[LT] Portcullises

[Anthony]

I feel like that sentence about the lifting mechanism is referencing text that no longer exists. In any case, a mechanism that allows lifting a 450 lb object at 1'/3s is doing about 200W of work, and with good axles could be done by one person.

[Ze'Manel Cunha]

Originally Posted by Anthony

I feel like that sentence about the lifting mechanism is referencing text that no longer exists. In any case, a mechanism that allows lifting a 450 lb object at 1'/3s is doing about 200W of work, and with good axles could be done by one person.

Sounds like someone dropped a factor somewhere...

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

If we were looking at a small portcullis bars which were say 2" on a side, we could calculate the weight of each 15' bar at:

15' = 180" = 450 cm
2" = 5 cm

5x5x450 = 11250cc x (Iron at 7.80 g/cc) = 87,750 grams

So each 15' bar would weigh 87.75 kg, or 193 lbs.

Each 10' bar at:

20' = 240" = 610 cm
2" = 5 cm

5x5x610 = 15250cc x (Iron at 7.80 g/cc) = 119,000 grams

So each 20' bar would weigh 119 kg, or 262 lbs.

If all the bars are 1' apart, and the gate is 20' wide, we'd have 21 bars which are 15' high, and leaving sharp ends we'd have 15 bars which are 20' long, that'd add up to:

(21x193) + (15x262) = 7,980 lbs ~ 4 tons


Having 8 men raise a portcullis which weighs 4 tons isn't unreasonable...

(Someone please check my math if you will.)

[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.

===================

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.

[Ze'Manel Cunha]

Originally Posted by Figleaf23

===================

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.

===

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.

===

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.)

===

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.

===

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.

===

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.