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MG MGA - MGA Has A New Home

At last my MGA has a new home. He (Marmaduke) is somewhere amongst the boxes. It's all got to be sorted before MG Live.

Steve


Steve Gyles

This is the house.


Steve Gyles

Hi Steve wow that looks lovely do you want a lodger lol


Gordon
g c pugh

Steve,
Welcome to your new home. We lived in Church Crookham back in the early 70s, when we had our previous 1500 Coupe. I had to cycle 6 miles to work so that the CinC had the MG. Good luck with the unpacking.
Shane
Shanerj

Steve................. Nice gaff mate !!..

re the lodger ...........X2

ken
k klay

Lovely house Steve, chocolate box!

N McGurk

Nice house Steve, looking forward to you getting the garage organized.

But with all that garden to maintain will you still have time for the MG!

Trust you and Mrs G will be very happy in your new home.
John Bray

I would have thought you would have renamed your car "Wilma" Steve.

What with all those "Flint-stones" on the front of your house! :-)

Sorry

Looks a great house though

Colyn
Colyn Firth

Lovely new house Steve - very envious - and the garage will be good when you get all the moving boxes out ! I hope you both ( or the 3 of you including the car) have a long and happy stay there - cheers Cam
Cam Cunningham

Question for one of you stress engineers (wood).

I am in the process of clearing the decks in the garage so that I can get access to the MGA once again. I intend to board in the centre section of the garage apex roof. I have 12 standard design transoms across the garage, each 18 inches (approx 45cm) apart. I want a decent access hole (say 3ft x 3ft) so I want to cut out a 2 to 3 foot section from one transom.

Question: Do I need to put temporary vertical supporting beams from the floor to the cross beam while I cut out the section, then strengthen to make the hatch or is there little load to worry about? I suspect the latter but just want to make sure. I don't want the roof coming down around me when I cut!

See photo

Steve


Steve Gyles

Steve not fully sure what your proposing to cut out but if you want a 3ft sq hole then this means your going to cut the bottom tie of one of your trusses that will be at 2ft crs. Trusses are designed as a structural system and if you intend cutting any member it will fail without strengthening. Its possibly to do but you might consider getting professional advice. You can get most things into a roof space with slightly less than the 2ft opening between trusses and it can be as long as you like. I've got 1/2 an mga in my roof space including a gearbox.
J H Cole

John

This is what I have in mind (see picture). I think I did it before in my old garage without problem. The trusses are 18" centre to centre, so the gap is only 16" which is a bit on the small size.

I can always screw cross braces in prior to cutting and then remove after fitting the cross beams.

Steve


Steve Gyles

Hi Steve,
That'll be fine. I'm sure it would stay up whilst cutting (no visible extra load in the roof space yet) but i would prop the trusses before cutting just to be safe (Just a simple timber prop wedged in place would suffice for half an hour or so) as i have seen some dodgy joinery in Charles Church houses. Joist hangers would work for the cross beams, but i would also put some additional beefy bracketry in place. Contact me off line and if you want, i can slip over and give you a hand at a mutually convenient time.

regards
Colin
C Manley

If I were you I would temporarily screw/nail a beam across and under the adjoining joists to tie them all together and outside the intended hole on both sides of the hole. There will be some tension load on the truss horizontal from the load on the roof so I would expect things to move if you don't restrain them.

That should hold everything in place until you fit cross braces where you want them presumably where your cross beams are shown. They might do for the cross beams anyway. No need for extra bracketry. Its just like a access hole to a loft.
John Francis

Thanks Colin and John. That's the way I will go, with 2 temporary cross braces. I will get added strength once I screw the tongue and groove boards in place, with extra screws into the cut joist and adjacent 2 joists.

Steve


Steve Gyles

Steve, you have to handle the tension in the bottom chord and nailing through the cross member into the end grain won't do it. As Colin says you need some form of bracket. I would favour galv strap 30x1.5 mm wrapped around the cross member and then taken above and below the bottom chord for say 150mm fully nailed so that the nails are working in shear and not in tension tending to pull out. Available in any builders merchant. I'll be interested in whether when you first cut the chord you get a small but permanent deformation. If you asked the truss manufacturer about this they'd say 'over our dead body'
J H Cole

I am reluctant to get involved here, but I don't like what I read above. Yes, there are lots of examples of bodged constructions not collapsing. Most likely a bit of sagging over time is all that will happen, if anything. Those gang-nail connectors are probably the weak points.

Are the trusses designed to support a ceiling load?

Are the trusses designed to support a storage load if that is contemplated?

Where is the proposed hatch to be located; mid-ceiling or at an end wall as the photo suggests?

What truss spans are we talking about?

