GRP/Fibreglass info needed

[irving2008]

I know there are a few here who know something about fibreglass/GRP. What I need help with is how to embed a mounting into a GRP sheet and work out how much load it can take.

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[EddyCurrent]

I don't know the answer but I imagine the strength would be increased if you also put rivets or bolts through the whole thickness each side of the lug, if that's possible in your application.

[irving2008]

Can't as need to preserve water tightness of lower/inner bit so can't penetrate it. If I could and know it was 100% guaranteed watertight I'd go that route and it'd be trivial. I need to figure out what size (area) of ali plate I need for a given force. So far haven't come up with a foolproof way to do this.

[EddyCurrent]

What about making some test pieces and applying weights to the mounting ?

[irving2008]

in an ideal world, but no time nor facilities.... was trying to model it with FEA but cant seem to get a meaningful answer...

[mekanik]

What is the substrate and the application ?
If substrate is fairly substantial the plate could be drilled and countersunk to take woodscrews, edges of plate need a good chamfer possibly 30 deg so that you can get a decent layup when bonding the cover layers of glass(will prevent air pockets you might get with 90 deg interface). bond the plate to existing fiberglass using your epoxy and screw it to substrate, then apply your epoxy and glass to cover the assembly.

[Neale]

This is the kind of thing that boat-builders are doing all the time, but I can't think of case where I've ever seen a plate embedded in laminate. For one thing, polyester resin isn't a very good adhesive so it won't bond to the plate - I suppose that you could put holes in so that the laminate is bonded through the plate, or use an epoxy laminate? Fixings to take rigging loads on a GRP boat would generally be through-bolted and use a sealant like Sikaflex which would give a good working life and if properly made, guaranteed waterproof.

[GEOFFREY]

There are purpose made fixings for this job called "big foot". They are available with male or female (stud or tube nut) protrusions and I think stainless would be a better option. The flange plate on these fixings is usually perforated to assist adhesion. I do not know how to calculate pull out strength, but would suggest the better way might be to rebate the the outer skin for the footplate. drill through for the stud bond the fixing in from the outside and then glass over the rebated area. The second skin of fibreglass applied to a precured skin does not adhere as well as a single skin laid up in several layers. If the studs you use are M6, I may be able to give you a few in stainless steel. If you google Bigfoot fixings they may be able to give you some technical advice. Good luck.G.

[charlieuk]

whats the application? if its taking any real load you are going to need a fair size plate and a good thick laminate to do it that way, i think you would be better off coming from behind if you can and then making water tight which should not be too hard and will give you far more strength.

[GEOFFREY]

Sorry Irving, "bigfoot" is just my nickname for these fixings, they are called "big heads"!!! G.

[irving2008]

Thanks all, but seems my idea is a non-starter. I need 750kN pull-out resistance and the best bigHead will manage only 1/20 of that.

The requirement is to tie down Solar PV panels to a GRP 'flat' roof without compromising water resistance or membrane integrity. Normally you ballast the panels down but that needs ~300kg ballast per panel and the roof simply wont take the load. and finding a way to fix through the membrane and insulation to the underlying joists hasn't, despite my best googling, come up with a solution. Ideas?

[Neale]

Will the GRP take the load anyway, assuming you could fix to it, or will you have to get a fixing to the underlying structure?
Maybe piece of angle screwed to side of joist and against bottom of GRP panel, hole drilled through both and then bolted together with external penny washer and appropriate sealant. Leaves protruding stud to take panel fixing bracket? My boat has plenty of examples of this kind of seal, underwater, and good for life of boat. I hope...

[irving2008]

If only it were that simple :(

This is a 'warm roof' so the GRP is bonded to the insulation, 120mm Kingspan polyurethane, which is itself pinned to 6mm marine ply over the joists. On the bottom of the insulation is a vapour barrier. any fixing to the joist would need to penetrate the ply and the vapour barrier and the insulation, and should have a thermal break in it to avoid cold spots/condensation forming...



