pacomartin
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October 20th, 2010 at 6:08:53 AM permalink
Quote: Wizard

I plan to see it for myself whenever it opens. I find it rather annoying that they can't give out an exact date when it will be open to the public.



The news article in the Las Vegas Review Journal says that they are afraid if they announce a date it will cause problems
because people will compete to be the first across the bridge. The intention is to open the bridge this week without an announcement.
Croupier
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October 20th, 2010 at 8:40:37 AM permalink
Well, the LV Sun says the bridge is open now, and opened around 10pm Tuesday night local time.

Source
[This space is intentionally left blank]
Wizard
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October 20th, 2010 at 8:53:24 AM permalink
Quote: Croupier

Well, the LV Sun says the bridge is open now, and opened around 10pm Tuesday night local time.



Thanks! I just might check it out tomorrow if the weather is good and I'm feeling better.
"For with much wisdom comes much sorrow." -- Ecclesiastes 1:18 (NIV)
teddys
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October 20th, 2010 at 8:56:33 AM permalink
I think one of the articles said you can walk across it in the "upstream" direction (whatever that means). I, for one, would be absolutely TERRIFIED to do that, but might do it for the adrenalin rush. (I get scared crossing the bridges over the Ohio River). It would be cheaper than the Stratosphere jump, and I'd get the same thrill.

I wonder who the first person will be to BASE jump from the bridge? It's too tempting for those people to pass it up ...
"Dice, verily, are armed with goads and driving-hooks, deceiving and tormenting, causing grievous woe." -Rig Veda 10.34.4
Nareed
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October 20th, 2010 at 9:04:51 AM permalink
Quote: teddys

I think one of the articles said you can walk across it in the "upstream" direction (whatever that means). I, for one, would be absolutely TERRIFIED to do that, but might do it for the adrenalin rush. (I get scared crossing the bridges over the Ohio River). It would be cheaper than the Stratosphere jump, and I'd get the same thrill.



How about the other rides at the Startosphere? I enjoyed the egg-beater over the side of the tower, until it spun too fast and made me dizzy. The other two are, respectively, terrifying and commonplace.

Quote:

I wonder who the first person will be to BASE jump from the bridge? It's too tempting for those people to pass it up ...



Someone with a higher-than-normal death-wish among such people. There are three walls too close to the jump.
Donald Trump is a fucking criminal
Wizard
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October 20th, 2010 at 9:23:59 AM permalink
There is still no mention of the bridge being open on the web site.
"For with much wisdom comes much sorrow." -- Ecclesiastes 1:18 (NIV)
pacomartin
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October 20th, 2010 at 11:33:09 AM permalink
Quote: Wizard

Thanks! I just might check it out tomorrow if the weather is good and I'm feeling better.


My understanding is that there are only 65 parking spaces for people who want to walk across the bridge.
It may make sense to take a cycle if you have to walk from a more distant parking lot.

A lot of people have speculated about BASE jumpers, but finding a place to land looks like it would be difficult.


Here is a guy jumping out of a helicopter into the ocean
with a windsurfer
. I can't imagine jumping off a bridge, dealing with high winds and canyon walls,
and hitting a fast moving river in a similar fashion. But if you aim for a road or other narrow flat place with those
winds and eluding police I think you stand a good chance of getting killed.

Most of the fatalities in BASE jumping occur on cliffs, because the wind blows you back into the cliff.
Lote
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October 21st, 2010 at 10:22:50 AM permalink
Quote: mkl654321

Quote: rxwine

The Bhakra-Nangal (gravity) Dam in India versus Hoover (arch/gravity) Dam

http://en.wikipedia.org/wiki/Hoover_Dam
http://en.wikipedia.org/wiki/Bhakra_Dam



Sure. There are dams with both designs. I'd like to know what the capacity of the reservoir behind the dam in India is vs. the capacity of Lake Mead.



The only thing that matters is depth of the dam and area of dam exposed to water. Capacity of the lake should have no impact on the amount of force pushing against the dam.
thecesspit
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October 21st, 2010 at 12:28:24 PM permalink
Really? Surely the more water behind the dam, the more pushing against the wall of the dam. If the lake was 1m wide there's going to be less force than if the lake behind the dam was 100km?
"Then you can admire the real gambler, who has neither eaten, slept, thought nor lived, he has so smarted under the scourge of his martingale, so suffered on the rack of his desire for a coup at trente-et-quarante" - Honore de Balzac, 1829
mkl654321
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October 21st, 2010 at 1:19:40 PM permalink
Quote: thecesspit

Really? Surely the more water behind the dam, the more pushing against the wall of the dam. If the lake was 1m wide there's going to be less force than if the lake behind the dam was 100km?



Yes. The longer the lake, the more force against the dam. One of many such forces is atmospheric pressure.

