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# Modeling techniques FAQ

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This page is devoted to frequently asked questions (FAQ) related to modeling techniques in SAFE.

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# Formulation

## Should I use thin-plate or thick-plate formulation during analysis?

Answer: The primary difference between thin-plate (Kirchoff plate theory) and thick-plate (Mindlin plate theory) options is that thick-plate formulation accounts for shear deformation, which may become significant when span-to-depth ratio is on the order of 5 to 10, where span represents the length of single-curvature deformation, or the distance between inflection points. Shear behavior is also important in regions of discontinuity and near slab openings, where shear deformation may be important in determining the stress concentration factors due to bending.

We recommend using the Mindlin thick-plate formulation since it accounts for shear behavior, without significantly increasing the time of computation. The Thin vs. Thick shells article provides additional discussion on the attributes and comparisons between both options.

## Why does SAFE restrain Rx and Ry on top of columns above slabs, but not walls above slabs?

Answer: For vertical systems that extend above slabs, restraints are applied to the top of columns, but not walls, because large moment values will then develop along the top of walls. We generally recommend that you do not apply restraints to the top of walls. To manually attach the top of walls and columns to a rigid diaphragm above, select Run > Advanced Modeling Options > Add Special Constraints.

# Modeling

## How is column/wall height changed within a SAFE model?

Answer: The height of columns and walls may be changed through the following process:

• Select all support points.
• Select Edit > Database Editing > Geometric Data > Point Coordinates.
• Specify new Z-coordinates for these points, which will increase or decrease the height as specified.

## How is the automatic column location changed?

Answer: To overwrite automatic column locations, select the columns to be edited, then select Design > Punching-Shear Overwrites. As illustrated in the Tooltip, the default configuration may be changed to any one of a variety of location types.

## How do I model a raft foundation?

Extended Question: To model a raft foundation, should I select the mat option, or should I assign the thick-plate formulation to a slab-on-grade?

ANSWER: A raft foundation may be modeled using either the mat option or a thick-plate slab-on-grade. The only difference in output involves slight variations in rebar detailing. Analysis and design is otherwise the same.

# Group editing

## How are section properties changed for all slab sections of a certain type?

Extended Question: I would like to define/change the rebar material type for many different slab sections which are of the same type. How do I do this without having to right-click, then edit, each individual slab section?

Answer: Select each slab section, then change the rebar material for all segments through Edit > Interactive Database Editing > Design Data > Finite Element Based.