Robot Structural Analysis

## Robot Structural Analysis

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Posts: 155
Registered: ‎10-08-2011
Message 1 of 9 (239 Views)

# Sum of Reactions

239 Views, 8 Replies
07-01-2013 08:17 AM

Good Morning - Evening

I m little bit confused about sum of Reaction values given by RSA.

In the example attached (xls from RSA), the sum of reactions for FZ are good

For MX and MY , I could not find the same values of sum of the reactions

I used simply the reaction values for all nodes ( support at the table ) and mutliply them by there positions ( arms ) + Moments on these nodes

Values given By RSA are :

MY =  68.28 t.m   - MX = 649.29 t.m

Values computed :

MY = 87.077 t.m  - MX = 826.089 t.m

Am I missing Something ?

Help Appreciated.

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Posts: 3,717
Registered: ‎04-26-2010
Message 2 of 9 (232 Views)

# Re: Sum of Reactions

07-01-2013 08:20 AM in reply to: faisal.aaouatif
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Posts: 155
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Message 3 of 9 (228 Views)

# Re: Sum of Reactions

07-01-2013 08:48 AM in reply to: Rafal.Gaweda

Thanks Rafal ,

Ive already read this topic and i think got the meaning of the sum of reaction and how obtained.

In the RSA help :

The sum of moments (for the sum of reactions and the sum of forces) is determined with respect to the point with coordinates (0,0,0). The moments and forces operating at appropriate distances (arms) are taken into account, and recalculated with respect to coordinate point 0,0,0.

By doing this approch , i found exactly the same values given by RSA for all load cases EXEPT modal ones ... as explained above....

Thank you for verifying

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Message 4 of 9 (201 Views)

# Re: Sum of Reactions

07-02-2013 12:30 AM in reply to: faisal.aaouatif

There are no such results for modal case

or in case of Modal taking into account static forces you get the same results as for simple case on which this modal case is based

Rafal Gaweda
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Posts: 155
Registered: ‎10-08-2011
Message 5 of 9 (183 Views)

# Re: Sum of Reactions

07-02-2013 07:34 AM in reply to: Rafal.Gaweda

Thank you Rafal.

Im sorry for not being precise when saying Modal Cases.

What i meant that the problem is raised for Seismic Cases when selecting Results by vibration Modes.

Of course for modale case there is no such results.

So if you take a look to the first post , they are two tables :

1 First one for the Seismic Direction Direction X - Mode 1 for which sum of reaction are indicated in the bottom of the table. ( All of nodes in this table are exactly support nodes of the model )

2.The second one is for the coordinates of the nodes indicated on the first table.

My concern is about the results of the "sum of reaction" given by RSA in this particular case ( Seismic case in a given vibration mode ) :

if a take Reaction values (FZ) on supported nodes in reaction table, and try to find the "sum of reaction" of there moment regarding (0,0,0) point ; i can not find the  "sum of reaction" value indicated by RSA.

I used the same method for all other static cases , i found excalty the value given by RSA.

I work on RSA2013 version 26.0.4.4461 (X64).

Thank you

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Posts: 3,717
Registered: ‎04-26-2010
Message 6 of 9 (149 Views)

# Re: Sum of Reactions

07-03-2013 02:11 AM in reply to: faisal.aaouatif

Example below:

Rafal Gaweda
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Posts: 155
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Message 7 of 9 (144 Views)

# Re: Sum of Reactions

07-03-2013 02:39 AM in reply to: Rafal.Gaweda

Thank you Rafal.

I have no problem with sum of Reactions ( FX Fy an FZ ) they are correct as i said earlier.

My problem is with sum of Reactions : moment MX , MY especially.

in the example attached , if i assume that ( shown plan XZ ):

Point 1 : x = +5.00

Point 5 : x = -6.00

Sum of reaction My = 0.41*5 + (-0.05)*(-6 )= +2.35KN.m instead of 2.06 KN.m

Thank you

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Registered: ‎04-26-2010
Message 8 of 9 (132 Views)

# Re: Sum of Reactions

07-03-2013 07:25 AM in reply to: faisal.aaouatif

You had no chance to compare results correctly

1. Z level not visible on screen shot so no contribution from Fx considered

2. My not considered in handcalcs

3. Point 1 : x = -6.00

Point 5 : x = +5.00

I made new example - screen shot of results:

Rafal Gaweda
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Posts: 155
Registered: ‎10-08-2011
Message 9 of 9 (103 Views)

# Re: Sum of Reactions

07-05-2013 08:35 AM in reply to: Rafal.Gaweda

Thank you Rafal

It was  good mistake in the way it makes look after the general definition of a moment which was not used on my first approach at Post 1.

general approch tested on other models and its ok.

Things getting clear now.

Thank you very much

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