A popular traditional design technique for slab systems is based on an equivalent frame method whereby a three-dimensional slab system is represented as a simplified series of two-dimensional planar frames. Although SAFE, with its object-based finite element methodology provides greater accuracy and superior modeling capabilities when compared to traditional equivalent frame techniques, it may be beneficial on occasion to have analyses performed and design results reported using the equivalent frame approach.
SAFE offers a tool whereby analyses can be performed for planar equivalent frames input directly by the user or from frames developed from existing strip design geometry. The equivalent frame method technique is well suited to slabs that are post-tensioned, where only column strips exist, and the strips extend to the centers of the adjacent bays.
The width of the slab within the design strip acts as the beam part of the planar frame, and this equivalent beam can include the effects of drops and capitals. If a beam is present in the strip, it is included in the equivalent frame cross-section. An equivalent frame cross-section is determined by the program for each mesh line; if the equivalent frame section is different at two adjacent mesh lines, the program assumes that the dimensions vary linearly between the two mesh lines. For an equivalent frame based on a strip object, the width of the slab is assumed to be the same as the design strip width, which may or may not be the same as the actual slab width. For this same frame, when multiple area objects exist at the same location, only the largest slab object will be used.
Columns can extend below and above the slab, and although equivalent frames typically consist of slabs and columns, transverse walls, walls parallel to the direction of the frame (longitudinal), and soil springs can all be included in the model.
To account for the rotation of the slab away from the column, equivalent frame techniques typically replace the actual column with an equivalent column that accounts for this flexibility. SAFE’s equivalent frame modeler handles this behavior by connecting the equivalent beams to the columns with torsional springs. The slab which lies directly above the column is assumed to be rigid, i.e., there is no deformation within this zone, and forces are reported at the column face.
Equivalent frames are analyzed for loads acting in the plane of the frames. Live pattern loading for strip object based frames will be applied if previously defined on the SAFE model, or may be defined directly while working on the equivalent frame. If tendon objects are present, the calculation of the required reinforcing includes the effects of post-tensioning. If a beam is present in the equivalent frame, the design reinforcing reported will be for the beam cross-section, i.e., the slab will not be designed separately. Loads, deformations, forces and reinforcing can be displayed on the frames.