Imagine a lake that was as deep as any other, but only a foot long. How much of a dam would be needed to hold it back?
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Doc
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October 21st, 2010 at 2:04:50 PM permalink
Quote: mkl654321

Yes. The longer the lake, the more force against the dam. One of many such forces is atmospheric pressure.

Imagine a lake that was as deep as any other, but only a foot long. How much of a dam would be needed to hold it back?

mkl, I am not sure whether this is one of your sarcastic posts or not. (Maybe I have been away from the forum too long.) If Lake Mead extended just one foot back from Hoover Dam, the dam would experience the same total hydrostatic force as it does now. A pressure/force sensor would not detect how far back the water extends. It is a simple physics problem that we could present here, if there are people who don't understand.

On the other hand, if the dam were to fail, the amount of water it was holding back would make a lot of difference in how much damage was done downstream.


Edit: to clarify, the width of the dam and the depth of the water are critically important; how far back the water extends behind the dam is not.
thecesspit
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October 21st, 2010 at 2:11:27 PM permalink
Quote: mkl654321

Yes. The longer the lake, the more force against the dam. One of many such forces is atmospheric pressure.

Imagine a lake that was as deep as any other, but only a foot long. How much of a dam would be needed to hold it back?



No idea why you are replying to me, except for my last question mark, which was rehtorical, as that's basically what I just said in reply to the previous poster.
"Then you can admire the real gambler, who has neither eaten, slept, thought nor lived, he has so smarted under the scourge of his martingale, so suffered on the rack of his desire for a coup at trente-et-quarante" - Honore de Balzac, 1829
thecesspit
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October 21st, 2010 at 3:05:16 PM permalink
Quote: Doc

mkl, I am not sure whether this is one of your sarcastic posts or not. (Maybe I have been away from the forum too long.) If Lake Mead extended just one foot back from Hoover Dam, the dam would experience the same total hydrostatic force as it does now. A pressure/force sensor would not detect how far back the water extends. It is a simple physics problem that we could present here, if there are people who don't understand.

On the other hand, if the dam were to fail, the amount of water it was holding back would make a lot of difference in how much damage was done downstream.


Edit: to clarify, the width of the dam and the depth of the water are critically important; how far back the water extends behind the dam is not.



That seems an unexpected answer to me... as it suggests a very thin sheet of water exerts the same pressure as a whole lake full. I'd not have been surprised if the length of water was some sort of inverse square law effect, but for it to have no effect... well, I'd like to see the math as this is new and interesting information!
"Then you can admire the real gambler, who has neither eaten, slept, thought nor lived, he has so smarted under the scourge of his martingale, so suffered on the rack of his desire for a coup at trente-et-quarante" - Honore de Balzac, 1829
pacomartin
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October 21st, 2010 at 3:07:12 PM permalink
Quote: Lote

The only thing that matters is depth of the dam and area of dam exposed to water. Capacity of the lake should have no impact on the amount of force pushing against the dam.



This statement is not true. The weight of the water (proportional to the volume of water) is the determing factor.


The Kariba Dam between Zambia and Zimbabwe
in Africa contains the largest reservoir by volume in the world. The lake is over 300 feet deep . While Lake Mead can be as deep as 500 feet , Lake Kariba
has 8.5 times the surface area of Lake Mead resulting in 5 times the volume of water.

There seem to be 4.5 million cubic meters of concrete in the Hoover Dam, compared to 1.0 million cubic meters in the Kariba Dam.
That would seem to go against the idea that the Kariba Dam must withstand more force. But the Kariba Dam opened 23 year
after the Hoover Dam, and may have a better arch design, lower safety factors, or simply have a more reliable natural wall
to take the weight of the reservoir.

It may have something to do with the Kariba Dam (1,900' by 420') not being as high as the Hoover Dam (1244' by 726.4' )
although it is longer. Also, the Hoover Dam project used 96 million pounds of steel, but there is no steel in the dam. The Kariba dam
used 11 million pounds of steel in the dam itself. The steel is probably taking a lot of the force,

Wizard
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October 21st, 2010 at 3:16:11 PM permalink
Quote: Doc

If Lake Mead extended just one foot back from Hoover Dam, the dam would experience the same total hydrostatic force as it does now. A pressure/force sensor would not detect how far back the water extends. It is a simple physics problem that we could present here, if there are people who don't understand.



Hmmm. Physics is not my strong suit, but that is not what I would have thought. I would think the dam would feel the greater pressure from more water behind it. Perhaps a submarine comparison is appropriate. Any given sub will be rated to only go so deep. At a certain point the sub is not strong enough to withstand the weight of all the water on top of it. What do others think?
"For with much wisdom comes much sorrow." -- Ecclesiastes 1:18 (NIV)
EvenBob
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October 21st, 2010 at 3:44:48 PM permalink
Quote: Wizard

I lived near the Patapsco River in Baltimore County from 1992-2001. There were four dams along that river, and I always wondered why. This web site says they built them for power around the turn of the century, and there are plans to remove them.



