Ensuring Deck Safety: Understanding Deck Torsion Resistance and Its Importance
This past week’s blog, we discussed some of the details related to lateral bracing and diagonal bracing. This week we’re going to look at torsional bracing and some of the minimum code requirements for deck building and structural ratings of support members and connections.
Torsional bracing, another type of common deck bracing, is intended to prevent the twisting of structural members. Cross-bracing (or cross-blocking is a common example of a type of torsional bracing, sometimes seen in the form of X-shaped supports, more often in the form of a solid cross-block. While there isn’t a single, simple formula for calculating torsional forces, we consider various factors such as eccentric loading (loads applied off-center), asymmetrical design elements, and the inherent properties of materials that might make them prone to twisting.
Although cross blocking and bridging or blocking between joist runs is a common type of torsional blocking, an additional common type of rack prevention on the entirety of the deck footprint might be strapping installed at the underside of the joist in a perpendicular v-shaped layout. These two different types of bracing are very different in scale, one applies to resisting the twisting forces in parallel joist runs and the other prevents or resist the racking force of the polygonal shape of the deck itself between rim joists and other perpendicular rim joists and ledgers. Both types of torsional bracing help resist both types of torsional forces.
This type of support is an auxiliary or additional method of reinforcement for a deck and can help in instances or areas prone to high wind or storm surges. The wind load calculation helps determine how strong this bracing needs to be for adequate resistance. You might choose to use thicker diagonal braces or increase the number of bracing elements based on this analysis.
Similarly, in a deck design where minimizing bounce or sag is needed for user comfort, based on span, vertical bracing adds support. By using a deflection calculation, you can get a sense of whether additional knee braces are needed or if the size of structural members needs to be increased to reduce deflection.
Torsional bracing becomes especially important in structures with irregular shapes or asymmetrical loads. For example, a deck with a cantilevered section might require consideration of torsional forces to prevent twisting that could compromise the integrity of the structure.
It’s important to note that all of these bracing systems often won’t work as effectively in isolation. A well-designed structure will incorporate elements of all three types of bracing to create a more robust and stable frame system. The specific combination and intensity of bracing will depend on various factors including the structure’s size, shape, location, intended use, and the materials used in construction.
One of the building code requirements looks at a combination of both wind and seismic forces as a horizontal force resistance. The impact of wind is measured both in wind load and wind velocity, based on geographic region and a horizont gust effect. From a perspective of resistance, the bracing analysis will factor the overall span, distributed load, the modulus of elasticity and the moment inertia.
The International Residential Code (IRC) provides specific guidelines and requirements related to deck construction and the associated structural calculations. Here are some relevant code references from the 2021 IRC:
- General Deck Requirements: Section R507 – Exterior Decks This section provides overall requirements for deck construction, including materials, fasteners, and general design principles.
- Load Requirements: Section R301.5 – Live Load Table R301.5 specifies the minimum uniformly distributed live loads for various components, including 40 psf for decks.
- Wind Loads: Section R301.2.1 – Wind Design Criteria This section references ASCE 7 or provides alternative provisions for determining wind loads.
- Lateral Load Connections: Section R507.9.2 – Lateral Connection Specifies requirements for lateral load connections to resist horizontal loads.
- Deck Joists: Section R507.6 – Deck Joists Provides span tables and requirements for deck joist sizing and spacing.
- Deck Beams: Section R507.5 – Deck Beams Outlines requirements for beam sizing, including span tables based on joist length and beam spacing.
- Deck Posts: Section R507.4 – Deck Posts Specifies requirements for post sizes based on deck height and beam span.
- Footings: Section R507.3 – Footings Provides requirements for footing size and depth based on tributary area and soil bearing capacity.
- Guards: Section R312.1.2 – Guard Height Specifies the minimum height for guards (36 inches for residential decks).
- Stairs: Section R311.7 – Stairways Outlines requirements for stair construction, including tread depth and riser height.
It’s important to note that while the IRC provides these guidelines, it doesn’t always specify the exact calculations to be used. In many cases, it references other standards or leaves the specific calculations to be determined as it applies to the specific circumstances on the unique characteristics of each deck layout.
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
