National Building Code of Canada NBCC 1995 Wind Load Provisions

The reader may be wondering why, after an arguably extensive coverage of the ASCE 7-02 wind load provisions, the author would burden the text with yet another building code provision. The reason is simple: Although extensive in its treatment of wind, the ASCE 7-02 does not provide an analytical procedure for estimating across-wind response of tall, flexible buildings. To the best of the author's knowledge, NBCC is the only code in North America that presents an analytical method for computing across-wind response. It is perhaps the most comprehensive standard for wind because it takes into consideration characteristics such as building dimensions, shape, stiffness, damping ratios, site topography, climatology, boundary layer meteorology, bluff body aerodynamics, and probability theory.

Figure 1.13b. Exposure B; urban area with numerous closely spaced buildings the size of single-family homes or larger.
Windbelasting Gebouw

Figure 1.13c. Exposure B; urban area with numerous closely spaced buildings the size of single-family homes or larger.

Figure 1.13d. Structure in the foreground is located in exposure B. Structures in the rear, adjacent to the clearing, are located in exposure C when wind flows from the left over the clearing.
Bluff Body Aerodynamics
Figure 1.13e. Structures on a shoreline with wind flowing over open water for a distance of at least one mile are located in exposure D.

Three different approaches for determining wind loads on buildings are given: 1) simple procedure; 2) experimental procedure; and 3) detailed procedure.

1.4.3.1. Simple Procedure

The simple procedure is applicable for determining structural wind loads for a majority of low- and medium-rise buildings and also for cladding design of low-, medium-, and high-rise buildings. The method is similar to other code approaches in which the dynamics action of wind is dealt with by equivalent static loads defined independently of the dynamic properties of wind.

The recurrence intervals used for evaluating wind loads are

1. 1 in 10 years for the design of cladding and structural members designed for deflection and vibration limits.

2. 1 in 30 years for the design of structural members of all, except post-disaster buildings, for strength.

3. 1 in 100 years for the design of structural members of post-disaster buildings for strength.

The external pressure or suction on the building surface is given by the equation P = qCeCgCp (1.50)

where

P = design static pressure or suction, acting normal to the surface: kilo pascals q = reference velocity pressure; kilo pascals

Ce = exposure factor that reflects the changes in wind speed with height and variations in the surrounding terrain: dimensionless

Cg = gust factor, with a value of 2.0 for the primary structural system, and 2.50 for cladding: dimensionless

Cp = external pressure coefficient averaged over the area of the surface considered: dimensionless

Reference Pressure q. The reference velocity pressure q, in kilo pascals, is determined from referenced wind speed V by the equation:

The factor C depends on the atmospheric pressure and air temperature. If the wind speed V is in meters per second, the design pressure, in kilo pascals, is obtained by using a value of C = 650 x 10-6. The reference wind pressure q, is given for three different levels of probability being exceeded per year (1/10, 1/30, and 1/100), that is, for return periods for 10, 30, and 100 years, respectively. A 10-year recurrence pressure is used for the design of cladding an for the serviceability check of structural members for deflection and vibration. A 30-year wind pressure is used for the strength design of structural members of all buildings except those classified as post-disaster buildings. A 100-year wind is used for the design of post-disaster buildings such as hospitals, fire stations, etc. The 10-, 30-, and 100-year mean hourly wind pressures in Montreal, Quebec are 0.31 kPa (6.5 psf), 0.37 kPa (7.72 psf), and 0.44 kPa (9.2 psf), respectively, with corresponding wind speeds of 22 m/ s (49.2 mph), 24 m/s (54 mph), and 26 m/s (58 mph).

Exposure Factor Ce. The exposure factor Ce is based on the 1/5 power law corresponding to wind gust pressures in open terrain. An averaging period of 3 to 5 seconds is used in determining the gust factor. It represents a 'parcel' of wind assumed to be effective over the entire building. For tall buildings, the reference height for pressures on the windward face corresponds to the actual height aboveground, and for suctions on the leeward face, the reference height is half the height of the structure.

The exposure factor Ce reflects the changes in wind speed and height, and the effects of variations in the surrounding terrain and topography. Hills and escarpments that can significantly amplify wind speeds are reflected in the exposure factor.

The exposure factor Ce may be obtained from any of the following three methods:

1. The value shown in Table 1.

2. The value of the function (h/10)5 but not less than 0.9, where h is the reference height above grade, in meters.

3. If a dynamic approach is used, an appropriate value depending on both the height and shielding.

Gust Effect Factor (Dynamic Response Factor) Cg. This factor accounts for the increase in the mean wind loads due to the following factors:

• Random wind gusts acting for short durations over entire or part of structure.

