Fundamental to ETABS modeling is the generalization that multi-story buildings typically consist of identical or similar floor plans that repeat in the vertical direction. Modeling features that streamline analytical-model generation, and simulate advanced seismic systems, are listed as follows:

Templates for global-system and local-element modeling

Customized section geometry and constitutive behavior  

Grouping of frame and shell objects

Link assignment for modeling isolators, dampers, and other advanced seismic systems

Nonlinear hinge specification

Automatic meshing with manual options

Editing and assignment features for plan, elevation, and 3D views

Elastic Shortening, Δ_fpES (S5.9.5.2.3)

The prestress loss due to elastic shortening in pretensioned members is taken as the concrete stress at the centroid of the prestressing steel at transfer, f_cgp, multiplied by the ratio of the modulus of elasticities of the prestressing steel and the concrete at transfer. This is presented in Eq. S5.9.5.2.3a-1.

Δf_pES = (E_p/E_ci)f_cgp   (S5.9.5.2.3a-1)

where:

Applying this equation requires estimating the stress in the strands after transfer. Proposed estimates for pretensioned members are given in S5.9.5.2.3a.

Alternatively, the loss due to elastic shortening may be calculated using Eq. C5.9.5.2.3a-1:

where:

The alternative approach is used for this example.

Δf_pES = 13.7 ksi

DynamicAnalysis.

After the 3 post major analysis and Modeling checks  have been  cleared, the designer should review the structural arrangement and the overall dynamic characteristics of the model. To ensure that our model complies with the code requirements, a separate check and review on the model should perform by the designer. Here are the 7 Dynamic Analysis Checks and review in an ETABS model that every structural design engineer should consider.

P-Delta effect, one type of geometric nonlinearity, involves the equilibrium compatibility relationships of a structural system loaded about its deflected configuration. Of particular concern is the application of gravity load on laterally displaced multi-story building structures. This condition magnifies story drift and certain mechanical behaviors while reducing deformation capacity.

P-Delta effect typically involves large external forces upon relatively small displacements. If deformations become sufficiently large as to break from linear compatibility relationships, then Large-Displacement and/or Large-Deformation analyses may become necessary. 

Pushover is a static- nonlinear analysis method where a structure is subjected to gravity loading and a monotonic displacement-controlled lateral load pattern which continuously increases through elastic and inelastic behavior until an ultimate condition is reached. Lateral load may represent the range of base shear induced by earthquake loading, and its configuration may be proportional to the distribution of mass along building height, mode shapes, or another practical means.

Output generates a static-pushover curve which plots a strength-based parameter against deflection. For example, performance may relate the strength level achieved in certain members to the lateral displacement at the top of the structure, or bending moment may be plotted against plastic rotation. Results provide insight into the ductile capacity of the structural system, and indicate the mechanism, load level, and deflection at which failure occurs.

When analyzing frame objects, material nonlinearity is assigned to discrete hinge locations where plastic rotation occurs according to FEMA-356 or another set of code-based or user-defined criteria. Strength drop, displacement control, and all other nonlinear software features, including link assignment, p-Delta effect, and staged construction , are  available during static-pushover analysis.

Buckling occurs physically when a structure becomes unstable under a given loading configuration, and mathematically when a bifurcation occurs in the solution to equations of static equilibrium. The two primary means for performing buckling analysis include Eigenvalue and Nonlinear buckling analyses. Buckling must be explicitly evaluated for each set of loads considered because, unlike natural frequencies, buckling modes are dependent upon a given load pattern . When evaluating buckling, any number of load case  may be defined, each of which should specify loading, convergence tolerance, and the number of modes to be found. Since the first few buckling modes may have similar factors, we recommend finding a minimum of six modes.

Additional information is available in the CSI Analysis Reference Manual (Chapter XVII Load Cases, Linear Buckling Analysis).

When using the ETABS software as your tool for analysis and design in your project on-hand, you must equip with thorough knowledge and understanding in every aspect and stage from modeling to analysis to design of each structural member. Each stage has its own technique and guidelines in order to make our ETABS model as accurate as possible and free of warning and error messages.

You probably knew the different considerations and techniques when modeling using ETABS as well as knew the analysis guidelines and checklists when doing the pre and post-analysis procedures. Well, this time we will tackle the Design and Check procedure options in ETABS particularly in column design. What are the design considerations to look at in order for us to be sure that the structural members that we are designing are passing and no single structural member fails?

There are two options to design the columns in ETABS. We can either use the “Reinforcement to be Checked” and “Reinforcement to be Designed” option. Reinforcement to be checked option is used when the designer inputs the number of rebars and the program will check if the inputted rebar is sufficient enough to carry such load. On the other hand, when using “Reinforcement to be Designed”, the program will generate the area of rebar needed in that column for the designer to interpret into how many numbers of rebar required.