Torsional Modes

[slickdeals]

Folks,
I have heard this many times that a first mode torsion is not preferred. Is that merely a better choice or does it have grave implications. Can anyone give me directions towards literature or reasons why such a mode is not preferred to be the fundamental mode?

I am designing a taller building where an offcenter mass on a few floors roof is causing the first mode to be a torsional mode.

Any thoughts/suggestions?

 

[Hanif08]

Hi,

Here are couple of simple reasons why atleast your first mode should be a translational as opposed to torsional:

- First/fundamental mode has lowest frequency and hence highest period. In a seismic event a structure will definitely experience it.

- Higher modes have higher frequencies and hence shorter period. In addition in a seimic event a structure may not even experience these modes.

Torsional modes are also coupled in which you have significant mass participation in more than one direction.

 

[slickdeals]

Hanif,
I understand that lower modes typically have higher participation. But would it not be prudent to designfor the torsional mode rather than trying to find expensive means to make that mode come lower down (with a lower time period).

For example, if you had a huge water tank on the roof of a structure (not centered obviously), then wouldn't we design for it rather than move the water tank to the center of mass?

I totally understand the logic behind it, but I want to understand if there are structural calamities waiting to happen if the structure has a first mode as a torsional mode?

Thanks

 

[Hanif08]

I don't know what you imply by 'expensive' means. Structural calamity can happen even if first mode is translational becuase there is a whole set of issues like strength of lateral resisting system, fatigue, detailing etc. In general though, yes torsion increases a structures vulnerability to damage.

Because torsion is as a result of distributed mass and/or stiffness which causes relatively large eccentricity between center of mass and center of rigidity; any steps you take to reduce this will also reduce torsional effects.

In my opinion it is prudent to avoid such a situation in the first place. It starts with the choice of structural system in conceptual phase itself should be so that it reduces the risk of finding out later that the fundamental mode is torsional.

Much of the research knowledge is from simple systems like one-storey because analysis of larger systems is extremely complex. Therefore using skilled judgement in the choice of lateral resisting system can go a long way, as was done by our predecessors prior to the age of fancy programs.