Given the importance of STCs in timber-framed shear walls, and given the fact that building materials are exposed to the weather during construction, it is somewhat surprising that relatively little research has been undertaken on the effects of weathering on the structural performance of STCs. Before looking at the literature on weathering of STCs, it is helpful to provide a broad overview of weathering of timber and wood-based composites.
2.2. Weathering of Sheathing-to-Timber Connections
There are a handful of studies on the types of weathering processes (i.e., flooding, wetting, artificial weathering, and natural weathering) and their effects on the structural performance of shear walls or sheathing-to-timber connections [
4,
5,
11,
13,
14,
15,
16,
17,
18]. The following review is presented in chronological order.
Leichti et al. [
13] tested nine (
)
timber-framed shear walls under three different scenarios (
): (1) dry then tested with a monotonic loading protocol, (2) dry then tested with a cyclic loading protocol, and (3) submerged in
of water for seven days, then air dried and tested with a cyclic loading protocol. The test panel framing comprised
Douglas fir double top plate, single bottom plate, studs at
spacings, and double studs at the ends of the test panel. The test panel sheathing was
Exposure-1 OSB panels fixed to the framing with
(
)
(
) 8d gun-driven nails at an unspecified spacing. The test panels in the monotonic and cyclic control groups achieved a mean ultimate capacity of
and
respectively. The test panels subjected to the simulated flood achieved a slightly higher ultimate capacity of
; however, the authors noted a moderate loss of initial stiffness (i.e.,
for the flooded test panels compared to
for the cyclic control group).
Nakajima and Okabe [
14] tested
nailed sheathing-to-timber connections in 4 different configurations, with 2 types of sheathing (i.e., plywood and OSB with thickness unspecified) fixed to unspecified timber using “common nails
in length (CN50)”, under 3 different climate conditions: (1) conditioned at 20 °C and
relative humidity (RH) for one week, (2) as per (1) plus conditioned at 20 °C and
RH for three weeks, and (3) as per (2) plus conditioned at 20 °C and
RH for three weeks. The results showed that plywood specimens lost approximately
strength when tested following high-humidity conditioning but regained that capacity after the additional three weeks of conditioning in low humidity. The capacity of OSB specimens was unaffected by humidity conditioning but failure modes of all specimens were significantly affected by climate conditioning, with a marked increase in the number of pull-through failures in the high-humidity specimens.
Beall et al. [
15] built an unspecified number of small
shear walls with two types of sheathing (i.e.,
Structural-1 Douglas fir plywood and
Exposure-1 OSB) fixed to 2″ × 4″ kiln-dried (KD) Douglas fir with 8d nails in various configurations under three different climate conditions: (1) standard “dry” condition, (2) conditioned at
RH for six weeks, and (3) wetting of the bottom plate to simulate green timber. The results showed moisture conditioning had minimal effect on the structural performance of small-scale shear walls.
King et al. [
16] built
small
shear wall test panels with
Exposure-1 OSB fixed to
KD Douglas fir with
(
)
(
) gun-driven Senco nails and tested under dry and wet conditions. Half the specimens were given fungal inoculations to promote decay. A cyclic wetting regime was implemented and
specimens were tested after
,
,
,
,
, and
days of wetting. Their study showed that the strength of wetted specimens was
to
higher than dry specimens. The result was statistically significant with
. The authors speculated that the higher capacity of the wetted specimens was due to corrosion of the connectors, which created more friction in the connection. As an aside, this hypothesis has been tested in nail withdrawal studies by Yermán et al. [
19], who conducted
nail withdrawal tests under several different scenarios and confirmed that corrosion of fasteners does, in most cases, increase nail withdrawal capacity after several cycles of wetting and drying (also correlating with increased corrosion); however, the capacity then declines with additional cycles of wetting and drying, while corrosion of the fasteners remained relatively constant.
Bradley et al. [
17] made some improvements to the methodology of [
13] and constructed nine
(
)
(
) timber-framed shear walls which were then tested with a monotonic loading protocol under three different scenarios (
): (1) dry, (2) wetted for five days in
of water, and (3) as per (2) plus dried at 38 °C and
RH. Test panel framing comprised
Douglas fir top and bottom plate and studs at
spacings. Test panel sheathing was
“Norboard” OSB/3 panels fixed to the framing, with
(
)
(
) gun-driven nails at
spacing around the perimeter of the OSB sheets and
spacing along internal studs. Test panels under the three scenarios achieved a mean ultimate load of
(dry),
(wet), and
(restored), respectively. These results differ remarkably from those of [
13], with the loss of ultimate capacity between the control group and the other two groups being statistically significant with
. The authors also noted a statistically significant loss of approximately
initial stiffness and a slight reduction in the ductility (not statistically significant) of the wet and restored groups compared to the control group. The authors note that their result is consistent with other studies on weathering of sheathing materials but not STCs (not reviewed here), which show that flooding has a negative effect on building materials, whereas Leichti et al. [
13] reached a different conclusion on the effect of flooding on the ultimate capacity of shear walls.