There are 75,000 dams in the US, most of them old hydroelectric's that were built in the first half of the 20th century to supply electricity to small communities. I know someone who's grandfather got rich by buying stock in hundreds of hydroelectric plants all over the country in the 20's thru the 50's. He never had a down year, even during the Depression. He felt people always need power and he was right. His family is still wealthy from his wise investments.
"It's not called gambling if the math is on your side."
pacomartin
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October 21st, 2010 at 4:04:41 PM permalink
Here is an excerpt for a science website aimed at a 5th grader.

•Why must an engineer know the volume of the water in a reservoir?
(Answer: Water creates large forces on dams and locks.
The greater the volume of water held back by a structure, the greater the pressure exerted on the walls of the structure.
Engineers must be able to calculate volume to know how big and sturdy dams and locks must be to withstand the forces.)


The submarine comparison is not exactly the same. The force on the submarine is proportional to depth, but it doesn't
matter if you are in the Meditteranean Sea or the Pacific Ocean. The volume of water is irrelevant.
thecesspit
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October 21st, 2010 at 4:27:21 PM permalink
It would make sense that the force on the dam is dependent on the area of the dam as percentage of the total area on which the the body of water is acting on (there will be a force acting on the reservoir bed and sides as well as the dam wall)....
"Then you can admire the real gambler, who has neither eaten, slept, thought nor lived, he has so smarted under the scourge of his martingale, so suffered on the rack of his desire for a coup at trente-et-quarante" - Honore de Balzac, 1829
Doc
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October 21st, 2010 at 4:33:50 PM permalink
Quote: pacomartin

Here is an excerpt for a science website aimed at a 5th grader.

•Why must an engineer know the volume of the water in a reservoir?
(Answer: Water creates large forces on dams and locks.
The greater the volume of water held back by a structure, the greater the pressure exerted on the walls of the structure.
Engineers must be able to calculate volume to know how big and sturdy dams and locks must be to withstand the forces.)

It is a real shame what foolishness is available on the web -- we could destroy some 5th graders' minds.

OK, since it appears that not everyone follows/agrees with my earlier comment, here is the simplified physics problem. To simplify, let’s consider a dam that is a plane wall, forgetting about the arc design used at Hoover Dam. Also, let’s treat water as a uniform, incompressible liquid, which is very, very nearly true. Finally, to perhaps make the visualization easier, let’s assume that at least the wet side of the dam is a flat, vertical surface of uniform width (shore to shore). I’ll use the following symbols:

W = width of the dam from shore to shore
w = weight density of water (mass density times gravity)
d = depth from the surface of the water to a point of interest
h = height of the water surface above the base of the dam (maximum value of d)
P = hydrostatic pressure at depth d
F = total hydraulic force against the dam

At any depth d, the hydrostatic pressure is:

P = w*d

That is, so long as water is incompressible (constant w), the pressure increases uniformly with depth.

The total force on the wet face of the dam may be calculated by multiplying this pressure at depth d times a differential area at that depth then integrating over the entire submerged area of the dam. Fortunately, because we assumed that the water was incompressible, the density w is constant, and the integration is quite simple (wish I knew how to draw integral symbols, etc. here). The result is:

F = (wWh^2)/2

This is the same result that would be obtained by assuming that the pressure at ½ the total depth is acting over the entire area of the dam, i.e.,

F = (P @ d = h/2) * (W*h) = (wh/2) * (W * h) = (wWh^2)/2

Note that the resulting force depends on the density of the water, the width of the dam, and the height of the dam below the water surface, but not upon how many feet or hundreds of miles upstream the water may extend.

Earlier, mkl654321 made a comment about atmospheric pressure being important. The pressure that I describe here is hydrostatic pressure. The atmospheric pressure would increase the pressure at all depths by one atmosphere, approximately 14.7 lb/sq. in. We could calculate the additional force this would place on the dam, but we don’t need to. So long as we haven’t created a vacuum on the downstream side of the dam, the atmosphere would be pushing back on the other side of the dam with the same force. This air-pressure on both sides might compress the concrete a totally insignificant amount, but it would neither tend to push the dam over nor help hold it up – only the hydrostatic forces are important. (OK, there are some hydrodynamic forces, too, due to water flow, but this is the simple version of the problem.)

There are lots of reasons that the extent of the lake upstream and the total volume of water in the lake are important. But the force on the dam is not one of those reasons. I wish that people who developed science educational materials for elementary schools had actually studied science themselves beyond the eighth grade.
Doc
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October 21st, 2010 at 5:30:09 PM permalink
It occurred to me that there may be some folks who do not want an answer based on a mathematical formula. Instead they just need to visualize why the extent of the lake is or is not important. For them, I offer this:

Suppose that by some miracle of civil engineering they were able to construct a dam right in Lake Mead, half a mile upstream of Hoover Dam. That dam could be operated to maintain the same water level upstream and downstream, so its presence would have no effect on the water flow or the forces on Hoover Dam.

