Got handed a project..snow weight

Hello again...I was offered the opportunity to design an early warning system for one of the buildings I work at. The building is used to store aircraft cargo and is several thousand square feet. The problem is the building is a steel type structure with limited supports inside (majority of the interior is open with no walls) and possibly prone to collapse due to excessive weight of snow on the roof. I was asked if I could come up with a way to measure the weight of the snow per square meter of roof and either display it remotely to the inside where a controller would sit or simply have an audible alarm go off when the weight was "x" amount. I think just an alarm is too easy and will use either a PIC controlled GLCD or LCD to display along with an audible alarm.

The question is whats a good way to accurately find the weight per square meter. A simple way would be to make a 1 square meter pan on the roof to collect the snow, attach it to some kind of scale to it (sensor of some kind) and have the PIC look for some value. Im being vague with values as I dont know what the weight per square meter is supposed to be...have to find out.

I was also told there are a few areas which have columns on the floor that dont quite go to the ceiling on the inside. Don't ask me why I haven't looked myself. Supposedly these areas have a gap of several inches where a pressure sensor could possible be attached to the top of the column and rest on the inside of the roof.

What sensors to you guys think would work best for this application? Any ideas welcome! THANKS!!!

Strokedmaro

What sensors to you guys think would work best for this application? Any ideas welcome! THANKS!!!

Click to expand...

A strain gauge for measuring weight, e.g. snow on top of the roof.

A pressure transducer, for measuring the pressure between the bottom of the roof and the top of the columns you speak of.

Also, you could simply measure the distance between the roof bottom and the top of the column (stress), however, you would have to know what kind of weight corresponded to a particular distance change or stress. That calibration alone, in my opinion makes it considerably more complex, and less accurate than a strain gauge (weight).

I would go with the strain gauge, into an A->D converter on your uC, and go from there.

strokedmaro said:

Hello again...I was offered the opportunity to design an early warning system for one of the buildings I work at. The building is used to store aircraft cargo and is several thousand square feet. The problem is the building is a steel type structure with limited supports inside (majority of the interior is open with no walls) and possibly prone to collapse due to excessive weight of snow on the roof. I was asked if I could come up with a way to measure the weight of the snow per square meter of roof and either display it remotely to the inside where a controller would sit or simply have an audible alarm go off when the weight was "x" amount. I think just an alarm is too easy and will use either a PIC controlled GLCD or LCD to display along with an audible alarm.

The question is whats a good way to accurately find the weight per square meter. A simple way would be to make a 1 square meter pan on the roof to collect the snow, attach it to some kind of scale to it (sensor of some kind) and have the PIC look for some value. Im being vague with values as I dont know what the weight per square meter is supposed to be...have to find out.

I was also told there are a few areas which have columns on the floor that dont quite go to the ceiling on the inside. Don't ask me why I haven't looked myself. Supposedly these areas have a gap of several inches where a pressure sensor could possible be attached to the top of the column and rest on the inside of the roof.

What sensors to you guys think would work best for this application? Any ideas welcome! THANKS!!!

Strokedmaro

Click to expand...

• What are the specs?
• How do you know when it is excessive?
• Can you modify the structure to accept a gauge?

I would go with the strain gauge (also called a "load cell") too. They make quite a variety of them, including some built into bolts -
**broken link removed**

If you can measure the strain on the roof, you are reading the snow load directly.

Here is another link: These record and can output your strain/load.

This will accept events as inputs, and auto dial/activate alarm under X condition **broken link removed**

Is the roof flat or pitched? If pitched, depending on the wind, one side can accumulate a lot more snow load due to drifting over the ridge. You may need more than one sensor.

I remember when inflated roof our Metrodome stadium in Minneapolis collapsed due to drifted snow load...I was sitting on a beach in Fort Lauderdale.

Ken

duffy said:

I would go with the strain gauge (also called a "load cell") too. They make quite a variety of them, including some built into bolts -
**broken link removed**

If you can measure the strain on the roof, you are reading the snow load directly.

Click to expand...

Im really digging these load bolts but I cant wrap my head around how I would use them. I imagine these read the "pull" on the bolt and would need to be mounted to something other than the roof (above the roof) and bolted to the roof so that the weight of the snow pulls the bolt causing a reading....correct? Or would these be mounted inside under the roof and the snow compacts the bolt causing a reading?

strokedmaro said:

Im really digging these load bolts but I cant wrap my head around how I would use them. I imagine these read the "pull" on the bolt and would need to be mounted to something other than the roof (above the roof) and bolted to the roof so that the weight of the snow pulls the bolt causing a reading....correct? Or would these be mounted inside under the roof and the snow compacts the bolt causing a reading?

Click to expand...

I'll ask again - what is the safe limit, and at what point is it dangerous?

I would supply the system and let the owners figure out what is safe.

I'm going to be a bit negative about this because I see some issues.

1. Did the building designers/contractors not allow for maximum snow-load for that area when it was constructed?

2. Since you are asking very basic questions here, what is your legal culpability if "your" design fails, and the roof collapses?

