Heritage Timber, Tree Rings, and Fire

Post and beam

Slow burning timber construction methods  circ. 1890

The past few days we have been sorting through the structures lab after the end of the last academic year. Among the materials which we were indexing and storing were the heritage timbers that we tested for the CSCE paper (posted below). The timbers were installed in a heritage building built approximately in 1890 or so. In a retrofit of a building they were removed. We tested the timber beam first in structural loading. The beam was tested for our second year undergrads to see. Then we extracted two planks from the timber as it only had moderate damage for flame spread testing (to be compared to modern engineered lumber of the same moisture content). Since then the planks have sat. Looking at the timbers myself and Mina Li, opted to count the tree rings this week to estimate the timbers age yesterday. Relating to Canada’s 150 we were in for a bit of a shock.  We counted approximately 180 rings on one of the pieces of lumber. In one sense because the lumber was cut from a solid tree and reduced to a square column we estimate that about 20 or so additional rings were probably present and unaccounted in our estimate. So in that sense this tree that this lumber came from was growing in the 1600s!! Amazing piece of Canadian history, and joy of this was that the students could see the lumber tested in both a ambient structural test and a flame spread fire tests. The timber will be preserved for future teaching demonstration exercises. In the mean time, a preliminary report of these tests has been prepared and is available in the public domain. You can download our paper here that we presented at CSCE led by student researcher Arlin Otto, who will be joining Arup UK this summer.

Beam comparison

Comparison of heritage versus modern engineered lumber in flame spread tests of representative and equal moisture content.

Above is the structural video of the timber beam being loaded only to peak load. Very little damage was instigated. The beam dimension were 290 x 240 mm. The largest beam we tested was 300 x 270 mm.

As we move towards tall timber, see my talk at ASTM last month, we undoubtedly will be left with the question of how engineered timber compares to the historical counterparts. And there I see a lot of confusion that we need to appreciate and consider – hence studies like this. Ill get into that at another time, but i do want to highlight the manufacturing differences as they really are not the same things in my opinion. More to come on that topic.

image6

I am often asked the differences between heritage and engineered construction. This exposed timber column can be found in Vancouver. It is about 400x400mm. These sizes are common to see in heritage construction. 200×200 can also be found as common sizes.  Today engineered timber is made up of thin laminations glued together to make larger sizes.

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