Then suppose these same miracle engineers build a bypass channel (and valves) from the portion of the lake upstream of the new dam past both dams to the river below. Then they close the gates in both dams and let water flow from the upper portion of the lake through the bypass channel. There would then be a stagnant pond between the dams, but from the perspective of Hoover Dam, nothing has changed except the water is no longer flowing through – that is, forces on the dam are the same as they are today.

Next, suppose the bypass channel was left wide open long enough to completely drain the portion of the lake upstream from the new dam. That would greatly increase the net forces on the new dam as the stagnant portion of the lake tended to push it back upstream. But so long as the new dam holds, there is no change in the forces on Hoover Dam. And the lake is only half a mile long.

Now reconsider all of the above but with the new dam constructed just one foot upstream of Hoover Dam and properly contoured to give a one-foot wall of water between the two dams. Even then, the forces on Hoover Dam would be the same as they are now; the major difference would be that the hydrostatic forces directed back upstream would be supported by the new dam instead of by the natural earth banks around the lake that support those forces now.

In the extreme, instead of building a new dam, the miracle engineers could perhaps fill in Lake Mead, right up to one-foot away from Hoover Dam. If they were able to do that, and still leave the one foot of water wall, the forces on the dam would be the same as now, with the earthen banks just a lot closer.

I know this description uses some unrealistic “supposes”, but does that help anyone better understand why the extent of the lake is not important, just the submerged depth and width of the dam?
thecesspit
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October 21st, 2010 at 6:32:48 PM permalink
What you're saying is effectively only due the column of water above it at the dam face? Interesting.... that's not what I expected, but it makes sense once I realise that the pressure exerted from the water is only due to depth of water, and that pressure is the same in all directions... meaning that the water 2 foot from the dam and 2 miles from the dam (at the same depth) all counter balances itself to not cause any net pressure on the dam.

I assume there's a limit to this where it doesn't work otherwise a thin film of water from the rain would cause pressure on a house that we'd not expect... it would have to be beyond the point where there is surface effects?

Thanks for the mathematical explanation.

I stand completely corrected.
"Then you can admire the real gambler, who has neither eaten, slept, thought nor lived, he has so smarted under the scourge of his martingale, so suffered on the rack of his desire for a coup at trente-et-quarante" - Honore de Balzac, 1829
mkl654321
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October 21st, 2010 at 6:47:40 PM permalink
Quote: thecesspit

I assume there's a limit to this where it doesn't work otherwise a thin film of water from the rain would cause pressure on a house that we'd not expect... it would have to be beyond the point where there is surface effects?

Thanks for the mathematical explanation.

I stand completely corrected.



I think what bothers people about the assertion that one foot and 100 miles of water would exert the same pressure against the dam, assuming the same depth of the water and surface area in contact with the dam, is that if that is true, why does the dam (or any dam) need to be as massive as it is? Wouldn't a sheet of aluminum foil do?
The fact that a believer is happier than a skeptic is no more to the point than the fact that a drunken man is happier than a sober one. The happiness of credulity is a cheap and dangerous quality.---George Bernard Shaw
EvenBob
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October 21st, 2010 at 7:06:53 PM permalink
Quote: mkl654321

I think what bothers people about the assertion that one foot and 100 miles of water would exert the same pressure against the dam, assuming the same depth of the water and surface area in contact with the dam, is that if that is true, why does the dam (or any dam) need to be as massive as it is? Wouldn't a sheet of aluminum foil do?



I always assumed dams were so big because they needed to be free standing and hold huge amounts of heavy machinery.
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cclub79
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October 21st, 2010 at 7:34:55 PM permalink
Quote: mkl654321

I think what bothers people about the assertion that one foot and 100 miles of water would exert the same pressure against the dam, assuming the same depth of the water and surface area in contact with the dam, is that if that is true, why does the dam (or any dam) need to be as massive as it is? Wouldn't a sheet of aluminum foil do?



I don't think he's suggesting that there isn't a tremendous amount of pressure (that foil couldn't contain). There is, because of the surface area of the dam being huge. But the tremendous pressure is the same if it's 2 feet of water or 1 mile of water. The water has to be completely stagnant though, right? If it was flowing, there would be additional force that the dam has to withstand beyond the surface area pressure.
pacomartin
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October 21st, 2010 at 8:10:09 PM permalink
By my estimate the concrete in Hoover Dam weight 13 billion pounds. A mere 1 foot length lake would be 43 million pounds of water.

The dam would weigh more than 300 times this small amount of water in this mini lake.
In comparison the water in Lake Mead weighs 200,000 times the weight of the dam.

I find it hard to believe that both the mini Lake and Lake Mead would require the same size dam.

