vertical accelerations - permanent or environmental? 2

[chaboche]

In DNV Part 1 Chapter 3, Design Loads, Section 3 Load and Load Effects, there is a definition of load categories.

It is stated that permanent loads include the weight of the structure.

It is also stated that environmental loads include the effect of waves.

If I know the acceleration acting on a component sitting on the deck of a ship, how should I class the vertical acceleration? Part will be the mass (gravity) and part will be action of the waves.

Thanks for your help.

 

[BigInch]

The Gravitational acceleration (negative down) component results in the weight load and the wave acceleration vertical component (results in the wave vertical(environmental) load, which is (positive) up when the wave surface is rising and (negative) down when the wave surface is falling.

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[chaboche]

Thanks BigInch,

Ok, so my understanding is . . .

The negative vertical gravitational acceleration is the weight, and therefore is a 'permanent' load.

The additional +/- vertical acceleration due to the waves is an 'environmental' load.


According to DNV:

load condition a
The load factor is 1.3 for permanent loads and 0.7 for environmental loads.

load condition b
The load factor is 1.0 for permanent loads and 1.3 for environmental loads.


The permanent (weight) loads and environmental (wave) loads have different load facors associated with them. Therefore I will need to consider the accelerations separately and ensure the correct factors are applied.

Is my interpretation correct?

 

[BigInch]

If the DNV definitions are as you say (I don't have a copy of it), that would be exactly how I would do it.

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[chaboche]

Thats great, thanks for your help.

Someone at work told me words to the contrary, ie all accelerations should have the environmental load factor applied. This is why I was confused.

 

[BigInch]

The load combinations you have,

Load Condition A
The load factor is 1.3 for permanent loads and 0.7 for environmental loads.

Load Condition B
The load factor is 1.0 for permanent loads and 1.3 for environmental loads.

A load "A" condition is generally used to give a very strong design for weight loadings, increasing (typically) any compression loads wherever the environmental loads cause overturning, but also reducing tension loads. That would be reasonable, given that compression load failures are typically more catastrophic, due to a rapid compressive failure of concrete or a very quick buckling of a steel column. Tension load failures are usually slower under a drawn out necking down process before failure occurs, so there is less need to increase tension loads to guarantee safety from rapid failures. The disadvantage is that it reduces compression when those loads are not causing overturning (such as a compressive member directly resisting a wave or wind load).

"B" will give a reasonably proportioned design, where the permanent loads are well known (can be well defined) and do not vary much (gravity is constant and well known) and the lesser well known and typically temporary loads from the environment, such as wind, snow, waves, earthquakes will be increased to compensate for their unknown and variable nature to be safely included in the design.

If some "permanent load" can vary considerably (as if there is such a thing as a "permanent" wave load), I don't think it would be reasonable to use a factor of 1.00 as that allows no real factor of safety for their variability, so those types of loads should indeed be classified as an "environmental" load for load condition "B" and for load condition "A", classifying a wave load as "permanent" load would not increase compression loads in overturning, so it should be an "environmental" load to correspond with the intention of the code (as I see it). Considering such a case as the compression member directly resisting a wind or wave load that I mentioned earlier, classifying the wind or the wave load as "environmental" would cause it to be increased by the 1.3 factor in Load Combination "B" resulting in a properly considered compression member design. So the resulting design would be OK for compression design too, thus covering the disadvantage of load case "A".

Considering component loads as we have above, should indeed result in a design complying with the intentions of the code (as I see it).

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[chaboche]

I see, so considering wind and wave loads as environmental loads ensures that their variablity is considered in the assessment.

For load condition A, the environmental loads are reduced to ensure that the maximum "compressive" stresses are considered for strength and stability purposes.

For load codition B, the the environmental loads are increased to ensure that the maximum "tensile" stresses are considered for safe design of the structure.

By considering the self weight as an environmental load would potentially make load condition A unconservative. This is because the environmental load factor would reduce the load due to self weight thereby reducing the compressive/overturning stresses.

For load condition B, the enviromental loads are increased to account for their variability, however the self mass does not vary and is well known, so it would be inappropriate in this case.

So to conclude, self mass should be classed as a permanent load. Only variable wind/wave loads should be classed as environmental loads.

Thanks, I understand these issues much better now. I also feel I understand the background to the code, which is key.

 

[BigInch]

Great! That's exactly why I tried to give you some background.

Yes, the "intent" of the codes should always be prevailent, and remember the codes only define the minimum requirements for typical design problems. They simply cannot cover all combinations and permutations. Conceivably, some particular structure and loading conditions might not fit into the neat code classifications for all situations, so with some understanding of the reasons behind the codes, you will be able to decide when more appropriate combinations might be necessary to guarantee a safe final design, even for unusual situations.

Also it is not the written load combinations and load magnitudes in the codes that are really so important to the word "minimum". It is the final design and construction that must meet a minimum strength, so you could if necessary, reduce or increase the weight load value (or any others) in special cases where weight or other loads may not be so well known, but ONLY if it results in a design that exceeds the Case Combinations written in the code. For example, if you think the weight could be lighter at some time in the future, it might be wise for you to consider that now by inventing a special load case for the overturning analysis using say only 85% permanent load with the same 130% environmental loads. As long as the final design can still meet 100% permanent load + 130% environmental, and 130% permanent + 70% environmental, the code would permit you to design for that special condition too. (Just be sure it is truely reasonable and necessary to consider such a deviation.)

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[chaboche]

Ok, that's good advise, thanks.

The structure is a reel which carries a certain amount of pipeline. I will therefore need to consider it with and without the pipe to ensure stability. Without the pipe in place, the reel is potentially more susceptible to overturning.

 

[BigInch]

You've got the idea now. You are absolutely right as there would be less weight holding it down.

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