8. Comparison of Two-Way Slab Analysis and Design Methods
A slab system can be analyzed and designed by any procedure satisfying equilibrium and geometric compatibility. Three established methods are widely used. The requirements for two of them are described in detail in ACI 318-14 Chapter 8 (8.2.1).
Direct Design Method (DDM) is an approximate method and is applicable to flat plate concrete floor systems that meet the stringent requirements of ACI 318-14 (8.10.2). In many projects, however, these requirements limit the usability of the DDM significantly.
The Equivalent Frame Method (EFM) does not have the limitations of DDM. It requires more accurate analysis methods that, depending on the size and geometry, can prove to be long, tedious, and time-consuming.
StucturePoint’s spSlab software program solution utilizes the EFM to automate the process providing considerable time-savings in the analysis and design of two-way slab systems as compared to hand solutions using DDM or EFM.
Finite Element Method (FEM) is another method for analyzing reinforced concrete slabs, particularly useful for irregular slab systems with variable thicknesses, openings, and other features not permissible in DDM or EFM. Many reputable commercial FEM analysis software packages are available on the market today such as spMats. Using FEM requires critical understanding of the relationship between the actual behavior of the structure and the numerical simulation since this method is an approximate numerical method. The method is based on several assumptions and the operator has a great deal of decisions to make while setting up the model and applying loads and boundary conditions. The results obtained from FEM models should be verified to confirm their suitability for design and detailing of concrete structures.
The following table shows a general comparison between the DDM, EFM and FEM. This table covers general limitations, drawbacks, advantages, and cost-time efficiency of each method where it helps the engineer in deciding which method to use based on the project complexity, schedule, and budget.
Applicable | Limitations/Applicability | Concrete Slab Analysis Method | ||
DDM | EFM | FEM | ||
8.10.2.1 | Minimum of three continuous spans in each direction |
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8.10.2.2 | Successive span lengths measured center-to-center of supports in each direction shall not differ by more than one-third the longer span |
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8.10.2.3 | Panels shall be rectangular, with ratio of longer to shorter panel dimensions, measured center-to-center supports, not exceed 2. |
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8.10.2.4 | Column offset shall not exceed 10% of the span in direction of offset from either axis between centerlines of successive columns |
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8.10.2.5 | All loads shall be due to gravity only |
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8.10.2.5 | All loads shall be uniformly distributed over an entire panel (qu) |
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8.10.2.6 | Unfactored live load shall not exceed two times the unfactored dead load |
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8.10.2.7 | For a panel with beams between supports on all sides, slab-to-beam stiffness ratio shall be satisfied for beams in the two perpendicular directions. |
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8.7.4.2 | Structural integrity steel detailing |
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8.5.4 | Openings in slab systems |
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8.2.2 | Concentrated loads | Not required |
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8.11.1.2 | Live load arrangement (Load Patterning) | Not required | Required | Engineering judgment required based on modeling technique |
R8.10.4.5* | Reinforcement for unbalanced slab moment transfer to column (Msc) | Moments @ support face | Moments @ support centerline | Engineering judgment required based on modeling technique |
Irregularities (i.e. variable thickness, non-prismatic, partial bands, mixed systems, support arrangement, etc.) | Not permitted | Engineering judgment required | Engineering judgment required | |
Complexity | Low | Average | Complex to very complex | |
Design time/costs | Fast | Limited | Unpredictable/Costly | |
Design Economy | Conservative (see detailed comparison with spSlab output) | Somewhat conservative | Unknown - highly dependent on modeling assumptions: 1. Linear vs. non-linear 2. Isotropic vs non-isotropic 3. Plate element choice 4. Mesh size and aspect ratio 5. Design & detailing features | |
General (Drawbacks) | Very limited applications | Limited geometry | Limited guidance non-standard application (user dependent). Required significant engineering judgment | |
General (Advantages) | Very limited analysis is required | Detailed analysis is required or via software | Unlimited applicability to handle complex situations permissible by the features of the software used | |
* The unbalanced slab moment transferred to the column Msc (Munb) is the difference in slab moment on either side of a column at a specific joint. In DDM only moments at the face of the support are calculated and are also used to obtain Msc (Munb). In EFM where a frame analysis is used, moments at the column center line are used to obtain Msc (Munb). | ||||
