GEOTECHNICAL IMPLICATIONS OF VCC 2021 ADOPTING ASCE 7-22
Chapter 20 of ASCE 7-22 does not strictly prohibit the correlation of average shear wave velocity with the use of N-Values derived from standard penetration tests (SPT) or cone penetration tests (CPT); however, this typical traditional approach does come with a “penalty.” The new revised ASCE 7-22 and Chapter 20 require a site class sensitivity analysis of shear wave velocity values with more complex calculations and more stringent parameters for site class designations. It would appear that throughout a large portion of the Mid-Atlantic region, the result of these code changes will probably often be traditional geotechnical services plus geophysical applications rather than just stand-alone geophysical applications. The new ASCE 7-22 code is not adopted everywhere, but most Mid-Atlantic jurisdictions will likely adopt it within the next eight to twelve months.
The 2021 Virginia Construction Code (VCC) has adopted ASCE 7-22 as its standard structural load design model code. The changes outlined in ASCE 7-22 are significant compared to the code it supersedes, ASCE 7-16. These major changes include tornado design provisions, snow load accuracy improvements, a new multi-period spectrum, and refinement of the seismic site class design bands.
These changes lead to the question, “What are the day-to-day implications of this code adoption in geotechnical practice?” In other words, how should the geotechnical engineer best determine a seismic site class using the refined Table 20.2-1?
Providing Site Class Using ASCE 7-22 Chapter 20 - Table 20.2-1
The previous code guidance (i.e., Table 20.3-1) pertaining to seismic site class included ranges of shear wave velocity (Vs) as well as ranges of average undrained shear strength (Su) and average blow counts (N-values), which all can be used to ascertain which band of site class to recommend. In common geotechnical practice, SPT and CPT data are often collected for each site to be used in the broader design of foundations. Therefore, designers typically have N-values and/or undrained shear strength values to band for assessing site class.
ASCE 7-22 removes the ranges of N-values and Su values from the governing design table (20.2-1). Geotechnical designers are now required to estimate or directly measure shear wave velocity for a site. One may infer that removing the SPT and CPT data used to correlate site class means governing code writers prefer direct measurement of shear wave velocity. Numerical estimations based on SPT or CPT-determined data appear to be permitted for predicting shear wave velocity but include additional steps to capture uncertainty and ensure the worst loading scenario is accounted for. The actual regression equations to use would be at the designer’s discretion.
Assessing Seismic Site Class Using Shear Wave Velocity Estimates
An iterative process begins once a designer has estimated an average shear wave velocity based on standard regression equations and/or regional correlations. The geotechnical engineer must evaluate a 30% increased value and a 30% reduced value unless authorities having jurisdiction have approved otherwise. For most scenarios, this will result in the evaluation of multiple site classes. The geotechnical engineer will be required to determine the worst-case ground motion for a given period associated with the structure being built. This presents a practical challenge for the geotechnical engineer, who often works with early structural estimates where final design periods may be unknown for a given structure. Therefore, the geotechnical engineer may need to provide multiple multi-period spectra for the site associated with multiple site classes to be used by the structural engineer once a structure’s period is determined.
The ASCE Hazard Tool
The ASCE Hazard tool can be accessed at ascehazardtool.org. The geotechnical engineer will need to know the following:
See illustration below.
The ASCE Hazard Tool can output a detailed summary, which includes a Multi-Period Design Spectrum for the selected Site Class. Individual plot data points can be downloaded in CSV format and compared.
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Comparing Multi-Point Spectra for Multiple Site Classes
Downloading the plot points in CSV format allows comparison of the multi-point spectra for different site classes. For many site structures, the lower site class in the comparison will be the controlling ground motion scenario. However, for short-period structures, these trends may deviate and should, therefore, be checked.
Direct Measurement of Shear Wave Velocity
Designers may wonder if there are advantages to directly measuring a site’s shear wave velocity. Estimating a shear wave velocity and assessing a value 30% higher or lower may require more iteration and could result in a more costly design. If a geotechnical practicing engineer wishes to directly measure shear wave velocity, what field methods are available and generally considered to be the “standard of care”?
Multi-Channel Analysis of Surface Wave (MASW) Surveys
MASW can be used to measure the shear wave velocities of on-site materials. A series of geophones along survey lines collect data as an impulse source (often a hammer strike) is placed at the end of the line. The travel time of shear sound waves (S-waves) is then used to determine the velocities of the on- site materials. MASW is an active method of data collection, but readings will require post-processing. Results can then be used to select the corresponding Seismic Site Classification in Table 20.2-1.
Seismic Cone Penetration Testing (SCPT)
For soil profiles without debris, cobbles, rock, and other potential tip-damaging impediments, a variation of the CPT sounding can be used to assess the shear wave velocity of the in-situ profile.
The idea would be to use a variation of the CPT probe that would allow the continuous collection of traditional CPT data and shear wave velocity readings. The drawback of this methodology would be if a 100-foot sounding could not be pushed. Where data does not extend to depths of 50 feet, or if data collected between 50 feet and 100 feet is such that site geology does not rule out the presence of soft layers, default site classes, as described in Section 20.1, should be assessed.
Other Points of Consideration
The adoption of ASCE 7-22 opens up a new dialog between the structural engineer and the geotechnical engineer. In practice, an early understanding of the structure’s estimated period and risk category is needed. In addition, the desire to directly measure the site’s shear wave velocity to refine site class may become more common. Project teams standing to gain significant construction cost savings with site class refinement will likely be willing to pay for higher-cost geotechnical exploration programs, which include direct shear wave measurement.
However, the traditional exploration methods using SPT drilling are not going away. Collection of site samples for laboratory testing, including undisturbed samples, rock cores, etc. will still be needed for site assessment in typical geotechnical practice. Therefore, the push for direct shear wave velocity measurement will be an additional cost to owners and project teams. Communication is key. Project teams will need to understand the reasons behind the code change and the benefits of direct shear wave measurement.
Speaking in Code thanks Erin Phillips, PE, and Steve Hjelle, PE, for their collaborative efforts in developing this technical article about the new ASCE 7-22, “Minimum Design Loads and Associated Criteria for Buildings and Other Structures.” Erin and Steve are both Senior Geotechnical Engineers in the F&R Roanoke, VA, office. This article was developed for Speaking in Code to alert readers to the dramatic code changes between the new Chapter 20 of ASCE 7-22 and Chapter 20 of all previous ASCE 7 codes.
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