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IS875 part III, Clarification.

 
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Aatif
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Joined: 26 Jul 2016
Posts: 14

PostPosted: Fri May 04, 2018 6:00 pm    Post subject: IS875 part III, Clarification. Reply with quote

Hello all sefians,

In calculation of design wind pressure, The pressure coefficient Cp which is used, after clause 6.2: pressure coefficients, in first of thrrt notes given reads "The pressure coefficients Cp given in different tables in the Codes are obtained from wind tunnel testing in conditions of relatively smooth flow.for rectangular buildings, values have been obtained to allow for turbulent flow".

Now I'm interested in knowing what were type of conditions in wind tunnel testing from where Cp was obtained.
By conditions I mean, velocity used in test, any wind incidence angle used, information regarding whether there was any inlet velocity profile maintained at inlet of tunnel, turbulent intensity used.

I need this information as part of my project where I'm comparing the wind load we get from codes with Wind loads we get from wind tunnel testing. By using Code Cp in former case and wind tunnel Cp in latter case calculations.

Any help on this is greatly appreciated.

Regards.
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sureshkumar_kumaresan
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Joined: 19 Nov 2012
Posts: 19

PostPosted: Mon May 07, 2018 8:22 am    Post subject: Re: IS875 part III, Clarification. Reply with quote

See if you are comparing Cp which is non-dimensionalized, then you do not need to know the wind tunnel speed. Further, in the code, the pressures are calculated based on mean Cp and gust dynamic pressure. The gust dynamic pressures is different on different terrains associated with turbulent intensities. So in my opinion, your wind tunnel results can be easily compared with the code calculated pressure. If you are comparing Cp's, understand the formulation of Cp's in the code, and change the measured pressures to this format for proper comparison.

On the other hand, if you are looking for the data on which the code Cp's are based, likely you are not going to find this.

Regards
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Aatif
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Joined: 26 Jul 2016
Posts: 14

PostPosted: Fri May 11, 2018 2:44 pm    Post subject: Reply with quote

Thank you for your response.

Sir I'm up to certain calculation of wind loads from Wind tunnel tests and also as per wind code, I wanted to get your views on that from the experience you have sir.
The following are the details regarding the type of wind tunnel we are having at our college.

-It's a subsonic wind tunnel.
- Max speed - 30m/s.
-No provisions for maintaining wind profile at upstream side in the wind tunnel. Just we have a honey comb mesh at inlet to make the flow uniform (laminar).
-We have used rigid building model. building shape is rectangular in plan(5cm*7.5cm*30cms , a 1:600 scaled model  of CAARC standard building 30m*45m*180 m) , No Openings in the model (Cpi=0) plane Exterior sirface (No curvature).
- we considered Reynolds number to Be insignificant.(as from literature for bluff bodies).
- The type of data that can be measured is Pressure heads in the manometers (Manual Recording of obervations) At different pressure ports on the Building. Cp is then calculated from those pressure heads as ratio of difference in Static pressure to dynamic Upstream pressure.

Now sir, from these details available, I intended to calculate wind loads on the building as per clause 7.3.1 of IA 875 part 3 (2015) from both from codes as well as from wind tunnel( in the same code equation, just changing Cp, for both the methods from code and Wind tunnel).

sir, Is My understanding and Approach Correct?

Also let me whether the meaurrmeasur I'm getting from wind tunnel in the form of Cp over surfave, Can it be used for calcuting design wind forces on building (As per above mentioned clause from code? Because I think the The data I'm getting from my wind Tunnel interms of surface pressure on cladding or surface, it's used only in design of facade and not in design of building (Because the pressure on cladding will varying and may be more than the load one overall Building).

Sir kindly let me know About these queries.
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