It smells burnt because it, of course, is a product of a high heat treatment of timber, commonly pine or fir, but also hardwoods, to impart various characteristics such as colour change, reduced brittleness and movement in response to changes in moisture content, but the big characteristic is improved durability in non-ground contact situations. A result of the treatment, particularly that for durability which requires high temperature (~230 deg C for extended periods) includes brittleness amongst other lesser inconveniences or disadvantages.
I've used high heat treated birch for furniture and brittleness wasn't a problem because the treatment was primarily undertaken for colour change, so the temperature used was lower and for less time, but my understanding is that material treated to impart durability characteristics (high heat, extended periods) should be evaluated carefully because of the resultant brittleness for situations where strength is important. Slainte.