Structural Design

Conventional structural design was based on satisfying two requirements, namely safety and serviceability. Safety relates to extreme loadings, which have a very low probability of occurrence, on the order of 2 %, during a structure’s life, and is concerned with the collapse of the structure, major damage to the structure, its contents, and loss of life. The major priority is on ensuring sufficient structural integrity so that the sudden collapse can be avoided. Serviceability pertains to medium to large loadings, which may occur during the structure’s lifetime. For loading, the structure must be designed well, must suffer minimal damage, and the motion experienced by the structure should not exceed specified comfort levels for humans and motion-sensitive apparatus mounted on the structure. Typical occurrence probabilities for service loads range from 10 to 50 % Safety concerns are satisfied by requiring the resistance, i.e., the strength of the individual structural elements must always be greater than the demand associated with the extreme loading. Once the structure is dimensioned, the stiffness properties are achieved and used to check the various serviceability constraints such as elastic behaviour. The same process is necessary for convergence to an acceptable structural design. This approach is referred to as strength-based design since the components are dimensioned initially according to strength specifications. Application of strength-based preliminary design is appropriate when strength is the dominant design requirement. Earlier, most of the structural design problems came under this category. However, certain developments have occurred in the recent past that limits the effectiveness of the strength-based approach. The requirement of a structure that is flexible like a tall building and long-span horizontal structures has resulted in more structural motion under service loading, thus shifting the emphasis toward serviceability.

Secondly, some new types of conveniences such as microdevice manufacturing centres and hospital operating centres have more severe design confinements on motion than the typical civil structure. For example, the environment for microdevice manufacturing must be essentially motion free. Thirdly, recent advances in material science and engineering have resulted in major increases in the power of conventional civil engineering materials. However, the material stiffness has not increased at the same rate. The lag in material stiffness vs. material strength has led to a problem that satisfies the requirements of the various motion parameters. Thus it can be said that, for very high strength materials, the motion requirements control the design. Fourthly, experience with recent earthquakes has shown that the cost of repairing structural loss due to inelastic deformation was considerably greater than envisioned. This finding has resulted in a trend toward decreasing the reliance on inelastic deformation to dissipate energy and shifting to another type of energy dissipating and energy absorption mechanisms. Performance-based design is an alternate design paradigm that addresses such issues. The manoeuvring takes as its principal aim the success of motion related design elements such as conditions on displacement and acceleration and has the optimal deployment of element stiffness and motion control devices to achieve these design purposes as well as satisfy the constraints on strength and elastic behaviour. Limit state design can be described as a form of performance-based design where the structure is allowed to experience a specific amount of inelastic deformation under extreme loading.