Strength of C section (Parallel Flange Channel) mis used as n-section

[Ginsambo]

Merry Christmas!

I need some help with strength/properties of parallel Flange Channel steel section.

I am building a new 1.5 (Metric) tonne capacity tipping body for a Transit.

Standard tipper body design has a captive 3m long ladder frame fixed to the chassis and a 3.3m long tipping frame which sits on top of it and tips. The two main beams for the captive and the two for the tipping frames are usually 80x40x3.2mm Rectangular Hollow Section (RHS) on edge.

In an effort to reduce loading height I am thinking about replacing the existing tipper beams with parallel flange channel ('PFC' or 'C section') laid on its side (So its n-section) and overlaps the captive RHS frame. There are a few extra mods, gussets and flanges etc but the crux of the thing is this:

I'm worried about the strength of the new two main tipper framing beams.

The existing 3.3m long 80mm x 40mm x 3.2mm Rectangular Hollow Section beams which are laid on their stronger (40mm) x-axis, would be replaced with 125mm x 65mm Parallel Flange Channel (PFC) laid on the weaker y-axis, in an inverted 'U' or 'n' fashion.

The thickness of the web of the PFC is 5.5mm and both the parellel flanges are 9.5mm.

The Second moment of area of the y axis of the PFC is 80 cm4, when used as n-section (Stronger x-axis, when used as C section, is 483 cm4)

The Second Moment of Area of the 80 x 40 x 3.2mm box section on the x axis is 57.2 cm4, when used on edge (Weaker y-axis is 18.9 cm4, when used flat)

So can I compare the valves of 57.2cm4 of the RHS on edge to 80cm4 of the PFC used as n-section directly and assume the PFC will be strong enough or is there more to it than that?


As I'm effectively using the C section PFC out of its design parameters, I don't know if the new 'n' beams be strong enough, or be as strong as the original Rectangular hollow section? I'm tempted to destructive test some RHS and some PFS sections on a loading bay, with a 3 tonne minidigger sat on them and load them up with two tonne of sand using dumpy bags.

Anybody know anything about strength of materials as my brain is feeling liquidized?

 

[gridley51]

You are only going to save 15mm in height,is it worth it?Big danger would be the channel legs opening out thus making it weaker.Additionally you are losing the flat 40mm face as a load bearing surface,replacing it with two 9.5 flats on edge and the top edge will be rounded.
On top of the risk factor you are adding weight and the skill in making bodies for three and a half tonners is having adequate strength at minimum weight in order to maximise payload.

 

[Ginsambo]

Tipper Body Construction

Thanks for the reply, it bucking is my fear too.

I've done a CAD section - Overall the height is reduced from current 206mm (80mm RHS top + 80mm RHS bottom + 40mm RHS Body rest + 6mm Mounting Plates) to 106.5mm (80mm RHS + 10mm Body Rest + 6mm Mounting Plates + 5.5mm Thickness of Web of PFC).

Weight wise it would add about 61kg (98kg - 37kg) (For replacing both tipping beams, without accounting for any extra gussetting needed etc.) (125x65x15mm PFC with a 9.5mm web and 5.5mm parallel flanges runs at 14.8kg a metre, 80x40x3.2mm RHS runs at 5.62kg/m)

Accepting that I'd be adding another 110kgs or so overall, could I not triangulate the flanges down their lengths with two 3.3m lengths of flat bar. and seam weld the lot, perhaps adding a few extra kg's?

.....OK SCRUB THAT IDEA!!....what about RHS used for both top and bottom section beams that are parallel at rest, as opposed to resting on top of one another as is normal? Eg - Use a deeper tipping beam to allow pivot to intersect both tipping and captive beams, whilst also allowing for some 30mm deep internal tipping frame crossmembers? and necessary top frame rests (Perhaps Angle iron welded to captive beams?). Also considering that the two tipping beams would be connected perpendicularly by the same RHS cross member, which would run the whole width of the body, at the cab end and at the tipping end, as these are only areas where interference issues allow clearance. The tipping beam would sit outside the captive beam and could have lightweight 75mm C section outriggers attached. Providing there is enough clearance with suspension etc.

Its a tough one as both tipping frame and captive frame interfere with each other to some extent when at rest.

Sometimes I wish I had something with a proper chassis like an Iveco instead of a Transit.

 

[scarecrow]

is it worth it just to drop by height by less than 2 in. you would probably need to double the amount of channel if you want to lighten the chassis you will need to spread the amount of lifting force by using two rams instead of one not exactly cost effective

 

[Ginsambo]

I'm trying to make a frame that will be sleeved with a Ingimex Ford Factory dropside body - That has alloy side raves and tail raves, plywood/possibly ally floor deck and ally dropsides. I'II be replacing the original tubular steel bulkhead with stainless tubing. I want the floor deck to sit as close to the original 75mm as found on the dropside as possible, really just cause it looks aesthetically pleasing, its for a custom modified Transit chassis cab (Long story, extra insurance, I wouldn't ask!). If the body sits 20cm above the chassis, well, I just won't be happy with it as it won't look right.

