A Simplified Design Approach - Simplified Design of Reinforced Concrete Buildings - page 3

Considering the responsibility of the engineer to provide a safe and cost-effective solution to the needs of the
building occupant and owner, it becomes clear that, for the vast majority of buildings designed each year, there
should be an extra effort made to provide for expediency of construction rather than efficiency of structural
design. Often, the extra time needed to prepare the most efficient designs with respect to structural materials
is not justified by building cost or performance improvements for low-rise buildings.

1.3 THE COMPLEX CODE

In 1956 the ACI 318 Code was printed in 73 small-size pages; by 2011, ACI 318 and Commentary contained
more than 500 large-size pages of Code and Commentary—a very substantial increase in the amount of
printed material with which an engineer has to become familiar in order to design a concrete building.
Furthermore, the code revision in 2002 has seen the most significant technical revisions since 1963. Several
new and sweeping changes were introduced in the load and resistance factors, unified design provisions for
reinforced and prestressed concrete design were introduced. For the first time, a new appendix on anchorage
to concrete is provided along with another appendix on the strut-and-tie modeling and design techniques.

To find the reasons for the proliferation in code design requirements since 1956, it is useful to examine the extensive
changes in the makeup of some of the buildings that required and prompted more complex code provisions.

1.3.1 Complex Structures Require Complex Designs

Advances in the technology of structural materials, new building systems, and new engineering procedures
have resulted in the use of concrete in a new generation of more flexible structures, dramatically different from
those for which the old codes were developed.

In the fifties, 3000 psi concrete was the standard in the construction industry. Today, concrete with 12,000 psi
to 16,000 psi and higher strength is used for lower story columns and walls of very tall high-rise
buildings. Grade 40 reinforcing steel has almost entirely been replaced by Grade 60 reinforcement. Today the
elastic modulus of concrete plays an equally important role to compressive strength in building deflection
calculations and serviceability checks.

Gradual switching in the 1963 and 1971 Codes from the Working Stress Design Method to the Strength Design
Method permitted more efficient designs of the structural components of buildings. The size of structural
sections (columns, beams and slabs) became substantially smaller and utilized less reinforcement, resulting in
a 20 to 25% reduction in structural frame costs. In 2002 the working stress design method, long in Appendix A as
an alternate design method was deleted.

While we have seen dramatic increases in strength of materials and greater cost efficiencies and design inno-
vations made possible by the use of strength design method, we have, as a consequence, also created new and
more complex problems. The downsizing of structural components has reduced overall building stiffness.
A further reduction has resulted from the replacement of heavy exterior cladding and interior partitions with