by Filippo Masetti, PE, Antonio De Luca, PhD, and Milan Vatovec, PE, PhD, LEED AP
In-situ load testing is a powerful tool to assess the performance of structures with respect to their ability to carry code-prescribed loads. The practice dates back to the late 1800s and has been used to verify the load rating of structures, as well as the expected performance of structures in retrofit, repair, and strengthening applications.
While not always considered the most cost-effective solution (often unjustly), these tests are especially powerful in situations where structures or components may be deemed inadequate to carry the intended loads and are slotted for potentially expensive retrofit or strengthening work. This is because other conventional and even sophisticated analysis methods may be unable to adequately assess reserve capacity due to the inherent inability to properly account for in-situ redundancy, load sharing, and effects of alternate load paths.
This article provides a historical review of the load-testing practice, a description of the state-of-the art protocols and methods for load testing (and their rationale), and a review of several related building codes and standards. It also provides a lessons-learned discussion based on real-life case study examples.
When can a load test be useful?
Load testing had been developed to either create a “confidence in the ability of a structure to perform” or “understand and define the mechanism of a specific structural behavior.” To this end, load test practices have been designed to determine whether the structure can safely support the proposed loading (i.e. building-code required loads), and are not aimed at providing an indication of the ultimate strength of the subject structure or its components, whose performance may be in question in a wide variety of situations.
Specifically, load tests can be used to evaluate structures exhibiting signs of deterioration and distress whose severity and extent is not fully understood. For instance, these authors worked on the evaluation of the concrete-bleachers structure of the concrete stadium of a prominent university in the Midwest. This investigation campaign included destructive sampling and non-destructive testing (e.g. impulse response, ground-penetrating radar) aimed at identifying extent of delaminated areas, followed by in-situ load testing of the worst-identified areas (which yielded sufficient load rating of the structure).
Similarly, load tests can evaluate the effects of design or construction defects, such as misplacement or omission of steel reinforcement within concrete elements, or use of lower-grade steel elements in construction. The authors were involved with a parking-garage project evaluating the negative-moment capacity of a flat slab in which the top reinforcing steel was misplaced (i.e. lower than specified in the construction documents). For this particular job, load testing was used to assess the performance of repair/strengthening details designed to address the slab’s reduced negative-moment capacity.
Moreover, load testing can be used to investigate performance of unique designs (i.e. proprietary systems) and/or historical/archaic structures where construction is not fully understood due to lack of documentation, concealed conditions, or inability to make exploratory openings. As an example, the authors worked on load testing of a late-1800s brick-masonry-arch floor systems in New York City.
Historical review of the load-testing practice
The practice of load testing has very deep roots, with its development dating back to the 1870s in Europe. A “very early example of proof load testing” was reported in New York in 1894. At that time, the concept was introduced to evaluate novelty and proprietary systems, which then largely consisted of reinforced concrete structures (such as the Turner system for flat slabs).
Although largely based on a rule-of-thumb approach, the first building-code reference to load-test protocol for reinforced-concrete structures appeared in the 1903 New York City Building Regulations. Five years later, a national code requirement for load testing for concrete structures was issued by the National Association of Cement Users (NACU), which became the American Concrete Institute (ACI).
The code was updated several times, but the general protocol remained conceptually unchanged:
- Threshold measurements of the structural parameters are taken before application of any load.
- The structure is loaded to a certain level, and structural parameters are recorded.
- Based on the measurements taken in the previous phases, the structure is evaluated.
Since the early days, as understanding of the relationship between the loads and reliability of structures increased, the load-test procedures in the ACI code also slowly changed. The main features that evolved over time can be summarized as:
- load test duration;
- establishment of test load magnitude (based on the design dead, live, and other loads required by the current code); and
- establishment of acceptance criteria to determine the outcome of the test (based on both maximum recorded deflections and recovery from maximum deflections).
Figure 1 summarizes the evolution of these features in the ACI load test provisions from 1920 to 2005.