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[N. Prabhakar]

Dear Sefians,

Wind tunnel studies have been carried out in the past on the interference effects of tall buildings at the University of Roorkey, University of Notre Dame, USA, and at other testing centres, both on rigid and aeroelastic models. (Ref.: Proceedings of International Symposium on Experimantal Determination of Wind Loads on Civil Engineering Structures, Dec, 1990, New Delhi).

These studies show that an upstream interfering building causes shielding effect on a downstream building. In the case of rigid model studies, the shielding effect reduces the force and moment coefficients of the downstream building. But, in the case of aeroelastic models, due to fluid-structure interaction, the interefere effect causes generally enhanced dynamic motion of a principal building in both along-wind and across-wind directions. This means that slender tall buildings on the upstream side, with building dimension ratio of height to minimum lateral dimension greater than 5.0 or whose natural frequency of the buidling in the first mode is less than 1.0 Hz as defined in IS : 875 (Part 3) - 1987, are prone to higher along-wind pressures and also subjected to cross-wind motions. Since numerous situations can arise amongst the possible surroundings of a building, generalisation of results for the effect of interference is difficult. For the design of such buildings, the code gives references to few publications which are listed under Note 8 of Clause 7.1.

codal规定不能迎合的结构all shapes, forms, sizes and topography of the site. For taller buildings with unconventional shapes and a combination of structural systems, it may lead to coupled lateral and torsional motions. In such cases and for those structures which do not fall in the category of those covered in the code, it is necessary to have aeroelastic model studies in a wind tunnel. There are now more than a dozen wind-tunnel facilities in India, located mainly at IISc, IITs and SERCs. Sometimes, getting the wind-tunnel studies done from these institutions at a short notice, as is the demand in most cases of commercial ventures, is a problem.

In design, most attention is paid to the influence of wind on the strucural stength and safety of buildings. The subject of occupancy comfort against wind induced acceleration has received somewhat varied attention over the years. However, methods have been developed now to give a designer some indication of how to avoid high acceleration levels in tall buildings. It is worth mentioning that in the 60-storey, 279 m high "Citicorp Center" in New York, tuned mass damper weighing 373 tonnes is loacted on a floor at 242 m above ground level to increase the damping from 1 to 4 percent, thus reducing wind-induced accelerations.

With best wishes,

N. Prabhakar
Chartered Structural Engineer
Vasai (E)

[Dr. N. Subramanian]

Dear Er Prabhakar,

I agree with you on the importance of wind tunnel tests for irregular tall towers. Even in the case of regular tall buildings, wind tunnel tests should prove to result in economical designs. Such tests should include the surrounding structures also, so that wind interference could be studied.





Of course the Draft code IS 875-Part 3 available in NICEE web site also gives methods to calculate interference effects.

www.iitk.ac.in/nicee/IITK-GSDMA/W02.pdf

Regards
NS

[ystr]

Sir,

Send me any PPT considering eartquake related steel structarl beams and fire resisting steel beams.

Posted via Email

[cckeshav]

Dear SEFIans:

In computing/Estimating interference effects of wind an imporatant aspect should be kept in mind. The building being designed can be designed for any level of interference and the enhanced wind forces for the same. But, what about the building which has already been built and hitherto had no interference effects? The new building may cause an increase in the wind pressures on the existing building which can not be strengthened for this effect. As such, the gap between these highrise buildings should be such that there is virtually no effect on existing buildings.

C.Channakeshava

Subject: [E-CONF] Re: Effect of wind on adjoining structures
From:forum@www.bussiapp.com
Date: Thu, 29 Nov 2012 03:20:10 +0530
To:econf34289@www.bussiapp.com

Dear Er Prabhakar,

I agree with you on the importance of wind tunnel tests for irregular tall towers. Even in the case of regular tall buildings, wind tunnel tests should prove to result in economical designs. Such tests should include the surrounding structures also, so that wind interference could be studied.

