### Traditional Design Teaching – Chief Concerns by Kotur Raghavan

**Design Approaches**

Broadly there are two classes of
approaches to Engineering Design.

The first is the traditional
approach. In this approach the designs are arrived at using the information available
in Codes, Standards and Handbooks. Industrial equipment like boilers and civil
engineering structures like bridges, transmission towers and buildings are
designed using this approach. As an example, we can quote the *ASME’s Boiler and Pressure Vessel Code,
Section VIII, Division One*. Whereas the codes and standards yield
satisfactory designs, the main objection will be that they are often too
conservative and the designs tend to be unnecessarily bulky and heavy. For
simpler designs the traditional method involves the use of formulae from
Applied Mechanics and Strength of Materials. Most of the Machine Design books use
this as basis. The approach is useful for simple mechanical components like
shafts, pulleys and springs. There are limitations and shortcomings in this
approach which will be discussed in the next section.

The second major category is
Design by Analysis (DBA). Usually this will not be a single step procedure.
Iterations will be involved. In today’s context, the only suitable method is the
finite element analysis. For boilers and pressure vessels, ASME prescribe the
guidelines available in in its *B & PV
Code, Section VIII, Division Two*. Similar guidelines are available for many
industrial equipment also. Design of engineering components using FEA calls for
in-depth engineering knowledge and a sound knowledge of strength of materials
and elasticity theory. If properly used,
the use of FEA will result in satisfactory optimum designs.

**Approach based on Strength of Materials – Main Concerns**

In the strength of materials
based approach, which is to be found in all machine design books, a common
feature is simplifications, often gross, are introduced in the geometry, loads
and load path. Frequently, a few assumptions are made with regard to the
structural behaviour of the component. These assumptions are needed so that
simple closed form expressions of strength of materials can be invoked. Quite
often, such assumptions result in statically indeterminate structure being
solved as statically determinate one. In effect, all the design problems
covered in the books can be considered to be “closed ended”. This means to say
that solution is obtained for N unknown parameters using N number of equations.
The simplifications may result in
conservative or non-conservative designs.

With regard to the treatment of
stresses, the main drawback is that only one component of stress, like direct,
bending or shear comes into reckoning. Consequently, equivalent stress, say von
Mises, or principal stress do not form the design basis. The effect of neglecting
the combined effect of several stress components is likely to result in
non-conservative designs. Consequently, the factor f safety will be effectively lower than what is
actually intended.

Another shortcoming is that the
stresses are considered at their face value only. The damage potential of
stresses is not accounted for. The recognition of damage potential is one of
the major aspects of ASME’s DBA rules. Stresses are assigned tags such as
PRIMARY, SECONDARY, MEMBRANE, BENDING and PEAK. The categories are dependent on
the spatial distribution of the stress and the type of load causing the stress.
The allowable stresses strongly depend on the category to which it belongs. For
example, the allowable bending stress is 1.5 times the allowable membrane
stress for the same material. By treating stresses of different categories on
equal footing, the designs tend to be conservative. If a bit too conservative,
the designs will turn out to be non-optimal.

**Design Evaluation Case Studies**

A series of case studies will be
published in order to demonstrate the foregoing aspects. The approach will be
analyse the component using a high fidelity models of the components concerned.
The models are going to be rigorous with recourse to any simplifying assumptions.
The design assumptions will be assessed based on the results of analysis.

Fig. 1

The contents of this article and
a few of the forthcoming case studies are based on a conference presentation shown
above.

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