Specifying the right framing connectors for deflection

by sadia_badhon | February 21, 2020 5:09 pm

by Chuck Webb, PE, CSI, CDT

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Cold-formed metal framing (CFMF) is one of the most widely used framing materials in commercial and residential construction because of its ability to provide strength and stability in a number of conditions. A well-detailed framing system can accommodate forces imposed on the structure by gravity, wind uplift, and seismic forces. A key element of this design is the system’s ability to account for vertical movement of structural elements like floor and roof systems by allowing them to deflect downward or upward without imposing axial loads to the wall stud members, gypsum wall boards, or other substrates.

When it comes to deflection, the performance of the connections between CFMF and the building structure is critically important, especially in coastal and high-seismic zones where structures face greater risk of movement, stress, and loading from natural events like earthquakes and high-velocity winds.

Compression and extension deflection

Section 1604, “General Design Requirements,” of Chapter 16 of the International Building Code (IBC) says structural systems and members shall be designed to have adequate stiffness to limit deflection including roof and floor members. Examples of these members are metal pan deck (fluted deck), composite deck, open web bar joists, steel I-beams, concrete beams and slabs, and precast or hollow-core planks.

Roof members typically move in two directions: downward (compression) or upward (extension). The compression movement of roof elements can be a result of one or a combination of the following:

• roof live load – during maintenance by workers, equipment, and/or material; movable objects such as planters; or by the use and occupancy of the roof whether it is a roof garden or assembly area;
• snow load – weight of the collected snow on the roof including snow drift;
• wind load – the pressure applied by the wind; and
• dead load – the weight of materials in construction incorporated into the building.

The upward movement of roof elements is a result of wind suction, where the pressure pulls away from the building, causing the elements to move in an upward direction.

Conversely, floor members typically move in one direction—downward. This is a result from live loads introduced by the use and occupancy of the building, as defined by table 1607.1 of IBC, and dead loads. However, if the structure has multiple floors, the downward movement of one level can cause an extension at the head-of-wall above.

When specifying deflection connectors, it is important to consider deflection and standoff distances. Deflection distance refers to the measure of the maximum vertical distance the primary structure is anticipated to move due to loading. Standoff distance is the space between the secondary and primary frames. This space allows contractors to install the CFS framing in a true line, while the structure of the building may be slightly out-of-plumb from floor to floor.

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Head-of-wall deflection joint/gap

Wall framing often bears on the top of the slab of one floor level and frames to the underside of the floor or member(s) above. This is called infill or slab-to-slab framing. Where this occurs, a deflection joint (or gap) is required if the wall framing needs to account for the compression or extension movement of the structure.

The deflection joint is the distance between the top of the stud framing to the underside of the substrate above (floor or roof member), as defined by the project’s architect or structural engineer. Often, this dimension is given in the project specifications—Division 09 22 16–Non-structural Metal Framing or Division 05 40 00–Cold-formed Metal Framing. It may also be in the contract drawings under the architectural or structural details. The top of the gypsum wall board to the underside of the substrate should also equal this distance.

Head-of-wall attachment methods

There are various framing methods to accommodate vertical movement at the head-of-wall. An option is head-of-wall track products with vertical slots in the track legs. The wall stud framing members are secured to the deflection track utilizing waferhead screws through the center of the vertical slots on both track flanges. This allows the primary structure to compress or extend without imposing axial loads on the wall studs. Otherwise, the addition of axial loads into the stud framing members could lead to buckling and performance failure of the wall system. Since the wall studs are secure at both flanges, stud rotation is prevented, thereby eliminating the need to add lateral wall bridging 305 mm (12 in.) down from the top of stud.

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A common method to account for vertical deflection in head-of-wall applications is a deep leg or oversized runner track. With this method, the top of the stud is held down and the required deflection gap is ‘free-floating’ in the track cavity. Since the stud is subject to rotation in this application, lateral wall bridging is required 305 mm down from the top of stud. This method allows for larger deflection gaps, where most slotted connectors are limited to 50 mm (2 in.) total deflection (25-mm [1-in.] compression, 25-mm extension). To calculate the deep leg track flange dimension, one must multiply the deflection gap by two and add 25 mm. For example, the structural engineer gives a required deflection gap of 19 mm (³/₄ in.)—the deep leg flange should be (2 x 19 mm) + 25 mm = 63 mm (2 ¹/₂ in.) flange. This ensures the top of the stud is not dislodged from the track due to upward movement.

