RETROFITTING WOOD FRAME CONSTRUCTION AGAINST WIND DAMAGE

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional Application 63/701,186 filed Sep. 30, 2024, the contents of which are incorporated herein in their entirety.

TECHNICAL FIELD OF THE INVENTION

This invention relates to strengthening wood-frame construction, specifically to systems and methods of reinforcing connections between structural members to increase their load-bearing capacity. Aspects of methods of the present disclosure may involve adhering wood blocks to wood framing members using a high-elongation elastomeric adhesive, in some aspects and embodiments, an elastomeric adhesive comprising polyether. This enhancement aims to improve the resistance of wood-frame structures to high winds.

BACKGROUND OF THE INVENTION

The present disclosure provides for reinforced wood structures including framing members as well methods to reinforce the connection between structural members to increase the load bearing capacity of framing member intersections by adhering wood blocks to the wood framing members on each using, in some embodiments, a high elongation elastomeric adhesive, in some embodiments comprising polyether.

An increase in uplift bearing capacity was demonstrated through a comprehensive testing program that included both single connection tests and full roof assembly evaluations. The application of wood blocks with adhesive has been shown to effectively retrofit wood structure connections, enhancing uplift capacity by up to three times compared to non-retrofitted connections using toenails only, and by up to one and a half times compared to connections utilizing hurricane straps.

Test results from dynamic wind uplift pressure on roof assemblies indicate that the wood block adhesive application increased the design uplift pressure by 50%, corresponding to an increase in design wind speed resistance from 137 mph to 176 mph. Moreover, the wood block adhesive application exhibited improved serviceability by enhancing energy dissipation, which helped control deflection. This performance mitigates the risk of crack propagation in non-structural components, such as drywall and ceilings, prior to reaching the ⅛-inch deflection criterion.

SUMMARY OF THE INVENTION

The present invention provides a solution by increasing the tensile and torsional bearing capacity of wood structures at framing member intersections. This is achieved by adhering wood blocks to the wood framing members using an elastomeric or high-elongation elastomeric adhesive, in some embodiments comprising polyether. These method significantly enhance the connection between structural members, leading to improved overall structural integrity.

By utilizing an elastomeric adhesive, the method avoids the high costs and application complexities associated with polyurea coatings. Unlike polyurea, which requires specialized equipment and controlled environments for application, the polyether adhesive can be applied without such constraints. This makes the method more practical and accessible for both new constructions and retrofitting existing structures.

Embodiments of the present disclosure may include a system for strengthening wood-framed roofs including: at least one rafter and at least one top plate, wherein said rafter is located above said top plate; and wherein said rafter and top plate are affixed to each other by a first block located above said top plate and adjacent to a first side of the rafter; and wherein said block is affixed to said rafter and said top plate by high-elongation adhesive.

In others, the high-elongation adhesive comprises a comprising silyl modified polymer. In others, the silyl-modified polymer comprises silyl-modified polyethers and/or silyl modified polyurethanes. In others, the high-elongation adhesive comprises a silyl modified polyether. In others, the high-elongation adhesive comprises an amino silane

In yet others, the block comprises wood. In others, embodiments may further include a second block affixed to a second side of said rafter and the top of said top plate, wherein said second block is affixed to said rafter and said top plate by high-elongation adhesive.

Some aspects of the present disclosure include a method for strengthening wood-framed roofs comprising, the method including: affixing a first block to least one rafter and at least one top plate, wherein said rafter is located above said top plate and adjacent to a first side of the rafter; and wherein said block is affixed to said a least one rafter and said at least one top plate by placing high-elongation adhesive at the junction of the at least one rafter and a first surface of the first block and the junction of the at least one top plate and a second surface of at least one block.

In others, the high-elongation adhesive comprises a comprising silyl modified polymer. In others, the silyl-modified polymer comprises silyl-modified polyethers and/or silyl modified polyurethanes. In others, the high-elongation adhesive comprises a silyl modified polyether. In others, the high-elongation adhesive comprises an amino silane

In yet others, the block comprises wood. In others, embodiments may further include a second block affixed to a second side of said rafter and the top of said top plate, wherein said second block is affixed to said rafter and said top plate by high-elongation adhesive.

DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts construction details of a typical rafter-to-top plate toenailed connection (N) used for testing;

FIG. 2 depicts construction details of a typical rafter-to-top plate tied connection (T) used for testing;

FIG. 3 depicts construction details of an example embodiment rafter-to-top plate wood block adhered (WBA) connection of the present disclosure;

FIG. 4 depicts an uplift test setup of rafter-to-plates connections;

FIG. 5 is a graph of the force-displacement relationship of a WBA embodiment of the present disclosure compared to N and T connections;

FIG. 6(a) is a graph of the strain Energy calculation (area under the curve) in lb-in up to 18 in. displacement (serviceability condition), (b) up to the peak load;

FIG. 7 is a diagram of a roof assembly specimen used for testing;

FIG. 8 is a diagram of a dynamic roofing facility for testing roof assemblies;

FIG. 9 is a graph depicting the loading protocol using the Standard Test Method for the Dynamic Wind Uplift Resistance of Roofing Systems (CSA A123.21-20);

FIG. 10(a) is a photograph of a failure mode in roof-to-wall connections for Substrate failure in WBA connections, and (b) Tension failure in hurricane tie;

DETAILED DESCRIPTION OF THE INVENTION

Roof damage due to high wind events, such as hurricanes and tornadoes, is a leading cause of significant property loss in residential buildings. This type of damage often begins at vulnerable points in the structure, particularly where the roof meets the supporting walls. When subjected to strong uplift forces generated by wind, these critical connections can fail, resulting in partial or total roof loss. The recurring nature of this damage has driven the need for effective retrofitting strategies, as outlined in the Federal Emergency Management Agency (FEMA) P-804 “Wind Retrofit Guide for Residential Buildings”, which provides guidance for improving wind resilience in homes through various strengthening methods.

In both coastal and tornado-prone regions, the threat of wind-induced damage has necessitated innovations in both construction techniques and retrofitting practices. Roof-to-wall connections are particularly susceptible to failure under wind uplift, a phenomenon in which strong winds create negative pressure on roof surfaces, pulling them upwards. Traditional methods for securing these connections, such as toenailing or the use of hurricane straps, provide some measure of protection but are often insufficient under extreme conditions. Research and post-disaster assessments have consistently shown that the roof-to-wall connection is a weak link in light-frame wood construction, leading to catastrophic structural failures during hurricanes and tornadoes.

Efforts to retrofit existing structures have typically involved two main approaches: mechanical reinforcement using metal straps and chemical reinforcement with advanced materials like polyurea coatings. Metal straps, commonly known as hurricane ties, are a widely used solution to improve uplift resistance in roof connections. These straps provide a direct mechanical link between the roof framing and the top plate of walls, helping to resist wind uplift forces. While they are relatively simple to install and cost-effective, their effectiveness can be limited by improper installation and material fatigue over time. In many cases, straps are inadequately fastened, or not installed at all, leading to failure at the very moment they are most needed.

Previous attempts to reinforce wood structures have involved coatings and adhesives. For example, applying closed-cell polyurethane foam over entire roof sheathing and sidewalls can increase uplift capacity moderately. However, this method requires a substantial thickness (typically 3 inches or more) and can be costly and impractical. Additionally, polyurethane foam may exhibit inferior elongation before failure and less heat stability compared to other materials.

Chemical reinforcement methods, such as polyurea coatings, offer another means of enhancing the uplift resistance of roof connections. Polyurea is a highly flexible and durable material that can be sprayed onto wood members, forming a protective layer that increases both the tensile and shear strength of the connections. This method is especially effective at reducing deflection and enhancing the energy absorption capacity of structural connections. However, polyurea applications are labor-intensive and expensive, often requiring specialized equipment and expertise, which makes them less accessible for widespread residential retrofitting. In addition, the high cost of materials and installation makes polyurea coatings less feasible for homeowners and builders aiming for cost-effective solutions.

Other methods, like those using multi-layer fiber-reinforcing materials with epoxy resins, can be expensive and challenging to apply, particularly when retrofitting existing structures. These approaches often require complex application processes and do not sufficiently address the need for increased energy absorption capacity without significant added weight or complexity.

