Structural Design

 Structural Design
Whenever a structure is designed, it is important to give consideration
to both material and load uncertainties. These uncertainties include a
possible variability in material properties, residual stress in materials,
intended measurements being different from fabricated sizes, loadings
due to vibration or impact, and material corrosion or decay.
- ASD. Allowable-stress design (ASD) methods include both the
material and load uncertainties into a single factor of safety. The many
types of loads discussed previously can occur simultaneously on a
structure, but it is very unlikely that the maximum of all these loads will
occur at the same time. For example, both maximum wind and
earthquake loads normally do not act simultaneously on a structure. For
allowable-stress design the computed elastic stress in the material must
not exceed the allowable stress for each of various load combinations.
Typical load combinations as specified by the ASCE 7-10 Standard
include
• dead load
• 0.6 1dead load2 + 0.6 (wind load)
• 0.6 1dead load2 + 0.7 1earthquake load2
LRFD. Since uncertainty can be considered using probability theory,
there has been an increasing trend to separate material uncertainty from
load uncertainty. This method is called strength design or LRFD (load
and resistance factor design). For example, to account for the uncertainty
of loads, this method uses load factors applied to the loads or
combinations of loads. According to the ASCE 7-10 Standard, some of
the load factors and combinations are
• 1.4 (dead load)
•1.2 1dead load2 + 1.6 1live load2 + 0.5 1snow load2
• 0.9 (dead load) 1.0 (wind load)
• 0.9 (dead load) 1.0 (earthquake load)
In all these cases, the combination of loads is thought to provide a
maximum, yet realistic loading on the structure.