Basic forces on a concrete dam says that the weight of the water is important.
cclub79
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October 21st, 2010 at 8:19:41 PM permalink
Oh, and the Wizard mentioned a submarine, which is a completely different situation. The gravitational constant is higher as you go deeper in the sub, meaning the pressure on the surface area of the sub increases as the sub submerges. The only way this would affect a dam is if it were being built at the bottom of a lake for some reason, or in a more practical sense, think of an aquarium where you can go in the basement and "look up" at the bottom of an exhibit. In that case, the amount of water does have a great effect on how much support you need to prevent a problem.
Doc
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October 21st, 2010 at 8:57:13 PM permalink
Responding to several recent comments:

Yes, there could be a number of dynamic forces on the dam, but the hydrostatic forces will dominate in a case like Lake Mead and Hoover Dam.

Yes, the weight of the water per unit volume is important. That is the weight density, which I represented as “w” in my earlier post. In the "Basic forces on a concrete dam" link that Mr. Martin provided, they use the term “unit weight of water”, which is the same as the weight density. That link shows a calculation of the total weight of the water (density times volume) but then says that this total weight should be disregarded. It is actually the total force that the ground underneath the lake must support, but that is irrelevant to the total load on the dam. At the same depth, the pressure on the floor of the lake is the same as the pressure on the base of the dam, but if the lake has a very large area, the vertical load just gets distributed over a large area of land.

And no, a sheet of aluminum foil would not do for a dam. It might withstand the forces at the surface, but not at depth. Let’s consider this with an example that considers vertical forces and later horizontal forces. Suppose you had a vertical pipe with a cross-sectional area of 1 sq. ft. and as tall as Hoover Dam (700+ feet?). You want to fill it with water, but you need to plug that 1 sq. ft. at the bottom of the pipe so the water won’t drain out. Whatever material you plug it with needs to support the weight of that entire column of water: (1 sq ft)*(700 ft)*(62.4 lb/cu.ft) = 43,680 lb (give or take). No, a sheet of aluminum foil won’t do the job, but a really thick concrete plug would do.

The horizontal pressure on the base of Hoover Dam is the same as the vertical pressure at the bottom of that hypothetical pipe. An aluminum foil dam wouldn’t hold, but a really thick concrete dam works quite well. And since the pressure is proportional to depth, there is greater need for thicker concrete near the base than near the top. I suppose they could have made the entire dam as thick as the base, but there was no need to do that.

Note: I’m not sure about the height of Hoover Dam.

As for a thin film such as rain running down a wall, thecesspit is correct that this is quite a different case. That is not a contained body of water exerting pressure in all directions. About the only force that water film imposes on the wall (after the impact of the droplet) is primarily vertical and due to some combination of surface tension, weight of the film, and viscous forces as the drops run down – basically negligible. If you had two walls very close together (1 inch?), with the sides and bottoms sealed and had enough rain to fill up the 1-inch gap, then the walls, bottom, and sides would have to be as strong as a dam of the same height; i.e., if these walls and the pool of rainwater were 700+ feet tall, then they would need to be as strong as Hoover Dam.
Doc
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October 22nd, 2010 at 1:05:26 PM permalink
This thread got diverted by a discussion of forces acting on a dam -- back to the earlier topic, does anyone know whether the bridge is actually open to traffic now? I plan to get out to see it when I visit in December. My last two attempts to visit Hoover Dam were frustrated by traffic jams so extensive that they convinced me to turn around and go back to Las Vegas.
RaleighCraps
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October 22nd, 2010 at 1:12:44 PM permalink
Quote: Doc

This thread got diverted by a discussion of forces acting on a dam -- back to the earlier topic, does anyone know whether the bridge is actually open to traffic now? I plan to get out to see it when I visit in December. My last two attempts to visit Hoover Dam were frustrated by traffic jams so extensive that they convinced me to turn around and go back to Las Vegas.



So the Bridge topic got diverted by a dam? How ironic, or is it apropos?
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Wizard
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October 22nd, 2010 at 1:32:44 PM permalink
I showed your original post to my dad, claiming that the distance the water extends behind the dam does not affect the pressure, for comment. As background, my father has a Ph.D. in physics from Yale, and was literally a rocket scientist his whole career. He agreed with you. Specifically, he said, "As for the dam, the fellow is correct. The pressure at each level of the dam depends upon only the height and density of the water above, not how big (or long) the reservoir is. The integrated pressure (force) on the dam depends only upon its area and the water height. " I would never question anything my dad said when it comes to science.

Given that, don't you think they made Hoover dam much thicker than necessary? It seems so much heftier than other dams.

Quote: Doc

This thread got diverted by a discussion of forces acting on a dam -- back to the earlier topic, does anyone know whether the bridge is actually open to traffic now? I plan to get out to see it when I visit in December. My last two attempts to visit Hoover Dam were frustrated by traffic jams so extensive that they convinced me to turn around and go back to Las Vegas.



