Questions about ETABS

[BONILL]

I have the following questions about ETABS:

1. Does it automatically include accidental torsion (5% of the plan dimension) or do I need to input this value myself? I have an illustrative model where the C.M. and C.R. coincide, and it shows no torsional effects for lateral loads.

2. Does it automatically take into account P-delta effects? What do I need to input for it to do so?

3. Does it take into account orthogonal effects? How does it handle biaxial bending of columns in case that it does?

4. Does it take into account special boundary elements? In my illustrative model, it seems that my shearwalls should have boundary elements. However, I see no information about this in my input.

I have a attached my illustrative model.


Thanks for your time.

 

[BONILL]

I forgot to ask one more:

5. ACI section 12.3.1.2 states that the clear span for the member, Ln, shall not be less than four times its effective depth. I have a situation where I have a collector beam to a shearwall that does not comply with this. ETABS shows it failing in flexure. What can I do in this situation?

 

[slickdeals]

1. In order to define 5% eccentricity, you will have to do it when you define the EQ static load. You have the option to do X +/- Yecc and Y +/- X ecc. In doing

2. No, it does not automatically take into account P-Delta. You will have to define P-delta under Analysis -> Set Analysis Options -> P-Delta

3. Biaxial moments are accounted by the program. It generated a 3D interaction diagram and checks for P, Mx, My

4. Boundary elements are designed for automatically and it should give the values in the design output for shear wall.

I suggest you go through the answers in the FAQ section as well

https://wiki.csiberkeley.com/display/etabs/Analysis

 

[slickdeals]

5. Are you sure you are quoting the right section? Which revision of ACI 318?

 

[BONILL]

slickdeals,

I meant section 21.3.1.2.

 

[PicoStruc]

IMPORTANT DESIGN PARADOX HERE :

Take note that P-Delta analysis add geometric stiffness to the axial/bending stiffness of the structure.

This means that your structure will be "more flexible" with the same mass.

Then, because design acceleration spectrum shape, by using p-delta with you seismic analysis, you will reduce seismic force. The period increase and the design spectral acceleration reduce accordingly.

Use you engineering jugement for the building you are analysing. ! This might not be a good idea of checking that checkbox in Etabs depending of the slenderness of your structural element.

I my case, what I do is :

1) Analyse with / with p-delta to check the effects (amplification on local moment in member (columns / walls) vs the reduction of global lateral load).

2) If the p-delta effect do not increase much local moment in members, Analyse WITHOUT p-delta to get maximum global forces (seismic effect)

3) Amplify the local forces from the previous analysis with the equation proposed by Pauley & Priesley.

By doing so, you get higher forces (p-Delta amplified force) instead of lower force (Reduced force coming from spectral shift in period)

 

[PicoStruc]

Note : it's seam you are refering to canadian code. then it's 10% eccentricity not 5% for accidental torsion

 

[jdgengineer]

The code section you are referencing is intended for the flexural members of special moment frames (i.e. beams and columns). The intent of the code provision is to prevent short members that are very deep. Deep short members are governed by shear and will therefore be dominated by shear inelastic actions instead of flexure. The hystersis loop for a shear dominated failure is not as stable as a flexural dominated failure and therefore the code is attempting to avoid it.

 

[BONILL]

Additional questions:

6. I have an underground parking garage. When specifying my seismic base in ETABS, when would it be safe to specify my seismic base as the top of my garage, instead of the true base of the model?

7. What is the best way to check for the 25% of the seismic forces that my moment frames should be able to take in ETABS? I'm removing my shearwalls and reducing by base shear, but I have the problem that some of my beams are supported by the shearwalls. How should I treat this condition?

 

[PicoStruc]

6) Basement generate shear amplification below ground level.
it's called Stay Back Effect and cant easilly double shear depending of your diaphragm/building geometry.

Then, basement story need to be modelled correctly.

7) Don't know what you are talking about !

 

[PicoStruc]

Forgot to mention about the probable shear reversal in basement due to diaphragm support at ground / basement levels.!

Check documentation for shear reversal and amplification at basement levels