IS 875 Wind Code Options - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

Specifies options for the Indian Standard, IS-875 (Part 3), wind code. IS-875 is the standard in India for design loads (other than earthquake) for buildings and structures.

Basic Wind Speed

Basic wind speed as applicable to 10 m height above mean ground level for different zones in the country can be directly calculated if the proper value is defined in the Wind Zone Number box. Alternatively, wind speed can be defined here. Basic wind speed should be based on peak gust velocity averaged over a short time interval of about 3 seconds and correspond to mean heights above ground level in an open terrain. This box is optional.

Wind Zone Number

Figure 1 of IS-875 shows different Wind Zones of the country. Various zone numbers and corresponding Basic Wind Speed values are:

Zone 1

33 m/sec 73.82 miles/hour

Zone 2

39 m/sec 87.25 miles/hour

Zone 3

44 m/sec 98.43 miles/hour

Zone 4

47 m/sec 105.15 miles/hour

Zone 5

50 m/sec 111.86 miles/hour

Zone 6

55 m/sec 123.04 miles/hour

The value typed here must be between 1 and 6. The zone the vessel is in is determined from a map of India showing the various wind zones.

Optionally, you can define the basic wind speed directly in the Basic Wind Speed box. If the wind speed is specified, it overrides the value of wind speed and zone based on the table above.

Risk Factor (K1)

Assuming the mean probable design life as 100 years, the corresponding risk coefficient values for various wind zones are:

Zone

K1

Zone 1

1.05

Zone 2

1.06

Zone 3

1.07

Zone 4

1.07

Zone 5

1.08

Zone 6

1.08

Terrain Category

Terrain in which specific equipment is assessed as one of the following categories:

Category 1 - Exposed open terrain with few or no obstructions and in which the average height of any object surrounding the equipment is less than 1.5 m. This category includes open sea - coasts and flat treeless plains.

Category 2 - Open terrain with well scattered obstructions having height generally between 1.5 to 10 m. This includes airfields, open parklands and undeveloped sparsely built up outskirts of towns and suburbs. This category is commonly used for design purpose.

Category 3 - Terrain with numerous closely spaced obstructions having the size of buildings and structures up to 10 m in height. This includes well wooded areas, towns, and industrial areas full or partially developed.

Category 4 - Terrain with numerous tall, closely spaced obstructions. This includes large city centers, generally with obstructions above 25 m, and well developed industrial complexes.

Equipment Class

Equipment and structures are classified into following classes depending upon their size.

Class A - Equipment and components having a maximum dimension (greatest horizontal or vertical dimension) less than 20 m.

Class B - Equipment and components having a maximum dimension (greatest horizontal or vertical dimension) between 20 and 50 m.

Class C - Equipment and components having a maximum dimension (greatest horizontal or vertical dimension) greater than 50 m.

Topography Factor

The topography factor ranges between 1.0 and 1.36. This factor takes care of local topographic features such as hills, valleys, cliffs, ridges and so on, which can significantly affect wind speed in their vicinity. The effect of topography is to accelerate wind near summits of hills or crests of cliffs and decelerate the wind in valleys or near the foot of cliffs. Effect of topography is significant if upwind slope is greater than about 3 degrees. Below 3 degrees, the value of K3 can be taken as 1.0. For slopes above 3 degrees, the value of K3 ranges between 1.0 and 1.36.

Use Gust Response Factor

If this box is checked, the software calculates the gust response factor per IS-875 and uses it in the appropriate equations. Experience has shown that these gust response factors are very conservative. Select this box only if the design specifications and the customer or owner explicitly require you to do so.