Someone quoted the LVRJ earlier that it opened to traffic around 10:00 PM last Tuesday. However, yesterday's paper said that it is still closed to pedestrian traffic.
"For with much wisdom comes much sorrow." -- Ecclesiastes 1:18 (NIV)
crazyiam
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October 22nd, 2010 at 2:15:32 PM permalink
Quote: Wizard

I showed your original post to my dad, claiming that the distance the water extends behind the dam does not affect the pressure, for comment. As background, my father has a Ph.D. in physics from Yale, and was literally a rocket scientist his whole career. He agreed with you. Specifically, he said, "As for the dam, the fellow is correct. The pressure at each level of the dam depends upon only the height and density of the water above, not how big (or long) the reservoir is. The integrated pressure (force) on the dam depends only upon its area and the water height. " I would never question anything my dad said when it comes to science.

Given that, don't you think they made Hoover dam much thicker than necessary? It seems so much heftier than other dams.



Someone quoted the LVRJ earlier that it opened to traffic around 10:00 PM last Tuesday. However, yesterday's paper said that it is still closed to pedestrian traffic.



I'm not an expert, but I believe over time concrete quality has improved. So the concrete used for the Hoover Dam might not have the same strength per weight as newer dams. Or maybe it was overbuilt so it would last forever. Just speculation.
Doc
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October 22nd, 2010 at 2:16:38 PM permalink
Quote: Wizard

I showed your original post to my dad, claiming that the distance the water extends behind the dam does not affect the pressure, for comment. As background, my father has a Ph.D. in physics from Yale, and was literally a rocket scientist his whole career. He agreed with you. Specifically, he said, "As for the dam, the fellow is correct. The pressure at each level of the dam depends upon only the height and density of the water above, not how big (or long) the reservoir is. The integrated pressure (force) on the dam depends only upon its area and the water height. " I would never question anything my dad said when it comes to science.

Given that, don't you think they made Hoover dam much thicker than necessary? It seems so much heftier than other dams.

First of all, please thank your dad for backing me up!

I have never designed or constructed a concrete dam in my life, but I do know a little bit about the science and engineering represented in my earlier post. Don't think it's necessary to cite credentials. I feel that I could explain this material fairly well in a direct conversation, but I may not have conveyed it properly in a forum post. I hope I did not offend anyone along the way. I have a feeling that Mr. Martin may still be a skeptic. I do understand that those who have never studied this field naturally have the intuitive feeling that a million acre lake must put more of a load on a dam than does a small pond, even though in reality the surface area and volume of water are irrelevant.

I have no idea about all of the design factors of the Hoover Dam. I think the concrete thickness at the base is highly related to the maximum water pressure at the base, which is determined by the maximum depth of water at the dam. There could be a number of other factors; among them might be (1) transitioning from the arc shape at the top of the dam to a straight-across profile at the bottom, (2) providing room for the various water channels, turbo-generators, and passageways, and (3) some characteristic of the profile of the narrows into which the dam base was constructed. There are probably a lot of other factors that don't immediately come to (my) mind. They certainly would have constructed the dam notably thicker/stronger than the minimum required to hold back the water -- there is the issue of safety factors and the general conservative nature expected in engineering design.

Quote: Wizard

Someone quoted the LVRJ earlier that it opened to traffic around 10:00 PM last Tuesday. However, yesterday's paper said that it is still closed to pedestrian traffic.

When someone actually sees traffic flowing and when someone actually sees pedestrians on the bridge, I hope they will post the word either here or in a new thread.
EvenBob
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October 22nd, 2010 at 2:18:03 PM permalink
Quote: crazyiam

I'm not an expert, but I believe over time concrete quality has improved. So the concrete used for the Hoover Dam might not have the same strength per weight as newer dams. Or maybe it was overbuilt so it would last forever. Just speculation.



Hoover dam isn't over built. Its a very high structure and needs a huge base to support it. It also contains hundreds of tons of running, vibrating turbines that would eventually shake the dam to pieces if it wasn't massive enough to handle it. As I recall, in EU they under built some dams in the early 1900's and the vibrating equipment caused huge cracks in the concrete in just a few years. Hoover dam was built to last hundreds of years.
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Wizard
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October 22nd, 2010 at 2:43:51 PM permalink
Quote: crazyiam

Or maybe it was overbuilt so it would last forever. Just speculation.



Funny you should mention that. There is a diagram of the brightest stars in the sky imbedded into the surface of the dam. The reason is that if we all die, and aliens find the dam millions of years from now, they will be able to date it, based on changes in the distances between the stars. For those who don't know, due to the expansion of the universe, the configuration of the stars changes over time.
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thecesspit
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October 22nd, 2010 at 2:54:13 PM permalink
It's not expansion of the universe... it's the rotation of the galaxy that causes the change as the sun moves at a relative different speed to other stars... there's also the progression of the poles and the equinoxial points due to the shorting of the earth's rotational axis over time as well.

