light reading, hey? ? I'll crack on with that. I know people on here have used I-beams and others have used Posi Joists. I like the idea of Posi Joists for the ease of first fixing but if there are potential issues using them then why do people? would this simply be a companies preference or (with my cynical hat on) a product they make a higher margin on? Once you read that document can you summarise for me please ? there’s just no time trying to work everything out with this building lark. We are using the same joists. I will have a butchers later tho ? Link to comment Share on other sites More sharing options...
Russell griffiths Posted March 9, 2020 Share Posted March 9, 2020 I have used i joists for a small section of upper floor as they came free when I ordered all my roof timbers i have considered cutting them out and throwing them on the fire, the trouble I’m having trying to get all my services running through them is untrue, by the time I’ve finished with ducts for mvhr pipes for water and cables for electric yhey will look like Swiss cheese. If you will have lots of services I would go with the metal web joists. 2 Link to comment Share on other sites More sharing options...
ProDave Posted March 9, 2020 Share Posted March 9, 2020 We have I joists downstairs (there are no services under the floor) and posi joists upstairs as there are plenty of services. Both are very good and solid. I believe the posi joists cost more. Link to comment Share on other sites More sharing options...
Thorfun Posted March 9, 2020 Author Share Posted March 9, 2020 49 minutes ago, ProDave said: We have I joists downstairs (there are no services under the floor) and posi joists upstairs as there are plenty of services. Both are very good and solid. I believe the posi joists cost more. Thanks Dave. I don’t mind the extra costs for an easier first fix. I’ll probably recoup some of the costs in saved labour costs anyway. Which is why I wanted the posi joists. Link to comment Share on other sites More sharing options...
joe90 Posted March 9, 2020 Share Posted March 9, 2020 I have I joists and “some” services including MVHR and found no problems but made sure we drilled holes where the regs allowed. Both my builder and I fitted the joists at 400mm centres even tho they were designed fir 600 centres to give a stiffer floor. Link to comment Share on other sites More sharing options...
Oz07 Posted March 10, 2020 Share Posted March 10, 2020 10 hours ago, joe90 said: I have I joists and “some” services including MVHR and found no problems but made sure we drilled holes where the regs allowed. Both my builder and I fitted the joists at 400mm centres even tho they were designed fir 600 centres to give a stiffer floor. Only problem is he got 3/4 of the way across the floor and ran out of joists! Ha 3 Link to comment Share on other sites More sharing options...
Thorfun Posted March 19, 2020 Author Share Posted March 19, 2020 so I finished reading (or flicking through) those 2 docs @PeterStarck linked (the Steico one was really dry and a tough read) but from what I can gather deflection isn't an issue unless you're getting to the limits of the specified beam and so I-Joists could also have deflection issues as well as Posis. It also seems that reducing the centres will reduce the deflection (is that correct?) as it makes a stiffer floor. so if one company is offering posi-joists at 600mm centres and another at 400mm centres then the floor with the 400mm centres will bounce less. is that also a correct assumption? The posi-joist brochure also talks about Strongbacks to reduce deflection so these can be added (and from the reading should be over certain spans) to assist. in fact, assuming I understand it all correctly, using Strongbacks on Posi-Joists results in less deflection ( span * 0.003 ) than the I-Joists (stated deflection of span * 0.004). this means for a 4.5m span the deflection of Posi-Joists with Strongback results in a 13.5mm deflection whereas the I-Joists would be 18mm. again, this is all assuming I understand it correctly! all of which means I'm feeling better about not having a trampoline on the first floor if I use posi-joists. ? Link to comment Share on other sites More sharing options...
Gone West Posted March 19, 2020 Share Posted March 19, 2020 19 minutes ago, Thorfun said: so I finished reading (or flicking through) those 2 docs @PeterStarck linked (the Steico one was really dry and a tough read) but from what I can gather deflection isn't an issue unless you're getting to the limits of the specified beam and so I-Joists could also have deflection issues as well as Posis. It also seems that reducing the centres will reduce the deflection (is that correct?) as it makes a stiffer floor. so if one company is offering posi-joists at 600mm centres and another at 400mm centres then the floor with the 400mm centres will bounce less. is that also a correct assumption? The posi-joist brochure also talks about Strongbacks to reduce deflection so these can be added (and from the reading should be over certain spans) to assist. in fact, assuming I understand it all correctly, using Strongbacks on Posi-Joists results in less deflection ( span * 0.003 ) than the I-Joists (stated deflection of span * 0.004). this means for a 4.5m span the deflection of Posi-Joists with Strongback results in a 13.5mm deflection whereas the I-Joists would be 18mm. again, this is all assuming I understand it correctly! all of which means I'm feeling better about not having a trampoline on the first floor if I use posi-joists. ? Reducing centres will reduce deflection of both types of joist as will increasing the cord width. Strongbacks are usually used with metal web joists and do reduce deflection. Link to comment Share on other sites More sharing options...
Thorfun Posted March 19, 2020 Author Share Posted March 19, 2020 thanks @PeterStarck for confirming that I did understand what was written! I've come to the conclusion that the company I spoke to that won't use Posi-joists as they've had bad experiences with deflection on them maybe didn't install them at the correct centres or didn't add strongbacks. either way, I'm convinced I'm happy to go with Posi-joists. 1 Link to comment Share on other sites More sharing options...
Russell griffiths Posted March 19, 2020 Share Posted March 19, 2020 Deflection can also increase if they are not fitted tightly against both ends, for example a tightly fitted joist has little space to deflect into, whereas a joist with a 10mm gap both ends has a large amount of room to deflect into. Hope that makes sense. Link to comment Share on other sites More sharing options...
Thorfun Posted March 19, 2020 Author Share Posted March 19, 2020 2 hours ago, Russell griffiths said: Deflection can also increase if they are not fitted tightly against both ends, for example a tightly fitted joist has little space to deflect into, whereas a joist with a 10mm gap both ends has a large amount of room to deflect into. Hope that makes sense. Yep. Makes perfect sense. I’ll make a mental note to check it when we get to that point in the build! Thanks. Link to comment Share on other sites More sharing options...
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