Design
Goal – Problem Statement
Design is said to be the essence of engineering. It is creation. It
starts with an idea – not well formulated at that, and then turns
into a set of drawings and instructions for the construction of an
engineering system. Analysis is done to determine the
parameters that best meet the goals.
Conceptual Stage
Candidate Concepts (Creation)
Analysis of Concepts (Evaluation)
Analysis and design are opposites of each other. Just look at the
problem-solving steps followed in analysis.
Set  up 
 Formulate
Transition 
Equation
 Answer
Knowledge
Conceptual Design Review
Detailed Stage


 Solve


Analysis (Specification & Evaluation)
Final Design Review
In analysis, you start with an engineering system. To analyze
the system, you formulate and solve a problem. At the end of
the process you gain insight into the behavior of the system.
But how was the system selected in the first place?
Somewhere, before the problem was originally stated, there was
a goal. The knowledge gained when solving the problem
describes the behavior that was originally sought. The end point
of the analysis is the starting point of the design.
Instructions
Drawings
Documentation
Some designs are prepared by a single individual while others
are a team effort. In either event, the analysis of the different
approaches requires a lot of calculations.
It’s perhaps interesting that analysis flows from left to right
when we really want to flow from right to left – from the
ultimate goal to the engineering system. Then, why do we
analyze from left to right and not the other way around? It isn’t
easy to answer this question. But, one reason is that the ultimate
goal is not precisely known ahead of time. The goal frequently
consists of competing objectives. The goal becomes clear as the
analysis is being performed.
Structural Design
This appendix contains a set of tools for structural design. The
specific purpose of the tools is to enable you to perform a lot of
calculations. The most powerful tool that enables you to
perform a lot of calculations is the computer.
It follows that design is inherently iterative. It’s an indistinct
process subject to unforeseen issues while the design is
developing. By its nature, a variety of approaches are compared
one against the other until a design is settled upon.
The next section introduces you to MATLAB programming.
It’s a tutorial that covers the basics of MATLAB programming.
You can read the tutorial and get started programming in
MATLAB in just an hour or two. The last section describes the
structural design process and the computer programs included
with the book.
Design possesses a back and forth character that is depicted to
the right.
1
Computer Programs for Structural Analysis and
Design
truss2
Finds the forces and displacements
in a planar truss
The three major functions of a structure are strength, stiffness,
and action. They serve as the structural designer’s primary
goals. When developing designs that achieve these functions,
the designer is faced with a set of secondary goals. They arise
from practical limitations of cost and time and they’re a result
of the physical limitations of materials. The secondary goals
can lead to a desire to minimize the structure’s complexity, for
example, the desire to use readily available materials and
components and the goal to minimize the number of different
types of structural elements and connections.
truss3
Finds the forces and displacements
in a space truss
frame2
Finds the forces and displacements
in a planar frame
frame3
Finds the forces and displacements
in a space frame
Structural Design Process
Section 5.5 shows how to uses these programs. To get a
complete list of the input parameters and the output parameters
in these programs, type help and program name.
The following outlines a typical structural design process that
would be followed by one or a team of engineers. The outline
doubles as a Table of Contents for a structural design report.
1.
2.
3.
4.
5.
6.
Section integrals are needed in stress analyses and mass
integrals are needed in dynamic analyses. The computer
programs listed below are used to calculate section integrals and
inertia integrals of composite bodies.
Goal – Problem Statement
The original goal is stated along with how the goal
evolved throughout the design process.
Concepts Considered
The different concepts are described.
Concept Evaluation
An analysis is performed of each concept. The
performance of each concept is evaluated in a way
that the degree to which the concept meets or does
not meet the goal can be understood.
Conclusions and Concept Selection
The constraints in the problem are described. A
winning concept is selected that best satisfies the goal
and overcomes the constraints. Unresolved issues are
identified.
Detailed Analysis
A detailed analysis is performed to overcome
unresolved issues identified in the previous step and
in order to complete the engineering specifications
needed later for construction.
Engineering Instructions
A set of instructions, drawings and tables specify the
engineering system so it can be constructed. The
tables can include vendor information, parts lists, and
a budget.
Composite2
Calculates the section integrals of a composite
body
composite3
Calculates the inertia integrals of a composite
body
These two programs are described in Section 6.5. They
assemble the composite body from the simple bodies cataloged
in Table 1 in the back of the book. Type help and program
name to get a complete list of the program’s input parameters
and output parameters.
Program List
The two major types of structures are a truss and a frame. The
computer programs listed to the right can be used to analyze
them.
2