Is it effective to use plates for designing slabs?

Hi Kaushal,

Yes, the descent mesh is necessary for proper load distribution and reinforcement zoning in plate design.
You will find a lot of useful information on concrete design here:

communities.bentley.com/.../2048.staad-pro-concrete-design-faq

Dear Sir,

Thanks for replying.

1. But this would result in more number of plates and take a lot of time to analyze the model. Isn't it?
2. Is this is the only reason why we always prefer to go for with FLOOR LOAD rather than defining the slabs by plates?

3. I have one more doubt, If we go with FLOOR LOAD, to account for floor stiffness we need to provide some specification. But there can be three options available I can think of listed as below:

i. Specs > Node > Floor Diaphragm (But this will make the floor rigid in all planes).
ii. Specs > Node > Master/slave > Rigid > and assign to all the nodes of the floor (this option will again make the floor rigid in all planes).
iii. Specs > Node > Master/slave > ZX > and assign to all the nodes of the floor. (this option will only make the floor rigid in ZX plane).

My question is that out of these three option which one define actual geometry more closely or if there is any other option available. Please provide an answer for all the questions.

Thanks in advance.

A1: Definitely yes ... this will take more time to analyze. On the other hand this will closely resemble to the actual structural behavior.

A2: A Floor Load command can only simulate the load but can not take care of the Slab Stiffness.

A3: In conjunction with the Floor Load command if you consider the option (iii) - Master Slave in ZX direction that will serve your intended modeling purpose.

Here is my views on this:

1. Increasing number of plate element would definitely increase the analysis time.
2. Discretization of large elements is a primary requirement(meshing) and it subdivides the beams with which it is connected. Thus, a member connected to a plate gets divided to smaller beams if you mesh the plates. This creates problem in analysis result interpretation and design.

This is why Floor Load is preferred. It maintains the gravity load path in the model.

3. If you use option (i) which is rigid diaphragm, creates the floor rigid only in ZX plane (similar to option (iii)) with and advantage that the master node it self will be the center of mass. Floor can undergo out-of-plane bending action.

The major limitations of Master-slave command from modeling point of view is the following:

1. Master joint needs to be physically present in the model in the form of analytical node, either connected to model or exists in the form of an orphan node.
2. Location of the master joint needs to be calculated manually.

The above two limitations are overcome by Rigid floor diaphragm feature. It performs the following functions.

a. It calculates the center of mass for each rigid diaphragm where master joint is to be located, considering mass model of the structure (as defined by REF LOAD MASS option)
b. It creates internally analytical node at the center of mass location to be included during analysis.
c. It searches all nodes available in the diaphragm and make them slave nodes with master node located at the center of mass for the diaphragm.

It provides advantages in seismic analysis. The lateral load is applied at the CM of the floor. The program is capable to compute actual storey stiffness and center Of Rigidity. The torsional effect for the seismic load on the floor can actually be computed if Rigid Diaphragm command is added.

If you assume that your in-plane floor action is infinitely rigid then you can use the Rigid Diaphragm or Master-slave without physically defining the meshed plate elements. But if you assume that the diaphragm will act as semi-rigid where the plate can go out- of plate bending( in reality) then you have to use the meshed plate elements inside the closed panels..