As mass timber members get larger and carry heavier combinations of loads, their connections must also scale up to meet that demand. Steel strong-back or splice plates are an increasingly common connection type for diaphragm chord force transfer or to handle the axial force in a beam-end to beam-end connection where moment continuity is required in these larger structures. Often, the steel plate required does not need to be very thick, but it is specified as such in order to accommodate the countersunk head of a self tapping screw. A special type of washer for each screw in the plate can be used instead so that an efficient thickness of steel is selected based on the load requirement only.
MTC cast iron 45° Wedge Washers are an engineered cost-saving solution for steel-to-wood connections. Available in three different sizes and compatible with 5/16” [8 mm], 3/8” [10mm], 1/2″ [12 mm] countersunk head fasteners [e.i: ASSY Ecofast & ASSY VG CSK], they are simply installed into machined holes in steel plates with an elliptical shape. Using a Pre-drilling Jig during the installation will not only accelerate even more the process, reducing the overall connections’ cost, but also, will ensure a successful and precise installation.
This post covers design considerations and installation instructions to avoid commonly made mistakes.
Approximately 8-minute read.
What Are the Benefits of Using The 45° Wedge Washers?
Designing connections using inclined fully threaded screws with the MTC Solutions 45° wedge washer into a steel plate is a great way to develop high-capacity connections. These fasteners have high tensile resistance and are available in relatively long lengths, which makes it possible to develop high withdrawal resistance. This format of steel to wood connection makes use of the axial properties leading to significantly higher capacities compared to lateral connections at 90 degrees.
The use of 45° wedge washers also eliminates the complex milling process, otherwise required to completely countersink a screw head in a steel plate. The 45° washer allow then for the use of a thinner machined steel plate leading to more cost-effective steel-to-wood connection solutions.
Design Considerations
To successfully install the 45° wedge washer, its geometric specificity must be considered. The washer’s compression lips must sit flush on the steel plate to ensure a proper tensile force transfer from the fasteners to the interior surface of the slotted holes of the steel plate.
The steel plate is sized to conform with the minimum and maximum thicknesses according to the size of the respective 45-degree wedge washer and the slotted hole dimensions, relative to the screw diameter. More information and design considerations for coated plates can be found in the Structural Screw Connection Design Guide. The following table presents the design requirements based on screw diameter:
How To Install The 45° Wedge Washer
The following steps highlight best practices to install a 45-degree angled screw using the MTC 45° Wedge Washer as outlined in the ICC ESR 3178, in a steel-to-wood connection configuration – in this case, the fastener used is an ASSY VG CSK 5/16″ x 6-1/4″ [8 x 160mm].
Tools and equipment
Preparation: Secure the steel plate’s position on the wood member
Plum and level the connector (i.e. MTS-I 30) in position on the wood member. Alignment is crucial for these types of connections, as the connection may be subject to high design loads and misalignment may result in reduced connection performance.
Step 1: Install Locater Screws
Install the first locator screw at a corner without fully setting the screw. Once the plate is in its final position, a second locator screw can be fully installed at the opposite corner and the previous screw can be tightened to fix the plate in position. Afterwards the two remaining screws can be installed.
Step 2: Optional Use of a Pre-Drilling Jig
In a previous blog post we explained predrilling & pilot holes to receive desired load limits and described the importance of an accurate installation, using the correct drill bit. Installation with a wrong screw angle can introduce an additional bending moment at the screw head. All inclined screws must be installed at the center of the hole in their steel plates to prevent the threads from clashing with any steel, which will significantly increase the torque required for installation leading to potential screw failure.
The Predrill Jig, as shown in the video, is a helpful tool to support the Wedge Washer installation with an accurate pilot hole, offering several benefits:
After drilling the pilot holes, it is important to clear any saw dust or drill shavings created. It may seem trivial but saw dust may retain humidity which has the potential to damage steel connections over time.
Step 3: Wedge Washer positioning
Place the 45° Wedge Washer firmly in position. Verify that the shoulder of the washer fits properly in the slotted hole. Avoid placing the Washer too far forward or backward, so that the Washer won’t lift on the opposing side.
Step 4: Fastener Installation into wedge washer
Position the screw in the wedge washer and drive the inclined screw into the wood. Once the threads are engaged with the wood fiber, avoid applying pressure on the drill. Further installation information can be found on our blog post about How to Install Structural Self-Tapping Fasteners?
Avoid over-torquing the screw by slowing down the rotation speed about 1” away from complete installation. Over-torquing can damage the screw or result in injuries to the installer. To avoid this, we suggest using a drill with a torque-limiting clutch or if not available, limiting the torque manually at the end of the installation, as shown in the video.
If you want to learn more about the 45° Wedge Washer and/or how to use them in high-capacity steel-to-wood connections, download our Structural Screw Catalog, and read through section Appendix E: Steel Plate Detailing.
If you have any questions, contact our Technical Service Team 🙂
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