# How to measure weight of a trap door?



## WYO (Dec 28, 2011)

Hi, new to the forum - hope you can help. 

I have a 30"x72" door in the floor of my mudroom that leads down to a cellar. The door is constructed of 2x4 pine stock, with approximately 1.5" of subfloor material on it (plywood and some particle board). The door weighs a TON and is very hard to lift up. I am trying to calculate the actual weight of it so I can install an appropriate sized gas strut to assist with the lifting up. 
- How do I estimate/calculate the weight of the door?
- Is this gas strut idea the best way to approach this? 

Many thanks
WYO


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## mdntrdr (Dec 22, 2009)

:smile:


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## WYO (Dec 28, 2011)

Scott - good idea, I thought of that, but I am trying not to take the door off. How would I suspend the scale in order to hand the door on it?? 

WYO


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## mdntrdr (Dec 22, 2009)

WYO said:


> Scott - good idea, I thought of that, but I am trying not to take the door off. How would I suspend the scale in order to hand the door on it??
> 
> WYO



Leave the door attached, lift it with the scale, there will be your reading of how much weight the strut/s will have to handle. :smile:


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## woodnthings (Jan 24, 2009)

*scottys right....*

there is some math involved, here's an example:


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## WYO (Dec 28, 2011)

Um, so this calc is saying the door weighs 320 pounds?!? It's heavy but I don't think it's that heavy. I can open it with one arm (as can my wife if she really gets hold of it.)


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## tcleve4911 (Dec 16, 2006)

woodnthings said:


> there is some math involved, here's an example:


That's some pretty fuzzy math there, Billy Boy......


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## tcleve4911 (Dec 16, 2006)

Take a scale down in the basement.
Cut a 2x4 to rest on the scale and hold the door open at the same time.
That should be the weight.

....or have Bill recalculate.....:laughing:


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## Longknife (Oct 25, 2010)

WYO said:


> The door weighs a TON and is very hard to lift up.


Ok, so you know it weighs a ton. What's the problem?



WYO said:


> I can open it with one arm (as can my wife if she really gets hold of it.)


Sounds like a contradiction to me

Jokes aside, the weight of the door is not really what you need to know. (Only way to establish that is to take it off and hang it in a scale). 
The interesting thing is to know how much of the weight will affect the struts. I guess that's what Bill tried to explain but I couldn't fully follow him on that. May be because I got tired from laying on my side reading.


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## rrbrown (Feb 15, 2009)

mdntrdr said:


> Leave the door attached, lift it with the scale, there will be your reading of how much weight the strut/s will have to handle. :smile:



I agree


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## Chaincarver Steve (Jul 30, 2011)

Lets see, we know that distilled water weighs approx 8 pounds a gallon. You can float the door in a bath tub and calculate how much water is displaced. From there you can easily extrapolate the weight of the door.

... or use a scale as mentioned above to see how much force is required to lift the door. It's really not rocket science.


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## benstark (Nov 6, 2011)

Hey all,
A few references may help: 2x4 weighs about 1 pound per linear foot (plf). 3/4 BC ply weighs about 2 pounds per square foot (psf).

Bill was right in his approach overall. I can't vouch for the math without some more effort than sitting here on my couch.

Think of it this way: what you are attempting to do is have the lid supported by some gas springs and the hinge. In this case the forces are balanced and static. Your effort to raise the lid would make the overall forces unbalanced so it moves whichever direction you move it, but when you stop, the lid returns to static.

The reason the gas springs need to be so strong is that (at least according to Bill's sketch) is that supporting the lid 18" from the edge, rather than from the end where you lift. It's similar to if you hold a bucket of water with you arm straight down by your side. Now take the same bucket and lift it so your arm is perpendicular to the ground. That's a hell of a lot harder (i.e. more strength to hold the same amount of weight.

This circumstance is also extra fun cause technically, as you raise the lid, more weight is supported to the hinge instead of the gas spring so it might work great for the first foot of lift, but the lid may shoot up as you raise the lid (and therefore harder to close).

In short, Bill's ideas are sound and good, but it gets complex. Physics is Fun!

Sorry I can't give you a simple answer or solution, but it does remind me I why I always try to avoid two things in construction: springs and magnets. They often require so much experimentation that it isn't worth it.


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## TimPa (Jan 27, 2010)

any strut you add will lighten the door load. I went to a junkyard and took one, with end fittings, off a hatch back car ($10). figuring the installtion for optimum travel is the toughest part, it will need to be about a 45 deg angle to closed door. try this first.

fwiw - your hinges will need to be very secure/strong, as this will add differant forces on them


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## cabinetman (Jul 5, 2007)

A suspension scale will tell you the weight, and how it changes as the door lifts. Spring or gas shocks used for lifting have some unique physical quirks.

When you first lift the door, the shock will have little to no effect, because it has to allow the door to close. The stroke of the shock (the length of it when extended) has to have relief, so the opening forces don't shoot the door open beyond necessary. It's at the top of the stroke that the door will be at it's lightest. 

The effectiveness of the shock can depend on where along the sides, or back (hinge edge) it is mounted. There is a leveraged efficiency that is commensurate with the action of the shock. I've made quite a few Murphy beds, and the spring action allows the bed to softly fall to a down position, and stay down, and when lifting, kicks in over enough of the lifting arc to carry the descending weight to an upright position and not snap the bed to the back wall.

I'm thinking that the struts you see on car hoods and back tailgates/windows would likely be too lightweight, and not have an adjustable stroke length to do a large hatch. You might check out companies like this that sell those basement access hatches for the correct hardware.










 







.


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## woodnthings (Jan 24, 2009)

*OK where do I begin.....*



WYO said:


> Um, so this calc is saying the door weighs 320 pounds?!? It's heavy but I don't think it's that heavy. I can open it with one arm (as can my wife if she really gets hold of it.)


NOPE!
320 lbs is the force need at 18" from the hinge....Not the weight of the door, which in my example which is 80 pounds. Got it?
Since there should be 2 struts one on either side to prevent it from racking each cylinder would have to exert 160 lbs at the fully closed position..... All this is assuming the door weighs/reqiuires 80 lbs to lift one end. the other end being hinged. We are not weighing the door here, only lifting one end. :yes:

If you have to totally remove the door that's a whole 'nother calculation and would require a chainfall on a skyhook :blink: bill


BTW the dead weight of lifting the door will diminish as it gets further angled upward and transfers some of the weight to the hinge....


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## WYO (Dec 28, 2011)

Holy cow - you guys are awesome. (Why are all my DIY projects much more complicated that they initially seem?!?)

I'd like to take a photo tonight and post tomorrow so you all can see exactly what I am dealing with. I like the ideas posted here but I am hesitant to spend the money on the strut(s) and time for install until I am confident on the how to. 

More to come tomorrow...

WYO


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## TimPa (Jan 27, 2010)

WYO said:


> Holy cow - you guys are awesome. (Why are all my DIY projects much more complicated that they initially seem?!?)
> 
> I'd like to take a photo tonight and post tomorrow so you all can see exactly what I am dealing with. I like the ideas posted here but I am hesitant to spend the money on the strut(s) and time for install until I am confident on the how to.
> 
> ...


$10 junkyard


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