Browse Forums Building Standards; Getting It Right! 1 Dec 04, 2014 7:28 pm Hey all. Frame is nearing completion on my house and I was there yesterday and noticed that the trusses where my AC indoor unit is to be installed are in the wrong spot per my plan. Now the unit weighs approx 85 kg and the truss beams are 35mm x 200mm approx and the std trusses are 35 x 90. Now where my AC goes I only have std trusses. the builder said they spoke to the truss company and it will be ok. All they are going to do is put some 40 x 90 cross beams across 4 trusses to spread the load. What are your thoughts? Do you think it will still be ok? Although they say it will be ok should they be giving me some structural documentation? Will the extra little bit of truss deflection affect my square set cornices? Re: AC Truss design 2Dec 04, 2014 8:01 pm In South Australia we have Minister's Schedule 5 - Roof Framing Checklists.
This is required in South Australia for every truss roof. It is normally provided by the truss manufacturer. It sets out the truss framing member sizes, the bracing & tie down requirements and also the allowances for additional roof loads (such as hot water systems, PV panels & roof mounted air-conditioners. You should request this written schedule or design drawings showing the design allowances for additional roof loads. Putting in spreaders to distribute the point loads across several trusses is a good idea but the trusses still need to be designed to be able to carry the additional loads. This came about and was legislated (in SA) because the truss roof of a golf club collapsed after air-conditioning units were installed and several persons (diners in the golf club restaurant) were killed. AC Truss design 3Dec 04, 2014 8:09 pm Thanks Beet. What he did say was that there is sufficient safety factor allowed in the design. My concern is, if this is the case, why wasn't it done that way in the first place? Do you think I would be within my rights to request a written statement from the truss manufacturer to cover the variation in design? Or would this be an engineer thing? Who actually designs the trusses? Re: AC Truss design 6Dec 07, 2014 12:33 pm Hi Crazyk, You have nothing to worry about that will be fine. Air con units that size and bigger are retro fitted into new homes every day with the motors susspended from trusses. Square set will be fine, there would be more chance of cracking with standard cornice. Re: AC Truss design 7Dec 07, 2014 4:02 pm Crazyk Hey all. Frame is nearing completion on my house and I was there yesterday and noticed that the trusses where my AC indoor unit is to be installed are in the wrong spot per my plan. Now the unit weighs approx 85 kg and the truss beams are 35mm x 200mm approx and the std trusses are 35 x 90. Now where my AC goes I only have std trusses. the builder said they spoke to the truss company and it will be ok. All they are going to do is put some 40 x 90 cross beams across 4 trusses to spread the load. What are your thoughts? Do you think it will still be ok? Although they say it will be ok should they be giving me some structural documentation? Will the extra little bit of truss deflection affect my square set cornices? Most of the time this will be ok. I would ask for a justification from the truss manufacturer to this end. It will be a diagram of the truss indicating the additional loads and may also include a layout indicating the trusses in question. The truss company should be able to readily provide this as it's a fairly routine question/modification. Square set cornices are a real problem as truss roofs do move over time which can cause continual cracking of square sets in particular if even moderate truss cambers are required. When I was designing trusses I would usually try to find some internal support in the middle half of the truss which stabilized the truss reducing movement to near zero and bought cambers down as well. That method was near 100% effective when it came to eliminating SSC cracking. In your case generally a larger dead load will improve resistance to cracking as the truss will settle to it's natural position quickly and require much more force (usually wind) to move. By movement I am only talking a few mm at the most however that's enough to get a hairline crack in plaster. Beetaloo In South Australia we have Minister's Schedule 5 - Roof Framing Checklists. This is required in South Australia for every truss roof. It is normally provided by the truss manufacturer. It sets out the truss framing member sizes, the bracing & tie down requirements and also the allowances for additional roof loads (such as hot water systems, PV panels & roof mounted air-conditioners. You should request this written schedule or design drawings showing the design allowances for additional roof loads. Putting in spreaders to distribute the point loads across several trusses is a good idea but the trusses still need to be designed to be able to carry the additional loads. This came about and was legislated (in SA) because the truss roof of a golf club collapsed after air-conditioning units were installed and several persons (diners in the golf club restaurant) were killed. Hmmm.... my recollection was incorrect bracing allowing a GT to move out of plane and collapse. Granted it's a few years since I saw the report which did the rounds of the industry Australia wide. Your first paragraph is correct in that it is generally required pretty much everwhere and most of the time are referred to as 'Truss Comps' and 