Floor Plate in Lateral Stability Model
Floor Plate in Lateral Stability Model
(OP)
Dear All,
I'm doing a lateral stability study on a 40 story building. Should I model RC floor plates in the lateral stability model? I know that there would be a beneficial effect by including the floor plate on lateral sway of the tower. But, is it a logical thing to do?
many thanks.
I'm doing a lateral stability study on a 40 story building. Should I model RC floor plates in the lateral stability model? I know that there would be a beneficial effect by including the floor plate on lateral sway of the tower. But, is it a logical thing to do?
many thanks.






RE: Floor Plate in Lateral Stability Model
If you're modelling the whole building then you need it to distribute lateral loads. If you're only modelling a single frame then you don't need it.
What beneficial effect would it have on drift?
RE: Floor Plate in Lateral Stability Model
Thanks for your post. Since, the plates are modelled as shell elements, their presence in 3D model will enhance the lateral stiffness of the tower. I have set up two models:
i)frames and shearwalls (i.e. w/o any fllor plate), which are part of the lateral stability system
ii)the whole building
the lateral displacement that I'm getting in the second model is considerably less than the first model.
is this rational?
thanks.
RE: Floor Plate in Lateral Stability Model
RE: Floor Plate in Lateral Stability Model
I apply the lateral loads on Diaphragm CM, either wind or seismic. I've also attached a section from "Reinforced Concrete Design of Tall Buildings by B. Taranath" that emphasizes on modelling the floor plate!!
RE: Floor Plate in Lateral Stability Model
RE: Floor Plate in Lateral Stability Model
Are you doing a wind tunnel study?
What kind of natural frequency do you have? I'm working on a 30 story building right now and we are kicking butt on drift (on the order of h/700 for a 50 yer wind), but are just squeaking by for accelerations at the top occupied story for a 10 year wind. Part of that is because we are using steel so we have a light structure and a smaller damping ratio. I'm just saying that you can work for drift but still fail accelerations.
RE: Floor Plate in Lateral Stability Model
Inclusion of the out of plane (flexural stiffness) of the floor plate will definitely stiffen the building up due to the outrigger action that results from coupling the shearwalls with the perimeter columns. If the floor plate was designed for the bending moments induced from this coupling action, then sure you can use it to check drift. If the floor plate was designed independently of the lateral loads (i.e. a gravity only floor plate design), then you should not include its beneficial effects for drift as the slab would crack under lateral loads since it is not reinforced to resist them.
Which way to design (as you asked which one is logical) is a matter of engineering judgment. By ignoring the slab flexural stiffness in the lateral model the design of the floors is a bit easier since you can do them independently, and the rebar will be a bit less. But the slabs are there, so with a bit more engineering effort you can use them to your benefit.
RE: Floor Plate in Lateral Stability Model
What do you mean by outrigger action and how is modelling a "plate element" different than designating it as a rigid diaphragm?
RE: Floor Plate in Lateral Stability Model
Perhaps I can help before Willis comes back to you. "Outrigger" is typical tall building speak for using the out of plane bending capacity of the floor systems to distribute loading, which otherwise would be taken by the cores, out to the perimeter. It usually involves stiff beams or floor to floor walls or trusses to make the columns work in push-pull like truss chords, but in some cases, some benefit can be shown just by using the slab bending capacity. I am not much in favor of using slabs as outriggers, but certainly stiff elements do a good job of reducing core requirements.
A plate element and a diaphragm have the same meaning to me as horizontal distribution members.
RE: Floor Plate in Lateral Stability Model
Presumably this would help to raise the natural frequency of the structure as well, since it is stiffening it, right?
RE: Floor Plate in Lateral Stability Model
For taller buildings it is very common to have one or two levels of deep outrigger beams (generally full story-height beams at the mechanical levels) connecting the center core to the perimeter columns. This helps widen the structural base of the building (same as it is harder to push a person over if they spread their legs out to widen the base of resistance vs. putting their feet together) to reduce overturning moments at the foundation and reduce drifts. It also reduces the natural frequency (helps with accelerations).
The difference betweeen a 4 and a 40 story building in this regard is that for a 4 story building with the same type force resisting system (shearwalls) the predominant resistance is from shear. For 40-story+ buildings this mode moves to flexural resistance of the core, so by engaging the perimiter columns you are providing a greater depth of flexural resistance for your cantilevered system, thus reducing deflections etc.
Other methods of helping tall building drift include the use of a hat outrigger system (basically outriggers at the roof) or using a virtual outrigger system by providing a belt of trusses or shearwalls around the perimeter of the building, which then engages the stiffness of the floor diaphragms in shear, though this is not as efficient as standard core to perimeter outriggers.
You can read some more about outriggers and virtual (belt) outriggers in attached file.
RE: Floor Plate in Lateral Stability Model
Hope Hokie's answer and my follow up above answered this - we are talking about the out of plane flexural resistance of the slab here - designating the slab as a rigid diaphragm for in-plane resistance has nothing to do with its out of plane flexural resistance.
Using a plate element in conjunction with a rigid diaphragm constraint will work for modeling out of plane stiffness and handling in plane load distribution, as will using a shell element and leaving the diaphragms as semi-rigid in-plane (you can use rigid constraint constraint with shells too - but then no reason to use shells in the first place as opposed to plates).
RE: Floor Plate in Lateral Stability Model
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RE: Floor Plate in Lateral Stability Model
Thanks for your response. I was actually wanted to point out this phenomenon at the beginning of my thread, that when you define floor plates, it starts to engage peripheral columns as outrigger action and stiffening up the building.
1) I agree with your approach as to design the floor plate for the moments induced by the lateral loads
2) In reality there would be a moment connection between vertical elements and floor plates which transfers some moments; How do you justify the approach of exclusion of floor plates and designing them just under gravity loads.
RE: Floor Plate in Lateral Stability Model
Just to add to valuable posts by WillisV and hokie66, if you build two models; one with floor plates included and the other one w/o floor plates; you will notice from reading the axial force in the peripheral columns that in the first model (i.e. floor plates included)you will get higher axial loads in the peripheral columns under lateral load cases, due to outrigger action.
RE: Floor Plate in Lateral Stability Model
RE: Floor Plate in Lateral Stability Model
Also, I don't know how it is for other software, but for RAM you would have to designate all fo the exterior columns as "lateral" columns.
RE: Floor Plate in Lateral Stability Model
RE: Floor Plate in Lateral Stability Model
My opinion is that a flat plate does not provide enough assistance to make up for the additional reinforcement required, and if outriggers are used, they should be major elements, with floor height trusses or walls being the most beneficial.
Connecting the braced frames with a stiff beam (I assume you mean steel) is similar to using concrete header beams to make two 'C' shaped lift cores work together.