[img]http://www.cadalyst.com/files/cadalyst/nodes/2007/3697/RWDI_Figure1.jpg[img]

Of course the Draft code IS 875-Part 3 available in NICEE web site also gives calculations to interference effects.

www.iitk.ac.in/nicee/IITK-GSDMA/W02.pdf

Regards
NS
N. Prabhakar wrote:Dear Sefians,

Wind tunnel studies have been carried out in the past on the interference effects of tall buildings at the University of Roorkey, University of Notre Dame, USA, and at other testing centres, both on rigid and aeroelastic models. (Ref.: Proceedings of International Symposium on Experimantal Determination of Wind Loads on Civil Engineering Structures, Dec, 1990, New Delhi).

These studies show that an upstream interfering building causes shielding effect on a downstream building. In the case of rigid model studies, the shielding effect reduces the force and moment coefficients of the downstream building. But, in the case of aeroelastic models, due to fluid-structure interaction, the interefere effect causes generally enhanced dynamic motion of a principal building in both along-wind and across-wind directions. This means that slender tall buildings on the upstream side, with building dimension ratio of height to minimum lateral dimension greater than 5.0 or whose natural frequency of the buidling in the first mode is less than 1.0 Hz as defined in IS : 875 (Part 3) - 1987, are prone to higher along-wind pressures and also subjected to cross-wind motions. Since numerous situations can arise amongst the possible surroundings of a building, generalisation of results for the effect of interference is difficult. For the design of such buildings, the code gives references to few publications which are listed under Note 8 of Clause 7.1.

codal规定不能迎合的结构all shapes, forms, sizes and topography of the site. For taller buildings with unconventional shapes and a combination of structural systems, it may lead to coupled lateral and torsional motions. In such cases and for those structures which do not fall in the category of those covered in the code, it is necessary to have aeroelastic model studies in a wind tunnel. There are now more than a dozen wind-tunnel facilities in India, located mainly at IISc, IITs and SERCs. Sometimes, getting the wind-tunnel studies done from these institutions at a short notice, as is the demand in most cases of commercial ventures, is a problem.

In design, most attention is paid to the influence of wind on the strucural stength and safety of buildings. The subject of occupancy comfort against wind induced acceleration has received somewhat varied attention over the years. However, methods have been developed now to give a designer some indication of how to avoid high acceleration levels in tall buildings. It is worth mentioning that in the 60-storey, 279 m high "Citicorp Center" in New York, tuned mass damper weighing 373 tonnes is loacted on a floor at 242 m above ground level to increase the damping from 1 to 4 percent, thus reducing wind-induced accelerations.

With best wishes,

N. Prabhakar
Chartered Structural Engineer
Vasai (E)

Posted via Email

[sureshkumar_kumaresan]

Hi All,

I thought of further discussing on this topic. The K factor in the code will account for the typical oncoming mean/turbulence profile of the wind. K factor for Cat 2 terrain is higher than Cat 3 terrain meaning peak wind in Cat 2 (more open) terrain is higher than Cat 3 (less open filled with subirban housing) terrain condition. This K factor doesn't cover the influence of immediate surroundings within 20times the building width of interest. Like in chimneys buildings do get buffetted/vortex shedding forces from upwind buildings at a particular distance. The effects will be severe when upstream building is at a distance of 3 to 7 building width. I do agree the buildings are typically heavy compared to chimneys and this will help in reducing the response, however the effect is still there. There are cases that I know the intereference effect induced double the load on a structure than one would expect.

None of the codes address this wake buffetting/interference issue. Probably generalizing the issue to come up with a recommendation for code is a tough task considering the numerous parameters involved. Note that the previous Australian wind code did include this in a simple form but the new version don't have this section.

Suresh

[swamikrishnan]

From the posts on wind excitation, I see a need for some clarity on the topic. I am going to try to briefly describe dynamic wind effects on tall buildings (you may find some of these points in other posts in the conference as well).

1. Wind loading is random in time. Can only be characterized in a stochastic manner. Wind speed is the critical parameter affecting this loading. Wind speed measurements during a storm or otherwise generally point to a mean value about which random fluctuations occur due to gusts. Mean wind speed increases with height above ground. Building codes usually specify the mean wind speed at a location and the associated pressures; the resulting quasi-static forces are amplified by a dynamic amplification factor, called the �gust response factor�.