Will the GRP take the load anyway, assuming you could fix to it, or will you have to get a fixing to the underlying structure?
Maybe piece of angle screwed to side of joist and against bottom of GRP panel, hole drilled through both and then bolted together with external penny washer and appropriate sealant. Leaves protruding stud to take panel fixing bracket? My boat has plenty of examples of this kind of seal, underwater, and good for life of boat. I hope...

[mekanik]

is the structure attached to your house or freestanding ?
If you have a vertical wall(if it's South facing) you could attach a framework to the wall to take the majority of the load and just have the bottom edge of the panels resting on a load bearing strip of ally or similar with a slayer of rubber between that and you FG roof. would need to box in the open ends to stop the wind getting under the panels and ripping them off

[Clive S]

I take it you are wanting to put pv panels on a flat roof. I was surprised just how light they are but the problem would be the wind. How about installing them in a frame and holding the frame down with very thin wire guys with the anchor points on brackets off the brickwork. A bit like they way they install aerials on to a flat roof. ..Clive

[irving2008]

Panels are on a flat crown roof, no way to have guy lines there's too much else in the way. The issue is wind uplift, the panel, even at 10°, acts as an aerofoil; it's about 3kN per panel on the 1 in 100y wind. I could take a risk of course...

[charlieuk]

in that case just fill the gap between roof and pannel if the wind cant get under it wont create any lift.

[GEOFFREY]

in that case just fill the gap between roof and pannel if the wind cant get under it wont create any lift.

I don't think that is quite true. G.

[irving2008]

I don't think that is quite true. G.
Its not.... you still get suction... but it will reduce it...

[EddyCurrent]

Ask a reputable company to give you a quote, when they come round to take a look, pick their brains, it works most times for me.

[GEOFFREY]

Its not.... you still get suction... but it will reduce it...

Not even sure that is true Irving. Obviously it depends on wind direction and strength, but according to Bernouli, (can't remember spelling) wind blowing between the surfaces will cause a reduction of pressure on the underside, so in some instances will actually create negative lift (downforce). Anyway that is all academic - try to pass all responsibilty on to the installer!!! Good luck. G.

[irving2008]

There's a proprietary product that claims to need only 1/10 of the ballast by directing the airflow to create negative lift, but none of the pics I've found give enough detail to see how its done :(

Tried the installer route... one suggested standard ballasted approach until I asked about loading then he wanted £700 up front for structural engineers report before he'd discuss further. Two others said they couldn't help....

Anyway, I don't want an installer, my sparky will do it, installers want to load the kit by 25% margin and charge £1500 for a days work roughly! Just so I can get the pointlessly low feed in rates that the gov now gives. Makes the ROI go from 3y to 8y...


Not even sure that is true Irving. Obviously it depends on wind direction and strength, but according to Bernouli, (can't remember spelling) wind blowing between the surfaces will cause a reduction of pressure on the underside, so in some instances will actually create negative lift (downforce). Anyway that is all academic - try to pass all responsibilty on to the installer!!! Good luck. G.

[GEOFFREY]

I suspect that the system you refer to will attempt to direct as much airflow as possible under the roof panel to create negative lift, and have some kind of spoiler to the perimeter of the upper surface to disrupt the airflow and form eddy currents(sorry eddy!!)and therefore prevent laminar flow or suction. Sorry to hear the installers don't want to play ball, but I do understand your wish to get a local sparky to fit, however I understand that to get the FIT benefits the panels and installer need to be MGCS (or whatever) registered. When I looked into it about a year ago I came to the conclusion that payback would be about 6 years and as I was 70, could not decide if it was worth it!!! G.

[irving2008]

I came to the conclusion that payback would be about 6 years and as I was 70, could not decide if it was worth it!!! G.that's exactly my issue, the kit is cheap and installation only a day at most, but knowing the savings you're going to make the MCS registered installers load the install costs. My 14 panel system kit cost is £3500, the quotes are £7500. With FIT that's about 7y ROI, but just saving on usage without FIT with £500 for the sparky to install gives me a 2.5y ROI.