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Wind Tunnel Testing

 
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sureshkumar_kumaresan
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Joined: 19 Nov 2012
Posts: 21

PostPosted: Tue Nov 20, 2012 11:56 pm    Post subject: Wind Tunnel Testing Reply with quote

Dear All,

Another topic of interest those who are working with tall buildings. I am working as a wind engineering consultant for the last 12 years and prior to this I did research in the wind engineering field for about 10 years. In my career as a wind engineer, I have used many approaches to calculate wind loads on any wind sensitive structures not just limited to tall buildings. The approaches that I have used are codes/standards, wind tunnel testing, analytical (stochastic) and full-scale measurements. I have to admit that out of all these approaches, wind tunnel testing approach becames the most sought out approach in the industry considering their accuracy and practicality.

There is always confusions with regard to when to carry out wind tunnel tests for tall buildings. The rule of hump is that one should be looking into this once the structure is above 35-40 floors and slenderness is above 5-6. The reasons for doing is multifold. The key reason is to obtain accurate loading on the structure and accelerations at the top of the building to evaluate human comfort. Without a wind tunnl test, there is no way of find realistic loading on the building unless the bulding is boxy and located without any immediate surrounding buildings. Mostly, the wind tunnel test will provide lower loads than an international code would provide. So here there will be huge saving for the owner. However, this is not the reason to do a tunnel test.

I welcome you to start discussing on this topic and I would be happy to share my experiences over here.

Thanks
Suresh
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Dr. N. Subramanian
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Joined: 21 Feb 2008
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Location: Gaithersburg, MD, U.S.A.

PostPosted: Wed Nov 21, 2012 6:08 pm    Post subject: Re: Wind Tunnel Testing Reply with quote

Dear Er Suresh,

You are absolutely right. In tall buildings we need modular plan and the elevation should be in such a way that it provides least resistance to wind loads. Of course, the current trend is to use the wind load acting on tall towers to run the turbines and generate the energy requirement of these tall buildings. See the link:
http://www.cppwind.com/support/papers/papers/windenergy/Building-Integrated_Turbines.pdf

In my opinion, Structural engineers have a higher role to play than architects in Tall buildings!

Best wishes,
NS
sureshkumar_kumaresan wrote:
Dear All,

Another topic of interest those who are working with tall buildings. I am working as a wind engineering consultant for the last 12 years and prior to this I did research in the wind engineering field for about 10 years. In my career as a wind engineer, I have used many approaches to calculate wind loads on any wind sensitive structures not just limited to tall buildings. The approaches that I have used are codes/standards, wind tunnel testing, analytical (stochastic) and full-scale measurements. I have to admit that out of all these approaches, wind tunnel testing approach becames the most sought out approach in the industry considering their accuracy and practicality.

There is always confusions with regard to when to carry out wind tunnel tests for tall buildings. The rule of hump is that one should be looking into this once the structure is above 35-40 floors and slenderness is above 5-6. The reasons for doing is multifold. The key reason is to obtain accurate loading on the structure and accelerations at the top of the building to evaluate human comfort. Without a wind tunnl test, there is no way of find realistic loading on the building unless the bulding is boxy and located without any immediate surrounding buildings. Mostly, the wind tunnel test will provide lower loads than an international code would provide. So here there will be huge saving for the owner. However, this is not the reason to do a tunnel test.

I welcome you to start discussing on this topic and I would be happy to share my experiences over here.

Thanks
Suresh
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sureshkumar_kumaresan
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Joined: 19 Nov 2012
Posts: 21

PostPosted: Thu Nov 22, 2012 10:45 pm    Post subject: Wind Tunnel Testing Reply with quote

Dear Prof,

In my opinion, there is a limit to structural engineer can play a role considering the restriction they do have in terms of the space utilization, framing patterns etc. When we are looking at tall/slender structures, the thinking for wind load optimization shall begin at the drawing board while suggesting a suitable geomtery. Otherwise, later in the process the retrofit could be an expensive proposition. In most of the existing tall buildings today, the geometry has been modified to attract less wind load.

Thanks for your comments.

Suresh
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Dr. N. Subramanian
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PostPosted: Thu Nov 29, 2012 11:27 pm    Post subject: Re: Wind Tunnel Testing Reply with quote


Wind Drift Design

In 1988 the ASCE Journal of Structural Engineering published a paper titled “Wind Drift Design of Steel-Framed Buildings: State-of-the-Art Report” (ASCE 1988). It was the culmination of four years of work by a task committee created by the ASCE Committee on Design of Steel Building Structures. The committee developed and conducted a survey to assess the state of the art of designing for drift of steel buildings. One-hundred and thirty-two firms were sent questionnaires and thirty-five responded. After the survey was conducted the committee published the above mentioned paper which summarizes the survey responses as well as opinions and comments provided by the task committee.

The objective of this project is to aid and assist in preparing a new survey to assess the current state of the art in wind drift design of steel buildings. This is necessary given the significant advances we have seen in the Field since the last survey, nearly twenty years ago, when the use of computers was just becoming commonplace:
- Powerful technological and computing resources have revolutionized the way structures are designed. Today including all sources of deformation, accurately modeling the panel zone regions, performing direct second order analysis and accounting for foundation flexibility are all within reach for the typical firm.

