how we decide?

[preceptor]

dear friends,

i wanna ask something,

example;

we have a structural problem,
but we can't decide this problem dynamic or not?

how we decide that any problem dynamic or not

regards

[mig21]

hi

the main concern is to decide if the loading to wich the structures will be subjected causes a structural response that varies in time.

For example for stiff structures ( low fundamental vibration periods - see your code for clarification) wind loading can be treatead as a quasi-static loading solutioned by applying static pressures on facade or internal elements. this is not true for very flexible structures where a dynamic analysis is in demand.

Another dynamic structural problem is the foundation of heavy machines with unbalanced or excentric masses that can cause ressonance or bad bheaviour of the machine if the problem is not well studied.

Earthquake action is obvious always dynamic, altouth in can be in most cases trated with a muilti modal response spectrum analysis that is not truly a dinamic analysis.

I thing in the future de capacity based proceder will be of great acceptance and will rule in the earthquake design.

if you could tell more about your structural problem maybe i could help more .

Keep up with the good work.

[yucapareja]

OK urbana....

First depend what you mean with "dynamic or not"?
You mean Modal, Pushover or Time History?

In the code of my country (Venezuela) there are only explain the use of static and Modal Analisys, But the results of both analisys is almost the same because the shear base are the same, the only benefit of Modal analysis is the show you the torsion effect in the structure.

I personally use Modal analisys because with the Structural Software is easy to apply.

Pushover and Time History analisys are more complex and I believe that should apply only and high rise building.

Regards,

[mig21]

OK, let me clarify my reasons :



1. Modal or multi modal response spectrum analysis.

it is an analysis which takes in account the basic dynamic properties of the structure but deals in a static procedure relatively to earthquake loading.


you perform a modal analysis with respect of the modes of vibration ,,, first mode ,,, second mode etc.....those you consider that are significant..

for each mode you go to a response spectrum and read a acceleration value... next ...

you combine the the response for each value using a combination rule CQC, SSRS ABSUM etc,,,
and you lose all the signs in the process,,, this is critical for columns,,,,,,,,,
this is the usual practice used in the world but must be used with caution!!! for some type of structures static analysis is more accurate for those whiteout torsional problems. keep in mind that design response spectrum are envelopes of spectrum's created from recorded or simulated earthquakes. There is "no dynamics" in multi modal response spectrum analysis ,, the only dynamic effect is the consideration of the vibration modes. nether less they dont account for degradation of stiffness due to hysteric behavior.


2.Pushover

this type of analysis is in the state of the art of structural analysis and its use is relatively low. basically it uses a monotonic increment of static analisys , lateral pushing over a structure regarding a determinate lateral distributional of inertial forces to a target displacement usually a top roof displacement and monitoring the formation of a collapse system since the model incorporates defined moment/rotation relationships ( plastic hinges) in the structural system. the dynamics in this procedure consists in the pre-definition of the target displacement and the definition of vibration modes.


3. Time History analysis.

This is a fully structural dynamic analysis both in terms of loading either in terms of the structural response of the structure. booth the loading and
response are varying in time and it can be considered true inelastic/hysteresis behavior. this is very time consuming, need very good computers and the interpretation of the results are difficult and error prone. also defining earthquake accelerograms either recorder or simulated or adjusted to ground site is still a matter of controversy.

i think steps 2 and 3 will be the future since modal analysis does not have a bullet proof creditability and 2 and 3 are based in capacity demand/deformation who take in to account the behavior and not a hypothetical base shear.

for machinery and other type of structures the design is very straight forward and with more or less work or more or less conservative design the problem will be solved.

i am Portuguese, last 1st of November it made 254 years that the city in which i live was destroyed by an 9.1 Richter earthquake . keep in mind that this discussion means nothing since the structures bheave as they are built not as thei are designed. i prefer a structure with a simple structural scheme designed with simple software than a complex structure designed with a very fancy computer in which i cannot control or feel the response of the structure.

let's keep the discussion on,,,, please comment !!!!!!!

[preceptor]

OK urbana....

First depend what you mean with "dynamic or not"?
You mean Modal, Pushover or Time History?
[/quote]

example
ground motion is a dynamic motion
may be solution is a static solution

dear buddy
my opinion

this is a relative problem,
we can easily understand that some conditions the best result is dynamic analysis ,some conditions the best result is pushover ,some conditions the best solution method static analysis,some conditions the best result is modal and/or multi modal response spectrum analysis,

i think,we should formulate this conditions,
we should consider all probabilities

because all probabilities,
directly related to structure's dynamic properties