2. Along-wind direction: In the along-wind direction, the building will experience buffeting, which can be defined as �repeated heavy blows� caused by the superposition of wind gusts on the background (constant) wind. The period at which the blows are delivered is called the gusting period. Gust periods are usually much longer (20s-70s) than building fundamental periods. As a result, the direct wind-loading spectrum has a peak at the gust period and decays smoothly for longer and shorter periods. Such loading will predominantly excite the fundamental mode of the building. So, generally speaking we don�t worry about higher mode effects when it comes to wind loading (unlike seismic loading where higher mode effects could be significant).

3.Across-wind方向:风方式round the building on either side of the building. As the flow goes past the two sides of the building, the streamlines separate from a substantial portion of the faces of the building; this leads to the formation of vortices. These vortices are shed at periodic intervals. The vortices, generated at the two (or more) leeward corners, impose alternating (periodic) suction forces on the building in the two directions orthogonal to the wind direction. This �dynamic� force has a predominant period equal (obviously) to the vortex-shedding period. The vortex shedding frequency (f = St*U/D) is dependent upon the Strouhal number St (which is dependent upon the shape of the building and the Reynolds number of the flow that characterizes the flow as turbulent or laminar), the across-wind dimension D of the building plan, and the wind speed U. Vortex shedding periods around buildings could be in the 5s-20s range, closer to tall building fundamental periods than gusting periods that could be in the 20s-70s range. Thus, for tall buildings the across-wind response may be far worse than the along-wind response (even twice or more as large). A vortex-shedding period close to the fundamental period of the building could lead to positive aeroelastic feedback (vortex lock-in) where the vibrational movement of the structure increases the aerodynamic load, i.e., the structural vibration leads to easier flow separation and aids in the formation of the vortices (flutter is an example).

4.流后在背风的一面building consists of a series of vortices lined up parallel to each other in a pattern known as the �Von Karman Street�. These vortices can remain coherent for significant distances in the wake of the flow behind a tall building if it has a uniform shape along its height and it is in a steady wind, i.e., one with less turbulence. As a result, these vortices can significantly impact nearby structures in less-dense environments, where the flow may not be as turbulent (in densely packed tall building environments, flow may be turbulent and the formation of vortices and the Karman Street can be impaired). In such cases, the vortices shed off a building can severely impact a neighboring building, especially if the vortex-shedding period off the first building happens to be close to the fundamental period of the neighboring building. While the first building may shield the second building from the along-wind forces, it can cause much greater across-wind forces. Thus, it is NOT always true that new tall buildings will lower wind forces on existing tall buildings. The new building can have shapes and dimensions that can result in vortex-shedding periods matching that of the existing building; this could lead to aeroelastic feedback in the existing building. One can think of several possibilities in this context that could result in greater wind loading in existing tall buildings as a result of new tall buildings cropping up in the neighborhood. Thus, in cities where tall buildings are just coming up, the possibility of the formation of Karman Streets (due to a future building some distance away) must be considered in the design.

5. Changing the cross-sectional shape of the building over its height can ensure that vortices are broken up and cannot be shed coherently over the entire height of the building, thus reducing across-wind loading. The Sears tower in Chicago and the Burj Khalifa in Dubai use this technique to great effect.

6. Three papers are must-reads if you want to understand wind loading:
http://www.annualreviews.org/doi/abs/10.1146/annurev.fl.16.010184.001211
http://www.cppwind.com/support/papers/papers/structural/240-2.pdf
http://www.math.lsa.umich.edu/~krasny/math654_irwin.pdf

Swaminathan Krishnan
California Institute of Technology
http://krishnan.caltech.edu

[baldevprajapati]

dear sir



here i attached my etabs file for design of 30storey building , in these file how to provide plate on the ISMB 600 section , please any buddy help to sole my these problem,my file more then 7 mb so file is not attached..so.. please give me email id
so i can send my file by email..

thank u

[thirumalaichettiar]