Vibration of the structure in response to ground shaking at its foundation is the concern of the structural engineer, and which is taken into account by codal provisions of the different seismic-resistant design codes. However, these codes do not include any provision due to other effects, which may even exceed that due to vibration, as the procedure of their estimation and the needed steps for the design are outside the scope of the structural engineering discipline. Even then, it is essential that the structural engineer should be aware of the different seismic hazards in order to advise the client of potential damage involved in selecting sites in such zones. Hence, the first step in the design procedure of a future structure should be the analysis of the suitability of the site selected with proper consideration for the potential of any one of the following types of damage.
The different ‘Direct’ and ‘Indirect’ seismic effects are as follows
• Ground failures, which include Surface faulting, Vibration of soil (or effects of seismic waves), Ground cracking, Liquefaction, Ground lurching, Differential settlement, Lateral spreading and Landslides.
(A) Damage due to surface faulting:
These damages to buildings and facilities along the fault scarps vary widely from completely demolished houses to rupture of the foundations, tilting of the foundation slabs and walls. Sometimes houses also have minor damage.
(B) Damage due to liquefaction:
The instability of the soil in the area affected by internal seismic waves can cause significant damage. The mechanical characteristics of the soil layers, the depth of the water table and the intensities and duration of the ground shaking influence the soil response. Deposits of loose granular materials if present in the site may be compacted by the ground vibrations induced by the earthquake. This will cause large settlement and differential settlements of the ground surface. Further, the compaction of the soil may result in the development of excess hydrostatic pore water pressures of sufficient magnitude to cause liquefaction of the soil, resulting in settlement, tilting and rupture of structures. The seismic-resistant design provisions of most codes only assure an effective design and construction of structures against damage due to the possible vibratory response of the structure to the shaking introduced at their foundation by the ground. However, it may not be possible to have success in all such cases. The only option remains in those areas is to prohibit the construction of building structures there.
(C) Damage due to ground shaking:
Integrated field inspection and post-earthquake analyses of structural damage due to earthquake shaking is one of the most effective means of having expertise knowledge on seismic response with a view to improving the state of the art and of the practice in seismic-resistant design and construction. Such integrated inspection and analyses revealed that in addition to the soil conditions mentioned above, the seismic performance of a structure is very sensitive to type of foundation; configuration of the structure; structural material; and design and construction detailing.
(D) Damage due to sliding of superstructure on its foundation:
One of the basic guidelines in the seismic-resistant design and construction of structures is that the whole structure-foundation system should work as a unit, and that the superstructure be tied or anchored properly to the foundation.
• Vibrations transmitted from the ground to the structure.
(E) Damage due to Structural Vibration:
I. Wood-Frame Houses:
The inertia forces develop during the vibratory response of a structure to earthquake ground shaking whose intensity depends on the product of the mass and acceleration. Hence, it is of the utmost importance to reduce the mass of the structure to a minimum. It is obvious that timber is the most efficient earthquake-resistant material for low-rise buildings among the traditional structural materials - timber, masonry, concrete, steel and aluminum. However, provision of proper lateral bracing and tying of all components together from the roof down to the foundation are to be followed.
II. Concrete structures:
Concrete is a comparatively heavy material and have a very good compressive strength. Due to its very small tensile and flexural strengths steel reinforcement is provided when used in structures. Such reinforced concrete can be used effectively in seismic-resistant construction. To overcome its relatively low strength per unit weight when normal weight aggregates are used, the use of lightweight aggregate concrete offers a significant advantage in seismic regions. members carefully: the proper amount and correct detailing of the reinforcing steel plays an important role in the seismic response of a reinforced concrete structure.
III. Steel structures:
Steel comes out of steel plants having excellent quality control. The stiffness per unit weight of steel is practically the same as of any other traditional constructional material. However, its strength, ductility and toughness per unit weight are significantly higher than concrete and masonry materials. Accordingly, the slenderness of steel structural members usually exceeds significantly that of other structural members. Hence, buckling becomes a serious problem. The danger of buckling becomes much more at higher yielding strength of the steel. Further, the plate elements used extensively to form the structural shapes are prone to local buckling, particularly when strained in the inelastic range. Accordingly, the compactness requirements for the cross section of the critical regions of structural members are more stringent in earthquake-resistant design than that of normal condition. Moreover, the field-connection of the structural members is another problem in achieving efficient seismic-resistant construction.
Indirect effects (or Consequential Phenomena):