A Beginner's Guide to the Structural Engineering

Basic Design Concepts

2006,2008 T. Bartlett Quimby

Introduction to Design Theory
Design Objectives

Limit State Concepts

Searching for the Best Design
ASD vs LRFD

Loads and Their Combinations

Example Problems

Homework Problems

References


Report Errors or Make Suggestions
Make Donation

 

Section DC.2

Design Objectives

Last Revised: 11/04/2014

Design objectives will change from problem to problem.  As a designer, the engineer must be able to identify the objectives pertinent to the problem at hand so that alternatives may be compared.  The problem objective can then be expressed as an objective statement.  In structural engineering, the objective statement can also be put in the form of an objective function.

Objective statements are generally expressed as "maximize" or "minimize" statements.  Objective functions return a single value that can be used to compare alternatives.  Some typical objective statements, and their associated objective functions, are:

Objective Statement Objective Function
Select the least cost member minimize(cost)
Select the least weight member minimize(weight)

Some objective statements contain multiple (and often competing) objectives.  An example of such a statement is:

  • Select the structural system that has the least cost while minimizing deflections.

In multi-objective statements, the objective function must have weighting factors to combine the various sub objectives into an single factor.  The weighting factors may have units to convert each of the terms to a common set of units before adding them together.  For the example previously stated, the objective function may take the form:

  • (Cost Weighting Factor)*(Structure Cost) + (Deflection Weighting Factor)*(Deflections)

Changing the relative values of the weighting factors will have a substantial impact on the final result, so they must be chosen carefully and with professional judgment.  It is common to refine these weighting factors as the design progresses, taking into account professional judgment and experience with the design at hand.