Similarly, the approach for duration of test load, as well as the extent and type of data acquisition system can be tailored to meet the needs of the project at hand. Due to time and access constraints, the ‘rapid hydraulic cylinder test’ approach was selected for a garage in Atlanta, Georgia. The test results were monitored in real time, and compared to the predicted finite-element displacement at key points of the structure. In testing the bleachers of the university stadium, the authors selected a 24-hour test, where the load was applied through 208-L (55-gal) water drums, and the structural response was monitored through an array of displacement transducers (i.e. linear variable differential transformers [LVDTs]) positioned in specific locations.
For this particular project, the authors had nearly unlimited access to the top and underside of the structure (and no schedule restraints), the opportunity to review the original structural drawings, and the ability to perform destructive and non-destructive testing to spot-check information shown on the drawings. FEM analysis was used to determine the amount and location of the water drums to induce in the tested seat rows the same internal forces (i.e. shear and moment) that would be caused by a test load distributed through the whole seating section.
Spatial, schedule, access, and other challenges are often encountered during the planning phases of load test procedures. For example, while planning the load test of an early 1900s masonry arch floor system (Figure 9), the authors did not have access to the original drawings, and only had limited information about the construction of the arch floor system (the thickness of cinder concrete fill on top of the structural brick arch was unknown). Further, existing piping and mechanical systems hung from the underside of the test slab, obstructing installation of the instrumentation.
The lack of information on the existing structural system was addressed with a large amount of LVDTs (accompanied by strain gauges) applied to the underside of the arch and to the tie rods, by performing several FEM parametric analyses, and by running a calibration cycle prior the actual load test. Once again, since the expected deflections were a small fraction of an inch, the authors considered temperature effects in their analyses. Other early 1900s systems, where drawings are usually unavailable and where the load-test development and implementation is generally challenging, include:
- terra-cotta arches;
- wood-framed floors;
- cinder concrete slabs with draped mesh; and
- other proprietary and archaic systems.
Conclusion
Where conventional assessment or analysis methods fall short for a project, load tests can often unlock and employ the much-needed reserve strength in the examined structural systems; alternate or supplemental load paths are discovered, unaccounted-for material strength is revealed, and the hidden effects of redundancy are taken into consideration.
When properly designed and implemented, load tests are a reliable, safe, and effective tool to assess the performance of almost limitless types of structures—whether complex or simple, modern or archaic. They allow engineers to effectively employ their judgment, experience, and outside-the-box thinking to effectively and definitively assess structural behavior of the examined structure, and, in some cases, achieve much-needed peace of mind.
Filippo Masetti, PE, joined Simpson Gumpertz & Heger (SGH) in 2005, and has been involved in design, investigation, strengthening, and rehabilitation projects involving concrete, steel, masonry, fiber-reinforced-polymer, and wood structures. He is an active member of American Concrete Institute (ACI) Committee 437 (Strength Evaluation of Concrete Structures) and load-tested concrete, masonry, and wood structures. Masetti can be reached at fmasetti@sgh.com.
Antonio De Luca, PhD, joined SGH in 2013. He received his PhD from the University of Miami, with focus on the use of fiber-reinforced polymers in new construction and for the repair and rehabilitation of existing concrete structures. De Luca has worked on load-testing of existing concrete, masonry, and wood structures. He can be contacted by e-mail adeluca@sgh.com.
Milan Vatovec, PE, PhD, LEED AP, is a senior principal at SGH and the head of structural engineering at the firm’s office in New York. In his more than 16 years with SGH, he has been involved with numerous design, investigation, forensic analysis, repair and rehabilitation, and research projects. He has worked on more than 400 different projects involving evaluation and structural design for repair or modification of various existing wood, concrete, masonry, and steel structures. Vatovec is a contributing member of the Wood Committee at American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) and of ACI Committee 440 on Fiber-reinforced Polymer Reinforcement. He can be reached at mvatovec@sgh.com.