This creates a significant gap in the market for retrofitting solutions that are both cost-effective and efficient. Mechanical solutions like hurricane straps are affordable and relatively easy to implement but fall short in terms of long-term durability and the level of protection they offer. Chemical solutions, while highly effective at reinforcing connections, come with prohibitive costs that limit their applicability, especially in large-scale retrofitting programs for older homes in high-risk areas. The need for a retrofitting solution that balances cost, ease of installation, and efficiency remains unmet in many regions vulnerable to extreme wind events.

In conclusion, the search for a cost-effective, efficient, and scalable retrofitting solution for roof-to-wall connections remains a key challenge in the field of wind damage mitigation. There is a need for solutions that bridge the gap between traditional mechanical methods, which are cost-effective but often underperform under extreme conditions, and advanced chemical reinforcements, which offer superior protection but are prohibitively expensive. Developing a solution that offers the best of both worlds durability, ease of installation, and cost-effectiveness-could significantly enhance the wind resilience of residential buildings in vulnerable areas.

To address this gap, there is a growing interest in innovative adhesive-based solutions that offer both the strength and flexibility of chemical methods, with the simplicity and affordability of mechanical reinforcements.

Modern elastomeric adhesives can potentially transform the realm of light-frame wood (LFW) construction, offering a cost-effective solution to increase strength, stiffness, and energy dissipation under lateral loads induced by wind as well as other stresses such as earthquakes.

Investigators have found that conventional adhesives, including water-based, solvent-based, and polyurethane-based (PU) adhesives, can significantly improve LFW strength and stiffness. However, concerns about volatile emissions, lack of durability, and brittleness limit their application in LFW structures, and they are currently not allowed in most seismic-focused construction codes. On the other hand, high-elongation elastomeric adhesives such as silyl-modified polyether (SMP) adhesives are gaining interest in construction for their moisture-curing, isocyanate-free, UV-stable, chemical-resistant, and high flexibility properties. Elastomeric adhesives are synthetic polymer-based. Of particular interest is silyl-terminated polyether adhesive, as it is thermosetting, moisture curing, and solvent-free.

In some aspects and embodiments, the high-elongation adhesive includes a comprising silyl modified polymer. In yet others, the silyl-modified polymer includes silyl-modified polyethers and/or silyl modified polyurethanes. In others, the high-elongation adhesive includes a silyl modified polyether. In yet others, the high-elongation adhesive comprises an amino silane.

Suitable blocks in aspects and embodiments of the present disclosure may be made of wood or other suitable materials of sufficient strength and fire resistance, such as PVC, MDF, masonry, metal and the like. Wood has an advantage of strength, overall cost, which may be quite low, as often smaller blocks of wood such as offcuts may be used. Typically, the dimension of the block should be sufficiently large to allow for sufficient contact with the gluing surfaces of the rafter and top plate. Lengths of dimensional lumber such as 2×4, 2×6 and the like may be used in some embodiments to reduce costs.

Example 1. Construction Adhesives

Table 1 lists construction adhesive properties of an example Silyl-modified polyether (SMP) used for further testing in Examples below.

Example 2. Testing Program for Single Connections

Sample preparation. Test rafter-top plate specimens were prepared as shown in FIGS. 1-3: two typical attachments of the prior art (FIGS. 1 and 2) for comparison to an example embodiment of the present invention (FIG. 3). Referring to FIG. 1, a typical toenailed connection (N) was prepared with a 2×4 Douglas fir rafter 1, attached tp with three 8d common nails 2 (2.5 in×0.131 in) to two stacked and attached 2×4 Douglas fir top plates 3.

Referring to FIG. 2, a typical tied connection (T) was prepared with a 2×4 Douglas fir rafter 1, attached to two stacked and attached 2×4 Douglas fir top plates 3 with hurricane tie H 2.5A 4. Hurricane tie 4 was attached with five 8d common nails 2 (2.5 in×0.131 in) each to the rafter 1 and top plate 3.

