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Engineers Also Act as Detectives

Maybe you have not thought of engineers as sleuths, but client questions such as “Can I add a story to my building?” or “Was that roof connection broken before the earthquake?” often place them in the role of detective. Starting with limited information, engineers apply their technical tools and deductive reasoning to uncover evidence and solve mysteries.

For example, a Terracon client asked if it was feasible to add a story to his pre-1974 building. Did the existing auger-cast-in-place pile foundations have the capacity to support the increased loads? To make it more complicated, there were no records of pile lengths or site soil characteristics available.

Terracon engineers, led by Senior Consultant Guoming Lin, Ph.D., P.E., developed a three-phase forensics program to recreate the missing information. The nondestructive methods used included parallel seismic tests, pulse echo tests, and magnetometer tests. The parallel seismic and pulse echo tests measured the velocity of impact waves generated by striking the piles with a hammer, and the magnetometer tests measured intensity of the magnetic field radiating from the pile reinforcing steel. Soil property information for evaluating pile capacity, once the pile lengths were known, was obtained with a seismic cone penetrometer (CPT) used to perform the parallel seismic testing.

Three independent estimates of the pile lengths were obtained with non-destructive test methods and were analyzed to provide a high-probability range of pile length. Pile lengths were estimated to be at least 44 feet long with an allowable capacity of 50 tons per pile, so enough to support an additional story.

Another practical example is after a magnitude 6.8 earthquake; City of Seattle employees noticed a damaged roof connection in one of their buildings and filed a damage claim with their insurance company. The insurer’s structural engineer asked, “Could the connection have been damaged prior to the earthquake?”

Senior Consultant David Baska, Ph.D., P.E., CEG, led Terracon engineers in the building performance evaluation by characterizing the ground shaking intensity. The first step was to properly characterize the soil profile overlying bedrock using conventional drilling and sampling methods, augmented with down-hole and seismic CPT soundings. The shear wave velocity measurements from the soundings allowed accurate estimation of material stiffness, and more importantly, the contrast in stiffness between the rock and soil. In the absence of strong motion recording devices at the building location, the ground response to earthquake excitation was modeled using numerical methods. This produced a suite of acceleration time histories representative of ground motions entering the building’s foundation during the earthquake.

Concurrent to Terracon’s modeling efforts, the structural engineer created a 3-D model of the shallow footing-supported, dock-high, tiltwall warehouse. The model allowed computer simulation of structural response to the suite of acceleration time histories.

The modeling efforts of Terracon and the structural engineer led to the conclusion that the building was likely not damaged by ground shaking, because the capacity of the lateral force resisting system exceeded the demand imposed by the earthquake. The insurer was only responsible for earthquakerelated damages; consequently, the cause of damage to the roof connection was not pursued further. However, consolidation settlement before the earthquake and/or a construction defect in the superstructure may have caused the observed damage.

The local knowledge of Terracon’s project engineers, complemented by the assistance of experienced senior consultants and technical leaders in the firm, make these challenging questions from clients possible to solve. Through our internal network, we link all of our engineers and scientists to extensive resources and specialized experience, like that of our senior consultants.