A Simplified Design Approach - Simplified Design of Reinforced Concrete Buildings - page 4

lightweight substitutes, which generally do not contribute significantly to building mass or stiffness.
In particular, the drastic increase of stresses in the reinforcement at service loads from less than 20 ksi to
nearly 40 ksi has caused a significantly wider spread of flexural cracking at service loads in slabs and beams,
with consequent increases in their deflections.

When structures were designed by the classical working stress approach, both strength and serviceability
of the structure were ensured by limiting the stresses in the concrete and the reinforcement, in addition to
imposing limits on slenderness ratios of the members. The introduction of strength design with the resulting
increase in member slenderness significantly lengthened the design process; in addition to designing for
strength, a separate consideration of serviceability (deflections and cracking) became necessary.

We are now frequently dealing with bolder, larger, taller structures, which are not only more complex, but also
more flexible. They are frequently for mixed use and as a result comprise more than one building and floor
system. Their structural behavior is characterized by larger deformations relative to member dimensions than
we had experienced in the past. As a consequence, a number of effects which heretofore were considered
secondary and could be neglected now become primary considerations during the design process. In this
category are changes in geometry of structures due to gravity and lateral loadings. The effects of shrinkage,
creep, and temperature are also becoming significant and can no longer be neglected in tall or long structures
because of their cumulative effects. Seismic codes continue to evolve and consolidate demanding consistent
risk assessment and demanding more aggressive design and detailing requirements.

Building and material codes are consensus documents written and edited by committees which can lead to
complications. Such committee is often hampered by the legal language these codes need in order to be adopted
as a law. This format restricts what can be said and how to say it, which results in a complicated document that
is not intended for easy reading and understanding.

1.4 A SIMPLE CODE

The more complex buildings undoubtedly require more complex design procedures to produce safe and
economical structures. However, when we look at the reality of the construction industry as discussed at the
beginning of this chapter, it makes little sense to impose on structures of moderate size and height intricate
design approaches that were developed to assure safety in high complex structures. While the advances of the
past decades have made it possible to build economical concrete structures soaring well over quarter mile in
height, the makeup of low-rise buildings has not changed all that significantly over the years.

It is possible to write a simplified code to be applicable to both moderately sized structures and large complex
structures. However, this would require a technical conservatism in proportioning of members. While the cost
of moderate structures would not be substantially affected by such an approach, the competitiveness of large
complex structures could be severely impaired. To avoid such unnecessary penalties, and at the same time to
stay within required safety limits, it is possible to extract from the complex code a simplified design approach
that can be applied to specifically defined moderately sized structures. Such structures are characterized as
having configurations and rigidity that eliminate sensitivity to secondary stresses and as having members
proportioned with sufficient conservatism to be able to simplify complex code provisions.