Estimating the ground motion

Estimating the ground motion

To construct a design spectrum for a particular site, an estimate of earthquake ground motion is necessary. This is based on seismic history in the vicinity of the site. Unfortunately in many regions this information is not available. Based on limited information, seismic risk procedures and attenuation relationship for estimating the peak ground acceleration (PGA) has been developed. From these studies it was recommended that the ratio of peak velocity to peak acceleration v/a be used to estimate the peak ground velocity and ratio of peak acceleration – peak displacement product to the square of the peak velocity (ad/v²) be used to estimate the peak displacement. A log normal distribution summary of v/a and ad/v² for records on four different soil conditions is presented in Table 17.6. In the table L, S and V denote the larger of two horizontal acceleration, the smaller of two peak horizontal accelerations and the vertical component of the earthquake motion. Using the seismal risk map and Table 17.6 one can estimate the maximum ground motion at a site. This information is used to construct a site-specific spectrum.

Example 17.9

Construct Newmark–Hall broad-banded design spectrum for a site. Develop spectrum curves using the program for damping factors 0.02, 0.05, 0.1. Estimate the maximum horizontal ground displacement assuming the maximum ground acceleration = 0.4g, assuming group L and 84.1% on 50 m alluvium by bed rock.

Solution

Maximum ground acceleration = 0.4g = 0.4 × 981 cm/s² = 392.4 cm/² From Table 17.6 v/a = 117/g; ad/v²= 7.8

v = 117 × 0.4 = 46.8 cm/s

d = 7.8 v²/a = 7.8 × 46.8²/392.4 = 43.53 cm

a = 392.4 cm/s²; v = 46.8 cm/s d = 43.53 cm

Construct the maximum ground motion polygon with the values of a, v and d. Apply appropriate amplification factors to the maximum ground motion components in a manner similar to that described in the next pages.