 

[gridley51]

I don`t rate the Ingemex bodies,since I watched a mate who has a tipper body on a Transit load an oxygen bottle one day.He put a ratchet strap over the bottle to rings in the floor and when he tightened it the floor curled up like paper.Hardly any crossmembers and only 12 or 14mm resin ply floor.
Adding 60+ kgs never mind 110kgs is a no no.What payload are you going to have left?
BTW,I thought from your op that you were replacing one 80mm box with a 65mm channel,hence the 15mm saving I got.
I`ve got a fixed flat back on an Iveco Daily.You can put a four post stillage with full payload on it anywhere and strap it and it doesn`t deflect,no matter how tight you pull the ratchets.
Long runners are 75 x 50 x 3 channels,crossmembers(lots of them) are 2" x 1" x 2mm ERW,bordered all round with 80 x 40 x 3 box and floored with 18mm resin bonded ply.
The ply is recessed in flush with the 80mm box,it`s now four year old and as good as new.Galvanised as well.
I know what you mean about looking right,Transit tippers never look right,not as good as a flat back.Bear in mind too that Transit tipper operators,especially builders don`t worry too much about overloading and you might find it got to carry and tip two one tonne packs of bricks or blocks.

 

[Ginsambo]

Thanks for the comments Gridley. To be honest I'm with you on Iveco's, at least the ones with a Tacho are unbeatable, not so good on the 3.5 tonne GVW ones due to weight of engine and chassis. Transit's failings, or strengths is the car like monocoque, tinfoil chassis which needs reinforcing for a tipper body. I agree even 20 tonner ally bodies get dented pouring bricks into them with a grab. Know of someone who even beat off all the cement on a brand new truck body using a metal hammer...

To be clear my Transit are 95-2000 era Mk5 smilies, so have the higher, and in my opinion stronger chassis than the MK6/7 which sits lower. But they are mint, customised and well worth the effort.

Ingimex bodies are pretty rubbish on there own, the ally side raves bend and permanently deform with the slightest ratchet strap, the crossmembers are 2mm 75x50 galvanised C section and will also bend pretty easy and even the tubular bulkhead is prone to bending if you knock it or ratchet it too much....BUT they look so nice and I'm a sole trader, with an occasional employee I really trust and a modified transit, hopefully with another on the way soon.

I'm also thinking about replacing the 14mm floor deck with secondhand ally deck or stretched and welded steel sheet as that plenobolic resin plywood floor is actually quite heavy.

Payload on my current transit is 1.5T (Just under 2T empty and useless in the snow!). Its got 80x40x3.2mm runners (Top and Bottom) with 75x50x2mm C section outriggers (Same as Ingimex c/members) and internal cross members butt welded flush to them, a one piece, a one piece 2mm floor bends around and under at side and tail to form side/tail raves, lightweight steel dropsides and a 50x50x4mm Bulkhead bar. To be honest its the lightest steel body I've ever seen - two people can lift it and carry it away! Praise to the company who built it, but it does sit high on the chassis, although lower than most I suspect as the outriggers/internal C members are butt welded flush to, rather than mounted on top of the runners.

I mostly carry green waste, bulk composts and mulches and blended manure, wood chip, plants, sand/10or20mm/gravel/Type 1, pallets of building materials etc. I try not to take loose rubble and waste too much these days, as although I have a carriers license its not economically viable with what you can fit in a Transit, weightwise, and being overloaded knackers the shocks and risks big VOSA fines. Anything small piles of dug up rubble I can't loose I load myself with the digger. Also, often, with the bulk stuff piled up, I have a very high centre of gravity.

To be fair I haven't knackered the tipper body yet, as the welded sheet steel floor, together with the shorter lengths of the cross members/outriggers spreads the weight quite well, and I've been known to load 3.5+ tonne of subsoil and rubble (5.5T GVW), not too carefully, on short offroad runs in the past. But its got so rusted now it has to be replaced.

The Ingimex dropside body relies on the load bearing, strength and weight distribution properties of the ply, so that the 2mm thick crossmembers/bearers aren't bent. The internal profile of the ally side raves has a recess for the 14mm ply floor and another recess for the 75mm bearers with a 5mm plastic spacer strip on the bearers to compensate for the difference.

I want to try and make my own low rise, tipper frame and sleeve it with the Ingimex side raves. Looking at 75+5+14mm = 94mm from chassis to floor deck to make it look like a dropside. I've seen a few Transit tipper bodies with shallow box section captively mounted on the chassis, but no matter how thick you go, or wide, shallow box just isn't as strong and it stresses the vehicles chassis out. That said 1.5T isn't a huge weight, so long as its not gold or lead tipped from a point load right up by the bulkhead.

50mm Box for both the top and bottom runners with a 10mm rest, 6mm mounting plates and 2mm sheet floor would be ideal = 118mm, but it just ain't strong enough unfortunately. Should think any runner would have to be a minimum of 75mm depth.

