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Spectral analysis

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Message 1 of 25
tony.ridley
1894 Views, 24 Replies

Spectral analysis

Hi,

 

When running spectral analysis;

 

I go to Spectral --------->direction definition---------->Resolution of a force into directions--------->Newmark Combination.  I'm only checking X and Y directions, so I choose Group 1 and Group 2.  

 

Document1_Page_1.jpg

 

Robot then automatically assigns 4 CQC combinations as shown in the next picture.  

Document1_Page_2.jpg

 

 

Questions:

  1. Why is only Y direction given a negative sign.  I would expect to see negative X somewhere too.  
  2. When the spectral analysis is run (say X direction only), does the active mass act on the structure in both +X and -X?
  3. If the mass in X acts both positive and negative, then I guess it's the same for the Y direction too.  So why the need for a neg Y case?
  4. If I double click on the generated combinations, I can choose "signed".  I guess this automatically runs the combination for a variety of -ve and +ve signs for X and Y.  If I guess coreectly, then I only need two linear signed combinations and not four, no? (+- 100%x +- 30%y, +-30%x +- 100%y).  If I guess wrong please can you explain it to me.  
  5. I read from help menu that combination sign "main mode" defaults to zero and thus all signs for all modes are taken into account.  By nominating a main mode, am I still getting +ve and -ve signing?
  6. Have I missed something and am completey on the wrong track here?

Tony

24 REPLIES 24
Message 2 of 25
tony.ridley
in reply to: tony.ridley

Question 7:

If I manually define a combination say 1.0x - 0.3y, will I get the same result as the automatically generated case from the Newmark combination?

 

Message 3 of 25

Tony,

 

Robot automatically generates only half of combinations (only these with positive X). As these combinations are then intended to be combined with other load cases it is assumed that they will be added with both positive and negative load factors e.g. 1.0* case 7 and -1.0* case 7. The same is done (automatically) when you use automatic code combinations. Masses act in both directions but only one load case for a given direction is generated (only one X or Y) therefore you have to create combinations that have a negative load factors for them. By default when you use quadratic combination formulas the signs of e.g. bending moments will be only positive. To have them 'behaving' as these from 'standard' load cases you have to decide which mode is to be reference for its signing.



Artur Kosakowski
Message 4 of 25



Question 7:

If I manually define a combination say 1.0x - 0.3y, will I get the same result as the automatically generated case from the Newmark combination?

 



Yes. There is no difference if a combination is created manually of automatically.

 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.



Artur Kosakowski
Message 5 of 25
Romanich
in reply to: Artur.Kosakowski

Artur,

 

As these combinations are then intended to be combined with other load cases it is assumed that they will be added with both positive and negative load factors e.g. 1.0* case 7 and -1.0* case 7. The same is done (automatically) when you use automatic code combinations.

 

Robot don't use cases 6...9 (linear combination in Tony's example). Only case 4 and 5 will be added to automatic combinations with positive and negative load factors. Maybe it's bug?

 

P.S. If I use quadratic combination, I have the same results.

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Message 6 of 25
Artur.Kosakowski
in reply to: Romanich

They should be



Artur Kosakowski
Message 7 of 25

To have them 'behaving' as these from 'standard' load cases you have to decide which mode is to be reference for its signing.

 

So if I choose a mode (mode 2 for Y direction in my model) in the analysis type---->combination sign, will my moment diagram behave as i would expect for a regular static analysis?

 

Thanks

Message 8 of 25

Correct. Usually the main criterion to select such modes is their contribution to participation mass for given direction.



Artur Kosakowski
Message 9 of 25
Romanich
in reply to: Artur.Kosakowski

Thank you, Artur. I've solved this problem.

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Message 10 of 25

Usually the main criterion to select such modes is their contribution to participation mass for given direction.

 

OK.  So when I put on the results, should I choose mode 2 only, or select CQC, as I have already chosen mode 2 as the main mode / signed mode?

 

Thanks

 

 

Message 11 of 25

 

OK.  So when I put on the results, should I choose mode 2 only, or select CQC, as I have already chosen mode 2 as the main mode / signed mode?

 

At the end you should focus on quadratic combinations (e.g. for code checking / design)
 

 



Artur Kosakowski
Message 12 of 25

thanks very much!

 

Message 13 of 25
willjvizcaya
in reply to: tony.ridley

Greetings!

 

I am using Spectral Analysis for South America, similar to U.S Code we get an Specter (period/Acceleration). along with a Reduction Response Factor (R).

Questions:

1) Colombian and Venezuelan Specters shows acceleration as a portion of gravity in "Y" Axis.. ¿Does Robot multiply these values with gravity so we get the true acceleration?? or should i multiply it myself?.

2)Same goes with R, is there any way of introducing it to Robot, of should i modify the Specter File (E.G 0,5 for R=2).

Message 14 of 25

1) Colombian and Venezuelan Specters shows acceleration as a portion of gravity in "Y" Axis.. ¿Does Robot multiply these values with gravity so we get the true acceleration?? or should i multiply it myself?.

 

In Robot you should enter 'manually multiplied' values of accelerations.

 

2)Same goes with R, is there any way of introducing it to Robot, of should i modify the Specter File (E.G 0,5 for R=2).

 

The later, yet you may find it easier to use the load factor (e.g. 0.5) for spectral case in the definition of combinations especially when you need to have reduced values of forces comparing to displacements (lad factor set a 1).

 

 

 



Artur Kosakowski
Message 15 of 25

1)In Robot you should enter 'manually multiplied' values of accelerations.

Yes it seems correct, Thx.

 

2)The later, yet you may find it easier to use the load factor (e.g. 0.5) for spectral case in the definition of combinations especially when you need to have reduced values of forces comparing to displacements (lad factor set a 1).

Is Just what im intenting to do, but cant do it correctly.

 

Thx for your response.

Message 16 of 25


Is Just what im intenting to do, but cant do it correctly.

 

What exactly do you find difficult?

 



Artur Kosakowski
Message 17 of 25
willjvizcaya
in reply to: willjvizcaya

The later, yet you may find it easier to use the load factor  (1)(e.g. 0.5) for spectral case in the definition of combinations especially when you (2)need to have reduced values of forces comparing to displacements (3)(lad factor set a 1).

1: This has to be apllied with manual combinations?.

2: For design forces, Seismic forces must be reduced, but displacement has to be calculated without R.

3. Where can be set lad factor?.

 

Sorry if the questions are too simple.. im a bit lost.

Message 18 of 25

1. Define a spectral case disregarding R (e.g. case 7)

2. Create two set of  manual combinations:

ULS (strength check) with load factor that corresponds to R e.g. comb1 = 0.5 * case 7

SLS (displacement check) with load factor equal to 1 e.g. comb2 = 1 * case 7

 

 

 



Artur Kosakowski
Message 19 of 25

Ok perfect!

Thank you so much!

Message 20 of 25
joverac
in reply to: Artur.Kosakowski

 


@Artur.Kosakowski wrote:

Tony,

 

By default when you use quadratic combination formulas the signs of e.g. bending moments will be only positive. To have them 'behaving' as these from 'standard' load cases you have to decide which mode is to be reference for its signing.


Could you explain that better? I don't understand what 'standard' load cases mean.

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