IBC/UBC 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 UBC and IBC wind codes.

Specifies the design value of the wind speed. This varies according to geographical location and according to company or vendor standards. Typical wind speeds in miles per hour are 85.0, 100.0, 110.0, and 120.0.

Typical units (depending on your unit system):

  • ft/sec

  • m/sec

  • mm/sec

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.

Exposure Constant

Specifies the exposure factor, as defined in UBC-91 Section 2312 or the IBC Exposure Constant:

Exposure B - Terrain with building, forest or surface irregularities 20 feet or more in height covering at least 20 percent or the area extending one mile or more from the site.

Exposure C - Terrain which is flat and generally open, extending one-half mile or more from the site in any full quadrant.

Exposure D - The most severe exposure with basic wind speeds of 80 mph or more. Terrain which is flat and unobstructed facing large bodies of water over one mile or more in width relative to any quadrant of the building site. This exposure extends inland from the shoreline 1/4 mile or 0 times the building (vessel) height, whichever is greater.

Most petrochemical sites use a value of 3, exposure C. This value is used to set the Gust Factor Coefficient (Ce) found in Table 23-G.

Importance Factor (IBC)

This is the Occupancy Importance Factor, IE, as defined in Section 1616.2 and shown in Table 1604.5. The calculated spectrum accelerations will be multiplied by this value to generate the shock spectra. Values range from 1.0 to 1.25 based on the function of the structure.

Importance Factor (UBC)

Specifies the importance factor. The software uses this value directly without modification. This value is taken from Table 23-L of the UBC standard. Followings are the context of Table 23-L:

Category

Value

I - Essential facilities

1.15

II - Hazardous facilities

1.15

III - Special occupancy structures

1.00

IV - Standard occupancy structures

1.00

Height of Hill (IBC)

Specifies the height of hill or escarpment value. This value is the height relative to the upwind terrain. It is used in calculations of the topographic factor of structures sited on the upper half of hills and ridges or near the edges of escarpments.

Typical units (depending on your unit system):

  • ft

  • m

mm

Distance to Site (IBC)

Specifies the distance upwind or downwind from the crest to the building site.

Typical units (depending on your unit system):

  • ft

  • m

mm

Crest Distance

Specifies the distance upwind of crest to where the difference in ground elevation is half the height of hill or escarpment.

Typical units (depending on your unit system):

  • ft

  • m

mm

Hill Type

Specifies the hill type. This value is the hill type is defined as follows:

0 - No Hill

1 - 2D Ridge

2 - 2D Escarpment

3 - 3D Axisymmetric Hill

Structure Damping Coeff. (IBC)/Beta (UBC)

Specifies the structural damping coefficient. Type the value of structural damping coefficient (percentage of critical damping) beta. The default value is 0.01.

This value is used to compute the dynamic gust effect factor G as outlined in the commentary section 6.6 page 158 of ASCE 95 or section 6.5.8 pages 29-30 of ASCE 98. For other design codes, then the software uses the damping coefficient for that wind design code.

If your design specification does not call out for a specific value of beta, then use the default value of 0.01. Other values of beta can be specified for the filled case and the empty case. Again, if the specifications do not supply these values for empty and filled, then leave these cells blank.

Natural Frequency (IBC)

Specifies the natural frequency used to calculate the dynamic wind gust effect factor for dynamically sensitive structures. (f < 1 Hz.)

If the natural frequency is not defined, CAESAR II uses a gust effect factor 0.85.