Mobile Access Platforms - Overturning Stability

[Rick_Australia]

Hi,
First post after really a long time.
I am designing an aluminium access platform usually called mobile platforms or truck access platforms specifically.
I have come across an interesting problem regardless of the actual load carrying capacity of the stairs or the working platform area.
The problem I have is the overturning stability of the whole structure under min. horizontal handrail/guardrail load of 0.6kN.
The whole platform weighs only 200kg, say 200/4 at each support point is 50kg.
The platform is 1.4m high above ground including castor height. The handrails are another 1m top of the platform.
AS1657 is the relevant standard for the Fixed platforms here in Australia.

The platform I have is not per se a fixed platform, it is movable but the castors will be locked on during operation - Still I don't think it is equivalent to a fixed platform attached to some permanent structure.

No code or the standard I have across talks about the platform stability under the handrail loads probably due to the statement above (Fixed to a structure, like you cannot overturn a concrete structure by just putting hand rail loads on a balcony).

But the structure I have is different. These units are quite extensively used through out the world and all have hand-rail safety features.
The issue is I don't understand what's protecting the unit to go off ground in case there is a code-prescribed hand load of 0.6kN?

The only reason I made up is that it is very difficult for a person of let's say 75kg to exert a horizontal force of 60kg.
Also, if there is an accident it is more likely an external accident, like a worker being hit by a steel beam while unloading it from the truck to this platform.
In this case, the hand-rails might protect the worker or at-least help reduce injury as the platform might want to topple over but may get stuck in something like with the body of the truck or something.
If I go with this logic, then hand-rails on this type of structure are a subjective matter, they might protect a falling worker, might not.

I cannot find any other relevant code for this type of structure. I guess it comes to the actual on-site risk assessment of the type of work, if it is risky, contractor will need to fix the platform with the ground slab through a scaffold tube and mechanical anchors or similar.

The codes only mention testing the hand-rail structure for the elastic deflections etc, so the hand-rails don't come off the structure they are mounted to. Like in a balcony or a fixed platform/ramp etc.

I have also attached an image of the forces involved to make my point that there is no way the stability is ensured with this type of loading.

Any thoughts or experiences or suggestion you would like to share?

 

[Lomarandil]

I haven't run the numbers (and suspect it won't make up all the difference) -- and it might be different in AS, but convention in US codes is that the overturning and restorative components originating from the same load source do not need to be factored differently (overturning at 1.5 and restorative at 0.9). You'd be able to use 0.6kN * 1.5 = 0.9kN as the opposite frictional force at the platform level, not only 0.54kN.

It isn't always a consistent convention, but at least in this case it provides a slightly more rational analysis with sum of horizontal forces = 0.

 

[JStephen]

One possibility: Take the whole assembly including people, treat it as a free-body, and calculate forces acting to overturn it. That would simply eliminate handrail forces from the overturning consideration.
If you're trying to model dynamic situations, that wouldn't work.
Or if you assume a user is on the platform, pushing/pulling on the handrail, while also holding onto, or pulling or pushing on some outside object.

 

[rb1957]

if someone, standing on the platform, is pushing on the handrail ... then there is an opposite reaction through their feet into the floor. So the moment effect is "negligible". If the top of the platform (on a scissor lift ?) is "wobbling" then there'll be a lateral load factor applied (to the person) and this'll create an imbalance moment. If the platform moves 6" (150mm for you in Oz) in one minute then you can calculate the acceleration ... assume different numbers as suits ...

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Rick_Australia]

Thank you guys. My comments below:

Lomarandil - Great idea. Thanks. Makes sense to use same factors. It did help but still net uplift

JStephen - Not sure if you had a look at the free body diagram image I attached?

rb1957 - The effect is not negligble as the hand and the foot are not at the same level. 1m difference is enough to create an over-turning moment against a small self-weight.

Additional, I don't think a 90kg person can push horizonally 60kg (600N as required in the code). So it is not a matter of a worker standing and leaning on to the guard-rails. The 600N in my post is more of an accidental fall protection load.

 

[JLNJ]

The last one of these I did, I used the 200# horizontal load. The logic was that whatever you place the mobile platform near is the stationary object and the platform occupant pushes on that stationary object. So, 200# load to the stationary object creates a 200# reaction on to the platform.

 

[canwesteng]

Statically, the effect is more than negligible, it is 0. The overturning caused by the lever arm between hands and feet is counter balanced by the persons selfweight being offset from the fulcrum (their feet). Dynamically, this isn't true. The force if they just run at the guardrail is real and can cause the lift to flip. So in your place I would just use the 600N force to be safe.

 

[rb1957]

"Statically, the effect is more than negligible, it is 0." ... is that (zero) more or less than negligible ? I think I know what you mean, but the language isn't very clear. But if we're worried by 90kg.m moments then I think there are bigger problems.

I think the critical situation is where there is a lateral load at the platform ... maybe the guy falls against the platform handrail, maybe the guy is pulling on a rope, but the guy pulling against the handrail is a non-problem.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[canwesteng]

Good point, that is definitely worded wrong... I guess less than negligible?

 

[Tomfh]

I agree with the comments saying the static effect is zero. It is a fundamentally a dynamic problem.

These platforms will not generally satisfy the static loads prescribed in the codes. A 60kg lateral load at the top will knock them over.

 

[Rick_Australia]

Thank you guys for udnerstandinf that a 600N load (pull/push) will knock the platform over.

And no, there are no bigger problems here, the platform is called "access platform", just to access stuff from a truck.

No horizontal loads to the main platforms, no pulling on ropes etc. The main platform is safe for the 2.5kpa loading.

The handrails are safe for the handrail loading of 600N if their connection is considered as fixed stand-alone.

The issue is only the stability of the whole platform on wheels with 600N handrail loads in case of accident.

And no, the weight of peron does help in counteracting some moment but not completely.

To sum it up:
This is dynamic problem, in case of an accident, the hand-rails won't servce the intended purpose and therefore the platform is not stable under these horizontal loads on its own. It needs to be fixed to ground or to a more permanent structure. Anyway, AS1657 Australian Standard is for fixed platforms so the provisions don't apply to mobile platforms.