Torsion in Buildings

[Kasim]

i have attached the photo of Modal mass participation Ratio of the Model, is The first mode is Torsional? As the RZ is 0.19 of the mass, and Can we allow Torsion in first mode? 

and is the First or the 2nd mode is the Fundamental one?

[Kasim]

@Rana Waseem @UmarMakhzumi need your Help as this is My Final year project , thanks

[WR1]

1. Yes we can allow torsion or anything in a building as long as you are designing for that (uneconomical).

2. Fundamental mode? What is a fundamental mode? A mode with highest mass participation.

3. Why you are just looking at Rz in first mode?

From the table you attached, in my opinion;

First mode = Mode2 of ETABS with 54.56% mass participation in X direction. (T=0.553s)

Second mode = Mode3 of ETABS with 36.33% mp, rotation in Z direction (T=0.505s)

Third mode = Mode1 of ETABS with 34.75% mp in Y direction (T=0.586s)

Remember that ETABS lists mode based on descending order of T and not mass participation but in my opinion we should sort modes based on mp. Moreover, You have to check that modes are not "CLOSELY SPACED". That means, if you look at T above, you will find that translational modes (T=0.553 & 0.505s) are very close to torsional mode (T=0.586s). This is bad. Try changing configuration of your building. You can still continue designing like that but ofcourse its more economical (bad design).

 

From the data you provided, it looks like your building is undergoing translational+rotational in a complex way all at once. The shape, geometry, configuration of shear walls etc is not regular or you are trying to model a very stiff and very flexible elements together like a concrete basement supporting a light steel roof/canopy.

[Kasim]

Thank you Waseem Sir, This is A four Storey Building having Basement Shear walls and Having Two Elevators Which  are not Symetric about X Axis, Will make it symetric and then will hear from you,

[UmarMakhzumi]

I will just add to what Rana has said. 

To control torsion, the best thing you could do is to have new shear walls at perimeter of your building. They would add the most value interns of controlling torsion.

Thanks.

[Kasim]

Manana  @UmarMakhzumi

 

[Ussama Waseem]

This topic is very informative. Thanks for the replies. But I think I am lagging interpreting the table. I have the following table and cant interpret. Can anybody help what does the table say and how does it effect the design?

Case	Mode	Period sec	UX	UY	UZ	Sum UX	Sum UY	Sum UZ	RX	RY	RZ	Sum RX	Sum RY	Sum RZ
Modal	1	0.719	0.3	0.1009	0	0.3	0.1009	0	0.0699	0.2062	0.2	0.0699	0.2062	0.2
Modal	2	0.702	0.0932	0.5007	0	0.3932	0.6015	0	0.3481	0.064	0.0103	0.418	0.2702	0.2102
Modal	3	0.642	0.2212	0.0081	0	0.6144	0.6096	0	0.0056	0.1487	0.3996	0.4237	0.4189	0.6099
Modal	4	0.217	0.0267	0.0377	0	0.6411	0.6473	0	0.0578	0.0441	0.0252	0.4814	0.463	0.6351
Modal	5	0.212	0.036	0.0492	0	0.6771	0.6965	0	0.0752	0.0581	0.0039	0.5566	0.5211	0.639
Modal	6	0.196	0.029	0.0037	0	0.7061	0.7002	0	0.0057	0.0445	0.0628	0.5623	0.5655	0.7018
Modal	7	0.119	0.0042	0.0131	0	0.7103	0.7133	0	0.007	0.0025	0.0103	0.5693	0.5681	0.7121
Modal	8	0.114	0.0163	0.0097	0	0.7267	0.723	0	0.0055	0.0096	0.0007	0.5747	0.5777	0.7128
Modal	9	0.108	0.0074	0.0037	0	0.734	0.7267	0	0.0022	0.0043	0.0177	0.577	0.5819	0.7305
Modal	10	0.092	1.318E-05	0.0046	0	0.734	0.7313	0	0.0072	2.301E-05	0.0021	0.5842	0.582	0.7326
Modal	11	0.038	0.2543	0.0002	0	0.9883	0.7315	0	0.0004	0.3844	0.0005	0.5846	0.9663	0.7331
Modal	12	0.037	0.0021	0.0009	0	0.9904	0.7324	0	0.0025	0.0032	1.603E-06	0.5871	0.9695	0.7331

[WR1]

1. Increase modes from 12 to have SumRx more than 90%.

2.

first mode is translation in x plus some torsion.

second mode is pure translation y.

third mode is torsional + x-translation.

 

it appears that you have irregularities in x direction (CM and CR are at different locations).

 

[Ayman]

Hello, I am doing modal analysis of an irregular building (mass irregularity and torsion). I am facing difficulties interpreting the table and what modes should be considered to reached 90% MPMR. I have the following table and i can't interpret. Can anybody helps me to obtain the appropriate mode shapes?  

Thnak you,

[UmarMakhzumi]

Ayman,

You will need to see the mass participation per mode and then make sure you have enough modes (sum up the mass participation for each mode) to meet your building code requirement (90% or something varies from Code to Code).  If you didn't have enough modes when you sum up the modal mass participation per mode, then you need to increase the number of modes thill you sum hits the magical number as specified in the code. The table above is showing displacements per mode is different than what you should be looking at.

Thanks.

[Jason]

I'm having a hard time understanding this table. My supervisor said the sum of Ux Uy Uz Rx Ry Rz for the first three mode should be 100% but now it is not. Also for the first three mode there should be at least one cell from Rx Ry Rz to be a relatively large number respectively. Could anyone help me with that please!