Overlapping Slab Objects in the ETABS and SAFE Programs 

In building modeling, it is often necessary to overlap different slab objects, either to represent different sections in the same plane or to define loads in a more flexible and reversible way. 

For a better understanding of the types of slab objects available in ETABS (and SAFE), an image of the slab section definition form is shown below. The Slab, Ribbed, and Waffle types are typically used to represent standard slab zones. The Drop type is generally used to represent drop panels or thickened zones, and finally, the Stiff type is used to simulate the stiffness of the column-slab (or column-drop) connection.

Additionally, there are Mat and Footing types that can be used to model foundations, such as mat foundations and isolated footings, respectively. 
 

When modeling overlapping slab objects in ETABS, only finite elements with the properties of one of the objects are generated. The choice of the object that generates finite elements is automatic and follows the following hierarchy: 

Priorities in Overlapping Objects

• Objects of multiple types: When the priority level of overlapping objects is different, finite elements will be generated for the object with the highest priority. ETABS follows this hierarchy: Stiff > Drop > Slab. For example, if a Drop object spans over multiple Slab objects, the properties of the Drop section will be used exclusively. 

• Objects of the same or similar types: If the priority level of overlapping objects is the same, ETABS uses the following criteria to select the priority object: 

  • When one slab object is completely inside another, ETABS uses the properties of the embedded (interior) slab object. 

  • When the overlap is partial, ETABS uses the properties of the last object drawn. 

Note that Slab, Waffle, Ribbed, Mat, and Footing objects all share the same priority level, which corresponds to the lowest. 

• Opening and None objects: In addition to the objects presented above, there are two additional types of area objects: 

  • The Opening object takes precedence over all other objects, ensuring that no finite elements are modeled over its entire area. 

  • The None object can be used, for example, to define a load in a region that shares two different sections. This object does not influence the generated finite element model. 
     

    

If you have any questions, just refer to the finite elements generated for analysis (View > Set Display Options > Shell Analysis Mesh + View by Colors of Section Properties). 

Additional Analysis and Design Considerations   

• Stiff Objects: The finite elements generated by these objects have a 100x modification factor internally applied to their flexural stiffnesses to simulate the increased stiffness in the contact zone with the columns. The forces of these elements are not used for slab design nor are they included in the Design Strips results. 

• Ribbed and Waffle objects: These objects can be used to model ribbed and waffle slabs. The generated finite elements have an orthotropic stiffness corresponding to the geometric data defined in the cross-section properties. The weight and mass of these objects are calculated excluding any lost formwork, so in these cases, the user must define mass and weight modifiers to account for the action of these non-structural elements. The design and detailing of these slabs also consider the geometric data of the cross-section. 

• Mat and Footing Objects: In terms of modeling, these objects generate finite elements with the same properties as solid slabs (Slab objects). The differentiation of these objects is solely for conditioning the automatic detailing of the reinforcement, considering that they are mat foundations and footings, respectively. 

Hierarchy in Load Assignment 

• Prevent duplication of loads: When multiple Slab objects overlap, the properties of non-priority objects are ignored, but their loads are not. This means that redundant load assignments in overlapping areas must be avoided to prevent unwanted duplication. Generally, loads should be assigned only to Slab objects, not to Drop or Stiff objects. 

• None area objects: These objects can receive loads, provided they are fully inserted within slab objects. The loads assigned to these objects will be added to the loads of the other overlapping objects. 

• Opening objects: Opening objects are treated as areas without loads; therefore, loads are not included in the calculations where these objects are defined. 

f you have any doubts, refer to the total loads applied by navigating to Display > Load Assigns > Shell > Show Contours in Plan View for Floor.
 

Attachments

Example.EDB Example.$et