Page 110 - Forensic Structural Engineering Handbook

DESIGN CODES AND STANDARDS 2.15

Association (NFPA). In 1993, NFPA merged with the American Paper Institute to form the
American Forest & Paper Association (AFPA).
The LRFD Specification for Engineered Wood Construction was developed by the joint
NFPA/ASCE Design of Engineered Wood Construction Standards Committee and adopted
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by the American Wood Council in 1996. The LRFD specification provides an alternative
design methodology to allowable stress design procedures specified in prior editions of the
National Design Specification for Wood Construction (NDS). There are two primary dif-
ferences between ASD and LRFD procedures for the design of wood structures. In ASD,
safety adjustments are applied to strength properties only, and in LRFD, safety adjustments
are applied to loads and strength properties. Load duration factors are used in ASD, and
time effect factors are used in LRFD.
The load duration effect, or the ability of wood to support greater magnitudes of load
for short time durations, has been an essential part of the ASD guidelines since the first edi-
tion of the NLMA design specification. The load duration effect is incorporated in ASD
procedures by publishing allowable strength properties (design values) based on a ten-year
normal cumulative load duration and providing load duration adjustment factors for shorter
load durations. Probabilistic analyses of load histories, load durations, and load combina-
tions, coupled with stochastic analyses of material properties, led to the development of
LRFD strength properties (resistance values) based on short-term (five- to ten-minute) load
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duration, and time effect factors (1.0) for longer load durations. Table 2.3 is a comparison
of ASD load duration factors with LRFD time effect factors.
Building Code Requirements for Masonry Structures. In the early 1960s, masonry
industry associations began development of a technological database of masonry materials
and performance through research and testing programs. The result of this effort culmi-
nated in design standards such as the Brick Institute of America’s (BIA) Recommended
Practice for Engineered Brick Masonry in 1966 and the National Concrete Masonry
Association’s (NCMA) Specifications for Loadbearing Concrete Masonry in 1970. 35,36
Each document addressed only selected masonry materials. In 1970, American Concrete
Institute Committee 531 published a report entitled “Concrete Masonry Structures—
Design and Construction,” and in 1976 it published Specifications for Concrete Masonry
Construction (ACI 531.1-76). 37,38 Both of these documents served as the basis for Building
Code Requirements for Concrete Masonry Structures (ACI 531-79), which addressed only
concrete masonry. 39
The American Society of Civil Engineers (ASCE) and the American Concrete Institute
undertook the development of a national design code in the late 1970s. An agreement resulted
in the ACI/ASCE 530 Masonry Structures Joint Committee, formed in 1978, to develop a
consensus standard for masonry design. A design code and construction specifications were

TABLE 2.3 Comparison of ASD Load Duration Factors with LRFD Time Effect Factors
Load duration factors, Time effect factors,
Design load C (ASD) (LRFD)
D
Permanent (dead) load D 0.9 0.6
Storage live load L S 1.0 0.7
Occupancy live load L 1.0 0.8
Snow load S 1.15 0.8
Roof live load L 1.25 0.8
R
Wind load W 1.6 1.0
Seismic load E 1.6 1.0
Impact load 2.0 1.25