Building Design and Construction Handbook - Merritt - Ventech!

STRUCTURAL THEORY 5.17
Building codes usually permit a smaller factor when the probability is small that
combinations of extreme loads, such as dead load plus maximum live load plus
maximum wind or seismic forces, will occur. Generally, for example, a factor of
0.75 is applied to load-combination sums (2) to (6). Such factors are equivalent to
permitting higher allowable unit stresses for the applicable loading conditions than
for load combination (1). The allowable stress is obtained by dividing the unit stress
causing excessive deformation or failure by a factor greater than 1.
In load–and–resistance factor design, the various types of loads are each multiplied
by a load factor, the value of which is selected in accordance with the
probability of occurrence of each type of load. The factored loads are then added
to obtain the total load a member or system must sustain. A structural member is
selected to provide a load-carrying capacity exceeding that sum. This capacity is
determined by multiplying the ultimate-load capacity by a resistance factor, the
value of which reflects the reliability of the estimate of capacity. Load criteria
generally used are as follows:
1. 1.4D
2. 1.2D � 1.6L � 0.5(L r or S or R)
3. 1.2D � 1.6(L r or S or R) � (0.5L or 0.8W)
4. 1.2D � 1.3W � 0.5 (L r or S or R)
5. 1.2D � 1.0E � (0.5L or 0.2S)
6. 0.9D � (1.3W or 1.0E)
For garages, places of public assembly, and areas for which live loads exceed 100
lb/ft 2 , the load factor usually is taken as unit for L in combinations 3, 4, and 5.
For roof configurations that do not shed snow off the structure, the load factor
should be taken as 0.7 for snow loads in combination 5.
For concrete structures where load combinations do not include seismic forces,
the factored load combinations of ACI 318 Section 9.2 shall be used.
For both allowable stress design and strength design methods, elements and
components shall be designed to resist the forces due to special seismic load combinations
a) 1.2D � 0.5L � E m
b) 0.9D � E m
For floors in places of public assembly, for live load in excess of 100 psf, and for
parking garage live load, the load factor is taken as 1.0 for L. E m is the maximum
seismic effect of horizontal and vertical forces.
5.2 STRESS AND STRAIN
Structural capacity, or ultimate strength, is that property of a structural member that
serves as a measure of is ability to support all potential loads without severe cracking
or excessive deformations. To indicate when the limit on load-carrying usefulness
has been reached, design specifications for the various structural materials
establish allowable unit stresses or design strengths that may not be exceeded under