Experimental and numerical investigation on the effect of gaps in mass timber connections in fire

The resurgence of timber as a structural material and increasing trend to leave mass timber members and connections exposed in buildings, for aesthetic reasons, makes the fire performance of mass timber connections an important research area to ensure structural fire safety. The influence of gaps in mass timber connections, and how they influence the fire resistance rating of a connection, has been the subject of limited research, and this information is paramount for reliable estimates of mass timber connections’ structural response in fire to prevent premature failure. This dissertation focuses on investigating the influence of gaps and the use of an intumescent fire sealant as a passive fire protection measure in mass timber connections.Two sets of furnace tests were conducted with the first being a simple timber sample configuration consisting of two glulam timber blocks with a steel component recessed in a gap between them. Gap sizes of 0 mm, 3 mm, 6 mm, and 10 mm were constructed with some gaps protected with an intumescent fire sealant and others left exposed. The temperatures in the timber around the gap and the steel component were measured and it was found that these temperatures were largely underpredicted when evaluated against the charring model of the forthcoming Eurocode 5 (3rd draft) and the requirements of the IBC 2021, with 75% of the temperatures underpredicted by prEN1995-1-2 at 60-minutes and 40 % of the temperatures failing the IBC 2021 connection temperature criteria. However, the intumescent sealant proved to be successful in limiting the temperatures when compared to the unprotected samples.A finite element model was created for two samples of each sample group to further quantify and investigate the heat transfer phenomena in the gaps. Convection in the gaps appears to be limited and not highly dependent on the gap width, especially deeper in the gaps. Radiation exposure is significantly underpredicted if it is based only on the calculated radiation dependent on the gap geometry. Radiation in the finite element models had to be calibrated to account for the combustion in the gaps, radiation between the faces of the gaps, the variable material properties and other complex heat transfer phenomena that occur during combustion in gaps. To obtain good agreement in the experimental and numerical models for the 3 mm gap samples, during the first 70-minutes of the test, the radiation onto the steel component had to be increased 5 times compared to the calculated radiation based on the geometry alone.The second set of furnace tests were conducted with a proprietary concealed beam hanger connection, with samples manufactured to include the same gap sizes as those previously tested. In these results it was clear that unprotected 6 mm and 10 mm gaps should be avoided. The unprotected 0 mm and 3 mm samples performed better but still showed larger variability in the experimental temperatures. The intumescent sealant typically performed well and limited the temperature development in the aluminium bracket significantly, with the increase in temperatures in the unprotected samples (on average at 60-minutes) ranging between 62 % and 258 %, and -4 % to 21 % for protected samples, when compared to the bracket temperatures in the 0 mm samples. In these connections the application of an intumescent fire sealant improved the predictability of the thermal development in the connections and in the 3 mm gap protected samples the lowest temperatures were recorded.Constructing concealed connections with no gaps is very challenging during construction, and therefore it is important to develop guidelines how to limit the influence of gaps on the thermal development in connections in a practical way.

随着木材作为结构材料的复兴，以及出于美学考量在建筑中裸露重型木结构（mass timber）构件与节点的趋势日益增长，重型木结构节点的防火性能成为保障结构消防安全的重要研究方向。目前针对重型木结构节点间隙及其对节点耐火极限的影响的研究尚少，但该信息对于可靠评估火灾下重型木结构节点的结构响应、避免过早失效至关重要。本论文旨在研究重型木结构节点间隙的影响，以及使用膨胀型防火密封胶（intumescent fire sealant）作为被动防火措施在重型木结构节点中的应用。

开展了两组炉内试验：第一组为简易木材试样构型，由两块胶合木（glulam）木块组成，钢质构件内嵌于二者之间的间隙中。试样间隙尺寸设置为0 mm、3 mm、6 mm及10 mm，部分间隙采用膨胀型防火密封胶进行防护，其余间隙则裸露处理。对间隙周围木材及钢质构件的温度进行测量后发现，与即将发布的《欧洲规范5（第三版草案）》（Eurocode 5）的炭化模型以及《国际建筑规范2021》（IBC 2021）的要求相比，实测温度大多被低估：在60分钟时，75%的温度值被预标准prEN1995-1-2低估，且40%的温度值未达到IBC 2021的节点温度判定标准。但相较于未防护的试样，膨胀型防火密封胶可有效抑制温度升高。

针对每个试样组的两个试样建立了有限元模型（finite element model），以进一步量化并研究间隙内的传热现象。间隙内的对流作用有限，且基本不受间隙宽度的显著影响，尤其是在间隙深处。若仅依据间隙几何形状计算辐射量，则会显著低估辐射暴露量。因此需对有限元模型中的辐射参数进行校准，以纳入间隙内燃烧、间隙表面间的辐射、可变材料特性以及间隙内燃烧过程中发生的其他复杂传热现象。为使3mm间隙试样的试验结果与数值模型结果达到良好吻合，在试验的前70分钟内，需将钢质构件受到的辐射强度较仅依据几何形状计算的辐射值提升5倍。

第二组炉内试验采用的是专属隐蔽式梁吊节点（proprietary concealed beam hanger connection），试样的间隙尺寸设置与第一组试验一致。该组试验结果表明，应避免采用未防护的6mm及10mm间隙。未防护的0mm及3mm间隙试样表现更佳，但实测温度仍存在较大离散性。膨胀型防火密封胶通常表现良好，可显著抑制铝支架的温度升高：与0mm间隙试样的支架温度相比，未防护试样的温度在60分钟时平均升高62%~258%，而防护试样的温度变化仅为-4%~21%。在该类节点中，使用膨胀型防火密封胶可提升节点热响应的可预测性，且3mm间隙防护试样的温度最低。

实际施工中，制作无间隙的隐蔽式节点极具挑战性，因此制定实用指南以限制间隙对节点热响应的影响至关重要。