Page 376 - Design of Reinforced Masonry Structures

WALLS UNDER GRAVITY AND TRANSVERSE LOADS 6.27

TABLE 6.1 Adjustment Factor for Building Height and Exposure, l ;
adapted from [6.19]
Exposure
Mean Roof Height (ft) B C D
15 1.00 1.21 1.47
20 1.00 1.29 1.55
25 1.00 1.35 1.61
30 1.00 1.40 1.66
35 1.05 1.45 1.70
40 1.09 1.49 1.74
45 1.12 1.53 1.78
50 1.18 1.56 1.81
55 1.19 1.59 1.84
60 1.22 1.62 1.87
Unit conversion: 1 ft = 0.3048 m.

The importance factor, I, accounts for the degree of hazard to human life and damage
to property. Its value, which varies between 0.77 and 1.15, depends on: (1) the occupancy
category of a building or a structure as listed in Table 1-1 of ASCE 7-05, (2) whether the
building or structure is located in a nonhurricane-prone region and hurricane-prone region
with the basic wind speed V (defined as the 3-s gust speed at 33 ft above ground in Exposure
Category C) of 85 to 100 mph and Alaska, or in hurricane prone regions with V > 100 mph.
ASCE 7-05 classifies various occupancies in four categories: Category I through IV (see
ASCE-05 Table 1-1):
1. Category I: Buildings in Category I are those that pose a low hazard to human life in
the event of failure, such as agriculture facilities, certain temporary facilities, and minor
storage facilities.
2. Category II: Buildings in this category are those that are not classified as buildings in
categories I, III, and IV.
3. Category III: Included in this category are such buildings and structures that represent
substantial hazard to human life in the event of failure. Such buildings include, amongst
others: schools and colleges, hospitals, and buildings and other structures in which more
than 300 people congregate in one area. Facilities that process; manufacture; handle;
store; and/or use or dispose of such substances as hazardous fuels, chemicals, hazardous

TABLE 6.2 Importance Factors I for Wind Loads
Occupancy Categories
I II III IV

Qualifying Regions Importance Factor, I PO
Nonhurricane-prone regions, and hurricane- 0.87 1.0 1.15 1.15
prone regions with V = 85–100 mph
Hurricane-prone regions with V = 85–100 mph 0.87 1.0 1.15 1.15
Unit conversions: 1 mph = 1.61 kmph.
Source: Adapted from Ref. 6.19.