Comparison of Concrete Chimneys designed to ACI 307-88 and CICIND Model Code
N.R. Bierrum
CICIND REPORT Vol. 11, No. 2, 1995
Order No: CR-039
The author reports that, when comparing designs of concrete chimneys carried out to the CICIND Model Code and ACI 307-88, he invariably found significant differences. He shows comparisons for 10 chimneys, varying in height between 80m and 250m, the comparison being in the adjustment necessary to the wind load factor in order to make the CICIND design total moment (Mw + M2) equivalent to that calculated using ACI 307-88. In all cases the adjusted load factor is less than required by CICIND, indicating that the CICIND Model Code is more conservative for wind moments than ACI 307-88. The major reason for the difference appears to lie in CICIND's consideration of second order effects and the use of the "Gliding Material Law". The differences become even wider if openings are considered and/or thermal effects are combined with wind moments.
In some cases (the larger chimneys with steel liners) chimneys designed to the CICIND Model Code do not meet the cross-wind requirements of ACI 307-88. It is possible that the latest version of ACI 307 will reduce these requirements, but the requirements of the CICIND Model Code should nevertheless be improved in this respect.