'Producer Statement'. Some councils also require a certificate of design &/or installation to be issued by a registered engineer. Complete and utter overkill for all but the most complex domestic roof structures but some councils require it none the less. I did have to supply one for a 6x6 hip end garage once - the paperwork cost nearly as much as the trusses Re: AC Truss design 8Dec 07, 2014 5:35 pm Our Build123 Hi Crazyk, You have nothing to worry about that will be fine. Air con units that size and bigger are retro fitted into new homes every day with the motors susspended from trusses. Square set will be fine, there would be more chance of cracking with standard cornice. Thanks. Yes they did say that to me also. Mike I have regretted not requesting centre load bearing walls as I do agree this would help stabilise my SSC. My truss spans are 12.8m with 30. Deg pitch. Massive suckers The green blue trusses should of been a further 10 trusses back. Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ Re: AC Truss design 9Dec 09, 2014 8:28 am Quote: When I was designing trusses I would usually try to find some internal support in the middle half of the truss which stabilized the truss reducing movement to near zero and bought cambers down as well. RHM, my experience with pre-made frames and trusses is that the trusses were all designed to be resting on the outside load bearing walls and the internal ( non load bearing ) walls were about 30mm lower to allow for movement. We then used slotted right angle brackets to secure the trusses in the middle to these NLB walls. I didn't know you could have trusses resting on these internal walls or have I got that wrong ? Are you a structural engineer as well or did you have a dedicated frame program that did all the heavy lifting in working out the truss design ? Stewie Re: AC Truss design 11Dec 09, 2014 6:51 pm Stewie D Quote: When I was designing trusses I would usually try to find some internal support in the middle half of the truss which stabilized the truss reducing movement to near zero and bought cambers down as well. RHM, my experience with pre-made frames and trusses is that the trusses were all designed to be resting on the outside load bearing walls and the internal ( non load bearing ) walls were about 30mm lower to allow for movement. We then used slotted right angle brackets to secure the trusses in the middle to these NLB walls. I didn't know you could have trusses resting on these internal walls or have I got that wrong ? What you are saying is correct. Most trusses are clear span (I've done them over 20m long before ) designed to be supported at each end (heal point where the bottom and top chords join) and they are usually built with a pre-camber of up to 20mm (We try not to make them go that high ) which is supposed to 'settle' down to close to 0 after the roof is loaded with no internal supports. With timber being a natural (ie not uniform) product, this settling is an educated guess at best and can happen when the roof is put on, once the ceiling sheets are fixed or gradually over a month or 2. TBH we get it right about 85% of the time (the software works it out) What can be done is to pick up an internal wall for support in the middle 1/3 (even outside that somewhat) which must be designed into the truss right from the start. These supports work wonders for taking (say) 12mm pre camber out of a truss down to 2 or 3 mm. It also stabilizes the roof over the internal walls making cornices etc far less prone to cracking. Typically these internal supports cost little as we manipulate the webs to land over the support. Smaller trusses may not require any extra webs at all. It's definitely not something you would do on every job but for SS cornices etc I'd look at it very closely if it were my house. If you get a set of truss comps from your manufacturer you can tell what the cambers are quite easily. If anybody want's to post up an image of a truss I can decipher it for you. The brackets you are talking about are "L" brackets or pryda PHL's (don't know what mitec or multinail call them) and are required minimum every 1800mm of un-supported wall, that is, every wall 1800 or longer without an intersecting wall to brace it laterally. Generally speaking many carpenters have nfi where they need to go because they haven't read AS4440 and inspectors are by and large the same. That said, if there are a few extras (correctly installed) it won't hurt. This is directed at any owners as industry people should (and mostly do) know: Nails on a L bracket to the truss should be at the top of the slot and not quite driven home. It's important as it allows the truss to drop to it's natural position. I was at a job 6 months ago where a massive crack in a wall eventuated (2 story house on stumps) because the nails were driven home and caused a un-intended internal support mid way on a floor joist. Quote: Are you a structural engineer as well or did you have a dedicated frame program that did all the heavy lifting in working out the truss design ? I have had some engineering training (but no degree) and a registered builder however none of that is required to design roof trusses these days. Most of it is done by the computer. The software will design any single truss near perfectly these days however, particularly on complex roofs it still gets confused making trusses wrong lengths or other stupid mistakes which sometimes are very difficult to pick up before they go to site where they suddenly stand out like the proverbial. From