BS-6399-2 Wind Code Options - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II Version
12

Specifies options for the BS-6399-2 British Wind Code.

Design Wind Speed

Specifies the design value of the wind speed. These vary according to geographical location and according to company or vendor standards. Here are a few typical wind speeds in miles per hour. Typical wind speeds are shown in Figure 6 of BS-6399-2. The wind speeds are only relevant to the United Kingdom. The wind speeds vary typically from 20 m/sec to 31 m/sec (44.7 mph to 69.3 mph).

Type the lowest value reasonably allowed by the standards you are following, because the wind design pressure (and thus force) increases as the square of the speed.

Site Elevation - delta s

Enter the site altitude above mean sea level (paragraph 2.2.2.2 of the code). Use this value plus the Base Elevation to calculate the height of each point in the vessel above mean sea level. For example, if the vessel is installed on a site that is 100 m (328 ft) above sea level, it is exposed to a higher wind pressure (P) than if installed on the beach (at mean sea level).

Upwind Building Height (Obstruction Height) - Ho

For buildings in town terrain, type the average height of the building upwind of the piping (as they tend to shield the piping from the wind). To be conservative, this value can be zero, so the piping takes the full force of the wind. Ho is used to modify the effective piping wind height (He) for any piping element. See paragraph 1.7.3.3 of BS-6399-2.

Upwind Building Spacing - X

For buildings in town terrain, type the average spacing of the buildings upwind of the piping (as they tend to shield the piping from the wind). If the buildings are closer together, they provide greater protection from the wind. See paragraph 1.7.3.3 of BS-6399-2.

Pipe Location

Specifies the location where the system is installed, either in the country, or in a town. The BS-6399-2 factors in Table 4 modify the wind velocity. The final wind pressure acting on any element of the piping is determined by the distance from the coast, whether located in the country or a town, and the effective height (He). This table derives Sb, which is calculated by internally.

Distance to Coastline

Specifies the distance the vessel is located from the coast in kilometers. This distance affects the corrected wind speed (Ve). The BS-6399-2 factors in Table 4 modify the wind velocity. The final wind pressure acting on any element of the vessel is determined by the distance from the coast, whether located in the country or a town, and the effective height (He). This table derives Sb, which is calculated by internally.

Factor Kb from Table 1 - Kb

Specifies the 'Building-type factor Kb' taken from Table 1 of BS6399. Choose from one of five values: 8, 4, 2, 1 or 0.5. CAESAR II sets the default to 2, but any other value may be chosen. Please note the following limitations of Kb based on the vessel height:

Kb

Maximum Vessel Total Height

8

23 m (75.4 ft)

4

75 m (246 ft)

2

240 m (787 ft)

1

300 m (984 ft)

0.5

300 m (984 ft)

Designing towers over 75 meters in height is unlikely and you would need to consider many other things.

BS 6399 Table 1. Building-type Factor Kb

8

Welded Steel unclad frames

4

Bolted steel and reinforced concrete unclad frames

2

Portal sheds and similar light structures with few internal walls

1

Framed buildings with structural walls around lifts and stairs only (e.g. office buildings of open plan or with partitioning)

0.5

Framed buildings with structural walls around lifts and stairs with additional masonry subdivision walls (for example, apartment buildings), building of masonry construction and timber-framed housing

Annual Probability Factor - Q

Calculates the final probability factor (Sp) associated with the likelihood of high velocity gusts occurring over certain periods such as 50 years. The default value is Q = 0.02. The code sets 0.02 as a standard value for a mean recurrence value of 50 years. Annex D of BS-6399-2 should be consulted for a fuller explanation.

Q

Explanation

0.632

NOTE 1: The annual mode, corresponding to the most likely annual maximum value. (Sp = 0.749)

0.227

NOTE 2: For the serviceability limit, assuming the partial factor for loads for the ultimate limit is f = 1.4 and for the serviceability limit is f = 1.0, giving Sp = Sqrt(1 / 1.4) = 0.845. (Sp = 0.845)

0.02

NOTE 3: The standard design value, corresponding to a mean recurrence interval of 50 years. (Sp = 1.000)

0.0083

NOTE 4: The design risk for bridges, corresponding to a mean recurrence interval of 50 years. (Sp = 1.048)

0.00574

NOTE 5: The annual risk corresponding to the standard partial factor for loads, corresponding to a mean recurrence interval 1754 years. This is back-calculated assuming the partial factor load for the ultimate limit is ?f = 1.4 and all risk is ascribed to the recurrence of wind. (Sp = Sqrt(1.4))

0.001

NOTE 6: The design risk for nuclear installations, corresponding to a mean recurrence interval of 10,000 years. (Sp = 1.263)

Seasonal Factor - Ss

BS6399 in paragraph 2.2.2.4 states: "...For permanent buildings and buildings exposed for continuous periods of more than 6 months a value of 1.0 should be used for Ss..." PV Elite uses 1.0 as the default value for this reason. Using a value of less than 1.0 is not recommended or should only be used with solid research.

Directional Factor - Sd

Taken from Table 3 of BS6399. Because a tower is symmetrical about its central axis, the default value has been taken as 1.0. It is recommended that this value not be reduced other than for exceptional circumstances. For other values, please consult Table 3. The values in that table range between 0.73 and 1.00.

Pipe Surface Type

Specifies the pipe surface condition. The three options are: 1 Smooth, 2 Rough, and 3 Very Rough.

Total Wind Height

Specifies the total height of the building or structure, which CAESAR II uses in wind force equations. For piping systems, consider using the maximum height of the piping system. Refer to the wind code standard for more information on this value.