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Structural Design Considerations




Structural Design, though reasonably scientific, is also a creative process.

The aim of a structural designer is to design a structure in such a way that it fulfills its intended purpose during the intended lifetime:



  • Safety
  • Serviceability
  • Economy
  • Durability
  • Aesthetics
  • Environment Friendliness
  • Functional Requirements


Safety:

Safety requirement is paramount to any structure-It ensures that the collapse of the structure (partial or total) is acceptably low, not only under the normally expected loads (service loads), but also under less-frequent loads and accidental loads.
Collapse due to various possibilities such as exceeding the load-bearing capacity, overturning, sliding, buckling, and fatigue fracture should be prevented.


Serviceability


Serviceability requirements ensure that the structure performs satisfactorily under service loads, without discomfort to the user due to excessive deflection, cracking, vibration, and so forth.

Other considerations of serviceability are durability, impermeability, and acoustic and thermal insulation.
A design that adequately satisfies the safety requirement need not necessarily satisfy the serviceability requirement.


Economy:



For the overall economy, the initial cost, as well as the life cycle cost and the long-term environmental effects, have to be considered.

Economy may not be achieved by minimizing the amount of concrete or reinforcement alone. It is because a large part of the construction cost involves a cost of labour, formwork, and false work.
    

Durability:



A durable structure is one that will continue to perform its intended functions, that is, maintain its required strength and serviceability in the working environment during the specified or traditionally expected service life.


Factors Affecting Durability:

  1. The environment
  2. Temperature or humidity gradients
  3. Abrasion and chemical attack
  4. The permeability of concrete to the ingress of water, oxygen, carbon dioxide, chloride, sulphate and other deleterious substances.
  5. Alkali–aggregate reaction (chemical attack within the concrete)
  6. Corrosion of reinforcement
  7. Shape and size of members
  8. Presence of crack



Aesthetics:


Aesthetics is important not only for structures of high visibility but also for all other structures, as it gives a sense of pride to the owner.

Aesthetic consideration may include a selection of shape, geometrical proportions, symmetry, surface texture, colour, and harmony. Aesthetics is an art and cannot be objectively quantified or subjected to fixed rules.

The structural engineer must work in close coordination with the architects, planners, and other design professionals to design aesthetic structures that are elegant and at the same time economical and functional. 


Environment Friendliness:



The construction industry consumes 40 percent of the total energy and about one-half of the world’s major resources. Hence, it is imperative to regulate the use of materials and energy in this industry.
CO2 is a major by-product of the manufacturing of the two most important materials of construction—Portland cement and steel. Thus, while selecting the material and system for the structure, the designer has to consider the long-term environmental effects.
By simultaneously using the following three tools, major reductions in concrete consumption and carbon emissions can be achieved:
1.Consuming less concrete by rehabilitating old buildings
2.Consuming less cement in concrete mixtures
3.Minimizing the quantity of cement in a concrete mix.


Functional Requirements:



A structure must always be designed to serve its intended function as specified by the owner and architect.

Constructability is a major part of the functional requirement and is also related to safety and durability.

During the planning and design stages, it is important and crucial to consider constructability, meaning, all the elements on the drawing board can be constructed practically.

One must not ignore the warning signals occurring during construction. These signals might be in the form of excessive deflections, vibrations, wrong construction practices, or the change in loading conditions during construction.

Job site visits by design engineers are crucial as they will confirm if the contractors are following the original details on the drawings.

Integrating Building information modelling (BIM) and constructability solves potential design issues and minimizes problems at the  construction site as construction knowledge is also utilized during the design process.  

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