Structural Steel

Determination of Wind Loads

Wind loading as described in Art. 9.1 is the basis for design wind loads specified in ‘‘Minimum Design Loads for Buildings and Other Structures,’’ ASCE 7-88, American Society of Civil Engineers. Model building codes specify simplified methods based on these provisions for determining wind loads. These methods can be used for most structures. One such method is incorporated in the ‘‘Uniform Building Code’’ (UBC) of the International Conference of Building Officials, Inc. (See Art 6.6 for ASCE 7-95.)

Wind-Load Provisions in the UBC

The basic wind speeds specified by the UBC for the continental United States and Alaska are shown in Fig. 9.3. The contours on the map indicate wind speeds that have a 2% probability of being exceeded in a year at a height 10 m above ground on open sites. (These are wind speeds that are expected to occur once in 50 years.) The effects of extreme conditions, such as tornadoes, hurricanes, or local wind currents in mountainous regions are not covered by this map. Further, special wind regions are identified in the map where local wind velocity may significantly exceed the indicated values for the location. The possibility of occurrence of these local variations should be considered in design.

The stagnation pressures qs [Eq. (9.3)] at a height of 10 m above ground are provided in tabular form in the UBC:

The UBC integrates the combined effects of gusting, changes of wind velocity with height above ground, and the local terrain or surface roughness of the earth in a coefficient, Ce.
Values of Ce are given in the UBC for specific exposure conditions as a stepwise function of height (Table 9.1). The UBC defines three exposure conditions, B to D. Exposure C represents open terrain (assumed in Fig. 9.3). Exposure B applies to protected sites. Exposure D is an extreme exposure primarily intended for open shorelines and coastal regions. Coefficient Ce as well as stagnation pressure qs are factors used in determination of design wind
pressures.

The UBC also specifies an importance factor I to be assigned to a building so that more important structures are designed for larger forces to assure their serviceability after an extreme windstorm. For most buildings, I = 1.0. For such buildings as hospitals, fire and police stations, and communications centers, and where the primary occupancy is for assembly of 300 or more persons, I = 1.15.
A final factor Cq depends on the geometry of the structure and its appendages and on the component or portion of the structure to be loaded. It is intended to account for the pressure distribution on buildings, which may affect the major load elements.
The design pressure p, psf, is then given by

system. Method 1 (Fig. 9.4b) is a normal-force method, which distributes pressures normal to the various parts of the building. The pressures act simultaneously in a direction normal to the plane of roofs or walls. In this method, Cq = 0.8 inward for all windward walls and 0.5 outward for all leeward walls. For winds parallel to the ridge line of sloped roofs and for flat roofs, Cq = 0.7 outward. For winds perpendicular to the ridge line, C = 0 7 outward on the leeward side.
On the windward side:
Cq = 0.7 outward with roof slope less than 2:12
= 0.9 outward or 0.3 inward with roof slope between 2:12 and 9:12
= 0.4 inward with roof slope between 9:12 and 12:12
= 0.7 inward with roof slope greater than 12:12.

Method 2 (Fig. 9.4c) uses a projected-area approach with horizontal and vertical pressures applied simultaneously to the vertical and horizontal projections of the building, respectively.
For this case, Cq = 1.4 on the vertical projected area of any structure over 40 ft tall, 1.3 on the vertical projected area of any shorter structure, and 0.7 upward (uplift) on any horizontal projection.

Individual components and local areas may have local pressure concentrations due to local disturbance of the airflow (Fig. 9.2). These normally do not affect the design of load frames and major load-carrying elements, but they may require increased resistance for architectural elements, local structural members supporting these elements, and attachment details. The UBC also contains values of Cq for these local conditions. Some of these component requirements for Cq for wall elements include:
1.2 inward for all wall elements
1.2 outward for wall elements of enclosed and unenclosed structures
1.6 outward for wall elements of open structures
1.3 inward and outward for all parapet walls
An unenclosed structure is a structure with openings in one or more walls, but the sums of the openings on each side are within 15% of each other. An open structure has similar wall openings but the sum of the openings on one wall is more than 15% greater that the sum of the openings of other walls. Open structures may accumulate larger internal pressures than enclosed or unenclosed structures (Fig. 9.1) and must be designed for larger outward pressures.

There are similar component requirements for Cq for roof elements. These include:
Cq = 1.7 outward for roof elements of open structures with slope less than 2:12
= 1.6 outward or 0.8 inward for roof elements of open structures with slope greater than 2:12 but less than 7:12
= 1.7 inward and outward for roof elements of open structures with slope greater than 7:12
= 1.3 outward for roof elements of enclosed and unenclosed structures with roof slope less than 7:12
= 1.3 outward or inward for roof elements of enclosed and unenclosed structures with roof slope greater than 7:12
Corners of wall elements must also be subjected to Cq  1.5 outward or 1.2 inward for the lesser of 10 ft or 10% of the least width of the structure. Roof eaves and other projections are also collectors of concentrated wind pressure (Fig. 9.2). Building codes require considerations of these local pressure distributions with
Cq = 2.3 upward of roof rakes, ridges, and eaves without overhang and slope less than 2:12
= 2.6 upward of roof rakes, ridges, and eaves without overhang and slope greater than 2:12 but less than 7:12
= 1.6 upward of roof rakes, ridges, and eaves without overhang and slope greater than 7:12
= 0.5 greater coefficient for overhanging elements and canopies.
These factors combine to produce a complex distribution of design pressures. Some of the distributions are illustrated in Fig. 9.5.
These localized distributions affect the strength of local elements and the strength of attachment details of local elements, but they do not affect the global strength requirements of the structure.

Other Provisions for Wind Loads

Alternative methods for determining wind loads, such as that in ASCE Standard 7-88, are available, and give more detailed provisions than those in the UBC (Art. 9.2.1) for defining and distributing wind loads. Tabulated data may be more detailed in these other methods, and more equations may be required. However. the pressure distributions are similar to that provided by the UBC.
These methods provide basic wind loads for buildings, but they do not specify how to estimate or control aerodynamic effects. Aerodynamic effects may result in interaction between the dynamic response of a structure and the wind flow around it. This interaction may amplify the dynamic response and cause considerable occupant discomfort during some windstorms.

Furthermore, local variations in wind velocity can be caused by adjacent buildings. The wind may be funneled onto the structure, or the structure may be protected by surrounding structures. Wind tunnel testing is often required for designing for these effects. Local wind variations are most likely to be significant for tall, slender structures. As a general rule, buildings with unusual geometry or a height more than 5 times the base dimension are logical candidates for a wind tunnel test. Such a test can reveal the predominant wind speeds and directions for the site, local effects such as channeling of the wind by surrounding buildings, effects of the new building on existing surrounding structures, the dynamic response of the building, and the interaction of the response with the wind velocity. The model used for the test can include the stiffness of the building, and wind pressures can be measured at critical locations. Major structures often are based on wind-tunnel-test results, since greater economy and more predictable structural performance are possible.
Special structures, such as antennas, transmission lines, and supports for signs and lighting, may also be susceptible to aerodynamic effects and require special analysis. Aerodynamic effects are beyond the scope of this section, but analytical methods of dealing with these are available. Wind tunnel testing may also be required for these systems. (E. Simu and R. H. Scanlan, Wind Effects on Structures, Wiley-lnterscience, New York.)

 

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