An option to accommodate vertical deflection for infill framing is to use a deflection clip with solid and slotted legs. The solid leg is affixed to the substrate and the slotted one is attached to the stud web. Some products use proprietary screws designed specifically to provide friction-free deflection.

Lastly, accommodating deflection in fire-rated wall applications presents a separate set of challenges. If deflection is required in a fire-rated wall assembly, then the head-of-wall deflection joint must be protected. The aforementioned framing methods can be used in conjunction with third-party, fire-rated products such as fire caulks or mineral wool and sealant. However, there are more labor-friendly options like deflection tracks incorporating an intumescent strip on each track flange that expands in a heat event to protect from heat and flame passage. One benefit of this approach is it allows for deflection of the primary structure while maintaining fire and smoke protection. This means the structure can compress or extend as needed by design and still maintain the integrity of the joint.

Deflection in bypass wall applications

Exterior, structural cold-formed steel framing must often accommodate vertical movement of the primary structure, similar to interior wall systems. The exterior wall framing will either be an infill condition like the aforementioned applications, or it will be a bypass condition where the wall framing members sit outside the slab edge and frame, past the floor or roof level. This is commonly known in the industry as ‘balloon framing.’

To accommodate vertical deflection of the primary structure for bypass applications, a deflection clip that can accommodate vertical movement in compression and extension is required. These clips have an extended leg with vertical slots that attach to the wall stud webs, and a solid leg that is anchored to the structure—either a steel member such as an edge angle or steel I-beam or a concrete member like a slab edge or concrete beam.

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Several versions of bypass slide clips are available on the market today, each with a specific purpose. Some are offered in both 14 and 12 gauge options, which are common for steel edge angle conditions, or with an oversized pre-drilled hole in the solid leg for anchor attachments to concrete elements. Others have extended legs with vertical slots for larger wall offsets from the primary structure edge.

Another option is to attach the deflection clip to the underside of a structural element instead of the face. Connectors for these types of applications have horizontal legs that extend from 305 to 610 mm (12 to 24 in.) with vertical slots along the face of the leg. The legs of these clips are intended to attach to the web of the stud members. These types of clips have a second leg that is solid and bent 90 degrees from the vertical, allowing them to attach to the underside of structural steel or concrete elements. However, it should be noted that rotation of the structural element is a possibility in this type of application, making it imperative the structural engineer review and approve this type of connection and ensure the stability of the structure.

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The last option for a bypass framing attachment is one that has a ‘tail’ feature on it so it sits flat to the underside of a structural element, or even the top side such as a concrete slab or steel I-beam top flange. This profile has a vertical leg with vertical slots to allow for compression and extension movement. This type of clip can also introduce rotation to the structural element, so the structural engineer of record should review and approve this type of connection, as well.

Other considerations for specifying deflection clips

It is common for CFS framing installers to fabricate clips and connectors in the field. However, there are many benefits to specifying pre-engineered connectors tested to perform exactly as required for each application.

Today’s high-performance buildings demand a certain level of assurance that each element in the structure is going to perform as expected. Pre-manufactured connectors are designed and tested for all allowable load capacities, allowing architects and engineers to ensure calculations are based on how the connector will actually perform. Additionally, these connectors often have third-party data backing up their performance and verifying the connector was manufactured according to applicable standards.

Another major benefit of specifying pre-engineered clips and connectors is their ability to save time and labor on a project by eliminating the need to cut, bend, and fabricate custom-made connectors in the field, which can consume many valuable hours on large commercial projects. Also, these handmade connectors often lack the pre-punched holes that help installers properly align the fasteners for achieving the intended design load, potentially compromising their performance.

Conclusion

Many projects require the cold-formed metal framing to allow for deflection of the primary structure in compression and/or extension. The movement and amount of movement should be specified by the architect or structural engineer of record. From there, and based on infill or bypass wall conditions, the appropriate framing product can be identified and installed to meet the project needs.

[6]Chuck Webb, PE, CSI, CDT, is the technical sales manager for ClarkDietrich in the Southeast and Mid-Atlantic markets. He has 15 years of experience as a cold-formed specialty engineer. In his current role, Webb advises architects, specifiers, and engineers on proprietary product application and works with contractors and sub-contractors on product opportunity and installation. He can be reached via e-mail at chuck.webb@clarkdietrich.com[7].

Source URL: https://www.constructionspecifier.com/specifying-the-right-framing-connectors-for-deflection/

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