But would like to help, for now.

Ken

Yes, they measure the "pull" (tension, as opposed to compression, torsion, shear, etc). If the roof's a welded transfer truss, it probably won't be real easy to implement in the bottom chord where the strain would be. Might be a truss plate on an upright or something with some bolts you could get at... seems kinda dicey. Why not investigate that column that supposedly doesn't go all the way up to the roof truss? You could maybe put a compression cell in there easy and safe.

If you want something that will actually detect roof sag, i.e. an actual displacement of the roof, rather than pressure, you might consider an LVDT:
Macro Sensors LVDT: Tutorial
The output is a linear AC voltage ....which you could measure with a meter display, or else link to something else .... maybe an audio tone.
... These devices are very reliable, no maintenance, essentially no complicated moving parts. .... Select one with the resolution which is appropriate for your application .... maybe something in the millimeter category.
.... Your final choice will probably have to consider unit costs, installation, and maintenance.

Yes, while loads cells are a very cool and simple to use sensor, this would be a challenging implementation. A roof is designed to transfer it's total weight to the supporting walls through the trusses so there is no discrete points to easily insert a load cell(s) to measure the total distrubuted weight of the roof plus added loads of snow/rain/wind. Plus adding the weight of a new 'outer' roof kind of goes against the goal of a warning system of too much weight on the existing roof.

A totally additional roof would have to be added above the existing roof with several interposing short support members that each have a load cell. Then all the load cells outputs would have to be added to get a total 'weight'. Even then I think wind might effect accuracy.

Having just a square meter pan and weighing it's contents might be more practical but again it might not be a accurate sample representation of the total additional roof snow weight loading. Measuring the height of snow in a pan is problematic also in that the density of the snow is a variable also I think ( I live where it doesn't snow, so what do I know of wet snow and dry snow

Your best bet might be to get the help and advice of a structural engineer to see if there is a way to measure truss strain with strain gauges at a few strategic points and use that as a early warning system of roof system loading.

Lefty

I agree with lefty.

Your best bet might be to get the help and advice of a structural engineer to see if there is a way to measure truss strain with strain gauges at a few strategic points and use that as a early warning system of roof system loading.

Click to expand...

On needs several sensors to take uneven loading in account.

Might be easier to build a robot that removes the snow

This idea sounds somewhat silly to me. So say you get an alarm at 4AM, then what? Is that to say someone will be alerted and have to run up on the roof in the wee hours with a snow shovel.
Aircraft parts are expensive are they not? Would it not be more prudent to just invest the dollars and make the building safe for use?

3v0 said:

Might be easier to build a robot that removes the snow

Click to expand...

+1

Now that is an idea.

Put this on the roof...eats snow, poops snow bricks.



Robot Snow Plow

That's awesome, it's a Pokemon that eats snow.

blueroomelectronics said:

That's awesome, it's a Pokemon that eats snow.

Click to expand...

And sh*ts ice blocks, now that is what the world was waiting for

Lefty

Great comments! Im not actually sure what the limitations of the roof are...I will have to find out tomorrow. This building is barely 4 years old and I would accept no liability in event of a failure. The area is also 24 hours manned so an alarm going off would allow those inside to at least have an idea that the roof might come crashing down. Im sure that the original contractors have been told and are working on a real solution and this project is simply to give everyone a heads up. I does not have to be super accurate and I was planning on possibly 10 sensors down each side of the roof. (Im pretty sure the roof is pitched but I will have to take a look again...if it is its a pretty low angle.

Here is what Ive come up with (attach a picture later) I was thinking of first attaching a 1 inch cube of wood to the ceiling on the inside with the roof mating surface cut so that the bottom of the cube is parallel to the floor. Then sandwich a sensor like this: SparkFun Electronics - Flexiforce Pressure Sensor - 100lbs. in between another 1 inch block of wood. Then the lower block would be attached to a threaded mechanism attched to columns. I could then screw the lower block up or down until the sensor reacted (completely engaging the upper block attached to the ceiling).

The resistance it had would be the starting point and I could then calculate the weight per square inch (once I know what it is per square meter) I could then simply put that amount of weight on the roof and capture that resistance on the sensor. Of course Im not looking to set it at the collapse point...maybe 80% of the maximum specs. With 20 or so sensor setups like this I think it would be cheap and fairly accurate...what do you guys think?

Your scheme will not work.

Yes there will be some load on the sensor as the roof is covered in snow, but you will not get the load per square inch, the roof material has stiffness and will deflect when a load is applied anywhere in the region of your sensor.

To be quite honest, this project sounds like managerial madness, a hare brained scheme dreamed up by a clueless suit, hell bent on having people carry on working in a potentially unsafe building.

I think that you will do just as well to measure the depth of snow at ground level and assume the same depth on the roof.
Knowing the depth of snow, you should be able to calculate the mass of snow on the roof once you know the density of snow.
Your next problem will be to find the density of snow which is far from constant.

JimB

Afterthought:
Why not measure the weight of snow on a 1sq metre board at ground level, and trigger your alarm from that.
(Still a daft idea).