You may be overloading the adjacent trusses by transfering the load from the cut truss to the adjacent ones. They should be reinforced. This will be difficult to do in place. A common method during original construction would be to double the adjacent trusses, but difficult now.

http://www.roof-trusses.co.uk/UploadedFiles/TechnicalInfo/pds7.pdf
John DeWolf

Bingo. John has it. Click on the PDF link and look at Figure 5, "Framing Around Hatch".

This discussion should raise some eyebrows. These are triangular trusses, and the bottom stringer is always in tension. If you cut the bottom stringer it will expand in length pushing out the eves while the roof above it will collapse.

To cut out a bottom stringer you have to eliminate all of the tension and compression loads first. The left and right adjacent trusses must be doubled up to support greater load. Then the "dummy truss" in the center that you want to cut can have the angle braces removed. The center rafters and stringers are then cross braced to the adjacent double trusses so the double trusses end up supporting the dummy truss sections to hold up the roof and the ceiling.

All said and done, the thin stringers on the bottom of the trusses are not strong enough to support any "normal" floor load. They are only intended to support the weight of the ceiling, some insulation, and some temporary light loading for maintenance purposes. The truss structure as a whole is intended to support the roof loading (like heavy snow load) and the weight of a ceiling, but no significant floor loading, not even if you laid a complete plywood floor up there.

You can step your body weight momentarily onto any stringer in any location, but that's about it. When you walk on the stringers you may notice a little bounce or jiggle as the stringer will bend downward a bit, and spring back when you step off.

Place a sheet of plywood or a couple of 2x4's across two or more trusses, and you can likely place a gearbox there with no ill effects, one place only, no more than 100 pounds supported (permanently) on two stringers. You definitely cannot line the entire attic floor with gearboxes, as that would cause sagging of the thin horizontal stringers, eventually resulting in a permanent downward set (like sagging leaf springs). So figure on limiting attic "floor" storage to relatively light objects.

The only high strength locations on a truss are at the apexes of the triangles where you find the joining "staple plates". Where the bottom apex of the "V" braces meet the ceiling stringer you can literally hang an engine on the truss, temporarily. But do not leave a load like that in place long term, especially if you have a heavy snow load on the roof at the same time.

If you want to place anything heavy in the attic for long term storage, then install a long 2x6 on edge in the base of the "V" and spanning across a number of trusses (preferably at least 4 trusses). Then place the heavy load on the new joist to spread the load across multiple trusses.

Whatever load you add to the truss in this manner is added to the snow load on the roof. If the roof structure is designed to hold 40 pounds per square foot (plus the drywall ceiling), and you add just 10 pounds per square foot in attic storage loading, this could overload the roof structure by 25% in case of heavy snow load.

In short, the attic space in a truss roof is not designed to handle any heavy storage load.
Barney Gaylord

John and Barney

Thanks for your observations. This is exactly why I asked the question, so all inputs most welcome.

I am glad I have learned over the years to ask the questions first and get the the deeper analysis before proceeding. A lesson for everyone in almost every task.

The trusses (12 in all including one at each end against the side walls) span 18 feet and each is 1.5 feet apart.

I am coming to the conclusion that it will be prudent to accept a smaller aperture and not risk cutting a joist. I just need to control my middle (old) age spread to avoid getting stuck!

It was never the intention to store heavy items up there, just things like my soft top, windscreen, sidescreens, a few boxes of lightweight material and wood offcuts etc. In fact, those MGA bits mentioned are already up there, resting on the joists. I don't want to risk their falling down on the car, hence the idea of some boarding.

Weight on those joists is obviously the issue, so I need to think a bit more about this one.

Steve
Steve Gyles

Steve, to give you a feel for how much area loading at ceiling level the trusses can handle: most domestic trusses are designed to accept 0.25 Kn/m2 dead load such as ceiling and floor finishes and 0.25 Kn/m2 live load such as moveable items, -your mga stuff. If you put down 18 mm flooring grade chip of 0.1 Kn/m2 then your live load allowance will be 0.4Kn/m2 or just over 8lb/ft2 over the whole floor. There is also one point load allowance for concentrated loads such as water tanks of 0.9 Kn -200lb anywhere along the bottom of each truss. As you can see the area loading is not that much and this is why most roofs owned by DIY chaps are overloaded sometimes excessively so. On the plus side history has shown that modern trusses can be overloaded with discretion and the fail criteria is the mid span deflection. just keep an eye on this- for a 6m span truss the allowance is 1/360 or say 17 mm.
J H Cole

