ACI - 437.1R
Load Tests of Concrete Structures: Methods, Magnitude, Protocols, and Acceptance Criteria
|Publication Date:||1 March 2007|
The provisions of Chapter 20 of ACI 318 have remained essentially unchanged for an unprecedented period of time since the publication of ACI 318-71, when the code was changed from working stress design to ultimate strength design. Before the 1971 code, the test load requirements or acceptance criteria were revised with almost every new edition of the code dating back to 1920. Chapter 3 and Appendix B of this report provide a detailed review of the history of the load test requirements and acceptance criteria in ACI 318. They also provide a discussion of other international standards and of significant research and reporting of other organizations on the subject of load testing.
The changes made in the load factors and load combinations of ACI 318-05 require a re-examination of the load test requirements of Chapter 20 of ACI 318-05. This report presents the recommendations of Committee 437 for revisions to the requirements of Chapter 20. Three key areas are addressed: load factors to be used in defining the TLM; the load test protocol; and acceptance criteria.
As will be discussed further in Chapter 4, the purposes of the recommended revisions to the TLM definition are twofold. The first purpose is to define a test load that will demonstrate a consistent safe margin of capacity over code-required service live load levels. Secondly, the definition of the test load primarily in terms of service live load rather than required (ultimate) strength is meant to emphasize the fact that load testing is (typically) a proof loading. In the experience of the committee members, most structures being load tested pass with small deflections. Load testing does not typically provide an indication of the ultimate strength of the structure, and that indication usually is not the goal of load testing.
Since 1920, the acceptance criteria used with load testing have incorporated a limit on measured maximum deflections after a 24-hour holding period of the total test load. The current criteria have not changed since ACI 318-63. Currently, the deflection limit is described by the formula Δmax ≤ lt 2/20,000h. The theoretical basis for this formula had its origins in the first decades of the 20th century. The committee has researched the origins of the formula and reevaluated its appropriateness. The committee recommends adopting other more meaningful deflection acceptance criteria.
Chapters 5 and 6 of the report discuss selection of a load test protocol and recommended changes to the acceptance criteria used in strength evaluation and load testing. Committee 437 in its report 437R-03, "Strength Evaluation of Existing Concrete Buildings," has discussed a cyclic load test method that offers advantages in terms of reliability and understanding of structural response to load when compared with the conventional static load test. Chapter 6 presents recommended acceptance criteria for both the 24-hour static test and for the cyclic test. Acceptance criteria for serviceability are also given