Finally, referring to FIG. 3, an example embodiment wood block adhered connection (WBA) was prepared with a 2×4 Douglas fir rafter 1, attached to two stacked and attached 2×4 Douglas fir top plates 4 via a Douglas fir wood block 2 (3.5 in×3.5 in×1.5 in), joined with adhesive layer 3 on contact points with rafter 1 and top plate 4.

Test Setup. The testing setup is shown in FIG. 4. Uplift load was applied via a top mount universal joint attached to a rafter clamp holding a specimen rafter. Load in the opposite direction (i.e. resistance against uplift) was applied to the specimen top plate held with a top plate clamp couple to a bottom mount capable of applying downward force.

Results. Experimental results are shown in Table 2.

The results of the uplift test for roof connections, comparing configurations of nails only (N), hurricane tie (T), and retrofitted wood blocks with adhesive (WBA), reveal significant differences in load-bearing capacity, deflection, and strain energy absorption. These findings are critical in understanding the effectiveness of each connection type in resisting uplift forces and maintaining structural integrity during wind events.

Maximum Load at P_max. Referring to Table 2, the maximum load capacity (P_max) is a crucial indicator of the strength of the connection against uplift forces. The complete curve is shown in FIG. 5. The Nails Only (N) configuration exhibited the lowest load capacity, with a maximum load of 430.99 lbs, indicating that nails alone provide limited resistance to uplift forces.

The Hurricane Tie (T) configuration showed a substantial improvement, achieving a maximum load of 888.44 lbs, more than doubling the capacity of the nails-only configuration. This highlights the effectiveness of hurricane ties in distributing forces across the connection and enhancing uplift resistance.

Surprisingly, the Wood Block with Adhesive (WBA) configuration outperformed both alternatives, reaching a maximum load of 1386.80 lbs. This configuration demonstrates the highest uplift resistance, providing approximately 3.2 times the capacity of nails alone and 1.6 times the capacity of hurricane ties. The addition of wood blocks with adhesive significantly reinforces the connection, suggesting its suitability for retrofitting to improve uplift performance in wood-frame structures.

Net Deflection at P_max. Referring again to Table 2 and FIG. 5, The net deflection at P_max reflects the deformation experienced by the connection at the maximum applied load. The Nails Only (N) configuration had a relatively low deflection of 0.1990 inches, which is indicative of its lower capacity to absorb energy before failure. This rigidity, coupled with low load capacity, suggests that the nails-only connection may be prone to sudden failure under uplift forces.

In contrast, the Hurricane Tie (T) configuration exhibited a much higher deflection of 0.6391 inches, indicating greater flexibility under load. While this higher deflection allows the connection to absorb more energy, it may also be a sign of reduced serviceability, especially in applications where excessive deflection could lead to non-structural damage. The Wood Block with Adhesive (WBA) configuration displayed a balanced performance, with a deflection of 0.3247 inches. This moderate deflection suggests that the WBA retrofit provides both strength and a controlled deformation, offering better serviceability by reducing excessive movement while still resisting high uplift loads.

Strain Energy to ⅛-inch Deflection. The strain energy to ⅛-inch deflection (see Table 2, FIG. 6(a)) measures the connection's ability to dissipate energy before reaching a critical deflection threshold, which is often associated with the onset of cracking or damage in non-structural components such as drywall and ceilings. The Nails Only (N) configuration had a strain energy of 39.08 lb-in, indicating limited energy absorption before reaching the critical deflection threshold. Surprisingly, the Hurricane Tie (T) configuration showed a lower energy dissipation capacity of 32.68 lb-in at ⅛-inch deflection, despite its higher maximum load. This result suggests that although hurricane tie increases strength, it does not perform as well in mitigating early-stage deflections, which could potentially affect serviceability

The Wood Block with Adhesive (WBA) configuration outperformed both other configurations with a strain energy of 67.95 lb-in. This higher energy dissipation at early deflection stages demonstrates that the WBA retrofit enhances the connection's ability to control serviceability-related deformations, reducing the likelihood of non-structural damage under uplift forces.