OK, I'll jump on!
Working off a ladder with bulky objects is dangerous, and through a too small hole much more so, therefore you are left with cutting a truss.
Once you cut the truss, it doesn't matter how long the absent section is, though you don't want to go beyond the adjacent truss diagonal intersections.
So, make the hole 3x4 or 3x5. (coming up, an advantage to more than 4ft!)
You need to carry the truss chord tensile load across the gap, and restore stiffness to the floor, so double the flanking bottom chords at least a foot (or two) past the cutout, and ideally to or beyond the diagonal truss intersections.
Put solid cross blocking between the doubled truss chords and the next ones either side, in-line with the hatch hole cross members.
Cut out your truss and install the hatch cross pieces. These should be double thickness.
At this point you have two methods to transfer the tensile load into the adjacent & doubled trusses.
First is to add solid cross blocking between the cut truss and the doubled ones, and between the doubles and the next set out, about 18"- 2ft back of the cut point, and put diagonals in so that it makes an arrowhead, vertex at the cut point, back ends in the corner of the blocking and the doubled trusses.
Second is to make it a stressed skin box, by using plywood as a skin, also becomes the floor. Glue and ring shank nails through the ply and into the chords and cross blocking will make it bulletproof. (the bigger than 4 ft hole is what makes it possible to get the ply into the attic!)
This will carry the truss tensile load just fine.

In floor systems, the governing criteria are total deflection and rigidity. An acceptable floor will be much stiffer for heavy use areas than for limited use areas, like this one. Catastrophic (collapse) failure is usually not a concern in published tables. First thing for floor stiffness is cross blocking between joists; blocking every 4 feet does amazing things. The loads are such that the top of the joist is in compression, but the bottom is in tension, and the joists try to fall over on their sides when loaded - this is the failure mode. Blocking eliminates this, works in the roof (rafter) structure also.

The floor skin needs to be sufficient that local deflection is acceptable, and I can assure you that even 3/8" ply is adequate for spans of 24", with blocking under all panel edges, or at least every 4 ft. Thicker is stiffer but not necessary for this kind of use, and it adds weight or total load. I used 3/4" in my house floor with blocking every 4 ft, and you can jump up and down all day with no noticeable deflection or bounce, despite the joists being undersized "by the book".

If you really want to go overboard, add a second skin on the bottom side. again glued and nailed. 1/4" ply is sufficient here. The stressed skins add far more strength than they add weight. I guarantee you could put the whole MGA up there.

Modern wood glues are such that you can glue even oily wood like walnut end grain to end grain, and the wood will break before the glue joint. Gluing all contact surfaces will add a lot of strength, even tensile.

I don't know what "chip" means there. We have "particle board" and "MDF", neither of which is allowed on my property, heavy and weak. "OSB" is just acceptable but heavy. I prefer real plywood, oriented face ply in direction of tensile load. Given your rather inconvenient truss spacing, you might find that you can save a bit on ply by using thin strips glued as splice plates to join pieces for better use/coverage.

Get done with this and you can build your very own Marcos or a variety of plywood aircraft or boats!

FRM
FR Millmore

Steve

If you still want to pursue the larger hatch, I think the simplest approach would be to convert the one truss in question to a rafter system. A 2X6 (38x140) properly shaped and nailed to the side of the top chord would do the job. Attention would have to be paid to bearing at the lower end. I don't know what the wall construction details are, but I don't think that would be difficult. You could then remove all of the truss except the top chord. Depending a bit on the wall construction, I think the two adjacent trusses would be sufficient for bottom ties, although that could be improved by nailing an additional member to the side of the bottom chord of the adjacent trusses. I notice that these have mid-panel splices with gang-nail connections, so stagger the joints. You could also add a collar tie further up. This is presuming a 4/12 or greater slope, which I think you have.
John DeWolf

All

I never thought a bit of woodwork would create such a response on a car BBS!

I have been in and out of the garage over the last day pondering the issue. The joists are are 4" x 1.5", so not the most substantial bits of timber. Obviously recent (last 25 years) stress studies have reduced timber sizes to the minimum acceptable. I have climbed up on to the trusses and had a jump about with no sign of any give (springiness).

The chippie who put the roof up was not good with the ruler (Colin was correct on that score) and I have found one truss nearly 2" wider apart on one side. On that basis I am not going to cut but make do and control my diet. I am now fully conscious of the weight issue, so any items I store will be light and well spread around the edges of the centre apex section. The spare engine, gearbox etc already have storage positions on the garage floor.

My last garage had larger timbers in the garage roof and, from new, was already fitted with a ceiling, floor above it and insulated. I did not appreciate at the time how fortunate I was.

Thanks for all the inputs.

Steve
Steve Gyles

This thread was discussed between 03/06/2011 and 16/06/2011

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