Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

5% Critical Damping

5% Critical Damping

5% Critical Damping

Can someone explain, in laymans terms, what 5% critical damping means; as it applies to Seismic Design Values (Spectral Values, Ss and S1).

RE: 5% Critical Damping

The Seismic Design Values (Spectral Values, Ss and S1) are based on a building with an assummed 5% of critical damping. Some buildings are designed with higher damping levels (say 10%), so that the building dissipates more of the seismic energy from an EQ, resulting in a reduction of Spectral Values, Ss and S1.

RE: 5% Critical Damping

Critical damping means that a system will not vibrate at all, on the other hand 0% damping means that the system will keep on vibrating without any reduction in the amplitude or frequency. 5% damping means that the damping is 5% of critical damping. 5% damping is low but still should reduce the amplitude of displacement with time of a system undergoing vibration. Hope this helps.

RE: 5% Critical Damping

How was the value of 5% selected?

RE: 5% Critical Damping

This has been in the seismic design code for sometime now. I believe, the value is selected from experience and testing based on material and structural elastic and post elastic behavior.

RE: 5% Critical Damping

5% is reasonable for many structures, as most will have more than 5% dampening except when empty or under construction.

It is an example of codified engineering judgement. I was fortunate enough to attend a lecture by the famous Prof. Tom Paulay where he openly admitted that the 0.6 term many codes still have in the base shear formula was added because the loads calculated when they first came up with the process for modern seismic design simply seemed too high...

So they just reduced them. *smiles*

The reality is that if the system is well designed with inherent ductility the the right places and over designed in areas of criticality, by which I mean a proper Capacity Design solution like we only rarely see in North America, the actual input load is much less meaningful than the system behaviour. That's pretty fortunate too, as I have yet to see any seismic record that looks like a code model loading! Lol....

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close