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.
Robot then automatically assigns 4 CQC combinations as shown in the next picture.
Questions:
Tony
Solved! Go to Solution.
Solved by Artur.Kosakowski. Go to Solution.
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?
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.
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.
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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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Roman Zhelezniak
Robot Evangelist & Passionate Civil Structural Engineer
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
Correct. Usually the main criterion to select such modes is their contribution to participation mass for given direction.
Thank you, Artur. I've solved this problem.
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Roman Zhelezniak
Robot Evangelist & Passionate Civil Structural Engineer
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
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)
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).
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).
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.
Is Just what im intenting to do, but cant do it correctly.
What exactly do you find difficult?
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.
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 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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