Historically, one of the biggest drawbacks in using timber and mass timber products for building’s main load bearing structure was the massive sections needed to overcome relatively long or medium long spans.
During the last few decades there have been significant improvements in the production of engineered wood products that have much better bending stiffnesses and bending strength (i.e hardwood glulam, hardwood LVL), but for these products shear strength has remained relatively low. Furthermore, this evolution of production methods of engineered wood products has led to complex beam shapes such as curved or tapered beams which have larger cross sections in mid span where the largest bending moment is expected and smaller cross sections at the supports.
As much as these evolutions in production made timber a more versatile building material, an essential problem remained which was the shear strength. Timber naturally having a lower shear strength meant that no matter how much improvement is made to the bending strength larger cross sections will be needed in order to not have shear failure at the supports.
To overcome this problem, research has been done on the use of fully threaded self-tapping screws and threaded rods as reinforcement elements. The analytical method for designing the screws as reinforcement has been presented by Dietsch, Kreuzinger and Winter in CIB-W18/46-7-9 “Design of shear reinforcement for timber beams”. In this paper are presented analytical methods for two situations: when the timber beam is in an unfractured state and when it is in a fractured state. Fractured or unfractured state describes the state in the beam before and after cracks develop due to excessive shear loads.
Analytical methods for designing shear reinforcement in both of these states are based on the theory for composite materials. In the unfractured state it is assumed that there is composite action between the timber and reinforcing elements. If glued-in rods are used as reinforcement than a rigid bond between the reinforcing element is assumed and if self-tapping fully threaded screws are used than a semi-rigid bond is assumed.
In the fractured state we assume a composite action between the timber parts above and below the longitudinal crack due to shear failure. Basically, the beam acts as a mechanically joined beam with the shear reinforcement acting as the connection between these two parts. Using the shear analogy, we can calculate the composite action between the two parts in the fractured state.
During destructive tests it was found that although a longitudinal shear fracture occurred, the reinforcing elements were still intact and able to carry loads. This means that shear reinforcement can be designed to carry the full shear stresses or tension perpendicular to grain in order to prevent the full separation of the two parts of the timber beam. This means that shear reinforcement acts as a second barrier against brittle failure of the beam and provides internal redundancy.
If all of this seems daunting and complex don’t worry because some screw manufacturers provide shear reinforcement design in their ETA’s which greatly simplifies the needed calculation.
This simple calculation is made even more accessible for engineers with CLT Toolbox’s Shear reinforcement calculator. Input the beam size, material, loading situation and select one of several screw suppliers which provide the design method for shear reinforcement in their ETA’s.
