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CTL | Thompson Completes Full-Scale Grillage Load Testing

by Chip Leadbetter

March, 2020

The Accredited Testing team at the Fort Collins, Colorado location of CTL|Thompson has completed full scale load testing on several models of steel pile cap products, often referred to as grillages.  CTL|Thompson conceived and designed the testing plan and unique equipment to apply the requested test loads.  

In this testing program, the piles used were 3 inch, hollow, square tube helical piles.  CTL|Thompson arranged and monitored the installation of the piles as a part of preparing for the grillage testing.  Samples of the grillage products were then mounted on the pile tops and instrumented with dial indicators and strain gauges.

Per the client’s request, target test loading included moments of up to 80,000 ft-lb with a static axial load of 4,000 lbs and keeping applied shear loads less than 2,000 lbs.  This loading specification of high moment and low shear required using load application “masts”. These unique testing masts were fabricated with 12 inch diameter pipe, 18 feet long, and filled with concrete to provide dead weight and stiffness.  While this pipe size didn’t quite have enough weight to meet the axial load requirement, the bolt pattern on the base plates prevented the use of a larger diameter pipe and for practicality reasons taller masts were ruled out.  The solution was 100 lb concrete ballast weights added to the sides of the masts.  

Two grillage frames were tested simultaneously by pulling the masts attached on each grillage frame together, essentially using each grillage frame as a reaction point for the other.  The client requested that loading forces be applied in four different directions on each grillage frame model tested, so the test samples were rotated, remounted and retested several times.  Testing was regularly paused to allow magnetic particle testing, and visual inspection of all welds to identify formation of cracks in the welds.  With over 8 dial gauges and multiple strain gauges installed on each grillage frame for each test, an enormous amount of data was collected, reduced, and provided to the client for their use.

Accredited as a 17025 testing laboratory in 2008 by the International Accreditation Service, the accredited testing laboratory team at CTL|Thompson has been actively involved supporting the fabricated foundation industries.  With their full complement of testing laboratory and full-service, full-scale field installation and testing combined with an experienced engineering design support staff, CTL|Thompson is unmatched in the testing industry.  For more information on CTL|Thompson’s accredited testing and inspection services, please visit their website at

About CTL |Thompson

CTL | Thompson is a full-service geotechnical, structural, environmental and materials engineering firm. Established in 1971, the firm currently employs 230 technical and non-technical employees and provides expertise in small and large-scale projects in all areas of construction. CTL|Thompson is headquartered in Denver and has offices in Fort Collins, Pueblo, Colorado Springs, Glenwood Springs and Summit County, Colorado, and Cheyenne, Wyoming. For more information, please visit

CTL | Thompson CTL | Thompson

CTL | Thompson

400 North Link Lane

Fort Collins, CO 80524