Performance of joints for glued laminated timber exposed to the ISO fire temperature curve

Abstract

As society becomes more ecologically conscious there is a push toward the use of materials with a smaller environmental burden, one of these materials is glulam (GLT), which has gained a lot of traction in recent years not only for ecological reasons but for its ease in manufacture, transport, noise dampening, etc.
The focus of this project was to evaluate the influence of different joint configurations on the fire performance of glulam members. To accomplish this 4 different samples were made each with variations in the type of fastener, plates, or protection used; these were fitted with a jack system to simulate the forces that glulam members experience during their use, and were afterwards exposed to 30 minutes of the ISO834 fire curve.
During the time of exposure, the size of the gap was measured as well as the temperature in certain relevant areas of the samples. For the unprotected samples tested the temperatures on the exposed side rose above 800 °C meaning a reduction of more than 90% of their yielding strength by the end of the test. In the case of the protected sample this meant only a 4.39% loss.
Additionally the affected areas were studied to determine the charring rate and burning patterns generated by the specific joint used, for the first joint this charred rate was 1.3±0.1 mm/min, on the second joint 1.16±0.07 mm/min, and 1.23±0.13 mm/min for the third. Finally, the protected joint did not have a measurable amount of charr on the joint itself. In some cases, these charring rates went above from what was expected using the Eurocodes safety factor for joint charring, which was a value of 1.14 mm/min or close to 14 % less than the one obtained in test 1.
The experiments showed the susceptibility of the unprotected joints under high temperatures, significant differences in the behavior depending on the configuration, and the efficacy of fire protection over the mechanical attributes of joints.

Popular Abstract

As the year pass, we are becoming a more ecologically conscious society, this means that there is a push for new buildings that put the wellbeing of our planet as a priority. For this reason, there has been a lot of interest in making new constructions made mostly out of wood products (as these are more sustainable), however legislators and concerned individuals have pushed back due to concerns about the fire safety. As anyone that has lighted a fireplace knows, wood can burn so these concern at not entirely unfounded.
To try to further the understanding of the fire behavior on timber buildings a study was done on an often-overlooked part of timber constructions, joints.
Joints are vital for any building, they are the glue that hold the structure together, and depending on the design of a building these joints can be very different from one another and so will their behavior against fire.
For this reason a study was done with 4 different configurations of joints in glue laminated timber to see the effects of different configurations such as: the size of the plate attaching the structural member (smaller vs bigger), the type of mechanical fastener used (shank nail vs self-tapping screw), and the protection of the joint. These configurations were then heated in very high temperatures for 30 minutes. During the testing a force was applied axially to simulate the forces that a timber member goes through during its regular use.
The results from the testing showed that the fasteners and the plates were transferring heat to the timber increasing the speed of the charring in the area under the joint. And although this is somewhat taken into consideration in the Eurocode regulations, the charring was seen to be more severe that the estimations provided by it, making designs following this regulation somewhat under conservative.
Another pattern that was seen was that the use of bigger plates leads to a more even charring and by consequence more endurance of the mechanical properties of the element under high temperatures.
This was also seen for the screws, since this type of fastener had a smaller amount of mass, the transfer of heat to the material was smaller than for the nails, showing that the use of slimmer fasteners could improve fire resistance.
The final test performed on a protected joint had the best mechanical performance, as the jointed area did not char significatively nor the temperature was able to alter the yielding strength of the metallic parts in a meaningful way.
The results from the tests were compared to those seen in previous research studies and the trends showed by this project were also observed in the literature giving some confidence that the findings from this project were heading in the right direction.
Although more testing should be performed to deepen our knowledge on the topic this thesis showed the importance on choosing the right configuration of fastener as a small variation on the type of fastener used can have a big influence on the performance and safety of a building.

• author: Gil, Jesus
• supervisor: Bjarne Husted
• organization: Division of Fire Safety Engineering
• course: VBRM05 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Glued laminated Timber, Structural fire engineering, Fire performance, standard ISO fire curve, Joints
• publication/series: LUTVDG/TVBB
• report number: 5638
• other publication id: LUTVDG/TVBB-5638-SE
• language: English
• id: 9048812
• date added to LUP: 2021-06-04 08:36:10
• date last changed: 2021-06-04 08:36:10