Tall Building - Modeling Survey & Punching Shear

[slickdeals]

All:

I am trying to do a quick survey of modeling approaches adopted by you/your firms for tall concrete buildings.

With central core and flat slabs.
1. Model the slab as shell elements with stiffness modifiers (0.25 to 0.5) and rigid diaphragm assumption. Account for lateral moments @ slab-column junction?
2. Model the slab as a membrane element with rigid diaphragm assumption. Ignore any slab-column moments?

With Central core + Beam & slabs
1. model the slab as membrane elements and beams as T-beams.
2. model the slab as shell elements and beams as rectangular elements. Can be tricky to capture actual beam moments...?

I know that a lot of people I have talked to prefer to model the slabs as membranes in order to ignore the slab's contribution to the lateral system and beef up the core walls. The slabs can have a good contribution depending on spans and research has shown in-situ behavior to be closer to FE models that have included slab's contribution.

However, regardless of how it's modeled, the slab-column frame will pick up lateral moments in real life.

1. Do the membrane modelers ignore that moment for punching shear design in flat slabs?

SIDEBAR (so that I don't have to start a new thread)

Punching Shear Design & Edge Columns:
1. In your slab analysis models, do you assume a column stiffness modifier to account for cracking, thereby relieving some of the moment?

2. Is it appropriate to look at your elastic floor analysis model, compare the column moment to its cracking moment capacity and reduce stiffness? In a PTI publication, it is recommended to use 0.33 to 0.5 stiffness modifiers for edge columns, with appropriate detailing of the column. I realize punching shear is a brittle failure mode and redistribution of moments is not advisable, although ACI (and other codes?) don't specifically prohibit this.

As always, appreciate the sharing of knowledge.

 

[rowingengineer]

slickdeals,
for flat slabs with columns and core (usually not a tall building) I use the plate analysis with stiffness modifications for complex buildings with heavy transfer systems. I do however segregate each system into its own analysis not relying on one model or calc procedure. I fail to see how a planar element could be used in this situation.

Generally for tall building you will need a lateral system that has an outrigger system or external load bearing arrangement.

I do assume a column stiffness modifier, depending on the situation this could reduce or increase the moment for shear design.

I use a few more factors than just the cracking moment of the slab to figure out the stiffness, don't forget that in the lower floors the columns will be in heavy compression.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[asixth]

I don't use Etabs for vertical design. Only need it to analyse laterally. Model as shell with modifers and semi-rigid. I cheat and use ram to mesh the slab then import into etabs (meshing and load takedowns are the only application I have found for ram, wouldn't use it to design concrete).

 

[slickdeals]

@asixth.......RAM Concept?
Just curious as to your reasons for not using it to design concrete?

 

[DST148]

We include structural elements as part of LLRS based on the structural requirements of the projects.
We are in a low seismic region. Typically we end up with SDC B, and in some cases category C. Wind governs for most of our 15+ story buildings. We model all the major structural elements as part of the LLRS with due consideration for cracking. Since we fall into intermediate seismic area, the only section of ACI 318-05 that is applicable is 21.12, which is more about detailing of frame members like beams, columns, and flat slab.
For one of our recent 20+ story reinforced concrete flat plate structure, we adopted the following methodology. For wind we modeled core, columns, beams and slab as part of the LLRS. For seismic, we threw everything at the core. Drifts were within permissible limits. We avoided Chapter 21 altogether.
Some times for low cost multi story housing projects we have just modeled the structural walls, both for wind and seismic. There is a considerable saving in time for detailing, preparation and review of shop drawings, fabrication and installation of rebars, inspections etc.