LECTURE SERIES on STRUCTURAL OPTIMIZATION

1. LECTURE SERIESonSTRUCTURAL OPTIMIZATION Thanh X. Nguyen Structural Mechanics Division National University of Civil Engineering thanhnx@nuce.edu.vn

2. LECTURE SERIESonSTRUCTURAL OPTIMIZATION LECTURE 2S.O. PROBLEM FORMULATION

3. CONTENTS Introduction Elastic Analysis of Framed Structures Elastic Analysis of Continuum Structures Plastic Analysis of Framed Structures

4. INTRODUCTION Structural Design Philosophy Determisnistic or probabilistic-based? Kind of failure modes? Service load and overload conditions? Ex. Design against Initial Yielding under Service Load Design against Collapse under Overload Design to Avoid Damage under Service Load and Collapse under Overload

5. INTRODUCTION

6. INTRODUCTION Traditional Approach Preselect the set of constraints active at the optimum Good when? [critical constraints are easily identified AND their number equals to the number of variables] Wrong solution awareness! [if wrong choice of critical constraints OR their number is less than that of variables]

7. INTRODUCTION Optimality Criteria Approach Assume a criterion related to structural behavior is satisfied at the optimum Ex. Fully stressed design; displacement constraints; stability … Iterative solution process, usually! Characteristics [simple; may lead to “suboptimal” sol.; not general as of math. programming]

8. INTRODUCTION Math. Programming Approach Oops, which of the constraints will be critical?? [no prediction on the set of active constraints  essentially use inequalities constraints for a proper formulation] Characteristics [no single method for all problem; no accurate method for large structures  often based on approximation concepts] Relative optima may occur [ repeat computations from different starting points]

9. INTRODUCTION Structural Analysis Core of any S.O. formulation and solution Acceptable analytical model [must describe physical behavior adequately] [yet be simple to analyze] Most common [elastic analysis of framed structures] [elastic analysis of continuum structures] [plastic analysis of framed structures]

10. ELASTIC ANALYSIS OF FRAMED STRUCTURES Frame structures Length >> sizes of cross-section Typically: beams, grids, trusses, frames Assumptions for linear elastic analysis Displacements vary linearly w/ applied forces All deformations are small Methods Force method Displacement method

11. ELASTIC ANALYSIS OF FRAMED STRUCTURES (1) (2) (3) Force method Compatibility equation Final displacement U and forces S If several loading conditions  all vectors are transformed into matrices

12. ELASTIC ANALYSIS OF FRAMED STRUCTURES Force method Brief procedure • Determine degree of statical indeterminacy; Redundant forces and primary structure are chosen. • Compute the entries of F, P, FU, UP, FS, USin the primary structure • Compute unknown redundant forces X • Compute desired displacements U and forces S in the original indeterminate structure Ex.

13. ELASTIC ANALYSIS OF FRAMED STRUCTURES (4) (5) (6) Displacement method Equilibrium equation Final displacement U (other than those included in q) and forces S If several loading conditions  all vectors are transformed into matrices

14. ELASTIC ANALYSIS OF FRAMED STRUCTURES Force method Brief procedure • Determine the joint displacement degree of freedom; Add restraints (= Establish restrained structure. It is unique.) • Compute the entries of K, RL, KU, UL, KS, SLin the restrained structure • Compute unknown displacements q • Compute desired displacements U and forces S in the original structure Ex.

15. ELASTIC ANALYSIS OF CONTINUUM STRUCTURES Continuum structures Often: plates, shells, membranes, solid bodies PDE Analytical solution or Numerical solution? For small problems For large problems Methods FDM FEM and many more …

16. ELASTIC ANALYSIS OF CONTINUUM STRUCTURES Finite element method See the structure as one compased of discrete elements connected together at a number of nodes PDE? Types of unknowns: Nodal displacements / Nodal stresses If “material nonlinearity” or “geometric nonlinearity”  using FEM with an incremental approach ODE

17. ELASTIC ANALYSIS OF CONTINUUM STRUCTURES Finite element method Brief steps • The structure is divided into finite elements • Compute stiffness matrix and nodal force vector for each element • Transform to the global coordinates • Assemble element stiffness matrices and nodal force vectors to have system stiffness and system force vector • Apply boundary conditions • Solve for unlnown nodal displacements • Compute back internal stresses (or other desired quantities)

18. ELASTIC ANALYSIS OF CONTINUUM STRUCTURES (7) (8) (9) (11) (12) Finite element method Equations at element level appropriate derivatives of N (10)

19. Moment Stress Yield stress Mp = fully plastic moment Strain Curvature PLASTIC ANALYSIS OF FRAMED STRUCTURES What for? Supplement elastic analysis by giving useful information about collapse load and the mode of collapse Limit design (in various codes) Plastic model

20. PLASTIC ANALYSIS OF FRAMED STRUCTURES (13) Kinematic Approach Equilibrium of possible mechanism i where Ei: external work of applied service loads i: kinematic multiplier Ui: total energy dissipated by plastic hinges

21. PLASTIC ANALYSIS OF FRAMED STRUCTURES (14) Kinematic Approach where ij: hinge rotations Mpj: plastic moments J: number of potential plastic hinges

22. PLASTIC ANALYSIS OF FRAMED STRUCTURES Kinematic Approach After the kinematic theorem of plastic analysis where : load factor p: total number of possible mechanisms (15)

23. PLASTIC ANALYSIS OF FRAMED STRUCTURES Kinematic Approach Independent mechanisms where NR: degree of statical indeterminacy All possible collapse mechanisms can be generated by linear combinations of m independent mechanisms! (15)

24. QUESTION?

25. Thank you for your coming and listening!