Sir,
When do we have to consider offshore wind velocity factor while calculating wind loads as per IS 875-part3 -2015

Question

Sir,&#010;When do we have to consider offshore wind velocity factor while calculating wind loads as per IS 875-part3 -2015

premjit · Accepted Answer

It is important to interpret all the codes in a logical manner.

IS 875 Part 3 -2015 has introduced a new factor called k4 which is a cyclonic factor.

This k4 is applicable only in cyclonic regions in India.The code clearly says that it is applicable in the east coast and the Gujrat coast.

The effect of cyclonic storms is largely felt in a belt of approximately 60 km width at the coast.This is all mentioned in the wind load code clause 6.3.4 in IS 875 Paer3 : 2015.The cyclonic effect needs to be considered 60km in to the land.

The k4 values are as below

Structures of post-cyclone importance for emergency services (such as cyclone shelters, hospitals, schools, communication towers, etc) - 1.30

Industrial structures -1.15

All other structures 1.00

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The exact question you have asked is about cl.6.6 which talks about cyclonic effect.

This needs to be logicaly interpreted.

The influence of wind speed off the coast up to a distance of about 200 km may be taken as 1.15 times the value on the nearest coast. The code also states that ''The factor 1.15 shall be used in addition to k4''.

This means that k4 x 1.15 is the value.

What needs to be carefully understood is that this 200km that code mentions is ''in to the sea'' and that is the reason that the wind is more.The 60km that code talks about is in to the land.

To clarify it even more, the origin of the wind is some where deep in the sea where the speed is much higher. As it nears the land, it reduces from the maximum. That is the reason a very high value is considered in the sea and only k4 in land and 60km in to the land. The title of the cl.6.6 also clearly says ''offshore structure''which are structures in the sea. An example can be some thing that is seen in the figure in this post or any oil rig or similar.

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shefalivthakkar · Answer

So for structures 5km away from land , what should we consider 1.15 or less?

premjit · Answer

@shefalivthakkar As mentioned in the above post aswell as in the code, the value is

K4 *1.15 up to 200km in to the sea. Logically thinking it can be less as it is more near to the land but then code has not mentioned any thing about any interpolation. You as an engineer can take a discretionary decision or you can get the actual wind data if available and design for it.

Sir, When do we have to consider offshore wind velocity factor while calculating wind loads as per IS 875-part3 -2015

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