My idea was to overlap the 80mm captive chassis mounted runner with Parallel Flange Channel (Like an '0' into an 'n'), with a 10mm rest, so that effectively height was reduced substantially due to the overlap, and 75mm outriggers and internal cross members could still be butt welded to it. This way, as the web of the channel is only 9.5mm, height would be 9.5+10 rest+80 + 6mm mounting plate + 2mm floor = 107.5mm.

Regarding the PFC, the dimensions of the channels itself are 125x65mm. The web is 125mm x 9.5mm and the two parallel channels are 65mm x 5.5mm thick. The quoted strength, in terms of second moment of area (Moment of inertia) when used as n section is 80cm4, compared to the 57.2cm4 of the original 80x40x3.2mm RHS (On edge) it would replace. Also PFC, like angle iron is extruded with square edges and corners with a small amount of gusset material left on internal corners, its quite different to C section in that respect.


Like you say Gridley, although technically the PFC should be slightly stronger than the 80mm RHS, one tiny stress fracture on the web of the PFC and its game over, quite dramatically too. That is what I fear might happen to be honest.


The other option, is the parallel runner idea using the same RHS as present but mounted in a side by side fashion, slightly offset in depth with one another to allow for shallow, internal SHS flush butt welded bearers to travel over the bottom runner, rather than the runners sitting on top on one another as is conventional. The tipping runners would sit outside of the captive runners so that the captive frame could be braced with anything and normal 75mm outriggers could be flush butt welded to the tipping runners. The compromise would be the internal cross members/bracing of the tipping runners would have to be shallow to travel over the captive runner.

Given that the top tipping runners would be braced/connected with a deep 80x40x3.2mm RHS crossmember section up at the the cab end AND at the tail end, butt welded flush to form basic 'II' or double 'H' shape for strength, although the inner flush butt welded crossmembers would have to be heavier grade and much shallower (30mm) to reduce height, with more of them for strength. that would give me 80 + 10mm rest + 30m internal SHS crossmembers + 6mm Mounting plate +2mm sheet deck = 128mm? Or 123mm if I used 75mm RHS. That's almost half what it is at present (208mm)

I could always add one or two more internal tipping runners for the ram to act against and reduce the span of the shallow internal crossmembers, if I lower the internal bracing of the captive runners by using triangulated gusset plates of the c member/runner joints? Will add weight but not as much I imagine.

 

[johnny max]

Why don,t you use 80x40 for your chassis runners and put the body rails in such a way so they sit side by side to chassis rail. Your crossmembers will then be cut into the body rails so as to leave frame flush on top for floor.
Chassis rails will be cut to accomadate crossmembers to sit into when body is flat on chassis. I assume you are using undermounted tipping gear.
Johnny

 

[Ginsambo]

You mean cut the captive runners, mounted on the chassis with slots to accommodate the tipper crossmembers? Won't that weaken the captive runners substantially when its tipping?

 

[johnny max]

You can weld a 30x6flat to the side below the cutouts to strenghten it,Also fill up all cutouts with 40x6mm bent into U shapes
I,v done it a few times and no probs
The body will sit only 80mm up plus wat ever flooring you use.
Johnny

 

[Ginsambo]

Great thanks,

Just nice to know its been done, I was thinking if I rounded and 'S' bevelled the cutouts and welded rolled steel plate in, it would be like the Olympic rather than the Titanic and releive the stress. As I said, I think the deeper chassis on a Mk5, with full top hat and bottom hat sections is stronger than modern Transits, but I could always double up on captive runners if I have any probs.

See if I can buy the steel before the Vat goes up, although I think they might be off this week.

 

[Ginsambo]

Problem

Found a problem with rear shock mount clearance issues. Basically rear shock mount, mounted on outside of chassis, rises above chassis by about 45mm. Can't reallly chop a chunk out of the tipper rails to accommodate this. The chassis is 60mm wide. 2 x 40mm wide parallel rails = 80mm + 20mm gap ideally = 100mm...maybe 90mm min. which still leaves a massive overhang.

I wonder if I offset the captive inner rails so they are slightly offset from the inside of the chassis? It will be mounted on 6mm plate anyway which is fixed by factory welded chassis mounts either side of chassis...Or should I be thinking of the reversing the two so that the inner rail tips? Thinking of using 42.4 x 3.2 or 4mm CH Section for the c/members that cut into the chassis rail, with 75 x 50 RHS or C section for framing the ends of the tipper rails.

Got a quote for the steel, excluding thick bent bits/angle iron for the top and bottom ram mounts and lightweight C section outriggers, much of which I can salvage. Roughly about £200 all in. Also have a recon ram and pump/res which is all ready to fit.

 

[derekm]

..
I'm also thinking about replacing the 14mm floor deck with secondhand ally deck or stretched and welded steel sheet as that plenobolic resin plywood floor is actually quite heavy.

..

Have you considered plywood with ally sheet expoxied on both sides?

 

[Ginsambo]

Have you considered plywood with ally sheet expoxied on both sides?

Have done yeah. Composite materials are the future. Only needs one side and an overlap/drip edge really, best of both worlds as that ply is pretty strong, if not heavy, and ali offer weather resistance and pretty good wear/abuse resistance if backed by the ply. Would need one giant Tig weld down the middle though = £'ss