and Randomness in Seismic Design
Randomness exists in both the demands on a structure and in its capacity to resist them. Earthquake motions are inherently random. Even with increased knowledge, there will be large randomness in both the excitation and response. Structural behavior is affected by random variations in material properties, deterioration, and construction quality. Capacity is also affected by loading history and duration which are both influenced by the randomness of the excitation.
Uncertainty in Demand
There are numerous sources of uncertainty in the expected demands on a structure. These sources of uncertainty include:
Uncertainty in Capacity
In the past, capacity was generally thought of in terms of strength, and the focus was on determining the strength capacity. Now, for seismic-resistant design, the focus is on deformation and energy dissipation capacity. Strength capacity is still widely used, though, because generally, we are able to predict the strength capacity of elements reasonably well, especially those controlled by flexure. Even for flexural strength capacity, however, there are difficulties:
Also, code and similar design equations are often lower bounds on capacity. The lack of consistency in developing theses equations makes it hard to determine the failure mode of a system. Even when more rational relations for determining capacity are used, system behavior is still uncertain because most capacity estimates focus on elements rather than systems. It is often unknown whether the "failure" of one or two elements will lead to the failure of the system.
way capacity testing is conducted is another source of uncertainty. Tests
are often terminated at some arbitrary level. For example, elements may
only be proof tested to a preselected drift or ductility. Many elements
have not been tested to failure. Also, many intermediate limit states
such as spalling, local buckling, or lateral buckling, have not been documented,
making limit states design or evaluations difficult.
of these sources of uncertainty in both demand and capacity show that
there is a strong need to make structural system response relatively insensitive
to the uncertainty in ground motion and structural characteristics.