05-12-2012, 06:35 PM
Commentary on Design and Construction of Reinforced Concrete Chimneys (ACI 307-98)
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INTRODUCTION
As industry expanded in the years immediately following
World War I and as a result of the development of large pulverized
coal-fired boilers for the electric power generating
utilities in the 1920s, a number of rather large reinforced
concrete chimneys were constructed to accommodate these
new facilities. A group of interested engineers who foresaw
the potential need for many more such chimneys and who
were members of the American Concrete Institute decided to
embark upon an effort to develop a rational design criteria
for these structures. The group was organized into ACI Committee
505 (this committee was the predecessor of the
present Committee 307) to develop such criteria in the early
1930s.
Committee 505 submitted to the Institute a “Proposed
Standard Specification for the Design and Construction of
Reinforced Concrete Chimneys,” an outline of which was
published in the ACI JOURNAL, Proceedings V. 30, Mar.-
Apr. 1934. This specification was adopted as a tentative
standard in February 1936. Although this tentative standard
was never accepted by ACI as a regular standard, it was used
as the basis for the design of many chimneys. As these chimneys
aged, inspections revealed considerable cracking.
When the industrial expansion began following World War
II, other engineers recognized the need for developing an improved
design specification for reinforced chimneys.
Synopsis of current revisions
Revisions to the ASCE 7-95 standard relating to wind and
seismic forces required that several changes be made to the
1995 edition of ACI 307. The following highlights the
changes incorporated into the current standard:
· Site-specific wind loads are calculated using a “3-sec
gust” speed determined from Fig. 6-1 in ASCE 7-95 instead
of the previously used “fastest-mile” speed.
· Site-specific earthquake forces are calculated using the
effective peak velocity-related acceleration contours
determined from Contour Map 9-2 in ASCE 7-95 instead
of previously designated zonal intensity.
· The vertical load factor for along-wind forces has been
reduced from 1.7 to 1.3.
· The vertical load factor for seismic forces has been reduced
from 1.87 to 1.43.
Deflection criteria
The incorporation of the strength design method into the
standard will generally result in chimneys with thinner walls
in the lower portion and with higher deflections. The Committee
felt that deflections under service loads should be
checked and that the deflections of chimneys designed by the
strength method should not vary greatly from the deflections
of existing chimneys designed by the working stress method.
Limiting deflections also serves to reduce the effects of secondary
bending moments.
However, the procedures in the ACI 307 1988 edition
were found to be too restrictive for shorter chimneys and
were modified in the 1995 standard. The deflection limit is
compared against the deflection calculated using uncracked
concrete sections and a fixed base.