- Optimization techniques are commonly integrated in design software, enabling designers to achieve a level of structural performance and economy that was previously difficult if not impossible.
- Wind loading in building codes has changed as has the typical return interval used in design for serviceability issues of drift.
- The complex nature of wind effects on structures is better understood. Wind tunnel testing continues to advance while becoming more affordable.

It is extremely important to gain an overview of the ways in which these advances have affected the state of the art and to bring these changes, positive and negative, to the attention of the community. Since serviceability limit states are not codified the way they are handled may differ considerably from firm to firm; these differences will be brought to light. Through the use of the internet as a distribution tool and the assurance of anonymity, the survey is expected to receive many more respondents than the first survey (thirty-five responses). This will provide for a more complete and accurate assessment of the art.

Read More: http://ascelibrary.org/doi/abs/10.1061/41171%28401%2945
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Dr. N. Subramanian
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PostPosted: Thu Nov 29, 2012 11:30 pm    Post subject: Re:A Case Study Reply with quote

I wish to present an interesting case study of Boston’s John Hancock Tower.

In early 1973 the John Hancock tower experienced 75 mph winds that were believed to cause over 65,000mpounds of double plane windows to crash to the sidewalks below. Due to an agreement between the involved parties nobody knows the exact reason why the windows failed, although it is widely speculated that the problems were due to a window design defect . In addition to the cladding issues the Tower swayed excessively in moderate winds, causing discomfort for occupants of the upper floors.

The unacceptable motion was solved by installing two 300 ton tuned mass dampers, which had just been invented for the Citicorp Tower in New York (LeMessurier 1993). Additional lateral
bracing was also added in the central core (at cost of $5 million) after it was determined that the building was susceptible to failure under heavy winds ( Sutro 2000).

It is interesting to note that prior to construction of the John Hancock Tower, wind tunnel tests on the design were conducted in a less expensive aeronautical wind tunnel, as opposed to a boundary layer wind tunnel, and the results did not indicate any problems.
The importance of modeling for the boundary layer, in which terrain, gustiness and surrounding structures all come into play, was suddenly obvious; the overall behavior and interaction of wind and structures was becoming apparent to the structural engineering
community. With proper wind tunnel testing, the John Hancock Tower may have avoided costly retrofitting.

References:

LeMessurier, W. (1993). “Breaking barriers.” Modern Steel Construction. Vol. 33, No. 9, pp. 26-33.
Sutro, D. (2000). “Into the Tunnel.” Civil Engineering Magazine, June 2000
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sureshkumar_kumaresan
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PostPosted: Sun Dec 02, 2012 3:36 pm    Post subject: Wind Tunnel Testing Reply with quote

Dear Er. NS,

Nice to see your thought provoking posting in the wind engineering area. Wind tunnel testing is so advanced today in comparison to few decades ago when mostly uniform tunnels (aernautical) have  been used for testing earth bound structures. The importance of boundary layer effects were revealed and there come the evolution of boundary layer long wind tunnels. Now we are looking at simulating different wind climate such as thunderstorms, tornadoes. Regardless of all of this efforts, like in any other science, wind tunnel testing is a tool to validate/investigate our our expectations on the wind loading/response/effects. One should be extremely careful about the simulations to get accurate model-scale data of the reality.

I think the most uncertainty is on the wind speed and directionality that we use to convert the model-scale values to prototype. We depend on the measurements at the airports for this and based on my little experience in India, this data is not complete and not measured properly.  More work in terms of measurements and simulations are required to overcome this issue.

Suresh
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Dr. N. Subramanian
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PostPosted: Sun Dec 02, 2012 5:42 pm    Post subject: Re: Wind Tunnel Testing Reply with quote

Dear Er Suresh,

Thank you for your updates on this issue. I feel that more and more tall buildings and towers havebto be instrumented in India also to get the real picture.

Due to (perhaps ) global warming, the wind speeds of tornadoes, hurricanes, etc are increasing and we need to modify the codes based on this.

Another issue that needs immediate attention is the flooding of coastlines due to the alarming snow and ice melting in the arctic regions, though it is not an issue in this e-conference.

Wish you all the best in your work in wind engineering.

Regards,
NS

sureshkumar_kumaresan wrote:
Dear Er. NS,

Nice to see your thought provoking posting in the wind engineering area. Wind tunnel testing is so advanced today in comparison to few decades ago when mostly uniform tunnels (aernautical) have  been used for testing earth bound structures. The importance of boundary layer effects were revealed and there come the evolution of boundary layer long wind tunnels. Now we are looking at simulating different wind climate such as thunderstorms, tornadoes. Regardless of all of this efforts, like in any other science, wind tunnel testing is a tool to validate/investigate our our expectations on the wind loading/response/effects. One should be extremely careful about the simulations to get accurate model-scale data of the reality.

I think the most uncertainty is on the wind speed and directionality that we use to convert the model-scale values to prototype. We depend on the measurements at the airports for this and based on my little experience in India, this data is not complete and not measured properly.  More work in terms of measurements and simulations are required to overcome this issue.

Suresh
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