Structural Modeling: Methodology and Geometry

Structural Modeling

• It is a mathematical or wireframe representation of a 3D model.
• Helps visualize each order of a virtual building structure.
• The analysis aims to investigate the most probable behavior of a structure when subjected to a range of applied loads.
• Results should convert into useful design data to evaluate the performance of a steel structure and to determine the appropriate actions needed for the most efficient design.
• Calculation of a structural system capacity is an essential aspect to determine the most reliable design.
• All work performed ensures that all analytical work enables designers to produce a construction document including plans, specifications & estimates.

Selection of Modeling Methodology

• The typical design process requires demand and capacity calculations

Geometry

• It is a behavioral characteristic of the structural component as well as the overall global structure.
• Considerations must include global & local geometric characterizations of individual component connections

Material and Section Properties

• The important aspect to capture proper behavior to determine the material and sectional properties of its components.
• Theories are based on homogenous material such as steel, meaning structural behavior is calculated with the material and section properties.
• Nonlinear behaviors in elements can be difficult, requiring constitutive relationships describing behavior calibrated by available test experiment data.
• Without understanding member behavior, it's easy to fall into the "garbage in, garbage out" mode, so engineers need to verify behavior with known material properties before production analyses.
• To verify the design with relevant material properties is very important

Boundary Conditions

• Examine column or abutment conditions at the support (or ground) and implement them into the analytical structural model, based on engineering assumptions.
• It's common to use a simple support representation (e.g. fixed, pinned, roller) for static analysis.
• Dynamic analysis requires soil/foundation system representations
• To model foundation or soil systems with greater detail use a nonlinear spring/damper representation, modeling subsurface elements and plasticity-based springs for the soil mass
• It is essential to set up small models to test behavior and cross-check results via manual calculations.

Loads

• Load pattern refers to the spatial arrangement of a specific collection of forces, displacements, temperature, and other related factors that impact structural behavior.
• A combination sums or envelopes analysis results of certain load cases
• A case defines how load patterns apply and how the structure responds (linearly or nonlinearly.

NSCP 2015 – 203.3.1 LRFD

• Combination 1: 1.4(D + F)
• Combination 2: 1.2(D + F + T) + 1.6(L + H) +
• Combination 3: 1.2D + 1.6(Lr or R) + (f1L or
• Combination 4: 1.2D + 1.0W + f1L + 0.50(Lr
• Combination 5: 1.2D + 1.0E + f1L
• Combination 6: 0.9D + 1.0W + 1.6H
• Combination 7: 0.9D + 1.0E + 1.6H

NSCP 2015 – 203.4.1 ASD

• Combination 1: D + F
• Combination 2: D + H + F + L + T
• Combination 3: D + H + F + (Lr or R)
• Combination 4: D + H + F + 0.75[L + T(Lr or l
• Combination 5: D + H + F + (0.6W + E/1.4)

NSCP 2015 – 203.4.2 Alternate

• Combination 1: D + H + F + 0.75 [L + Lr (0.60
• Combination 2: 0.60D + 0.6W + H
• Combination 3: 0.60D + E/1.4 + H
• Combination 4: D + L + 0.60W
• Combination 5: D + L + E/1.4

Common Combination for LRFD

• Combination 1: 1.2D + 1.6L
• Combination 2: 1.2D + f1L ± 1.
• Combination 3: 1.2D + f1L ± 1.
• Combination 4: 1.2D + 1.0W
• Combination 5: 1.2D + f1L ± 1.

Structural Analysis

• Linear studies can be static or dynamic.
• Nonlinear studies can be static or dynamic.
• Simplest simulation is static.
• A more complex study is Dynamic, includes dynamic input
• Nonlinear static is more complex, it includes non linearity
• The study which includes non linearity and Dynamic is the most complex.

Static Analysis

• Static analysis is used for structures subject to constant or slowly varying loads where impacts are negligible.
• Linear static analysis assumes material remains elastic and deformations are small.
• Nonlinear static analysis accounts for material nonlinearity (e.g. yielding), geometric nonlinearity (large deformations), and boundary nonlinearities (e.g. contact, uplift).

Dynamic Analysis

• Dynamic analysis is used when loads change with time, such as in seismic or impact scenarios.
• Modal Analysis determines the natural vibration modes & frequencies of a structure.
• Response Spectrum Analysis (RSA) uses predefined seismic response spectra to estimate peak responses.
• Time History Analysis (THA) applies actual or synthetic acceleration/displacement time records to analyze the structure's response over time.

Buckling Analysis

• Used to determine the load at which a structure becomes unstable.
• Linear (Eigenvalue) predicts the theoretical buckling load assuming small deformations
• Nonlinear considers large deformations and material yielding for more realistic results.

Pushover Analysis

• Is a nonlinear static analysis method used in seismic design.
• Simulates progressive damage and plastic hinge formation under increasing lateral loads.
• It helps assess structural capacity and performance during earthquakes.

Soil-Structure Interaction Analysis

• It's used when foundation flexibility & soil behavior influence structure response.
• This analysis models springs, dampers, or detailed finite-element representations of the soil.

Fundamental Assumptions of Structural Analysis

• It consists of Elastic analysis.
• Material of the structure should be continuous & absolutely elastic.
• Relationship between stress and strain is to be linear.
• Deformations of a structure caused by applied loads are small & do not change the original design.
• Superposition should always be applicable.

Computer-aided Structural Analysis

• A analysis program depends on if it can provide a suitable model to replicate the structure and how easy it to use.
• Two main categories of analysis are: General purpose programs & Design oriented programs.
• General programs are suitable for advanced analyses. It is helpful for special engineering problems (blast and fire) with material models, elements & different solvers like explicit solvers
• A disadvantage is its design checks against code are not always available.
• Abaqus & ANSYS are examples of these
• Design oriented programs programs work best for conventional structure issues. Their capability to model structural types are limited plus limited finite elements and disadvantage can be its design checks against certain standards.
• The advantages are capacity to carry out design checks based on pre- programmed pre-existing other guidelines for the analysis.
• SAP2000, Staad.Pro, Robot & ETABS are examples of design oriented programs.

Basic Analysis

• The process requires engineers to understand structural behavior and the feasibility of results.
• Engineers must be able to judge if the analysis results are correct based on their engineering knowledge.
• The stages of operation include: creating a model of the proposed structure by assigning geometry,types of finite elements and material properties for each element. Then job should now be analysed by the solver. At this moment,unknown values, can now be determined, depending on the type of problem, different types of analysis procedures and solvers (such as implicit or explicit) will be used in order to solve the problem. Lastly, after determining the solutions to the problem, we can analyse, interpret the result of the finite element problem, these may include strains, stresses and bending moments plus axial forces together.