For anyone whose design intuition is based In steel construction, bolts are the boss. The project strength, certainty, and their familiarity make them an obvious choice for structural connections. But in mass timber — a material with vastly different properties — this intuitive response doesn’t hold true.

While they look robust to the untrained eye, bolts can introduce issues if used indiscriminately. The material behaviour of timber subverts many of the reasons bolting is so effective in steel design - and that’s why we should drill down to better understand why screws are so effective in mass timber design.

Let Timber be Timber

Both steel and timber are "strong" and this is why they are part of the primary materials utilised in construction. At the scale of structural elements such as a beam it can be argued that timber is "stronger" - on a mass basis timber can perform better than steel. But when considering connection design relative strength is dictated far more by local areas of compressive strength and there is a large disparity between steel and timber at this scale.

Steel is as hard as, well… steel - which is really hard. Even highly focussed forces that generate points of high compression in steel rarely result in stresses that require consideration in design. Normally, bolts or connection plates will yield long before this becomes a factor. What this means in practice is that if your bolts aren't big enough for design loads, you just make your bolts bigger - M16 bolts are not working. Up them to M24s, do a check and move on.

Timber is strong but not nearly as hard (as anyone with Baltic pine floorboards would know after someone walks on them with stilettos would know). Timber's relatively low compressive strength completely changes the dynamics of connection design. Faced with the same design problem - my bolted connection isn't strong enough - increasing the bolt size seldom solves the issue and can sometimes even reduce the strength of the connection.

Some see this as a weakness in timber, but I say timber isn't the problem, you just need to let "timber be timber".

Timber's softness can be beneficial and I will explore this in Pt. 2.