Hi all,
I’m working on a timber connection where a steel plate is bolted through the timber with four through bolts arranged in a square. The plate hangs about 2 feet below the timber, and the load is applied at the free end of the plate — so the load is purely axial (vertical) but eccentric relative to the bolt group.
My understanding:
This eccentric axial load causes a moment that tries to rotate the plate. As a result, the bolts on one side compress against the bolt hole edges, and the bolts on the other side experience tension. This rotation induces lateral bearing forces between the compression bolts and the timber hole walls, potentially causing localized crushing perpendicular to grain in the timber.
Concern:
While the axial load in the bolts is clear, I believe these lateral bearing forces caused by rotation can be significant and may govern the design because timber is weak perpendicular to grain. This means bolt hole crushing can be a real failure mode even though there is no direct shear force applied to the bolts.
Counterpoints I’ve encountered:
My thoughts:
I agree the plate bears against the timber surface, but the eccentricity induces rotation, which creates lateral bearing forces at the bolt holes independently from the plate bearing. These lateral forces are not shear in the traditional sense but still cause pressure on the timber perpendicular to grain at the bolt holes.
My questions:
I’d appreciate any insights, references, or practical experiences related to this issue. Thanks in advance!
I’m working on a timber connection where a steel plate is bolted through the timber with four through bolts arranged in a square. The plate hangs about 2 feet below the timber, and the load is applied at the free end of the plate — so the load is purely axial (vertical) but eccentric relative to the bolt group.
My understanding:
This eccentric axial load causes a moment that tries to rotate the plate. As a result, the bolts on one side compress against the bolt hole edges, and the bolts on the other side experience tension. This rotation induces lateral bearing forces between the compression bolts and the timber hole walls, potentially causing localized crushing perpendicular to grain in the timber.
Concern:
While the axial load in the bolts is clear, I believe these lateral bearing forces caused by rotation can be significant and may govern the design because timber is weak perpendicular to grain. This means bolt hole crushing can be a real failure mode even though there is no direct shear force applied to the bolts.
Counterpoints I’ve encountered:
- The force on the bolts is purely axial and perpendicular to the plate, so no shear force exists to cause bearing failure.
- The compression force is mainly transferred from the plate bearing on the timber, not from the bolt pressing into the hole, so bolt hole crushing isn’t a critical concern.
My thoughts:
I agree the plate bears against the timber surface, but the eccentricity induces rotation, which creates lateral bearing forces at the bolt holes independently from the plate bearing. These lateral forces are not shear in the traditional sense but still cause pressure on the timber perpendicular to grain at the bolt holes.
My questions:
- Is it valid to consider bolt hole bearing (crushing) as a design concern under eccentric axial loading in timber connections?
- How can the lateral bearing forces caused by the moment be quantified in the absence of direct shear?
- Are there established design guidelines, references, or examples that address this phenomenon?
I’d appreciate any insights, references, or practical experiences related to this issue. Thanks in advance!