• Fluctuating pressures induced in the wake of a structure, including vortex shedding forces.

• Fluctuating forces induced by the motion of a structure.

Pressure Coefficient Wind Buildings
Figure 1.14. External wind pressure coefficient Cp; flat-roofed buildings H > W (Adapted from NBCC 1995.)

All buildings are affected to some degree by their dynamic response. The total response may be considered as a summation of the mean component without any structural dynamic magnification, and a resonant component due to building vibrations close to its natural frequency. For the majority of buildings less than 120 m (394 ft) tall, and with height-to-width ratio less than 4, the resonant component is small. The only added loading is due to gusts that can be dealt with in a simple static manner.

For buildings and components that are not particularly tall, long, slender, lightweight, flexible, or lightly damped, a simplified set of dynamic gust factors is given as follows:

Cg = 2.5 for building components and cladding

Cg = 2.0 for the primary structural system including anchorages to foundation

Pressure coefficient Cp. Cp is a nondimensional ratio of wind-induced pressure on a building to the velocity pressure of the wind speed at the reference height (see Fig. 1.14). It depends on the shape of the building, wind direction, and profile of the wind velocity, and can be determined most reliably from wind-tunnel tests. However, for the simple procedure, based on some limited measurements on full-scale buildings supplemented by wind-tunnel tests, NBC gives the following values of Cp for simple building shapes:

Windward wall: Cp = + 0.8 (positive pressure)

Reference height = Z aboveground

Side wall and roof: Cp = -1.0 (negative pressure, suction) Reference height = H aboveground

Leeward wall: Cp = -0.5 (negative pressure, suction) Reference pressure = 0.5H aboveground

1.4.3.2. Experimental Procedure

The second approach is to use the results of wind-tunnel or other experimental procedures for buildings likely to be susceptible to wind-induced vibrations. Included in this category are tall, slender structures for which wind loading plays a major role in the structural design. A wind-tunnel test is also recommended for determining exterior pressure coefficients for cladding design of buildings whose geometry deviates markedly from more common shapes for which information is already available.

1.4.3.3. Detailed Procedure

In this method, a series of calculations is performed to determine more accurate values for the gust factor Cg, the exposure factor Ce, and the pressure coefficient Cp. The end product of the calculations yields a static design pressure, which is expected to produce the same peak effect as the actual turbulent wind, with due consideration for building properties such as height, width, natural frequency of vibration, and damping. This approach is primarily for determining the overall wind loading and response of tall slender structures, and is not intended for determining exterior pressure coefficients for cladding design.

The code gives procedures for calculating the dynamic effects of vortex shedding for slender cylindrical towers and for tapered structures. Since the available data are limited for slender structures with cross sections other than circular, wind-tunnel tests are recommended for estimating the likely response. To limit the cracking of masonry and interior finishes, the total drift per story under specified wind and gravity loads is limited to 1/500 of the story height, unless a detailed analysis is made and precautions taken to permit larger movements.

The code recognizes that maximum accelerations of a building leading to possible human perception of motion or discomfort may occur in a direction perpendicular to the wind. A tentative acceleration limit of 1 to 3% of gravity for a 10-year return wind is recommended to limit the possibility of perception of motion.

Exposure Factor Ce. The exposure factor Ce is based on the mean wind speed profile, which depends on the roughness of terrain over which the wind has traveled before reaching the building. Three wind profile categories are used in building design.

Exposure A. This is the exposure on which the reference wind speeds are based. The exposure is defined as open, level terrain with only scattered buildings, trees or other obstructions, and open water or shorelines. Ce is given by

Exposure B. Suburban and urban areas, wooded terrain, or centers of large towns with terrain roughness extending in the upwind direction for at least 1.5 km. Ce is given by

Exposure C. Centers of large cities with heavy concentrations of buildings extending in the upwind direction for at least 1.5 km, with at least 50% of the buildings exceeding four stories in height. Ce is given by

Exposure factor Ce can be calculated from Eq. (1.54) or obtained directly from the graph in Fig. 1.15.

Gust Factor
Figure 1.15. Exposure factor Ce as a function of terrain roughness and height aboveground. (From NBCC 1995.)

Gust Effect Factor Cg (Detailed Procedure). A general expression for the maximum or peak load effect, denoted Wp, is given by

where

1 = the mean loading effect a = the root-mean square loading effect gp = a peak factor for the loading effect

The dynamic gust response factor is defined as the ratio of peak loading to mean loading,

The parameter a/1 is given by the expression s m

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