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Engineering question about load bearing

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#1 ·
I'm building a shelter and I have questions about loads and load bearing. If a piece of plate steel will handle 2000 lbs per square foot and concrete is 200 pounds per cubic foot (I know it's less, just say it is) does that mean I can have 10' of concrete overhead?
Is that based on beams spread at 1 foot intervals?
What happens if I spread the beams to 2 foot interval and so on, 3', 4', 6' etc?
 
#4 ·
Are you saying that you are going to take a flat piece of plate steel and pour 10' of concrete over the top of it? You are going to die.

Please get some real (non-internet) help before you hurt someone. Something like that needs to be professionally designed. I'd start with a corrugated arch design. They are pre-engineered to be buried. You can bury them under concrete.
 
#5 ·
Pervious concrete, prestressed concrete, blended concrete, recycled concrete, reinfoced concrete, fly ash concrete? What kind of concrete are you talking about and why would you need 10' of it.

What kind of steel plate, how thick the steel plate, hot rolled, stainless steel, carbon steel, iron. single layer, single sheet, layered and loose or layered and bolted, riveted, or welded together.

What are you going to store if your support beams are 2. 3, 4' apart, how high is your ceiling and what are your support walls made of. Come on young feller you need a shelter but what are you sheltering. Just a wee bit more data is needed.
 
#6 ·
As a general rule, floor/roof decking materials don't hold didly without a network of closely spaced joists and beams to support them. To say a piece of plate steel will handle 2000lbs per square foot makes no sense. 2000 lbs/ft^2 may be the design load for a particular building which will dictate the design of the supporting structure. If you are building an underground shelter, your design load will take into account the weight of the concrete, dirt, snow, rain, ice, people, vehicles, etc. overhead. Don't forget the weight of the structure, itself.

Beam calculations can also apply to decking materials when you plug in the appropriate moment and modulus values because decking material is basically a really crappy beam. If you double the spacing between beam supports you will get 8 times as much (L^3) deflection for the same weight. If you double the width of a beam it only holds twice as much weight but if you double the height of a beam it holds 8 times as much weight for the same deflection or deflects 1/8 as much for the same weight. Since decking materials have very little vertical height, they are very flimsy. Load bearing doesn't go down quite as fast as deflection would indicate because the amount of deflection tolerated with load is proportional to length so load bearing goes down with the square of length rather than the cube. But that is total load. Distributed weight per square foot increases with length so you may be back to length cubed. Allowed deflection: Rafters length/180, floors length/360 (but often spec'ed at length/480 because people want floors). It also matters if the beam is allowed to flex or is rigidly supported at the support points. In practice, a 1" decking plate probably supports 16 times as much as a 1/4" decking plate. As you spread the supports, the thickness of steel plate required will rapidly increase to 1" and beyond. But you can get much more support for the same weight of steel by putting a bunch of beams under it rather than using thick plate.

For purposes of calculation, you should assume your beams/decking are not ridgedly supported unless you can prove otherwise. If a beam sits on top of a support column, it is not rigidly supported. If the bottom is attached to the support column but not the top, it is not rigidly supported. If the top and bottom are welded to the support column, then it is rigidly supported but there is a catch. The rigid support reduces the bending of the beam but it transfers those bending stresses to the column causing the column to bow and thus buckle easier. If the beam or decking is welded both sides of a rigid support column, it is also rigidly supported.

Strength mostly comes from having a top and bottom skin of the material with an internal structure that keeps them from shearing relative to one another. Imagine taking a ream of paper. support it on the edges with some bricks. Now press down on the center. It bends easily because the sheets of paper are free to shear. Now glue all the sheets together. Now you effectively have a piece of wood and it is much stronger. Hollow beams (rectangular tubing or wooden box beams) and I beams take advantage of the fact that you get more strength by taking the material and moving it away from the center, within reason, while still having a web of material to lock the top and bottom together.

You can model a simple floor as two layers. One layer is the joists and the other is the decking. Do the beam calculations for both. If your joists are themselves supported on beams between upright supports, then you have another layer to calculate. You also have to consider the compressive strength and buckling of your uprights or walls. You wouldn't want cinderblock walls crumbling under the weight of your joists and the weight they are carrying, for example.

You also have to consider concentrated weights. A car tire. The leg of a person, the leg of a piece of equipment, or a vehicle tire which puts the weight of a heavy object in one or a few places. This has to be added to your worst case design weights and you have to consider if you can punch through thin decking.

The taller the support columns (or walls) are, the more prone they are to buckling. Double the height and it will only take 1/4 as much weight to buckle them.

Trick question: how much does a 200lb person weight? If they are walking or jumping on your floor, it is way more than 200lb. How much more depends on the rigidity - how fast is their momentum decellerated?
In the event of an earthquake effective weights, such as the weight of dirt on the roof, could also be greatly increased because it is in motion. In an earthquake, your structure may not be evenly supported from below. As it rides the wave, it could be effectively supported only from the ends one instant and only supported in the center the next. The mass of your structure is also trying to stay still while the ground underneight it is moving side to side. If the structure is buried, you also have the mass of the earth beside the structure pushing it sideways. Most of the assumptions on which your calculations and design were based are invalidated.

If your structure is above ground, there are wind load issues. On a windy day, in a microburst, in a hurricane, or in a tornado there are substantial wind loads on your walls and other parts of your structure trying to topple it or buckle your walls. A tsunami or flood can be even worse. What I wrote about loads on roofs/floors also applies to walls. Walls are generally supported by the walls they join with perpendularly plus some floor and ceiling. Double the length of a wall and it may only take 1/8 the wind load for the same construction if you only consider support of adjacent walls. And wind load itself increases with the square of wind speed. This is one reason you see hurricane/tornado shelters having very small dimensions, such as an 8 foot cube. The walls and ceilings are supported by adjacent walls/ceilings such that the maximum distance to a support is 4 feet. Even so, each wall can see a force of 5 tons. In a 200mph wind, each 4x8 sheet of plywood on your exterior walls sees a force of about 5120lbs. A 32 foot long wall 8' high sees 40960lb and a double story structure twice that. Not counting roof.

Engineers and architects also multiply design loads by safety factors.

Crappy welds or inadequate fasteners can also cause collapse. Even good welds/fasteners if the design puts too much stress on them.

Here are some sites that have much of the formulas and data you need to do some basic calculations:
http://www.forestryforum.com/members/donp/beamsizing.htm
http://www.faztek.net/technical.html
http://www.engineersedge.com/calculators/section_square_case_4.htm
http://www.engineersedge.com/beam-deflection-menu.htm
http://en.wikipedia.org/wiki/Buckling
http://en.wikipedia.org/wiki/Deflection_(engineering)
http://bulk.resource.org/codes.gov/

If you do your own design and calculations, have it reviewed by someone qualified. Structural collapse is a deadly issue and a serious possibility, even in less than disaster situations. The information here can help you evaluate a concept or cost out a project. But there is a lot that has to be considered in an actual design before it is built.

Consider for a second an intermodal cargo container. Properly stacked, these can support hundreds of thousands of pounds. But only if the weight is where they were designed to take the weight. Piling dirt on top, does not cut it. Something many people don't realize when they think about burying them.
 
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