At least as I understand it, the space between the stars in our galaxy is not changing due to expansion, but the space between our galaxy and the next is...
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Wizard
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October 22nd, 2010 at 4:03:03 PM permalink
Quote: thecesspit

It's not expansion of the universe... it's the rotation of the galaxy that causes the change as the sun moves at a relative different speed to other stars... there's also the progression of the poles and the equinoxial points due to the shorting of the earth's rotational axis over time as well.

At least as I understand it, the space between the stars in our galaxy is not changing due to expansion, but the space between our galaxy and the next is...



I think you're right too; I just forgot that.
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October 22nd, 2010 at 4:16:20 PM permalink
Quote: RaleighCraps

So the Bridge topic got diverted by a dam? How ironic, or is it apropos?



It's not ironic, it's coincidental.

Now, had discussion fo a dam been diverted by a river, that would have been ironic; I mean given how rivers are diverted to build dams and all.
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Ayecarumba
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October 22nd, 2010 at 5:27:17 PM permalink
A link to a great article on the construction of the dam Here


Stats:
All in all, Hoover Dam stood 725 feet high, is 1244 feet wide, 660 feet thick at the base, tapering to a thickness of 45 feet at the top. It cost a total of $165 million to build and was completed in four and a half years.
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mkl654321
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October 22nd, 2010 at 5:37:53 PM permalink
Quote: Wizard

Given that, don't you think they made Hoover dam much thicker than necessary? It seems so much heftier than other dams.



Someone else suggested that it may be because so much of the operating machinery of the whole complex is actually inside the dam. That said, it is puzzling why the bloody thing is so thick, if what your dad says is correct. It could be that it was meant to withstand some kind of other, nonconstant force, like an earthquake. I doubt very much that it was simply overengineered--even in 1926, poured-form concrete construction was a mature technology.

Re the bridge: they should have left the middle section out, so motorists could have had the thrill of accelerating up one side and jumping the gap, like Sandra Bullock driving a bus. Whee!
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Doc
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October 22nd, 2010 at 5:46:19 PM permalink
Quote: mkl654321

Re the bridge: they should have left the middle section out, so motorists could have had the thrill of accelerating up one side and jumping the gap, like Sandra Bullock driving a bus. Whee!

I understand your comment is for amusement purposes, but are you aware that there really isn't an "up one side" issue? I think the roadway is basically flat; the arch is just the support structure, not a place to drive.

Ahh! Another potential engineering discussion -- the use of arches and their appropriate shapes!
EvenBob
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October 22nd, 2010 at 5:53:45 PM permalink
Quote: Ayecarumba

A link to a great article on the construction of the dam Here


Stats:
All in all, Hoover Dam stood 725 feet high, is 1244 feet wide, 660 feet thick at the base, tapering to a thickness of 45 feet at the top. It cost a total of $165 million to build and was completed in four and a half years.



Its almost as thick at the base as it is tall. Probably not an accident.
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mkl654321
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October 22nd, 2010 at 5:57:00 PM permalink
Quote: Doc

I understand your comment is for amusement purposes, but are you aware that there really isn't an "up one side" issue? I think the roadway is basically flat; the arch is just the support structure, not a place to drive.

Ahh! Another potential engineering discussion -- the use of arches and their appropriate shapes!



Yeah, I know, I probably should have said, "along one side". Though it does look like the approach from the north would be more like "down one side". I realize the arch is just the supporting structure.

It also looks like it would be one of the world's best places for bungee jumping. I put the over/under at ten days before someone tries it, hefty fine or no. In fact, maybe they can sell permits and pay for the bridge that way....
The fact that a believer is happier than a skeptic is no more to the point than the fact that a drunken man is happier than a sober one. The happiness of credulity is a cheap and dangerous quality.---George Bernard Shaw
EvenBob
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October 22nd, 2010 at 6:00:18 PM permalink
I enjoyed Chevy Chase's adventure at Hoover Dam in 'Vegas Vacation'. Like the dam security would ever allow any of that to happen.
"It's not called gambling if the math is on your side."
Ayecarumba
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October 22nd, 2010 at 6:26:42 PM permalink
Quote: Wizard

I showed your original post to my dad, claiming that the distance the water extends behind the dam does not affect the pressure, for comment. As background, my father has a Ph.D. in physics from Yale, and was literally a rocket scientist his whole career. He agreed with you. Specifically, he said, "As for the dam, the fellow is correct. The pressure at each level of the dam depends upon only the height and density of the water above, not how big (or long) the reservoir is. The integrated pressure (force) on the dam depends only upon its area and the water height. " I would never question anything my dad said when it comes to science.

Given that, don't you think they made Hoover dam much thicker than necessary? It seems so much heftier than other dams.



