Tall buildings - using slab as a linking element

[slickdeals]

Folks,
I am reviewing a tall building ETABS model.

The building has a central core and perimeter walls

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These perimeter walls are connected by extremely fine meshed slab elements to create what I assume is almost frame action on every floor.

I have never seen this done in the few buildings that I worked on. Is this common? Are you really gaining tremendous frame action from say an 8 or 10" slab spanning about 18 feet between the core wall and the perimeter wall?

Do you engineers use a rigid diaphragm constraint or model shell elements explicity to engage perimeters walls or columns?

Any input is appreciated.

Thanks

 

[civilperson]

The moment capacity of the column wide strip of the slab is the most available to produce frame action. (i.e. transfer of moments between columns and slabs).

 

[slickdeals]

A slab element will obviously have more stiffness than a equivalent frame element. How do you go about modeling an equivalent frame element?

I am looking for information regarding what other engineering firms do in these situations when are trying to engage frame action to squeeze every bit out of the system.

 

[Teguci]

It is typical for a 2-way slab system to use frame action from the slab column interface for moment resistance. However, at least for seismic loads, mixing a core wall (read - shear wall) with a very flexible moment frame won't be acceptable per IBC.

As for modeling it, the slab is useful for transferring any lateral loads from the outside wall to the inner core and vice versa.

 

[slickdeals]

@Teguci,
I understand the concept, however, my questions are more pertaining to structural modeling.

Examples of such questions are:
1. Do you use shell or plate elements for your slab modeling?
2. How fine is the meshing for the same?
3. Do you use a rigid diaphragm constraint and not model the slab?
4. Do you use equivalent beams (effective slab width) for modeling?
5. What is the correlation between simple frame models and full 3D models?

I am sure the answers to these questions are going to be very varied, but I would appreciate all input nonetheless.

 

[Teguci]

1. Shell will be fine
2. Start with course meshing. After analysis look at the mesh. If the contours jump from one mesh to the next, refine that area (I beleive for ETABS you have to turn off an automatic meshing refinement procedure)
3. For a tall building, I would be tempted to model the actual slab. I might start with a rigid link to get an idea first but then continue to an actual flexible slab model.
4. I would consider the design of the building separate from the design of the floor slab. When modeling the whole building, the floor slab is used in diaphragm action. After getting the results for the lateral system, I would come back and model a typical floor (or just do the floor design calculations by hand)
5. Not sure. Isn't simple frame a 2D model?

Been out of the ETABs - SAP world for some time so some of my responses might not be entirely pertinent.

 

[hokie66]

Many tall buildings use outriggers to engage the external columns as part of the lateral resisting system. But thin floor slabs are not very effective as outriggers.