For anyone whose design intuition is based In steel construction, bolts are the boss. Their familiarity make them the first choice for structural connections. But in mass timber - a material with vastly different properties to steel - 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.

Go With the Grain

Certainly — here is a slightly more formal version without any formatting:

Both steel and timber are considered strong materials, which is why they are among the primary choices in construction. At the scale of structural elements such as beams, it can be argued that timber is even stronger on a mass basis, often performing better than steel. However, when it comes to connection design, relative strength is influenced far more by localised compressive capacity, and at this scale there is a significant disparity between steel and timber.

It is common knowledge that steel is extremely hard. Concentrated compressive forces rarely cause stress levels in steel that require detailed consideration. In most cases, bolts or connection plates will yield well before compressive failure becomes a factor. This means that when bolts are under-designed, the typical solution is simply to increase their size - for example, from M16 to M24 - then recheck the calculations and if all is OK move on.

Timber is strong, but not comparably hard. As anyone with Baltic pine flooring will know after a guest has walked across it in stilettos, timber has a relatively low compressive strength and this changes the behaviour of connections entirely. Faced with the same design issue in a timber connection - an underperforming bolted connection - increasing bolt size seldom resolves the problem and can, in many cases, actually reduce the connection’s capacity.

This is sometimes viewed as a weakness of timber, but the material is not at fault. Rather, it requires a different design approach. We need to let timber be timber.

Where bolts can fall short in timber, screws often excel. This is because screws are able to engage timber fibres along their full length, rather than relying on concentrated bearing at a limited number of points. Screw connections reduce localised high stress concentrations and distribute load more evenly, engaging a larger volume of timber.

Additionally, screws can be installed at angles to better align forces with the stronger directions of both the timber elements and the tensile direction of the screws (steel performs excellently under tensile forces).

Screws can then allow for an approach harmonises better with the material characteristics of timber. Rather than forcing the material to accommodate standardised connection logic from steel design, screws allow us to tailor the connection to timber’s strengths - dispersing forces and engaging with the grain better.

The result can be more robust connections with better stiffness, strength and resilience when subjected to extreme or unexpected loading.

The subject of screw connection in mass timber is a vast and complex field of knowledge that would require several careers to fully understand, so we are going to share some of our insights and experiences in some further pots to help promote what is a very important area of mass timber design.