Reuse of Timber Screws for Lifting: From Jobsite Practice to Verified Reuse

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Lifting in mass-timber construction is a high-consequence operation: a single connection failure can lead to a dropped load, with potential for severe injuries, fatalities, and major property damage. Industry safety rules therefore treat suspended loads as a critical hazard and require exclusion zones and rigorous controls. In this context, any proposal to reuse removable fasteners in lifting must be grounded in demonstrable mechanical performance and conservative screening rules.

Despite that need, the sector routinely reuses screws used to install temporary lifting devices. This practice persists even though, as a general state of the art, wood screws are not sold with a “reuse” designation for lifting; technical documentation commonly prescribes single use. The implication is not that reuse is always unsafe, but that end-users are currently assuming the risk without an objective method to screen used screws and without clear alignment between manufacturers’ assumptions, standards, and field practice.

A second driver is sustainability. Single-use assumptions for removable fasteners can inflate both material demand and the embodied carbon attributed to lifting operations. If reuse is already happening, a verified reuse pathway becomes relevant not only for safety, but also for life-cycle assessment (LCA) calculations that seek to represent real practices.

A dedicated research program was therefore set up to (1) characterize the mechanical behavior of used screws in lifting applications and (2) translate test outcomes into a practical, objective reuse protocol. The experimental work combined campaigns at the University of Maine and the University of Bologna with additional testing at Rothoblaas (an International company that designs, manufactures and distributes high-tech solutions for timber construction) laboratory facilities, with the explicit aim of checking whether resistance degradation occurs after simulated reuse and whether specific damage modes control the outcome. In parallel, a dedicated (Life Cycle Assessment (LCA) study on cross-laminated timber (CLT) lifting operations was carried out in collaboration with Politecnico di Milano (Polytechnic University of Milan) to quantify the environmental implications of reuse scenarios for lifting screws.

The test campaign was structured in three phases: 1) single‑screw analysis carried out to assess geometric and mechanical parameters under simulated reuse‑cycle conditions; 2) system level analysis of the lifting hook configuration comparing new and used screws to evaluate global behavior, ultimate resistance, and failure modes under representative loading; and 3) system analysis with controlled deformation or damage introduced to the screw/system to understand sensitivity to realistic (but extreme) misuse conditions.

System-level tests were performed on the lifting-hook assembly under pure tension, pure shear, and combined tension–shear loading; combined loading was applied using a dedicated jig set to 45 degrees, as shown in Figures 2 and 4.

The experimental matrix was designed around the factors most likely to influence the performance of screws in lifting applications. It therefore included reference tests, reuse-cycle simulations, and system tests in which “representative damage mechanisms” (RDM) were intentionally induced (Fig. 3). The underlying premise of the RDMs is the hypothesis that the primary concern is not reuse per se (Phases 1 and 2), but reuse combined with plastic deformation or damage introduced by installation errors and/or overload events, which can alter resistance and failure mode (Phase 3).

The experimental campaign included both a short-screw series and a long-screw series (lengths: L = 80 mm (3 1/8 inches) and L = 180 (7 1/8 inches), respectively) both with diameter 10 mm (0.40 inches), to capture sensitivity to embedment and withdrawal contribution.
Three RDMs were investigated based on their potential to reduce the capacity of the lifting system:

At the system level, the study then evaluates how reuse and these damage mechanisms correlate with mechanical performance under pure tension, pure shear, and combined tension–shear loading.

Across the system tests, reuse cycles alone had limited influence on ultimate resistance when compared with reference assemblies. The largest variability tended to appear in pure shear —especially when simulated cycles were applied at load levels high enough to occasionally approach or exceed plastic limits in the connector or plate—yet results remained above the system’s declared maximum operational load capacity. This supports a key distinction: reuse itself is not the dominant risk driver; rather, plastic deformation and damage to the screw and/or the timber substrate are.

Results were analyzed at CIRI-University of Bologna to quantify the effect of each damage mechanism on load-carrying capacity. As shown in Figure 5, the damage effect was first expressed as the percentage difference between the mean capacity of the reference tests (FRef) and the mean capacity of the damaged configuration (FDam), and summarized in a matrix using discrete ranges mapped to a color scale.

A second matrix (Fig. 6) compares the mean force at the end of the linear branch of the load–displacement curve (FElastic) with the declared maximum operational load capacity (RWLL/WLL) used for lifting practice (Machinery Directive 2006/42/EC, which lays down health and safety requirements for machinery in the European Union market). This comparison supports the safety interpretation because FElastic > RWLL indicates elastic-range operation under correct working conditions also under a deformation mechanism.

The damage-mechanism campaign helped distinguish which misuse scenarios actually control performance—and which are secondary:

A final step in the analysis was to plot the tension–shear interaction domains for each damage mechanism. Figure 7 summarizes the normalized interaction domains for the tested configurations and relates the experimental envelope to both the maximum operational domain (RWLL/WLL) and the certification-level limit domain (4×RWLL). This comparison helps distinguish cases that remain within the elastic-range operating region from intentionally severe misuse cases that approach limit conditions.

