Cracked sections for gravity elements under seismic action 2

[LJ_]

I am confused about the property modifiers for the cracked sections under seismic analysis. Could someone shed some light in this matter:

1. I think only the elements part of the seismic resisting system are going to go under in inelastic deformation thus are the ones that crack.

2. Gravity elements, specially gravity beams should remain with uncracked sections and thus the property modifier should be 1. Maybe the columns could crack a little due to diaphragm action but definitely not the gravity beams.

3. However in the perimeter things get confusing. Say you have the frame in the figure, where only the blue moment frame is special, while in the other sections are gravity elements. Would you apply the property modifiers as well to the gravity frames? I am confused here.

Thank you!

 

[Agent666]

Any member that the moment is over the cracking moment should be reduced.

It really has no bearing on whether you've arbitrarily designated a frame as gravity or seismic resisting.

Deformation compatibility means they need to achieve the same drift performance for both frame types hence gravity frames will still carry some degree of seismic actions, and will crack accordingly, so you should be applying appropriate stiffness reductions to all members. In your picture if the columns and beams are all the same size then they should all have the same reduction factor applied.

 

[LJ_]

Hello Agent666, yes I was wondering about this. It is kinda arbitrary what is special and what is ordinary. However, the special detailing requirements should only be satisfied for those elements part of the seismic resisting system.

 

[HTURKAK]

I fully agree with Agent666 and a pink star to his respond.


It is not an arbitrary process to choose some SFRS elements are special , others ordinary. You should choose type of SFRS depending on the SDC. For instance, SMF can be choosen for all SDC category but OMF can be chosen only for SDC A, and B.

I do not know your local code. Some codes allow the use of gravity columns which are not part of SFRS. Still these elements shall be detailed to achieve the same drift performance .

For example, EC 8, distinguishes the structural members that have a secondary contribution to earthquake resistance from the primary members . The contribution of ‘secondary’ members to the lateral stiffness and earthquake resistance of the building is not taken into account in the analysis for the seismic action. Only ‘primary’ members are designed and detailed for earthquake resistance with
the rules of Eurocode 8. However, the secondary members SHALL be designed to maintain support of gravity loads under the most adverse displacements and deformations imposed on them in the seismic design situation .

 

[LJ_]

@hturkak however do you apply the same cracked factors for all the elements gravity and seismic resisting elements ?

The source of my confusion is R. With special moment frames we use R=8. Now, those elements part of the SRFS will dissipate the energy which will allow R to be 8. But it is not all of them. For moment frames, it is the perimeter frames that do most of this work. These members will crack much more than the gravity yet we are using the same stiffness modifiers to all the elements in the building.

 

[r13]

LJ,

I think HTURKAK's last sentence has answered your question - consider deformation compatibility of all members in the bundle, regardless of individual strength requirement/capability. You should check the state of the stress in the secondary members after deflection, to determine whether the cracked property is appreciate or not.

 

[HTURKAK]

1- Yes.. the cracked stiffness of ‘secondary’ members is used.

2- How did you model the secondary elements ? Modelling of gravity columns somehow simple.. Either assign pin connections at top and bottom joint or set bending stiffness to zero. However, modelling of gravity only beams would need some work. If the beam is continuous and supported on gravity columns , no need to modify any property or connection.

In order to find the design displacements, you SHALL multiply the displacements (found with R value applied ) with deflection amplification factor Cd or maybe with R depending on your local code.

If you post the structural plan with primary SFRS elements and Gravity elements, the discussion may be more helpful.

 

[HuynhKeshley]

For the gravity column which is not part of the lateral resistance system, is it checked under combination of gravity load and lateral deformation in ETABS?

 

[hardbutmild]

The design displacement is probably nowhere near the actual displacement during an earthquake action. Another thing is the fact that no one takes reinforcement into account when determining stiffness (and it significantly influences it). Since primary elements usually have more reinforcement, they'll have even greater stiffness than in the case you're comparing non-reinforced stiffness's. Also, neither the earthquake nor the structure know what is laterally resistant and what is "gravity" so it'll crack at the beginning of an earthquake at latest.

In my opinion because of all this you should always consider secondary elements to have the same stiffness modifiers as the primary ones (anything other than that is just fooling yourself that you actually know what's happening).

 

[HTURKAK]

How did you model the gravity columns? The last time i used ETABS, around twenty years ago.. IMO, you shall develop two models;

In the first model , you may model the gravity columns with assigning pin connections at top and bottom joints or set bending stiffness to zero.. The primary SFRS shall be designed for the effects found with this comb.

The second model, delete pin connections and run again.. gravity columns shall be designed for the effects found with this comb.

 

[HuynhKeshley]

The second model, delete pin connections and run again.. gravity columns shall be designed for the effects found with this comb.

I have modeled as the second model.
However, as the response modification factor was assigned, so the deformation output the model is not correct. It should be multiplied by the deflection amplification factor.
Therefore, I would like to understand if ETABS taking into account the deflection amplification factor in design of gravity column.

Do you know which software considering this?

Thanks.

 

[HTURKAK]

I proposed two models.. the first model for designing the primary SFRS.. and the second model for gravity columns..
If you perform the analysis with R= 1.0 you will obtain elastic forces and displacements.
In order to obtain design forces, you will perform the analysis with dividing R.

If you perform the analysis with Cd/R , you will obtain the design displacements..