Characteristics of Climate Loads

Wind driven rain is necessarily characterized by two basic climate parameters: wind or wind speed, and rainfall intensity, the latter parameter typically re­ferred to as the rainfall rate and reported in mm/h. Wind driven rain events are associated with tropical cyclonic events such as tropical storms, typhoons (in the Western Pacific), and hurricanes (in the Eastern Atlantic). Table 1 provides the range of wind speeds and related velocity pressures in relation to different categories of tropical cyclonic events.

The values provided in Table 1 are those that occur at a height of 10 m, the height at which meteorological data are collected. Thus these would suffice for estimating the wind loads for a residential building of three stories or less. Wind loads atop buildings of greater height can be estimated by using the following equation for velocity pressure, qz, given in ASCE-7 [8]:

qz(Nm-2) = 0.63[2.0(z/zg)2/a]KztKdV2I (1)

where z = height above ground (m); zg = 213.36 m (atmospheric boundary layer reference height); and assuming, « = 7 and «=11.5 for exposure categories B and D, respectively, and I (Importance factor); Kzt (topographic factor); Kd (wind directionality factor) =1.

For example, using this relationship, and assuming wind velocities at 10 m consistent with the range of speeds associated for a severe tropical storm (i. e., 69-103 km/h), information on velocity pressures at different building heights for different exposure categories such as urban (Exposure Category B) and flat,

TABLE 2—Velocity pressures at different building heights associated with severe tropic storms for buildings subjected to ASCE Exposure Categories B (urban, suburban) and D (flat, unobstructed).

Velocity Pressure (Pa)

Height

(m)

Storya

Exposure Category B

Exposure Category D

10

3

183-417

258-587

30

10

250-571

313-711

75

25

326-742

367-834

150

50

398-904

413-941

aEstimate assuming a 3-m story height.

open terrain (Exposure Category D) can be estimated as given in Table 2. The information is important because it shows that the pressures are not linearly proportional to the height but vary exponentially in height, thus the severity of the wind can be significantly more important atop tall buildings as compared to low-rise structures. This may be self-evident, however, it bears directly on the risk to water entry through small openings in exposed joints of tall buildings as the risk to entry is itself proportional to the pressure difference across the joint.

Rainfall Intensity during Storms—An example of the amount of rainfall that can occur during a typhoon is provided in Fig. 3 [9] that shows the temporal

image257

FIG. 3—Temporal rainfall variation during Typhoon Gloria and Herb, 1996 [10].

variation in rainfall intensity for an occurrence of two typhoons in Taiwan in 1996. Taiwan receives an annual rainfall of 2500 mm of which 80 % of the annual rainfall occurs in May to October and especially during typhoons. Rain­fall intensity during some typhoons may exceed 100 mm/h and 1000 mm over a 24-h period with the recorded maximum one-hour and 24-h rainfall before 1996 being 300 mm and 1672 mm, respectively [9]. As can be seen in Fig. 3, the recorded 96-h rainfall for Typhoon Herb was accompanied by heavy rain that reached 1994 mm over this period, the bulk of the rain falling in a 48-h period (1987 mm) with maximum rainfall rates attaining values in exceedence of 110 mm/h.

Relating Test Conditions to Weather Parameters—A summary of extreme wind-driven rain (WDR) conditions in relation to the return period in years is provided in Table 3 for different locations across the United States including: Boston, MA, Miami, FL, Minneapolis, MN, Philadelphia, PA, and Seattle, WA [11]. Information on rates of wind-driven rain (L/(min-m2)) and driving rain wind pressures (DRWP) are given as average extreme hourly values. The DRWP is the velocity pressure exerted on a surface (e. g., wall) normal to the wind direction during rain. The analysis from which these values were determined also indicated that the wind driven rain values provided for Miami, FL would be overestimated for shorter return periods (i. e., <10 years), and underesti­mated for the longer return periods (> 20 yrs); the extent of under or over estimation has not yet been determined as this would require a more detailed analysis of the effects of tropical cyclonic activity on the wind-driven rain and driving rain wind pressures.

In the United States, design wind pressures are derived from information based on a 1 in 50-year return period [12]. With reference to this return period, extreme WDR conditions in Miami, FL indicate a water deposition load of 3.9 L/(min-m2) and a DRWP of 553 Pa, whereas in comparison, Seattle, WA provides for much reduced WDR loads; 0.3 L/(min-m2) and a DRWP of 198 Pa. It should be emphasized that these values represent extreme values associated with each individual driving rain parameter and are unlikely to oc­cur coincidentally.

This implies that testing at conditions in which both extremes are used would subject a specimen to an event that would have a much heightened return period as compared to the return period associated with a particular extreme WDR parameter. Typically, for nontropical cyclonic events, at height­ened rates of wind-driven rain, the corresponding DRWP are lower than those of the extreme values shown in the table and likewise, rates of WDR are lower when extreme values of DRWP are evident.

However, what is evident from this information is that tests undertaken at the 700 Pa level and 3.4 L/(min-m2) adequately represent expected extremes for the different locations of the United States provided in Table 3. The WDR rates at which tests were conducted (i. e., 0.8, 1.6, and 3.4 L/(min-m2)) may be slightly higher than that provided in Table 3 for Seattle and Boston; however, the threshold value for WDR rate of 0.8 L/(min-m2) is reflected in values given for Minneapolis at a 1 in 10-year return period (0.76 L/(min-m2)), Philadel-

Characteristics of Climate Loads

phia at a 1 in 20 year return period (0.78 L/(min-m2)), and every other year in Miami (1-in-2 yr: 0.88 L/(min-m2)). It is cautioned that for Miami, FL the val­ues provided are likely an overestimate of WDR.