Tipping Hazard 2

[drawoh]

I am starting work on the design of an optical fixture that is going to be around 5m high, with a mass of hundreds of kilograms, perhaps in the low thousands. I have a requirement for part of this to be 300mm thick, although I can mess with the configuration of the base. Probably, I can put anchor studs in the floor.

My ideal case is that this thing will sit on the floor of the lab, held in place by gravity.

Is there a formal standard somewhere on tipping hazards, specifying the base configuration, and/or tie-downs?

JHG

 

[ishvaaag]

Gooogle shows some literature on these things, you can search.

Anyway, selfweight plus stiffness of some structural support would lead to some deformation, some of which migh come out of required tolerance.

By including sensors for temperatures and distances, and pressure or mehcanical/electrical jacks in your support structure you can monitorize your structure is keeping the mirror as wanted. Furthermore, it could even correct somewhat some fabrication geometrical imperfection of the mirror in the weak deformation ranges since applied to the combined stiffnesses of part of the structure and the mirror.

 

[MiketheEngineer]

Assuming no wind or seismic events - I like at least a 2:1 overturning factor - preferably 3 or 4:1 if possible.

 

[racookpe1978]

A tipping hazard is a dynamic hazard, almost always coming from impact (from something/somebody running into the target) or from a collision (a forklift or crane load or laundry basket) being driven into the target, or from a movement of the object - then not being properly supported again when its being lowered back down. If your experiment platform is moving (has moving parts) or can be climbed on to adjust or install material - then the dynamic accident is both more serious and more likely to occur- people are going to do stupid things at random times - despite all the signs and barriers you can think of.

A stand-alone, static, not-being-moved, protected "thing" will rarely fall over - unless your region is subject to periodic earthquakes. Even a two meter long, 150 mm diaa pipe could stand n end forever on the lab floor if nothing ever touched it,

Problem is, you can't prevent all these hazards in the real world and so can't pretend to just "leave it alone" and "nothing will run into it" as a adequate solution.

Have you seen the photo of a multi-hundred million dollar JPL satellite tipped over inside the lab? That's what I mean be by this kind of "moving" hazard even in a "safe" lab enviroment with highly-trained people.

You need a risk assessment and hazard analysis document to present to your boss (the design team): What things at what weight (what energy) could hit this platform at what elevtion above grade? What is the probability of impact, and how can I design things to prevent/mitigate that impact? What is the most severe "routine" movement or impact (a broken brake could jar the mirror when it hits the stops, or a sudden startup of the mirror motor will create dynamic forces each time an adjustment is made, what if four people are standing on the same side of the platform when they are installing the mirror -> this will create an unbalanced load up high on the platform) that is expected? Then, assign a safety factor on that load, and design the bottom of the mirror platform (size of baseplate, legs, and max width of the legs away from the CG, and size and number of anchor bolts) to resist that maximum expected dynamic force.

you can't protect against everything: But you must make reasonable assumptions about reasonably-expected hazards, chose a reasonable safety margin, then design against that. Not all things are in the OSHA and design books. A guard rail around the base might be cheaper than an elaborate foundation and leg assembly.

But, if a tornado tears the roof off of the building, or a semi-truck runs through the parking lot and hits the building, that mirror will fall down. 8<)

 

[civilperson]

Seismic design is required in most locations. (Maybe not Florida).

 

[drawoh]

racookpe1978,

This device will sit in an optical lab where high powered lasers will be firing. Access is restricted accordingly. Outside of lasers, I am not sure of the safety discipline in the lab. I am strongly tempted to install bollards around the structure just in case somebody does go in there with a cart. A small impact that does not tip it could still mess up the optical alignment. Earthquakes are rare up here, but not unheard of.

The structure will have to be accessed for assembly and alignment, and then for optical testing. If I separate the access platform from the structure, I eliminate a source of disturbance that might cause misalignment or tipping. If I integrate the platform, I wind up with a fatter, more stable base.

I just want to make sure I understand overturning factor.

Structure mass m=1000kg. Structure breadth b=300mm. Structure height h=5m. I assume centre of gravity is in the middle. An assembler might exert f=200lb at the top.

Gravity moment M_g = wb/2 = mgb/2 = (1000kg [&times;] 9.81m/s² [&times;] 300mm [&times;] .001m/mm) / 2 = 1470(kg.m/s²).m = 1470N.m

Overturning moment M_O = fh = 200lb [&times;] 4.45N/lb [&times;] 5m = 4400N.m

Overturning factor: M_g/M_O = 1470N.m / 4400N.m = .33 (it is tipping over)

Is this correct? (the .33, not the tipping over) If the assembler at the top, uses whatever force is required to push it 300mm, it is falling over. It will take 294N (66lb) to start tipping it.

I am just punching in numbers here. The 300mm is an optical requirement. It does not have to control the base breadth.

JHG

 

[ishvaaag]

Well if a thing is 5 meters tall and 300 mm diameter at the base and simply supported certainly is not well conceived for overturning. Trees have roots just for that. So you can go directly to the instrument creator and ask him where are the strong points you can hold his device from, and if none, what way he thought it was going always afoot without falling. You will need some points of fixity or restraint.

 

[MiketheEngineer]

What about installing a safety line from top of mirror to ceiling.....

 

[drawoh]

MiketheEngineer,

I have to fatten the base to the point that I have an acceptable overturning factor. I am just trying to verify that I understand overturning factor.

I have considered tying the top of this to structure overhead, but I am not sure of where I can attach. Also, it has to be stable while we erect it.

JHG

 

[ishvaaag]

You understand well. As long your base is rigid enough (must be for such precision instrument) and rigidly attached to the base, you can use the new B/2 of the overall base with the total instrument plus base weight to state the restoring moment. So as you put,

Safety factor against overturning=W·(B/2)/(F·5m)

This was once considered for ***walls*** be better around 3, then later reduced to 2, and now 1.5 safety factors start to seem acceptable. If the forces atop can vary wildly because the use I would take a high value; otherwise, in a lab environment 1.5 might be fine.