Most importantly, two safety-relevant observations emerge when results are interpreted against the operational and limit conditions used in lifting practice. First, for all damaged configurations the mean force at the end of the linear branch of the load–displacement curve (elastic limit) exceeded the maximum operational load capacity declared for the lifting system (RWLL, i.e., the rated Working Load Limit used as the maximum allowable service load). This indicates that when the lifting operation respects the RWLL, the connector works in the linear elastic range and is not expected to accumulate use-induced damage; therefore, correct installation becomes the governing prerequisite for any verified reuse pathway. Second, the tension–shear interaction domains lay outside the declared operational domain and were also outside the 4×RWLL limit condition that reflects the safety factor typically applied for certification under the Machinery Directive (Directive 2006/42/EC), except in a small subset of intentionally severe cases (notably over-torque for short screws and a combined case involving thread damage). Reaching this threshold is an important milestone: it places the experimental evidence within the same certification-level safety framework used to qualify lifting equipment, providing a clear and internationally recognized benchmark for interpreting reuse conditions.

The mechanical results alone are not a permission to reuse; they are inputs to a conservative decision framework. The practical challenge is that the most consequential damage mechanisms are not always obvious to a jobsite crew and can accumulate across repeated installation/removal. A reuse protocol therefore must be simple, objective, and biased toward discard when uncertainty exists.

The protocol developed from this project follows three principles:

In addition to pass/fail screening, reuse is governed by a bounded reuse count, implemented through an objective coating-wear indicator checked against an acceptance region, as described in the next section.

Only if all checks are satisfied can the screw be reused. The protocol is paired with process controls—controlled installation/removal torque, dry and segregated storage for used screws, and immediate discard after any abnormal event (suspected overload, impact, or loss of control of the suspended load). Finally, a bounded reuse count or traceability method further reduces uncertainty about cumulative damage.

Even when “reuse alone” is not strongly correlated with strength loss, a bounded reuse count reduces uncertainty about cumulative damage and procedural drift. A practical way to implement this control is to use coating wear as a traceability indicator by confirming whether the wear line/area falls within an acceptable region.

For example, VGS PLATE screws (the reusable lifting screw from Rothoblaas) use a coating system composed of a chrome-passivated electroplated zinc layer, a black e-coating topcoat, and an additional wax layer. Beyond corrosion protection and reduced insertion friction (and easy identification as lifting screws), these outer layers exhibit a progressive wear-off after repeated insertion/removal, as Figure 9 shows. When linked to predefined acceptance criteria, this provides an objective visible threshold to control maximum reuse count. As displayed in Figure 10, in practice, after the screw passes the straightness check, it is inserted fully into the reuse jig and the wear indicator is checked against the acceptance region; if the threshold is exceeded, the screw is discarded.

The sustainability benefit of reuse is straightforward: distributing the production impacts of the screw across multiple lifting operations reduces the impact per use. An LCA performed by Politecnico di Milano confirms that the global warming potential per use decreases with increasing reuse count, tending toward an asymptote as the number of uses grows (Fig. 11). The highest marginal benefit is realized in the first few reuses; additional reuse still helps, but with diminishing returns.

In this case, impacts are allocated per use by distributing the production stage across multiple cycles and by accounting for deconstruction/removal and end-of-life assumptions (including mixed scenarios intended to represent realistic waste management).

A verified reuse protocol therefore has value beyond safety: it provides the documentation needed to justify a multi-use assumption in LCA models, procurement decisions, and potentially in future product declarations.

For lifting connectors installed in timber elements, reuse of screws is already occurring in the field. The critical question is not whether reuse happens, but whether it can be made measurably safe. The test evidence indicates that, when installation and use are controlled so that screws operate within the elastic range, a limited number of reuses does not compromise system strength. At the same time, installation errors and overload-related damage mechanisms can markedly reduce resistance and induce premature failure—especially over-torque on short screws, which primarily damages the timber support (fibers/counterthread) rather than the connector itself, and damage modes that reduce withdrawal capacity.

A practical reuse protocol, supported by objective screening tools and conservative rules, is therefore the essential bridge between mechanical evidence and safe practice. In parallel, the existence of a verified reuse pathway enables more realistic LCA assumptions for lifting operations and supports future work toward product and application guidance that recognizes reuse in a controlled manner. ■

About the Author

Andrés Reyes is Product Line Manager at Rothoblaas. He also serves as an industrial expert at CEN/TC 124/WG 4, the European standards working group on fasteners and connectors for timber structures, contributing to the development of European standards for the timber sector. Reyes regularly presents technical topics related to timber connections and fastening systems at seminars and international conferences.