I need to design a platform that will enable a fat* man to stand on the end of it, 1.2 m-ish off the ground.

I have a design in mind, details omitted, and it will use a pair of supports made of pin-jointed members as sketched below ie 2 of these structures with a platform between them. However I am using the full weight of the fat man on each support, as he may well stand over to one side of the platform.

Can someone check my maths for the force in member A – it’s been far too long since I did this and no-one here know anything about it!

H in the lower breakout triangle is A in the drawing, and I forgot to draw a pinpoint at the bottom of A, but there is still one there.

Thanks for giving me a hand 🙂

*200Kg!

I’m not going to comment on the maths as yet as I’ve only had one coffee this morning, but won’t you have a massive bending moment on the 890 element?

yes – it’s 76.2mm Square 3.2mm Wall 6082 Al per side.

This is a concept for the item, It needs to be feasible with some tweaks.
Sadly I’m on my own here, and I don’t don’t know how to calculate these forces 🙁

also your angles are wrong if top and bottom are parallel

the top angle should be 75 (as at the bottom) not 68 😕

honeybadgerx – Member
I’m not going to comment on the maths as yet as I’ve only had one coffee this morning, but won’t you have a massive bending moment on the 890 element?

+1

also your angles are wrong if top and bottom are parallel

the top angle should be 75 (as at the bottom) not 68

All the PJ’s on the longest member are attached on the centerline with the exception of the topmost which is shifted to the right [see pic]

TBH AFAIK the top triangle doesn’t do anything WRT this lower support, but I included it in case I was wrong and it’s needed.

I could be wrong but I make it to be 9152N

That’s very kind.

I’m really beginning to think I’ve screwed up here – this is not going to sustain the forces is it…? I previously thought the A member was 1″ square Al, now it’s looking like it needs to be a huge piece like the others…

Bugger.

And it was all going so right before “Just one more little project before you leave” 🙁

Are you sure this isn’t for some funking looking gallows?

for the fat guy? Naah – he’s the relation of the 2nd most senior staff member…. 😉

I would suggest not having anything to do with this as I have a feeling that someone’s going to get hurt. 3″ Aluminium SHS? That’s not going to cope with a quarter of the load that you’re looking to support.

With any cantilevering structure like this the governing aspect is normally deflection.

The element labelled 890 and 700 or both going to deflect far too much with the material/sizes you’re looking at using.

Edit: Not implying they’ll work in compression either though

At a glance the 328 long piece up near the top is the only thing that can be fairly slim as it’s in tension. Everything else is in pretty heavy duty bending or compression.

oh wow – well I was asked to give them a rough prototype concept and they will test it. I came up with the idea for a cantilevered structure as resting on the top of the variable surfaces was unlikely to allow the structure to be deployed and adjusted quickly. However since starting this I have been told we’ll get it tested with a 200kg sandbag and if it passes we’ll try rolling them out.

I am quite concerned as I thought the next step would be to an engineering co to calc the tubes etc required, but it seems not…we don’t have FEA plugins for Solid Edge so I really don’t know what the forces are, as I said I thought it was going to be worked-on after me.

I don’t know what to do – this is supposed to be fabricated ASAP

crap crap crap – this placement has been a nightmare and I’m shit 🙁

EDIT – I have asked everyone around that seemed to know what they’re doing and the fabricators and we all thought it would be in the ballpark. AFAIK if it deflects it’s going to fatigue and break anyway even if it hold the static weight.

We had to do a similar thing at the last place I worked, but it was a cantilevered gantry for holding a printing system above a conveyor. It had to be cantilevered to fit over the conveyor and the legs had to slide in underneath.

I think the guy who designed it used welded mild steel box section and then added welded on triangular gussets in the corners.

Why does your design need to cantilever? Adding a big X to your design from the corners would strengthen it massively. Then whack a few welded gussets in and bob’s your uncle….

if theres access why not use a counter balance like a fork lift

Here’s a pic of the whole thing – I’m severely limited as to it’s size and weight because it needs to weigh around 30kg and be around 4-500mm max depth from the pivot of the top platform away from it [ie the space the steps take up]

it also has to be carried in by one guy and unpacked in a space 700mm wide 🙁

Thanks for all your help guys I’m srsly pooping in my pants about this.

Counterbalance would be great, but no space

has anyone looked at the euler buckling load for the little supports at the bottom?

it’s been a few years since i did this kind of thing, but the fea program that lives in my head says: ‘er… seriously?’

No – I seem to have totally misunderstood the forces – I basically thought it was a massive compressive load on the 3″ 1500 strut and didn’t realise the 15 degrees would generate such a load on those small beams. Just calculating I now…

So why does is have to be cantilevered?
Is it collapsible and carried around or something too?

Some more info might enable people to give you some ideas…

From your rendering…..what’s stopping the lower triangulation points being further up the ladder and further along the feet?
And can’t you extend the top of the ladder uprights a bit further past the platform so your brace can be located further from the platform (vertically) and be connected to it further along?

