<p> Design of 2-hour fireproof wooden beam protected by wood-based fire protection barrier is studied. The lightness of the structure, one of the most important features of the wooden construction superior to the reinforced concrete and steel constructions, becomes more significant as the buildings become taller. The study aims at minimizing the total weight of the fire protection barrier, which is composed of the Phosphorus fire-retardant treated wood layer and, if necessary, the surface sacrificial layer and is notably heavier than the load-bearing part, assuming Japanese cedar as the material. Through review of previous studies, it has been expected that the sacrificial layer with the universal optimum thickness for Japanese cedar(25mm) can substitute for the 1.5 times heavier treated wood of the same thickness and the bottom and the lower corners of the beam may need thicker barriers than the vertical sides for possible dropping of the sacrificial layer during fire heating and the increase of the relative heating surface especially at the corners.</p><p> The following three series of furnace tests are conducted to clarify the necessary performance and thickness of the treated wood for the barrier on the vertical sides, the bottom and the lower corners.</p><p> Test I) Bench-scale test on specimens reproducing the barriers of vertical side part in order to explore the suitable treatment and thickness of the treated wood barrier for the 2-hour fireproof wooden beam</p><p> Test II) Unloaded full-scale test on an entire beam cross section to verify carbonization of the bottom and lower corners</p><p> Test III) Bench-scale test on specimens reproducing a lower corner with different treated layer thicknesses</p><p> Test I has revealed the followings:</p><p> ①Whether the wood within 25mm from the surface is treated or not, there is no influence for the achievement of the 2-hour fireproof performance. Use of 25mm thick of sacrificial layer is effective to reduce the weight of the total assembly.</p><p> ②The target amount of the fire-retardant agent of 120kg/m³ is found to be not enough to suppress glowing combustion against the 2-hour ISO834 standard fire heating, while sufficient performance is demonstrated for 180kg/m³. The test result suggests the effectiveness of the fire-retardant treatment as the barrier for 2-hour fireproof performance would reach the upper limit at around 150kg/m³.</p><p> Test II has revealed that:</p><p> ①Using an extra lamina(25mm thick) treated wood for the bottom is effective to prevent the dropping of the charred protection layers during the fire heating and secure the 2-hour fireproof performance.</p><p> ②The lower corners can be critical for the achievement of any design fireproof performance, due to the largest relative heating surface area during the fire heating and the reduction of the heat exchange performance after the heating by the dropping of the edge surfaces. Design of the lower corners is found to be the key for the fireproof performance of wood-based fire protection layers.</p><p> The result of the Test III suggests the following specification as the minimum weight 2-hour fireproof wooden beam with totally wood-based fire protection: i) 25mm thick sacrificial wood, ii) 75mm thick treated wood on the vertical sides and 100mm thick treated wood on the bottom as fire barriers, and iii) target amount of Phosphorus fire retardant agent of 180kg/m³.</p>

<p> The concept of fireproof construction in Japanese building regulation was redefined in 2000 to enable large wood based buildings to sustain permanent load bearing capability even after enduring the effects of a fire. The target of this study is the fireproof layered structure composed of:</p><p> a) a load bearing part at the core</p><p> b) a barrier (fire-retardant impregnated wood)</p><p> c) a sacrificial layer</p><p> This type of layered structure is the furthest behind in terms of development in respect to other types of wood-based fireproof construction.</p><p> The authors clarified the condition of ability to self-extinguish, to the point of no further glowing combustion, after the fire heating of this test target. In the previous report, by designing the thickness of the sacrificial layer so as to reach the pyrolysis temperature of the fire retardant agent during heating, the fire retardant performance of the barrier can be minimized. From the pyrolysis temperature of the fire retardant agent, the sacrificial layer thickness may be increased in proportion to the fire resistance time.</p><p> However, if there is an optimum thickness of the sacrificial layer at each fire resistance time then the sacrificial layer, which is exposed to fire heating shorter than the designed fire resistance time, cannot self-extinguish. Therefore, the following 2 tests were conducted in order to clarify that the sacrificial layer thickness does not depend on the designed fire resistance time.</p><p> (1) Bench-scale tests were conducted using small specimens modeling the wooden fireproof structural elements in order to confirm whether they self-extinguished when they were heated for a time shorter than the designed fire resistance time.</p><p> (2) From series (1), it was estimated that 25mm is the optimum sacrificial-layer-thickness independent of the designed fire resistance time (the specification had fire resistance for 1 hour). Therefore, the specification was confirmed through further testing that it self-extinguished even when exposed to heating for less than 1 hour.</p><p> From these tests, the following conclusions can be drawn.</p><p> 1. The sacrificial layer, with a thickness designed according to the fire resistance time, may not be self-extinguishing in some cases if the heating time is shorter than the designed fire resistance time. The reason are as follows. First, since the sacrificial layer is too thick to allow the heat to penetrate into the barrier layer by the end of heating, meaning the barrier layer will not reach pyrolysis temperature. Second, the sacrificial layer is kept hot by the carbonized layer on the surface, meaning heat loss on the surface does not cause it to self-extinguish.</p><p> 2. Regardless of the heating time and type of the fireproof performance, the optimum thickness range of surface layer is 25 mm or less. In the range where carbonization is completed during heating, fire proof performance is not greatly affected without using fire retardant wood as the sacrificial layer. Therefore, an optimum design method for wooden fireproof structural elements is to set the sacrificial layer thickness to the upper limit value so that it does not burn by glowing combustion after the end of heating.</p>

Strategy for the development of wooden structural elements with self-extinguishing performance based on a sacrifice-layer concept was studied through a series of bench scale tests. The typical structural elements composed of two layers and structural part: the first is sacrifice layer (chemically untreated wood), the second is fire barrier (fire retardantand untreated load bearing wood. It has revealed that the thickness of sacrifice layer is important to achieve self-extinguishment of timber elements. All the sacrifices layers should be burnt and glow at around the end of the heating period following the ISO 834 standard curve. It has resulted in the development of Japanese-cedar based 1-hour fire-resistive beam. only with small test apparatuses available almost everywhere, demonstrating its advantage for the technical development in the districts not accessible to large furnaces.

Strategy for the development of wooden fireproof structural elements based on the sacrifice-layer concept through bench-scale tests is studied through a series of bench scale tests on the design composed of sacrifice layer (chemically untreated wood), barrier (fire retardant treated wood) and loadbearing part (untreated wood). It has revealed importance of the design of the sacrifice layer to reach the glowing combustion temperature at around the end of the heating for the achievement of self-extinguishment. It has resulted in the development of Japanese-cedar based l"hour fireproof beam only with small test apparatuses available almost everywhere, demonstrating its advantage for the technical development in the districts not accessible to large furnaces.