Could it be that Hoover Dam is thicker at the base due to dynamic (as opposed to static) forces. Holding back water that is moving requires a stronger structure, than water that is still.

Interestingly, there is also an "uplift" force acting on the dam which is caused by the water and silt trying to "pick up" the base of the structure. Dams have drainage channels built into them to try to counteract this force.

Here is a cross section of the dam. When you appreciate how tall the structure is (and remember that it was orginally designed to be placed in another location that would require it to be even taller), the big base doesn't look too out of proportion for the giant wedge that we all imagine it to be:



This image is from the University of Virginia's The Hoover Dam: Loney Lands Made Fruitful website.
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pacomartin
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October 22nd, 2010 at 8:20:38 PM permalink
Quote: Ayecarumba

Could it be that Hoover Dam is thicker at the base due to dynamic (as opposed to static) forces. Holding back water that is moving requires a stronger structure, than water that is still.



I am sure that dynamic forces are a huge factor. They are in almost all engineering systems.
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October 23rd, 2010 at 11:06:47 PM permalink
I read on a government link (sorry not sure what it was now) that the dam has paid for itself. So...does the upkeep and saleries, and lower power rates reflect that?

I was curious, because eventually, these solar projects (reflected light thermal in desert) will all be paid for too one day. So, will Nevada Power (or others) keep raking in the money once they've paid most of the intial costs?

(I don't know if anyone knows the answer -- just thought I'd ask.)
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mkl654321
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October 23rd, 2010 at 11:49:55 PM permalink
Quote: rxwine

I read on a government link (sorry not sure what it was now) that the dam has paid for itself. So...does the upkeep and saleries, and lower power rates reflect that?

I was curious, because eventually, these solar projects (reflected light thermal in desert) will all be paid for too one day. So, will Nevada Power (or others) keep raking in the money once they've paid most of the intial costs?

(I don't know if anyone knows the answer -- just thought I'd ask.)



Bureau of Reclamation dams, Army Corps of Engineers dams, etc. that were built primarily as cash flow generators were justified by using extremely unrealistic cost-benefit calculations. Virtually no dam built in the period 1920-1990 justified its cost, even over the long term. Hoover/Boulder was supposed to have paid for itself in twenty years. It took over eighty.

As to whether power rates will now be reduced....HAHAHAHAHAHAHAHAHAHAHAHAHAHA.....sorry. The Golden Gate Bridge was paid for about thirty-five years ago. The toll is now $5.00. The Bay Bridge to Oakland was paid for in 1964. The toll is now $3.00 (maybe more; I haven't been there in a few months). The cash register dams on the Columbia have been supplying cheap power to the Pacific Northwest for decades, but power rates in Seattle and Portland have been steadily climbing, because that power is now being sold to California through the Pacific Intertie. It's easy to make one generation pay for public works and then deprive the next generation of the benefits.

Nevada Power will no doubt use Chinese accounting to say that its amortized costs (of the solar arrays) won't be recovered until the year 2635, so they will get virtually cost-free power and sell it at the going rate.
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Doc
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October 24th, 2010 at 7:07:48 AM permalink
Just curious -- suppose you constructed a manufacturing plant, competed well in the market, and eventually not only made enough to pay for your plant but fully depreciated it for tax purposes. Would you then sell your product at a price based on your continuing costs (and assumed the facility was free), even if all your competitors still had to pay for their facilities and even if the market placed a much higher value on your product?
rxwine
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October 24th, 2010 at 9:48:46 AM permalink
Well, I don't know. There's a reason (although it's not the same thing) why patents run out. And books go into public domain. Anyway, thers's something about power sources, water, and air (what was the movie where one person controlled the air on Mars)

It begins to feel like the Corleone family has a resource tied up, and were all prisoner to their whims.

I'm still thinking about it though.
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mkl654321
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October 24th, 2010 at 11:02:15 AM permalink
Quote: Doc

Just curious -- suppose you constructed a manufacturing plant, competed well in the market, and eventually not only made enough to pay for your plant but fully depreciated it for tax purposes. Would you then sell your product at a price based on your continuing costs (and assumed the facility was free), even if all your competitors still had to pay for their facilities and even if the market placed a much higher value on your product?



Irrelevant comparison. Dams are (and were) public works. Government agencies are not supposed to construct public works for profit--once costs are paid, they are supposed to be operated revenue-neutral. In this case, the government HAS no competitors--there are no private companies building dams on main-stem rivers.

If a dam generates saleable power, and its costs are fully amortized (amortization, not depreciation, is what is relevant), then that power should be sold at cost. That benefit is what was supposed to be the rationale for asking the taxpayers to fund the dam in the first place.
The fact that a believer is happier than a skeptic is no more to the point than the fact that a drunken man is happier than a sober one. The happiness of credulity is a cheap and dangerous quality.---George Bernard Shaw
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