Usage scenario – I’ve been a bit shy to show the whole thing, I have never screwed up this bad before and I feel bad asking for so much help…

Yes it needs to be carried in [or al least handleable – spot the handles on the two separated sections]

In this pic you can see that some units it’s designed to go over are quite deep, so the attachment points are limited by the space available along the feet. There is also the issue of how much space it takes up under the units – there is <100mm available there too

Just to add to the tide of rising panic (don’t worry, we’ve all been there, even if we are ‘Engineers’), but as the designer don’t you also need to consider the safety of the person stood on the top at height, i.e. safety rails, etc?

yes – rails are on the way – I think I have a way of adding them without changing the rest of it.

Dammit why does this work with 2000kg?!

Not inside 30kg you don’t, and certainly not if the bloke falls or leans on them (Transferring the load through the rail) 😳

Tricky problem

Steel will help with the loads if not the weight

You need to draw a free body diagram. I suggest drawing a FBD for each portion of your structure. i.e. draw a FBD for the upper cantilever then use the reaction shear forces and moments from that to draw a FBD for the upper triangular pinned structure, and so on until you have your forces reacted at the floor.
Once you have your FBD you then need to draw a shear force and bending moment for each member which can then be used to size the tubes. The long slender tubes will need to be checked for column buckling and local crippling.

Also check your load case of a ‘fat man’ of 200kg applied to one side. This should be considered the ‘limit’ or safe working load upon which the proof and ultimate factors are applied. We generally use 1.125 and 1.5 as our respective factors in our industry (aerospace)in general no permanent deformation shall occur at proof load and no failure below ultimate loads.
The stability of the sections will also have to be assessed against buckling and crippling, the pivots will be assessed against shearing and bending of the pin/bolt and bearing of the pin/bolt in the frame.
The completed design should then be proof tested by an accredited test company (Briggs, LLyods) to demonstrate the structural integrity of the design, this will be required for insurance purposes.

RE Proof testing – is this the case even if it’s for the companies own engineers – we’re not going to sell it? What’s the ballpark price for that? I don’t think the boss knows!

Aww hell – this would be much less stressed wouldn’t it – though they would probably be better if they were vertical.

I can’t believe this, I should have never said I’d stay on.

I’d just buy a ladder and tell him to stand on the boxes.

😆 OK but that’s what we’re tying to get away from.

I’m going to have a smoke and get my head together.

Just realised – your ‘fat man’ weighs 200kg! That’s 31 stone….

Pretty sure you aren’t gonna get too many 31 stone chaps ascending those stairs.

I know you need a safety factor, but even a 15 stone bloke with a safety factor of 1.5 would only be 143kg.

If it’s for internal stuff, can’t you just put a label on saying ‘not to be used by anyone over 15 stone/XX Kgs?

Reducing the loading by 25% would make things easier….

Usage scenario – I’ve been a bit shy to show the whole thing, I have never screwed up this bad before and I feel bad asking for so much help…

I wouldn’t say you’ve screwed up. It’s not an easy scenario to resolve. Sounds like you haven’t been given the support you need to get the job done.

Does it need to be ‘carried’ in one lump? If you could disassemble it, could you make it out of sturdier materials and have sections that could be carried to the location and bolted together?

hehe – The fat man weighs 23 stone and he might have several tools with him or a 30kg bottle of liquid – I just rounded up.

the unit does come apart – but I am worried about it taking too long to assemble. The job the platform is required for will only take 20 minutes to complete, so adding much assembly time is a -ve point for the design goals.

OK – I need a plan.

The boss loves the look of this concept, so:

*I’m going to finish adding the rails to it.

*Check the loading on the supports and beef them up as much as possible in the space.

*Get it made and tested.

*Meanwhile, I’m going to work it over seeing if I can do a better job of the engineering calcs, as I clearly need to have a more accurate grasp of the forces if it is going to be much cop without having a firm re-engineer it. I’d better go and explain to the Director.

WRT Support – no they haven’t been able to give me any time or help, but I am also a bit to blame for not asking for more I guess. Maybe I was foolish, proud – I really thought I could do this on my own [esp as we have a couple of access platforms which hold 150kg and they weigh about 10kg in Al. But then again, they are made with only vertical or horizontal members…]

Please feel free to suggest any changes to Mk2 🙂

Not gone into any calcs or anything but basic principles i would think you need to add a second diagonal main brace opposing the original, creating an x shape between base and platform.
Extend the main rails of your platform back within your given width to meet this second support.
This would give you a main support in tension and the original in compression which i should help to distribute the loads better within your restrictions.
Thoughts?

Sounds fair to me. I sketched it with a pair of parallel rising members on the other page – looking at the 2Ton crane I posted I figured out why it was strong, as you said it has a pair of members, one in compression [beam>bottom of rising beam>ram>bottom] and one in tension [top of rising beam/supports] – much as you said if you view it in 2D.

Yeah it was the crane that made me think. By increasing the length of the top platform member backwards it allows it to pivot about the top of your steps with the tail being restrained by the tension member rather than purely trying to rotate and bend the sections with the steps.

Oh damn – I know now, I should have used “WEAPONS GRADE ALUMINIUM”

I think you need to invent a man portable AG unit