Key Techniques for Effective Deck Bracing
This past week we started a multi-part series which will continue today, looking at the structural elements of diagonal deck bracing and other types of deck support to resist the structural loads and forces of both the live loads, dead loads, and external forces of the environment.
The core of deck shift resistance is centered around load path distribution and shift / sway stability. A simplified calculation follows, for a 16′ x 10′ deck, like the one shown in the photo below:
- Assumed Dead Load = 16 psf
- Live Load = 40 psf (per typical residential code)
Total Load = (10 psf + 40 psf) x 16′ x 10′ = 8,000 lbs. That’s a lot of weight and the load path must be stabilized to support that load.
This load is transferred through the deck’s structural elements to the foundation, but each system of components along the way from the deck walking surface through to the foundation must be able to support that load, continuously. Each component plays a role, to work together, like the set and series of bones in a body. The series of items below work in an interconnected linear fashion:
- Decking: Distributes loads to joists
- Joists: Transfer loads to beams
- Beams: Carry loads to posts
- Posts: Transmit loads to footings
- Footings: Distribute loads to subsoils
Bracing systems increase the overall stability by stiffening in opposing or alternating directions, resist several directional forces therein, enforcing the overall rigidity of the structure. There are three main types of bracing systems: lateral bracing, vertical bracing, and torsional bracing. Each serves a unique but related purpose in reinforcing and stabilizing the overall structure.
Lateral bracing
Lateral bracing is designed to resist horizontal forces, primarily those caused by wind and seismic activity. A common example of lateral bracing is diagonal bracing, shown here, which forms triangular shapes within the structure to distribute lateral loads by additionally connecting the structural members at intermediate points. When we talk about alternating or opposing directions in stabilizing structural forces, diagonal braces also change the direction of stability by reinforcing and supporting the rigidity of the structure at opposing angles.
To analyze the load and support, calculations can be used to determine the wind load. The formula for wind load (W) takes into account several factors: velocity pressure (qz), gust effect factor (G), force coefficient (Cf), and the projected area normal to wind (Af). Here, there are a variety of different factors, it’s more than just the simple context of wind speed.
The velocity pressure (qz) represents the force exerted by wind at a specific height and is determined by typical, expected, local wind speed data. The gust effect factor (G) accounts for the turbulent nature of wind and how it can create sudden, strong gusts. The force coefficient (Cf) is related to the shape of the structure and how it interacts with wind. Finally, the projected area normal to wind (Af) is simply the surface area of the structure facing the wind. By multiplying these factors together, you can calculate a total wind load the structure should be designed to withstand.
Vertical bracing
Vertical bracing, in contrast, focuses on reducing deflection and increasing the load capacity of the structure. Knee braces are a common form of vertical support bracing, often seen connecting posts to beams in deck construction, shown here. To understand deflection, a calculation can be used that considers the distributed load (w), span length (L), modulus of elasticity (E), and moment of inertia (I) of the structural member. The deck shown in the picture below includes just vertical posts without angled intermediate bracing. With a short span as shown in this example, depending on a variety of factors, the deck, built without intermediary support may be sufficient.
The deflection calculation (Δ = 5wL^4 / (384EI)) provides insight into how much a beam or joist might bend under a given load. The distributed load (w) represents the weight spread across the member. The span length (L) is the distance between supports. The modulus of elasticity (E) is a measure of the material’s stiffness – how much it resists deformation. The moment of inertia (I) relates to the cross-sectional shape of the member and how it resists bending. This formula can predict how much a structural member will deflect and design bracing systems to minimize this deflection. All of these details may sound complicated, but in some cases, deck building just requires understanding the points or thresholds required to increase the support capacity.
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
