A six-story medical office building near Lake Eola required a Site Class determination before the structural engineer could finalize the lateral load path. The geotechnical boring logs showed loose to medium dense sands down to 45 feet, but the IBC Section 1613.2 demands a measured Vs30 value, not an assumed one. We mobilized a 24-channel seismograph with 4.5 Hz geophones along the proposed building footprint and ran two orthogonal MASW lines. The survey took four hours on a Tuesday morning in August. The resulting shear wave velocity profile—Vs30 of 238 m/s—placed the site firmly in Site Class D, avoiding the costly detailing requirements of Site Class E that a nearby property triggered. When you need a defensible MASW / VS30 (shear wave velocity) measurement for an Orlando project, the method must account for the shallow limestone pinnacles and sinkhole-prone sands that define Central Florida geology. We often pair the survey with CPT soundings to verify the depth to competent bearing strata, especially east of I-4 where the Hawthorne Formation creates abrupt impedance contrasts.
A measured Vs30 profile replaces the default Site Class D assumption with a defensible number, often saving structural steel tonnage on mid-rise projects across Central Florida.
Top questions
How much does a MASW / Vs30 survey cost for a typical Orlando commercial building lot?
For a standard single-building lot with two orthogonal MASW lines totaling approximately 300 linear feet, the survey cost ranges from US$1,620 to US$3,280. The price depends on site accessibility, surface conditions (asphalt versus grass), and whether existing boring logs are available to constrain the inversion model. Sites with heavy vegetation or active construction traffic fall at the higher end due to additional setup time.
How does Orlando's karst geology affect the MASW results?
The alternating sequence of sands, silts, and limestone pinnacles in the Floridan aquifer system creates high lateral variability in shear wave velocity. A limestone pinnacle with Vs above 800 m/s can sit adjacent to a raveled zone with Vs below 200 m/s. The MASW dispersion curve averages over the geophone spread length; we mitigate this by running multiple overlapping spreads and comparing the inverted Vs profiles against any available SPT data. When a velocity inversion appears—soft material beneath stiffer material—the team evaluates whether it represents a true karst feature or a processing artifact.
What is the difference between Site Class D and Site Class E in Orlando?
Site Class D corresponds to Vs30 between 180 and 360 m/s—most Orlando sites with medium-dense sands and shallow limestone fall here. Site Class E applies when Vs30 is below 180 m/s, typically found in areas with thick organic or loose alluvial deposits, or when more than 10 feet of soft clay exists. The structural impact is significant: Site Class E triggers higher seismic design category coefficients, which can increase lateral force demands by 30–50% compared to Site Class D. A measured Vs30 value can sometimes reclassify a site from E to D when the default assumption is overly conservative.
How long does it take to receive the final Vs30 report after the field survey?
The standard turnaround is five to seven business days. The processing involves dispersion curve picking, inversion with multiple starting models to test sensitivity, and integration with available boring logs. Expedited two-day delivery is available for projects on a tight permit deadline. The final report includes the processed Vs profile, Vs30 calculation, IBC Site Class letter, and a summary of the field acquisition parameters.