Strain Energy to P_max. The strain energy to Pmax represents the total energy absorbed by the connection up to failure (see FIG. 6(b), Table 2). The Nails Only (N) configuration absorbed 69.60 lb-in of energy before failure, consistent with its lower maximum load capacity and limited resistance to uplift forces. The Hurricane Tie (T) configuration absorbed significantly more energy, 375.84 lb-in, before failure, underscoring its superior performance in terms of overall toughness and ability to handle uplift forces. Interestingly, the Wood Block with Adhesive (WBA) configuration absorbed 316.23 lb-in of energy, slightly lower than the hurricane tie. However, the WBA retrofit demonstrated a more balanced performance, with high load capacity and moderate energy absorption, which is sufficient for achieving high uplift resistance while maintaining better control over deflection.

Example 3: Testing Program for Roof Assembly

Next an entire roof assembly was tested using an example WBA connection and compared to a tied (T) connection test specimen. A diagram showing the overall contraction of both test specimens is shown in FIG. 7 (top). FIG. 7 and Table 3 provide the details of the test specimens. Each test specimen was placed on a wind uplift table frame with interior under the test specimen exposed to allow attachment to 4×10 inch I-beams on the uplift table and 500 lbs load cells (see FIG. 7 (bottom)). attached to measure uplift forces.

Test Setup. FIG. 8 shows the test setup, where a blower fan 1 is connected to top chamber 2 with an adjustable gust simulator opening 3. The top chamber 2 can be placed atop the adjustable bottom frame 4 upon which the roof specimen 5 may be placed. A good seal between top chamber and bottom frame may be maintained to allow for wind generation above the roof specimen, with a good seal between roof specimen and bottom frame to allow for the creation of uplift forces.

The loading sequence used in some tests, i.e., a loading protocol using the Standard Test Method for the Dynamic Wind Uplift Resistance of Roofing Systems (CSA A123.21-20) is shown in FIG. 9.

Results

Results are shown in Table 4 and discussed below.

Failure mode. The adhesive connections exhibited either adhesive failure or wood fiber failure (see FIG. 10(a)). Adhesive failure occurs when the failure plane is within the adhesive itself, while wood fiber failure occurs when the fibers of the connected members are pulled apart but remain bonded to the adhesive. The majority of adhesives tested failed through wood fiber tearing, which is the preferred failure mode. In this case, the strength of the connection is governed by the tensile (splitting) strength of the wood members (see FIG. 10(b)).

Utilizing a load-distributing object, such as wood blocks, increases the load-bearing area and consequently enhances uplift capacities. Due to this benefit, wood blocks with adhesives were explored as a potential retrofit connection solution. In cases where hurricane ties (straps) are used, failure typically occurs through tensile failure, where the strap itself tears under load. In some instances, the nails securing the strap are pulled out, particularly during the application of static loads. This form of failure is more common when the nails are subjected to high tension forces, which can compromise the connection integrity. Both types of failure highlight the importance of ensuring that the connection between the strap and the structural members is robust enough to withstand the expected load conditions.

Conclusion. Comprehensive testing has demonstrated substantial benefits:

Uplift Capacity Improvement: The application of wood blocks with the polyether adhesive has been shown to effectively retrofit wood structure connections, enhancing uplift capacity by up to three times compared to non-retrofitted connections using toenails only, and by up to one and a half times compared to connections utilizing hurricane straps.

Enhanced Wind Resistance: Dynamic wind uplift pressure tests on roof assemblies indicated that this method increased the design uplift pressure by 50%. This corresponds to an increase in design wind speed from 137 mph to 176 mph, significantly improving the structure's ability to withstand high-wind events.

Improved Serviceability: The reinforced connections exhibited enhanced energy dissipation, which helped control deflection under load. This performance mitigates the risk of crack propagation in non-structural components, such as drywall and ceilings, prior to reaching the ⅛-inch deflection criterion.

Aspects and embodiments of the present disclosure offers a practical and efficient way to retrofit existing structures or enhance new constructions. By focusing on the critical connections between framing members and utilizing an adhesive that is cost-effective and easy to apply, it addresses a primary source of structural failure during high-energy events. This approach provides increased safety for building occupants and reduces potential damage without the need for expensive materials or specialized application environments.