Maqsood et al. [
18] constructed twenty
(
)
(
) timber-framed shear walls using two different sheathing types, described simply as OSB and high-density fibreboard (HDF), which were then tested with a cyclic loading protocol under two different scenarios (
): (1) dry and (2) immersed in
of water for four days and allowed to air dry for six weeks. Timber framing was described as MGP10 with studs at
spacings; however, the sheathing thickness and timber framing sizes were not provided. Sheathing was fixed to the framing with
(
)
(
) nails. OSB test panels had nails at
spacing along the top and bottom plates,
spacing along the vertical edges of the sheets, and
spacing along internal studs. HDF test panels had nails at
spacing on all studs and plates. OSB test panels achieved a mean ultimate capacity of
for the dry group and
for the wet-then-dry group. HDF test panels achieved a mean ultimate capacity of
for the dry group and
for the wet-then-dry group. The difference in ultimate capacity was statistically significant for the HDF test panels, with
on a one-sided rank-sum test; however, there was no significant difference between the two groups of OSB test panels.
Way et al. [
11] tested the lateral nail resistance of
sheathing specimens (i.e., not connected to a timber substrate) with nails driven near the edge using 2 different sheathing types,
thick aspen OSB and
thick Douglas fir plywood, of 3 different sheathing widths (
,
, and
) under 4 different scenarios (
): a control group tested dry and 3 different methods of accelerated weathering. Nails of
(
)
(
) were driven into the sheathing and tested in lateral shear. The results showed no statistically significant difference in capacity between the control group and weathered specimens.
Poletti et al. [
5] tested the performance of nine traditional half-lap joints using
Pinus pinaster timber connected in three different configurations (
): (1) unreinforced, (2) reinforced with four screws, and (3) strengthened with steel plates. One specimen in each group was subject to wet/dry cycles for
days and one specimen in each group was subject to the same wet/dry cycling for
days. The last specimen in each group was subject to the same wet/dry cycling for 4 days followed by submersion in water for 7 days. All specimens were allowed to dry for 7 days prior to testing. The results were compared to previous testing of sound connections [
20] showing that all groups experienced a decrease in capacity of
to
due to weathering.
Way et al. [
4] built 12
timber-framed shear walls using 2 different sheathing types,
thick Exposure 1 OSB and
thick Exposure 1 plywood, and tested using a monotonic loading protocol under 2 different scenarios (
): (1) dry and (2) with sheathing which had been subjected to accelerated weathering over a 28-day period followed by undercover storage for
days prior to assembly. Test panel framing comprised
KD Douglas fir double top and bottom plates, studs at
spacings, and double studs at the ends of the test panel. Test panel sheathing was fixed to the framing with
(
)
(
) 8d nails at
spacing around the perimeter and
spacing along intermediate studs. The results showed that test panels constructed with weathered OSB lost
ultimate capacity and
in energy dissipation (statistically significant with
), whereas those constructed with weathered plywood experienced a small, but statistically insignificant, gain in ultimate capacity of
and loss in energy dissipation of
. Curiously, test panels with weathered sheathing had higher initial stiffness, although this result was not statistically significant.
To summarise the literature on weathering of STCs, some studies show that weathering can lead to a loss of up to
in the structural performance of traditional joints and timber-framed shear walls [
5,
17]; others show that weathering/flooding can improve the structural performance of STCs and timber-framed shear walls by as much as
[
13,
16]; most studies show that weathering/moisture conditioning has no significant effect on the structural performance of STCs and sheathing [
11,
14,
15]; one study showed that flooding had no effect on the structural performance of timber-framed shear walls with OSB sheathing, but did have a
lower capacity on walls with HDF sheathing [
18]; and one study showed that weathering did not affect the structural performance of timber-framed shear walls with plywood sheathing, but did have a
lower capacity on walls with OSB sheathing [
4]. Importantly, all these studies used artificial weathering techniques. None of the studies in this review adopted a natural weathering methodology.