EXPERIMENTAL STUDY TO IMPROVE THE THERMAL MECHANICAL PROPERTIES OF GLUED-IN ROD JOINTS FOR WOODEN STRUCTURAL MEMBERS

Abstract

　The purpose of this experimental study was to understand and improve the heat resistance of the glued-in rod (GIR) method of joining wooden members. Experiments were carried out with the conventional GIR method (Con), in which the inner surface of the reinforcing bar insertion hole is smooth, and an improved GIR method (Imp), in which the inner surface of the insertion hole is grooved with the expectation of improved mechanical bond strength.

　The experimental procedure can be broadly divided into three steps, as follows.

　Step 1: Assuming a building containing wooden members jointed using the GIR method, the internal temperature of a GIR joint in the event of a building fire was calculated by unsteady heat transfer analysis. This temperature, 80℃, was then set as the target temperature for pull-out experiments.

　Step 2: The heating required to achieve the target 80℃ boundary temperature between adhesive and wood in the GIR joint specimen was determined and experimentally verified by analytically examining the temperature-time relationship. It was determined that the temperature in the electric furnace used to heat the specimen should be raised to 180°C over a period of 35 minutes.

　Step 3: Based on the determinations made in steps 1 and 2, the following pull-out experiments were carried out:

　•　Experiment I: Pull-out from unheated specimens at room temperature.

　•　Experiment II: Pull-out under constant load from specimens during heating.

　•　Experiment III: Pull-out from specimens when the boundary temperature reaches the target of 80℃. The loaded or unloaded state during heating was a parameter in these pull-out experiments.

　•　Experiment IV: Pull-out from specimens after initially raising the boundary temperature to 80℃, holding it there for about 2 to 3 hours, and cooling again.

　The results obtained from these various pull-out experiments were as follows.

　•　Experiment I: The pull-out strength of Imp was lower (about 90%) than that of Con. The cause of this has not been clarified, and further experiments and data accumulation will be required in the future.

　•　Experiment II: The boundary temperature at the time of pull-out failure was 96°C for Imp and 61℃ for Con. The loading in this experiment was fixed at 37% of the room temperature pull-out strength, so further experiments and data accumulation will be required for other loadings.

　•　Experiment III: The average pull-out strength of Imp was 97.7 kN for unloaded heating (49 kN for Con) and 74.5 kN for loaded heating. Thus the pull-out strength of Imp under unloaded heating was about twice as high as that of Con. The pull-out strength of Imp under loaded heating was about 1.5 times as high as the pull-out strength of Con under unloaded heating. The high-temperature pull-out strength of Imp is clearly an improvement over that of Con.

　•　Experiment IV: The pull-out strength after heating and cooling greatly exceeded the high-temperature pull-out strength in both Imp and Con, and was almost equal to the unheated room-temperature pull-out strength. One of the reasons for the exhibited strength recovery may be that the moisture content near the boundary between adhesive and wood after heating and cooling was almost the same as that of the unheated wood.