The Structural Engineer's Corner

Eng. Onorio Francesco Salvatore

The Dynamic Response Factor as per Australian Standard 1170.2

Written By: Lexatus - Sep• 14•13

First-mode Onorio

In order to evaluate the design wind pressure on a structure, the basic pressure has to be multiplied by an Aerodynamic Shape Factor (as per Section 5 of Australian Standard AS 1170.2) and by a Dynamic Response Factor (as per Section 6 of Australian Standard AS 1170.2).

The Standard specifies that the value of the Dynamic Response Factor Cdyn is 1.0 except where the structure is dynamically wind sensitive. This coefficient is variable with the natural first-mode fundamental frequency. The cases are:

Greater than 1 Hz => Cdyn = 1.0;

Less than 1 Hz => Cdyn to be calculated.

When the natural first-mode fundamental frequency is less than 1 Hz, we need to make a distinction between:

  • tall buildings and freestanding towers;
    • less than 0.2 Hz is not covered by the standard and detailed analysis are needed;
    • between 1 Hz and 0.2 Hz, Cdyn shall be defined as per Clause 6.2 for along-wind response and Clause 6.3 for cross-wind response;
    • where the frequencies of vibration for the two fundamental modes of sway are within 10% of each other and are both less than 0.4 Hz, this is not covered by the Standard and requires detailed analysis.
  • cantilever roofs;
    • less than 0.5 Hz is not covered by the Standard and requires detailed analysis;
    • between 1 Hz and 0.5 Hz, Cdyn shall be as defined in Paragraph D5, Appendix D.

The detailed analysis required are, for example, the wind-tunnel testing.

In the next post, an Excel tool to evaluate the along-wind response.

For any issues or questions, you can contact the author at:

Eng. Onorio Francesco Salvatore

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