Wind Loads

Wind Pressure

The design of the structure's primary lateral force system, as well as its elements and components, shall be based upon wind pressures determined from the following equation:P = CeCqqsIwhereP = design wind pressure in psf.

a. Qs = wind stagnation pressure in psf at a height of 33' above the ground. This wind force is a function of wind velocity: Qs = .00256 V2Wind Stagnation PressureWind stagnation pressure is based upon Bernoulli's equation and its a function of the wind velocity squared and the mass density of air. The UBC equation assumes a standard air density of 0.0765 pcf and does not account for density changes as the result of altitude, season of the year, weather, and latitude.

where V = windspeed in mph found from UBC Figure 16-1. The applicable wind speed map can be found as Figure 2-1 in the article Nature of Wind Can also find Qs using UBCTable 16-F. The minimum design speed is taken as 70 mph. Refer to Wind Speeds for information relating windspeeds in mph to observations. V is based upon the 50-year mean recurrence interval for the fastest-mile wind speeds.50-Year Mean Recurrence IntervalThis means that the probability of experiencing a basic wind speed faster than the indicated value in any one-year period is one in fifty of 2%.Fastest-Mile Wind Speeds

In the U.S. wind speeds are measured by recording the time a constant number of turns of the anemometer occur equivalent to one mile of wind passing by.Other countries use different windspeed measuring methods. This results in different reported values for similar intensity winds.b. Iw= Importance factor taken from UBCTable 16 - K. Essential facilities and hazardous facilities are designed to withstand higher forces by setting Iw= 1.15, which is approximately equivalent to a 100 year recurrence interval. Otherwise, Iw= 1.0

c. CE = combined height, exposure, gust coefficient found inUBC Table 16 - G. Wind pressure increases with building height: For the windward wall, UBC uses a stepped pressure diagram as f(height). For the leeward wall, pressure taken as constant over the full wall height and is calculated using the mean roof height for CE determination.

Turbulence (gusting) = f(terrain), is quantified in terms of an exposure rating:GustingThis component accounts for the effects of air turbulence and dynamic building behavior. The underlying gust response assumptions are only appropriate for structures that have stiffness, mass, and damping characteristics similar to ordinary buildings with a period of vibration less than one second. More flexible structures may experience wind effects that are substantially greater than that predicted by UBC; particularly with respect to the across-wind response.

Exposure EffectsGround surface irregularities and flow obstructions such as trees and other buildings reduce wind forces. The UBC broadly categorizes these terrain influences through the use of exposure ratings.

Although a building site may have different exposure in different directions, the most severe exposure governs for all wind load calculations. B - urban, suburban, closely spaced obstruction the size of single - family residences. C - open country and grasslands that are generally flat. D - unobstructed flat terrain facing a large body of water extending inland 1/4 mile or ten times building height, whichever is greater. UBC recognizes only 3 of the 4 possible exposure classes. It has not adopted Exposure A - building sites in large city centers.

d. Cq= Pressure coefficient. Found in UBCtable 16 - H. Cq= f (structure or part of, analysis method, openings) Structure or part of: Cqfor primary LFRS.LFRSThe lateral force resisting system (LFRS) is that part of the structural system designed to resist lateral loads. For bearing wall type building considered here, the horizontal diagrams and the shear walls make up the LFRS. Considering the whole structure as it resists lateral forces. Applied to horizontal diaphragms and shearwalls. Cqfor elements and components: Accounts for higher pressures that occur locally. Taken at locations away from discontinuities, and At discontinuities. Primary frame analysis methods: Method 1: normal force method

A more accurate description of wind forces. Windward and leeward forces acting normal to all exterior surfaces simultaneously. Method 2: the projected area method. Carry over from earlier codes. Simpler than method 1. Generally more conservative. Cannot be used for gabled rigid frames or structures greater than 200' in height. Simultaneous application of horizontal pressures on the vertical projected area and vertical pressure on the horizontal projected area. Openings: Open (known as partially enclosed in newer codes like the '94 and '97 UBC's) structures have higher outward pressures than enclosed or unenclosed structures. Doors and windows are considered openings unless protected. Account for the effect of openings in partially enclosed buildings by using a bigger Cqcoefficient according to footnote 1 Table 16 - H.Footnote 1, Table 16-H1For one story or the top story of multistory partially enclosed structures, an additional value of 0.5 shall be added to the outward Cq. The most critical combination shall be used for design. For definition of open structures, see section 1613. This effect could be simply thought of as what happens to the surface of a balloon as it is being blown up. For primary LFRS, method 1: increase Cqfor roofs according to footnote 1 of UBC 16-H. For elements and components not at discontinuity: select Cqaccording to the element under consideration, and if building is unenclosed, enclosed, or partially enclosed. Definitions, according to '97 UBC 1616, for enclosed, partially enclosed, and unenclosed: Aoi= area of opening on projected side i. Ai= total projected wall area of side i. Aoj= area of opening on projected side j. If Aoi/ AI0.85 for all sides, then structure is unenclosed. If Aoi/ AI > 0.15 for side i and ifAoj< .5 Aoifor all other sides j, then structure is partially enclosed. All other cases, consider the building as enclosed.

Wind Loads

Building Stability

In addition to the design of the primary load-resisting system and the individual elements with respect to wind pressures, the entire structure must be checked for overturning. This phenomenon can be simply described by considering an empty cereal box located in front of a household fan.

