Deck Torsion Connections and Linkage: Key to Stability and Safety
Over the past few weeks we’ve looked at several different deck layouts with reinforcement in different directions of the structural load path. A quick summary list of some of the different types of framing bracing that we’ve looked at follows below:
- Cross bracing
- Joist blocks
- Joist hangers
- Knee bracing
Today we’re going to take another look at structural hardware connectors used to support the joined locations of framing lumber. The picture below shows knee bracing used in a part of a deck, underneath of the girder, but set off from the area where a car will enter under the deck into the garage of the back of the house.
The next picture below shows v-bracing (a type of cross bracing) installed underneath of the deck floor joist, ran diagonal to the run of the joist to prevent racking and stabilize the joist system. Cross bracing run in a diagonal direction to the regular joist run is stronger than an intermediary support run perpendicular.
The joist hanger shown below is similar to the typical galvanized finish joist hangers used in rough framing, but the one shown below is prefinished with a black paint to give an aesthetic design touch that looks better when left exposed. Unlike the typical joist hanger which has a flange on the outside of each side of the vertical corner connection between the joist and the supporting rim joist or ledger, this particular type of joist hanger is fastened with the flange is bent in behind the edge of the joist cross grain. It looks more concise and neater bilit lacks a bit of the strength of a outward bend because it concentrates the connection in a smaller area.
Larger structural corner connectors like the one shown in the picture below can also be used in lieu of regular joist hangers for stronger heavier load connections.
In the next picture below, we show a girder that sits on top of a post, but unlike a typical shoulder connection from a beam to a post, this post is cut off flush where the perpendicular joists meet at a tee connection on top of the post. These structural ties allow fasteners to connect the joists to the post in a stronger configuration than simply toe-nailing / toe-screwing.
Toe nailing is a relatively common method of hammering or shooting nails, with a nail gun, through the flat side of a board at an angle which allows it to peirce through the cut edge, and the end grain of one board into a flat edge of another board. This type of fasting connection is acceptable by the building codes but only in certain types of configurations, such as nailing studs to plates at a limited type of framing connections. To connect the end grain of a joist to a rim joist or ledger, a stronger connection is required.
Toenailing is generally considered a weaker connection in stick framing primarily because of the nature of how the nails are driven. In toenailing, nails are driven at an angle through the end of one framing member into another. This angled approach reduces the surface area of wood that the nail engages with the joined perpendicular member, thereby lessening its holding power compared to nails driven perpendicularly.
The connection relies more on friction and shear strength, which makes it less secure. As well, toenailing increases the risk of splitting the wood, particularly if the wood is dry or if the nails are not placed precisely, which can further weaken the joint. This method also provides less resistance to lateral forces, such as those which cause movement such as wind wind or seismic activity, making the joint more prone to shifting or loosening over time. Toenailing also limits the number of nails that can be used due to space constraints. As well, achieving consistent angles and depths can be challenging, resulting in varying effectiveness of the nails.
The efficacy of bracing systems to support the overall frame rigidity depends on the connection layout. Some considerations include:
- Bolted Connections: Shear strength calculation: V = Fv * Ab * n Where: Fv = allowable shear stress Ab = bolt area n = number of bolts
- Nailed Connections: Withdrawal resistance: W = G * D * L Where: G = specific gravity of wood D = nail diameter L = penetration depth
- Joist Hangers: Load capacity determined by manufacturer testing and engineering analysis
Like the tee connection support shown above, the pair of L-plate connector ties supports a similar layout but at an outside corner where the run of a rim joist changes direction, such as the outside corner shown below.
With all of the connected pieces of deck framing, including the entirety of the deck structure, all of the different elements have characteristics which affect their longevity and strength. Some of the examples follow below:
- Wood:
- The anisotropic nature of wood members affects strength in different grain directions
- The moisture content influences dimensional stability
- Steel:
- Steel has an innate high strength-to-weight ratio
- The ductility of steel allows for energy dissipation in dynamic loading
- Composites:
- Composite board members are engineered for specific strength and durability characteristics, but not always as durable to withstand moisture
- Composites are increasingly used in modern deck construction but still have limitations
Bracing Techniques in Traditional Stick Framing
The list below include some of the typical basic structural support configurations in stick framing, used in all types of frame constructions:
- Moment Frames: Resist lateral loads through rigid connections Moment capacity: M = F * d Where: F = applied force d = distance from pivot point
- Shear Walls: Provide lateral stability in multi-story deck structures Shear capacity: V = v * A Where: v = allowable shear stress A = shear area.
Effective deck bracing is a combination of structural principles, material science, and design configuration. By understanding some of these main forces at work and coordinating appropriate bracing techniques, you can create deck structures that are not only stable and strong but also functionally effective. We are happy to help.
Use a contractor who understands and cares about doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios. We are happy to help with almost all steps of the deck building and design process. Let us know about your ideas and talk to us if you have questions about possibilities . You can call us at (202) 774-9128. You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com/contact-us
