Determination of tall structure response properties induced by close-in construction blasting in urban environments

Vibration measurements of three tall structures were performed during close-in rock blasting in New York City. Two high-rise structures (20 and 25 stories) and one mid-rise (3 to 5 stories) structure were instrumented with geophones to measure structure response to blasting in the hard Manhattan schist. All structures represented historic or Landmarked structures and protected with a 0.5 in/s ground velocity limit by the City Department of Buildings. Compliance with this limit was monitoring using one tri-axial geophone that was buried 15 ft in the ground (GV) at the bottom of each blasted lift adjacent to each structure. Structure-mounted bi-axial geophones were placed at the street level (S1) and at the upper structure (S2). The purpose of this instrumentation was to evaluate structure response to ground vibrations and determine if close-in, high frequency rock blasting resulted in upper structure free response to enable calculations of natural frequency and damping.;A single, 4-channel seismograph was used to measure time-correlated upper and lower horizontal structure velocity time histories. All structure-mounted transducers were connected to cellular modems for remote control and uploading of data. The in-ground geophones were not time-correlated and data were uploaded in the same manner.;Several methods were employed in an attempt to determine the dynamic properties of the structures by assuming a single-degree of freedom model (SDOF). Methods used to establish natural frequency and damping included visual inspection of upper structure motions, the use of Pseudo-velocity response spectrum (PVRS) plots to compare the expected structure differential motions generated by the ground time histories with the actual measured values, the use of Fast Fourier Transform (FFT) ratios dividing the upper structure FFT by the FFT of the ground motion, and comparing theoretical amplification plots as a function of frequency over a range of damping values with the measured amplification.;Natural frequency and damping values could not be determined for most blasts because free response could not be detected in the upper structure. This was due in part to low displacement amplitudes generated from high frequency ground motions and measurement limitations of blasting-type seismograph systems. The amplitudes of upper structure motions were within the low-end resolution bit range of seismographs where one bit is equivalent to 0.0025 in/s. The expected natural frequency of high-rise structures was 0.5 Hz and 2 Hz for mid-rise structures. These frequencies were below the reliable frequency response of 2 Hz for blasting-type geophones.;Visual analysis of vibration time histories proved to be the most useful method to evaluate free response and calculate natural frequencies and damping coefficients. This analysis was limited by the lack of upper structure resonance after ground excitation creased within the upper structure. Estimated natural frequencies for all structures ranged from 1.9 to 5.5 Hz. However, based on well-established observations for tall structures, natural frequencies were expected to be 0.5 Hz and 2 Hz for high-rise and mid-rise structures, respectively. Calculated damping ranged from 2.9% to 6.5% and centered on the expected 5% value typically used in structural design. Without the ability to improve the low-end gain of seismographs and increase the accuracy of geophone below 2 Hz, other methods used to compute the dynamic properties of structures proved unsuccessful. This study concluded that the SDOF model may not be a good predictor of damping and natural frequency for tall structures.;Peak ground vibrations for all blasts ranged from 1.04 to 5.35 in/s and were higher than the 0.5 in/s regulated limit imposed for historical structures in New York City. Based on small differential displacements over building heights, wall cracking was not possible. This fact was verified with parallel studies that evaluated global wall strains. Therefore, increased ground vibration limits should be implemented as an allowance for ultra-high frequencies expected during close-in, urban blasting in New York City.