The HDUE Holdown represents the latest milestone in a long history of innovative products at Simpson Strong-Tie. Our journey began in 1966 with the introduction of the first HD welded holdown, a robust, raw steel piece designed to secure posts to foundations (figure 1). Over the decades, we have continually refined and improved our designs to meet evolving industry needs and standards. 

In 1986, we automated the production process, creating a single-piece, non-welded HAD series holdown that offered higher load capacity and reduced installation errors (figure 2). The predeflected holdown (PHD), introduced in 1997, incorporated bending the steel into its design, significantly reducing deflection and improving performance (figure 3). 

By 2006, our HDU models further streamlined manufacturing and installation processes (figure 4). However, with advancements in manufacturing and evaluation technology, we saw an opportunity to innovate once again, leading to the development of the HDUE Holdown. This new product line replaces our entire HDU series, bringing modern enhancements and superior performance to one of our flagship products. 

How Did the Development of the HDUE Holdown Come About? 

The development of the HDUE holdown was driven by two primary factors: the increasing load demands of the market and advancements in manufacturing and technology. Since the last major development in 2006, there have been significant improvements in both areas, creating the need and opportunity for this innovation. Our goal was to create a product that not only meets current demands but also leverages the latest technological advancements and extensive research efforts to provide superior performance and reliability. 

Technical Specifications of HDUE Holdown Models 

To provide a clear comparison, Table 1 shows the technical specifications of the new HDUE holdown models versus the old HDU models: 

New Features and Reasoning Behind Them

Here are the new/upgraded features and the reasoning behind them:

Angled Screws: Enhancing Load Distribution

Angled screws have been a game-changer in our design. By directing loads more efficiently, they significantly reduce stress concentrations in the wood. This prevents splitting and enhances overall structural integrity. The introduction of angled screws marks a critical improvement in our holdown technology, ensuring our structures are safer and more reliable than ever before.

To better understand the critical role of angled screws, as the post experiences a tension load, the load transfers to the perpendicular screws as a shear load. This shear load can lead to screw bending and failure, creating high-stress areas in the wood, resulting in wood splitting (figure 5).

On the other hand, angled screws utilize both the shear and withdrawal capacity of the screws while the tension load of the post transfers to the screws. This distribution of the load reduces the chances of screw failure and minimizes stress concentrations in the wood (figure 6).

Combining Angled and Perpendicular Screws

To provide flexibility for installers to mount the new holdown both raised off the sill plate and flushed with it, we ensured ample height for the angled screws in larger models and did not incorporate them in smaller holdowns. On smaller models like the HDUE3 and HDUE5, height limitations presented a unique challenge. Therefore, we conducted rigorous testing to determine the most effective screw formation for optimal load distribution (figure 7).

For taller holdowns that incorporate angled screws, it is crucial to add perpendicular screws at the top (figure 8). The load eccentricity on the holdown creates a moment, making the top screws necessary to secure the part to the post. These screws act in withdrawal and placing them perpendicularly enhances their effectiveness in securely holding the part (see figure 9).

The Introduction of Teardrop Holes

Larger models also feature teardrop holes (figure 10). Unlike round holes, teardrop holes have a limited stress area, which helps the screws engage later in the loading phase, after the angled screws have been engaged (figure 11). This design provides better load distribution and relieves the perpendicular screws in the critical lower part of the post, where there is limited end distance for screws.

Addressing Load Limitations with Extruded Holes

One of the new features in the HDUE Holdown is the introduction of extruded holes at the base of the holdown (figure 12). These holes strengthen the seat around the hole by adding material, thereby enhancing its strength and load-bearing capacity. This modification ensures that our holdowns can handle greater loads without compromising on safety.

Full Overlap for Integrity and Easy Installation

One of the standout features of the HDUE Holdown is the full overlap design. This feature prevents splitting, ensuring the post maintains its integrity. Additionally, it simplifies installation. By slightly shifting the back plate, angled screws are forced to enter at an incline, preventing misinstallation and ensuring a secure fit (see figure 13).

Upgraded Washers for Superior Performance

In our pursuit of excellence, we’ve upgraded the washers used in our holdowns. Smaller models now feature washers that are over 50 percent stronger, and the HDUE17 is equipped with a solid aluminum washer (figure 14). These enhancements significantly improve the performance and durability of our holdowns, ensuring they meet the highest standards of safety and reliability.

Detailed Installation Guidelines for the HDUE Holdown

One of the key advantages of the new HDUE Holdown is its ease of installation. In fact, there are no specific guidelines required for installing screws in the HDUE. The precisely designed angled holes, along with the shifted and raised back plate, prevent misinstallation of screws at different angles. Additionally, the generous height provided allows for the installation of angled screws with various equipment on construction sites. Installers can rely on the HDUE just as they did with the old HDU models, ensuring a smooth and hassle-free installation process.

How Does the HDUE Holdown Contribute to Sustainable Building Practices?

The new HDUE Holdown aligns with our company’s commitment to supporting sustainable building practices, and various state and city-specific green building codes. The HDUE uses a lighter gauge in some models compared to the old HDU models and requires fewer screws, which leads to lower material usage in some models and reduces installation energy. Despite these material reductions, the HDUE still increases the load capacity of the part, ensuring both sustainability and performance.

Engineering Documentation for HDUE Holdowns

For those looking to validate or specify HDUE products in place of discontinued HDU models, two engineering letters are available:

• Engineering Letter: HDUE™ One-for-One Replacement for Discontinued HDU (L-C-HDUECONV25) outlines how each HDUE model exceeds the performance of its corresponding HDU model, enabling direct substitution.
• Engineering Letter: HDUE™ Holdown with Shorter SDS Screws (L-C-HDUESDS25) provides additional allowable loads and installation options for HDUE using shorter Strong-Drive® SDS Heavy-Duty Connector screws.

These letters support the HDUE as a high-performance, code-compliant replacement and provide designers with added flexibility across applications.

The Evolution and Impact of the HDUE Holdown

The HDUE holdown is not just another product launch; it’s a significant milestone in our history of innovation. Since the introduction of our first holdown in 1966, we’ve continually evolved our designs to meet the changing needs of the construction industry. From raw steel welded pieces to sophisticated automated designs, each iteration has incorporated feedback from the field and advancements in technology. To find more information about our HDUE holdowns, please refer the HDUE.