# Test Loading of Structures with a Suspect Resistance
## 1 Introduction  
The trustworthiness of a structure is normally secured through codes, a competent structural design and proper execution control. Skilled test loading is an alternative technique that may be used as a partial or full replacement of special code stipulations, comprehensive design calculations or special quality-control measures. The current codes, however, lack detailed instructions for test loading. The Eurocodes [1, 2, 3] include general guidelines for test loading without the reliability survey, which is the key focus of this article. Apparently, for this reason, test loading is seldom applied in structural practice, and its full potential is not normally understood. The testing may occur before the assembly of the structure (e.g., grading timber boards to various strength classes) or after the assembly. The theory and the principles presented here apply to both cases. Test loading is, nevertheless, sometimes an outstanding and even a superior alternative to reliably verify the load bearing of a structure. Currently, it is applied in the following exceptional cases only: (1) when the reliability of the structure is doubtful; for example, due to a defect, it can be a feasible alternative to strengthening, demolishing or reduced load; (2) when the reliability demand is especially high or when test loading is easy, for instance, bridges are often test loaded; (3) when the material of the structure or the joints have exceptionally high variability, for example, glue lam beam lamellas, I-joist flanges and glued timber trusses are often test loaded. Video [4] discloses that I-joist flanges are test loaded before the joist assembly. In North America, glued timber trussed joists have been exploited extensively for several decades, and these joists are always test loaded [5]. (4) Some geotechnical codes include rules for test loading; pull anchors, forinstance, are always test loaded [2; 3; 6]. However, these test loads seem high, about twice the characteristic load, and are set without a comprehensive probability examination. Furthermore, the process of test loading involves several open research questions, including: What is the required test load in comparison with the expected maximum service time load or the characteristic load? How can the instantaneous test load be compared with the actual long-term service time load, and is the test loading harmful for the structure? The authors have successfully test loaded several long-span girders e.g., the 50-year-old, 55 m span sports hall in the city of Lappeenranta, Finland, for which demolition had been suggested by previous studies but which, through test loading, has been verified for sufficient load-bearing capacity without strengthening (Figures 1 and 2). Based on these experiences and on the following arguments, we see that test loading has the potential for more extensive practical usage. The governing assumption for the combination of permanent and variable loads is the independent load combination and semi-probabilistic mathematics [1; 2; 3; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. The authors have disclosed a new simple procedure and equations for combining permanent and variable loads independently [18; 19]. These equations are modified here for the test  
Figure 1: Principle of the roof of the Lappeenranta sports hall: tension rods with pitched glue lam beams loaded by 10-point loads. Figure 2: View of the test loading, with weights on the floor lifted up. loading. In the authors' opinion, the loads should be combined dependently [18; 20]. The test loading equations are presented here for the dependent load combination as well. Test loading is a supreme technique in some cases due to its unique characteristics:  
First, test loading ensures that the structure does not fail due to lack of strength; i.e., that the strength corresponds to the assumptions of the design.