Seismic loads Selectivity

Equating the appropriate stress equation from equations 4-17 to 4-20 to the allowable stress, a solution for distance X may be obtained for a selected shell thickness. Because the stresses from both wind loads and seismic loads increase with X, the resulting equation is a binomial. 

For the lower portion of tall towers, where the combined axial stress controls the design of the shell, there is the problem of selecting the maximum allowable axial compressive stress. The combined axial tensile stress presents no problem. The tensile stresses produced by internal pressure, bending stress of wind loads or bending stress firom seismic loads may be combined by simple addition of the stresses. The thickness of the shell may be calculated so that the combination of axial tensile stresses is equal or less than the maximum permissible value specified by the ASME Code. 

The enhancement of strength and stiffness is alternatively achieved by selective or large-scale strengthening of structural members or by the addition of new elements that can resist either partially or fully the seismic actions (e.g., reinforced concrete shear walls, steel trusses, infill walls, etc.). In this case, particular attention should be given to the design of the foundation due to the increase of both the structural mass and the seismic loads. 

Selecting the seismic loading for design and/or assessment purposes is not an easy task due to the uncertainties involved in the very nature of seismic excitations. One possible approach for the treatment of the seismic loading is to assume that the structure is subjected to a set of records that are more likely to occur in the region where the structure is located. 

Selection of foundation type must be based on an assessment of the magnitude and direction of loading, depth of suitable bearing materials, seismic activity, evidence of previous flooding, potential for liquid action, undermining or scour, swelling potential, frost depth, and ease and cost of construction. 

The inelastic static pushover-based methods, which are typically used for the assessment of the existing structures, are presented. Some of the basic concepts that are nowadays used are presented in the example of the four singlemode pushover-based methods included in the codes. Some parameters that influence the accuracy of these methods are presented selection of the lateral load pattern, idealization of the capacity curve, numerical models, influence of the higher modes, and changes of the shapes of the vibration modes depending on the seismic intensity, in-plan torsion, strength degradation, and soil-structure interaction. 

A method to be selected for evaluating the foundation stiffness must adequately reflect the shape of the foundation-soil interface the amount of embedment the nature of the soil profile and the mode of vibration and frequencies of excitatiOTi. The uncoupled spring approach satisfies all these conditions. It is accomplished through determining the dynamic impedance functirais for the foundation. This method is adequate if the seismic foundation loads are not expected to exceed twice the ultimate foundation capacities. As illustrated in Fig. 2, the dynamic impedance model is an uncoupled single node model that represents the foundation element. An upper and lower bound approach to evaluating the foundation stiffness is often used because of the uncertainties in the soil properties and the static loads on the foundations. As a general rule of thumb, a factor... 

Selected key elements of modern seismic codes are reviewed here, along with a brief review of their historical development. What can be considered a single seismic code pertaining to a nation or a subdivision thereof consists of dozens of pages of provisions and by reference includes thousands of pages of standards that relate to loads, testing procedures, manufactured... 

The selection of appropriate load combinations for seismic assessment is a relevant issue addressed by ISs and CGs. In general, they are recommended based on the observations that follow. 

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