experience, a good truss detailer is worth their weight in gold. These days a lot of it is being exported to SE Asia with mixed results. For regular project homes etc they do a very good job but a lot of the custom stuff that requires interaction between the design office and builder is not so successful. Most of the companies don't tell their clients where stuff is detailed (not that it matters) but when it comes to complex stuff there is nothing like talking direct to the person designing it IMO. Re: AC Truss design 12Dec 09, 2014 8:11 pm Quote: The brackets you are talking about are "L" brackets or pryda PHL's (don't know what mitec or multinail call them) and are required minimum every 1800mm of un-supported wall, that is, every wall 1800 or longer without an intersecting wall to brace it laterally. Generally speaking many carpenters have nfi where they need to go because they haven't read AS4440 and inspectors are by and large the same. That said, if there are a few extras (correctly installed) it won't hurt. We weren't sure either so we used to nail a bracket on every truss ( just to be sure ). Mind you we used to nail a triple grip on every truss/top plate junction too. Overkill for some jobs maybe but I used to sleep OK at night. I guess with the trusses resting on the top plates mid span I guess anything that cuts out untoward movement in the framing or at least reduces it as much as possible has to be a good thing. Thanks for the additional info. Stewie Re: AC Truss design 13Dec 13, 2014 10:04 am Got another question. I went for a 30deg pitch roof and there is tons of space up in the roof. The truss is a full single span 12.8m with no internal load bearing walls. So is there a way that I can put some flooring up there without affecting my square set cornices? Will the ceiling deflect if I walk on the rafters and crack the corners? Re: AC Truss design 14Dec 13, 2014 10:31 am I'b careful CK. The roof trusses would have been been designed with mostly dead load and a bit of live load thrown in. One person walking around up there probably would cause any issues but put flooring down and you'd be tempted to store stuff up there. That's OK if it is only light stuff like empty boxes, anything more could cause cracks in your plaster. I had a call once from a mate of mine doing some internal renos including a new bathroom for a client who couldn't understand why all her ceilings were sagging and the cornices were cracking in her hall, bathroom and dining room ( center rooms in a Californian Bungalow - 35º roof pitch). Turns out she had gotten someone in to install an attic ladder and lay down some flooring sheets for storage straight over the ceiling joists " just for a few empty boxes ". The old girl had decided the space was too good not to use so had stacked the place floor to underside of the rafters with boxes of books, crockery - you name it. There must have been a couple of tons of extra weight up there at least and I'm surprised the ceilings didn't actually collapse. Stewie Re: AC Truss design 16Dec 13, 2014 7:48 pm Stewie D I guess with the trusses resting on the top plates mid span I guess anything that cuts out untoward movement in the framing or at least reduces it as much as possible has to be a good thing. Thanks for the additional info. Stewie Just remember that supporting a truss where it wasn't designed to be supported can be just as bad as leaving out a support that was supposed to be there. Crazyk Got another question. I went for a 30deg pitch roof and there is tons of space up in the roof. The truss is a full single span 12.8m with no internal load bearing walls. So is there a way that I can put some flooring up there without affecting my square set cornices? Will the ceiling deflect if I walk on the rafters and crack the corners? Plenty of people do. Stuff like Christmas decorations and the like which is very light will most likely be fine. Don't store paper and books up there - they get *real* heavy *real* quickly. Of course once your builder realizes what you have done he'll just walk away from any ceiling or roof related defect that eventuates, as will the truss company. That said, square set cracks are very easily fixed with a tube of salleys no more gaps. Re: AC Truss design 18Dec 13, 2014 8:40 pm The only way to use your roof for storage without risking overloading the trusses is to install floor bearers/joints supported off the walls. (not off the bottom chord of the trusses). It probably is not practical to install large long timbers through the existing man hole. It may be necessary to remove part of the ceiling. This starts to make it a BIG job. Re: AC Truss design 19Dec 13, 2014 9:43 pm Beetaloo The only way to use your roof for storage without risking overloading the trusses is to install floor bearers/joints supported off the walls. (not off the bottom chord of the trusses). With a bit of forethought it's easy enough to design a truss to take a floor load in some degree right the way up to full blown attic trusses. I've done plenty of clear span trusses with a fully engineered attic floor load in the middle. Of course after the roof has installed is a different story entirely. It's hard to comment as the photos area bit dark ( you might need a new 15 - just got one and wow ) Jokes aside, I can see one member that is cracked. I would find a… 2 2213 Thank you so much for the effort. We will use it to talk with builder. We also had idea of building duplex instead and seeking suggest ions. viewtopic.php?f=31&t=106744 11 13833 |