[LT] Portcullises

[Lord Carnifex]

I don't know about 8 men, but I would imagine a well constructed mechanism is going to have a ratchet system installed, so that if someone slips, the whole thing doesn't slide down. That will add some mechanical resistance, although not four people's worth.

[Figleaf23]

Originally Posted by Ze'Manel Cunha

Sounds like someone dropped a factor somewhere...

15 lbs for a 10 foot square grate of iron also seems a factor too light.

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I was confused by that at first too. However, upon rereading it refers to 10 square feet (3.16' x3.16'), not 10 feet square.

[Figleaf23]

Originally Posted by Lord Carnifex

I don't know about 8 men, but I would imagine a well constructed mechanism is going to have a ratchet system installed, so that if someone slips, the whole thing doesn't slide down. That will add some mechanical resistance, although not four people's worth.

Any kind of decent counter-weight system would mean that the only work the crew needs to contribute is control and overcoming the resistance of the mechanism itself.

[Figleaf23]

So is this the sort of thing I should submit as an erratum?

[DanHoward]

Wait till next week when Kromm and P.Kitty are back and see what they say.

[Ze'Manel Cunha]

Originally Posted by Figleaf23

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I was confused by that at first too. However, upon rereading it refers to 10 square feet (3.16' x3.16'), not 10 feet square.

Yes, I did the numbers, and at 15 lbs that's a very low weight unless the bars are only half an inch thick.
For clarification, I do further calculations below.

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Now, a portcullis doesn't need to have 2" wide bars like I calculated out earlier, but even so you don't make a portcullis out of rebar.

Take a piece of grate which is 10 square feet (3.16' x3.16') with bars 1' apart, we have 4 vertical and 4 horizontal bars, by itself that piece of grate would weigh:

Grate with 2" wide bars:

3.16' = 38" = 96 cm
2" = 5 cm

5x5x96 = 2400cc x (Iron at 7.80 g/cc) = 18,720 grams

So each 3.16' bar would weigh 19 kg, or 42 lbs.

4 vertical and 4 horizontal bars, would mean a 10 square foot grate of 2" thick bars weighs in at 336 lbs.

If part of a larger piece you wouldn't count the outside bars each time, so 3x3 you would be adding 252 lbs for each additional 10 square foot of 2" grates.

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Grate with 1" wide bars:

3.16' = 38" = 96 cm
1" = 2.5 cm

2.5x2.5x96 = 600cc x (Iron at 7.80 g/cc) = 4,680 grams

So each 3.16' bar would weigh 4.75 kg, or 10 lbs.

4 vertical and 4 horizontal bars, would mean a 10 square foot grate of 1" thick bars weighs in at 80 lbs.

If part of a larger piece you wouldn't count the outside bars each time, so 3x3 you would be adding 60 lbs for each additional 10 square foot.
(That's close to the numbers for the Extra Heavy Grate in LT.)

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To get to a 15 lbs grate, which didn't count the outside bars, we'd end up with:

Grate with 0.5" wide bars:

3.16' = 38" = 96 cm
0.5" = 1.25 cm

1.25x1.25x96 = 150cc x (Iron at 7.80 g/cc) = 1,170 grams

So each 3.16' bar would weigh 1.17 kg, or 2.5 lbs.

If part of a larger piece you wouldn't count the outside bars each time, so 3x3 you would be adding 15 lbs for each additional 10 square foot.

The actual weight of a single standing grate composed of 4 vertical and 4 horizontal bars, would mean a 10 square foot grate of 0.5" thick bars weighs in at 20 lbs.

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Rebar #4 weighs in at .668 lb per foot.

3.16'x.668lb/'= 2.1 lbs per bar

If part of a larger piece you wouldn't count the outside bars each time, so 3x3 you would be adding 12.6 lbs for each additional 10 square foot.

The actual weight of a single standing grate composed of 4 vertical and 4 horizontal bars, would mean a 10 square foot grate of #4 rebar weighs in at 16.8 lbs.

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So the numbers listed for Average Grate in LT correspond to half inch wide bars, or half inch rebar, (Rebar #4), something too thin to ever build a portcullis out of.

[Anthony]

Originally Posted by Ze'Manel Cunha

Yes, I did the numbers, and at 15 lbs that's a very low weight unless the bars are only half an inch thick.

Or are not metal, though even then it's unduly light for something intended to be a credible barrier. 1.5 lb/sf is an average thickness of 0.19" / density, so metal-clad wood could probably manage an average thickness of 0.25". Average thickness is (board width * 2 * board depth / board spacing), so 1" boards at a spacing of 8" will result in the desired areal density. That's pretty thin, you probably can't actually run through it, but it's not very durable.

For comparison, average (non-armored) doors range from about 2 lb/sf for a hollow core door (which is not a credible barrier to anyone determined to get through) to 8 lb/sf for 2" oak.

[Ze'Manel Cunha]

Originally Posted by Anthony

Or are not metal, though even then it's unduly light for something intended to be a credible barrier. 1.5 lb/sf is an average thickness of 0.19" / density, so metal-clad wood could probably manage an average thickness of 0.25". Average thickness is (board width * 2 * board depth / board spacing), so 1" boards at a spacing of 8" will result in the desired areal density. That's pretty thin, you probably can't actually run through it, but it's not very durable.

For comparison, average (non-armored) doors range from about 2 lb/sf for a hollow core door (which is not a credible barrier to anyone determined to get through) to 8 lb/sf for 2" oak.

Well, many types of small internal portcullises were straps of metal bolted onto wooden bars, so we could calculate out 2" oak bars with 0.25" metal cladding on each side, but the weight of the metal on those alone would be 10lbs per bar, not counting the wood, and the section on p.119 of LT being discussed is labeled as metal:

Grate (TL1)
A grate is a metal grille that covers an opening.

[johndallman]

Originally Posted by Figleaf23

Any kind of decent counter-weight system would mean that the only work the crew needs to contribute is control and overcoming the resistance of the mechanism itself.

Of course, you don't want to make your portcullis too easy to open; the enemy are not supposed to be able to lift it. That argues for some way of blocking it, and against a counterweight system.

[DouglasCole]

Perhaps useful illustration:

http://en.wikipedia.org/wiki/File:Po...cassonne.2.png