Wind

Front 1

Response to wind loads

Back 1

Buildings respond to send forces in a variety of complex and dynamic ways. Direct pressure of the wind acts on the Windward side of the building. Suction is created on the leeward side such as the side walls and the back wall. Uplift is created at any horizontal or sloping surface such as the roof. The structure also vibrates because of the gusting effects of the wind, similar to the way a building responds to earthquake motion. As the wind passes over the building, friction causes drag forces to act in the direction of the wind. The cornete, edges, and eave overhangs of the building are subjected to especially complex forces as the wind passes these obstructions, causing much higher local suction forces than those on the building as a whole. Another important factor affecting wind velocity and pressure is the terrain surrounding the building. If is built-up with buildings or features the resulting turbulence reduces wind velocity and pressure.

Front 2

Stagnation pressure

Back 2

Stagnation pressure, in pounds per square foot, on a vertical surface is related to the 3 second gust velocity (mph) by p= 0.00256V².

Front 3

Code Requirements

Back 3

Methods of analysis:

Simplified Procedure - Wind pressured can be selected from tables in the code with minimal calculation when the building meets all the requirements specified in the code. This procedure is used primarily to design regularly shaped low-rise buildings.

Analytical procedure - Wind pressures are determined using formulas, tables and figured provided in the code. This method is used to design many types of structures.

Wind tunnel procedure - this procedure provided general guidelines for conducting a wind tunnel test. Such tests may be performed on any building to determine wind pressures. Typically performed only on irregular, very tall, slender buildings due to costs.

Minimum design value of 10 psf regardless of method used.

Front 4

Site exposure

Back 4

In order to determine the site exposure we must know surface roughness on site (as surface roughness impacts wind velocity). Once a roughness category has been selected, each wind direction is assigned an exposre category. Exposure categories may be different in different wind directions depending on terrain surrounding. However the exposure that results in the highest wind pressure is usually specified.

Front 5

Importance factor

Back 5

The importance factor varies between 0.77 and 1.15 depending on occupancy categories and whether the building is in hurricane-prone areas. The 1.15 factor is also used for hospitals, fire, and police station as they are expected to be safe and usable following a severe windstorm. Also when using the 1.15 factor is not inside hurricane-prone areas (and not an emergency building) represents building for a 100 year storm and 0.87 and 1.0 represent a 25 and 50 year storm.

Front 6

Method 2 - Analytical Procedure.

Back 6

The analytical procedure is used to determine wind pressures or forces for many types of buildings. Wind pressures are calculated on Windward, leeward, side, and roof surfaces using formulas, tables, and figures given in the ASCE 7. On the windward face, wind acts toward the surface of the building (pressure) and varies with respect to roof height. On the leeward face and side walls and roof, pressure acts away from the surface (suction) and is assumed constant with respect to the mean roof height. Suction occurs on the side walls and leeward side of the of the roof, while either pressure or suction occurs on the Windward side of the roof depending on it's slope.

Front 7

Method 1 - Simplified Procedure

Back 7

Method 1 is acceptable primarily in the design of regularly shaped low-rise buildings. In order to use this method the following condions must be satisfied.

1. The building must be a simple diaphragm, where a building in which both leeward and windward wind loads are transmitted through floor/roof diaphragms to the same wind-force-resisting system. Cannot be used on buildings without a diaphragm.

2. The mean roof height must be less than 60 ft and cannot be greater than the least horizontal dimension of the building.

3. The building must be enclosed. Buildings in wind-borne debris regions must have special impact-resistant glazing.

4. The building must be regularly shaped; it cannot have unusual geometric irregularities.

5. The building cannot be flexible; it's natural period must be one second or less.

6. The building cannot have characteristics that make it subject to across wind loading and other higher order response modes, and cannot be located where there are channeling effects of wind.

7. The building must have an approximately symmetrical cross-section in each direction and must have either flat, Gable, or hip roof with angle less than or equal to 45 degrees.

8. The building must be exempted from torsional load cases due to building symmetry and stiffness, or have flexible diaphragms, or be free from the control of torsional load cases.

Method 2 or 3 must be used if any of these conditions are not met.

Front 8

Lateral load resisting systems

Back 8

The lateral load resisting systems are the same used in earthquake load resisting systems, moment-resistant frames, shear walls, and braced frames. However, the type of lateral load resisting system does not affect the magnitude of the wind load, also ductility or the ability to absorb energy is not very important in wind design.

Front 9

Overturning

Back 9

Buildings must be designed to reisit overturning moments caused by wind forces. The wind overturning moment cannot exceed 0.6 of the dead load resisting moment, unless the structure of anchored to resist the excess moment. The factor of safety for overturning is equal to the dead load resisting moment devided by the wind overturning moment. The factor of safety must be greater than or equal to 1/0.6 = 1.67 for the building to be adequate. Tall slender buildings are more likely to overturn compared to short squat, or pyramidal buildings.

Front 10

Deflection and drift

Back 10

The deflection and drift of a building under wind loading must be limited in order to prevent damage to the brittle elements of the building and to minimize discomfort to the building's occupants. The code does not provide limits on drift between stories for wind. Care must be taken to adequately separate adjacent buildings, and non-structural elements should be detailed to allow for movement under wind loads.

Front 11

Diaphragms, collectors, and torsion

Back 11

Wind forces are transferred to the vertical resisting elements (shear wall, braced frames, and moment-resistant frames) by diaphragms, in exactly the same way as earthquake forces. Collector members may be used in wind design, just as in earthquake design. Torsion occurs in a rigid diaphragm when the center of mass first not coincide with the center of rigidity. Under wind loading, torsion occurs in a rigid diaphragm when the center of the applied wind load does not coincide with the center of rigidity. See earthquake design

Front 12

Elements and components of structures

Back 12

The wind pressures on components and cladding are determined from essentially the same badig formulas used to design the main force-resisting system. At any given instant, the wind pressures over the surface of the building vary greatly, and gusts cause the pressures in localized areas to exceed the average pressure on the entire building. Furthermore, observed wind damage to buildings indicate that very high suctions or negative pressures occur at the building's discontinuities, such as eaves, ridges, and wall corners. So higher values for the external pressure coefficients are used in the design of components and cladding.

Front 13

Combined vertical and horizontal forces

Back 13

All building components must be designed for load combinations using either (1) strength design or load and resistance or (2) allowable stress design. The appropriate set of combinations usually depends on the material specified.

Strength design load combinations:

1. 1.2D + 1.6(Lr or S) + 0.8W

2. 1.2D + 1.6W + f1 + 0.5(Lr or S)

3. 0.9D + 1.6W

ASD

1. D + W

2. D + 0.75W + 0.75L + 0.75(Lr or S)

3. 0.6D + W

The code also gives a set of alternate basic load combinations for allowable stress design. Load combinations reflect probabilities of various loads acting on the structure at the same time.