Frequency for Wind tunnel
Frequency for Wind tunnel
(OP)
thread507-242316: Dynamic Properties - Wind Tunnel
Dear Experts,
Could someone shed light on the matter that the time period to be sent for wind tunnel should be based on uncracked (SLS) structural frequency or based on cracked section (ULS) to determine the Wind forces.
If it should be uncracked (SLS), is there any reference other than the ACI 318 commentary R10.11.1 which hints of using SLS?
What is the logic of using uncracked Time period for estimating Wind forces from Wind tunnel since we know greater time period will yield greater wind forces.
Dear Experts,
Could someone shed light on the matter that the time period to be sent for wind tunnel should be based on uncracked (SLS) structural frequency or based on cracked section (ULS) to determine the Wind forces.
If it should be uncracked (SLS), is there any reference other than the ACI 318 commentary R10.11.1 which hints of using SLS?
What is the logic of using uncracked Time period for estimating Wind forces from Wind tunnel since we know greater time period will yield greater wind forces.






RE: Frequency for Wind tunnel
As an aside I asked them if they have any good data about actual vs reported periods. He told me that they only really get that on buildings where they are doing supplemental damping and need to verify/calibrate it. On those he said that the estimated/analysis period is not that close to what they see.
RE: Frequency for Wind tunnel
There is a paper on this. Please see link below for the paper. Refer to Table 2 of the paper, there is a recommendation of structural properties for various limit state.
We will provide the wind tunnel specialist with 2 set of dynamic properties.
For Service - Partially Cracked (SLS), 25 Years Return Period, Damping about 1% to 2%, Applicable for drift and acceleration check
For ULS - Fully Cracked, 50 Years Return Period, Damping about 3%, To be combined with code defined combinations for ULS design.
Link: http://www.cppwind.com/wp-content/uploads/2014/03/...
RE: Frequency for Wind tunnel
You mean that the Wind tunnel guys asked for ULS periods?
I think the practice is to provide SLS dynamic properties for Wind tunnel. But i wanted to ask for reference and the logic behind using SLS properties.
The posts in the thread "thread507-242316: Dynamic Properties - Wind Tunnel: Dynamic Properties - Wind Tunnel" do not provide convincing logic.
Could you please share your ideas?
RE: Frequency for Wind tunnel
Our wind tunnel report includes the forces corresponding to the SLS dynamic properties (uncracked frequency) along with forces corresponding to 5% higher and 5% lower than the SLS frequency.
Our SLS model loads are ok as it is not changed during detailed design, however the ULS model shows frequency much lower than the frequency provided for Wind tunnel. It's about 10% lower than the SLS frequency.
Could someone shed some light that forces for ULS calculations be based on what frequency.
RE: Frequency for Wind tunnel
When I say service vs strength that's not necessarily uncracked vs cracked, depending on your building you may still have cracking at service level - so it might be cracked vs more cracked.
RE: Frequency for Wind tunnel
Then what is the intent of ACI 318 commentary R10.11.1, if Wind tunnel forces should be based seperately for SLS and ULS.
ACI 318 commentary R10.11.1:-
"Section 10.10 provides requirements for strength and assumes frame analyses will be carried out using factored loads. Analyses of deflections, vibrations, and building periods are needed at various service (unfactored) load levels to determine the serviceability of the structure and to estimate the wind forces in wind tunnel laboratories."
RE: Frequency for Wind tunnel
It is usually considered that the commentary is not code.
Keeping in view the responses from wind tunnel experts I came to know that the ACI commentary R10.11.1 is outdated and removed in latest version ACI 318-14.
It turn out that it is usual practice to provide one set of frequencies for wind tunnel test. This can be based on SLS frequency.
The SLS wind tunnel analysis (forces, acceleration etc) should be based on SLS frequency. However it should be requested to the wind tunnel testing company to provide sensitivity analysis for the variation of frequency and forces. Based on this the ULS model forces must be determined corresponding to the ULS frequency (Wind speed and damping).
RE: Frequency for Wind tunnel