Without proper connection to the table, the box will act like a rigid body; overturning by rotating and falling about its far corner. Similarly, you need to ensure moment stability of the entire structure and those elements that are part of the LFRS, (e.g. shearwalls). According to '94 UBC 1619.1, or '97 UBC 1621.1'94 UBC 1619.11619.1 General.The primary frames or load-resisting system of every structure shall be designed for the pressures calculated using Formula (18-1) and the pressure coefficients, Cq, of either Method 1, or Method 2. In addition, design of the overall structure and its primary load-resisting system shall conform to Section 1603.

The base overturning moment for the entire structure, or for any one of its individual primary lateral-resisting elements, shall not exceed two thirds of the dead-load-resisting moment. For an entire structure with a height-to-width ratio of 0.5 or less in the wind direction and a maximum height of 60 feet (18,290 mm), the combination of the effects of uplift and overturning may be reduced by one third. The weight of the earth superimposed over footings may be used to calculate the dead-load-resisting moment., the overturning moment (OM) must be2/3 of the resisting moment (RM) due to dead loads. But if h/b0.5, h60', and the entire structure OM accounts for both lateral and uplifting loads, then may use OMRM. Consider the simple model of our building in terms of the cereal box.OM = Ph + U b/2RM = D b/2If OM2/3 RM, then hold corner A down with an anchor capable of resisting a tensile force of:T = (OM - 2/3 RM) / b.

In addition to checking the entire structure, you need to also check each shearwall as modeled below.

If all shear walls are adequate wrt OM, surely the entire structure will be adequate. With other structural systems it is common practice to check overturning of the entire building, but with a conventional shearwall building the practice is to check each shearwall instead.

Wind Loads

Building Drift

Another important consideration in building design is to examine the structure's potential for movement due to lateral loads. This is known as building drift,s, or more specifically story drift. UBC doesn't limit story or building drift due to wind, but does for seismic in UBC '94 1628.8 or '97 UBC 1630.10 Story drift - lateral displacement of one level relative to level above or below.

General guidelines suggest limitingwind:(.0025) h as applied to masonry shear walls. Shearwall behavior is a function of shearwall length, L, to height, h, considerations.

Wind Loads

Example

Develop the applicable wind forces for a one-story, box-type industrial complex located in Southern California, in a suburban area with roof D = 12 psf and 16" o.c. partially grouted CMU walls.

1. Design pressure for primary LFRS:

For this example, the pressure coefficients are the same regardless of load direction.

P = CeCqqsIwqsV = 70 mph (Figure 16-1, UBC '97)qs= 12.6 (UBC '97 Table 16-F)

Iw= 1.0 (UBC '97 Table 16-K)

CeExposure B.62(0 - 15')(UBC '94 Table 16-G)

.67(20')

CqMethod 1(Table 16-H).8 windwardAssumingflat roof and closed building.

-.5 leeward

-.7 roof

The resulting pressure diagram for wind loading in the transverse direction:

2. Diaphragm Loading:. Showing transverse loading here.. Consider a 1' width of wall that is simply supported at foundation and roof.. Examine the normal wall load effect on the supporting diaphragm:. Since the diaphragm must support both, simultaneously, the inward and outward pressures, model these effects as a resultant pressure.

The roof diaphragm must resist 125.4 lb/ft (or, in other words, each foot of wall produces a force of 125 lbs on the diaphragm.)

Diaphragm to shear walls for flexible diaphragms: Showing only the transverse loading. We will only work with flexible diaphragms in this class.

Diaphragm Plan View:

In this example, the shear wall at each end must resist a resultant load of 7837.5 lbs.

Upon development of the shear wall loads, examine shear wall drift and overturning. Shear wall drift:. L/h = 66/17 = 3.88, drift dominated by shear deflection.. Therefore during wall design, ensure that1.2 P h / G A.0025 h Overturning (OM). Since this is a shear wall building, every shear wall should be checked for OM.. Check all shear walls, in lieu of an overall building OM check (In other words, if the shear walls are adequate then the overall building is adequate. But, remember that overall building stability does not ensure that all shear walls are ok).

RM = Resisting MomentRM = 87120 (33) = 2,874,960 ft-lbsOM = Overturning MomentOM = 7837.5(17) = 133,240 ft-lbs. If the shear wall supported a significant % of the roof framing, then a portion of roof uplift should be included in OM. Likewise, a portion of the roof dead load should be included in RM.. 2 / 3 RM = 1,916,640 > > OM. No special anchorage required for the shear wall, which is typical for masonry walls that support wood roof systems. (This is not, however, true for all wood buildings with wood shear walls. Very often, these lightweight walls will need special hold down anchors to resist the overturning potential.)

. Lateral forces normal to the wall. In addition to acting as shear walls, these walls must be designed to resist forces acting perpendicular to them. These perpendicular forces do not come from the primary LFRS analysis as shown in step 1. Develop these forces by considering localized, higher loads taken on portions of the building as reflected in Cq. A careful reading of '97 UBC Table 16-H indicates two load cases here.. One for all structures and the other for enclosed and unenclosed structures.. For outward (negative) forces, use Cq/Cebased on mean roof heights.. For inward (positive) forces, use Cq/Cebased on the actual height of the element.

Repeat wind load development and distribution for wind in longitudinal direction.