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In architecture or structural engineering, a girt, also known as a sheeting rail, is a horizontal structural member in a framed wall. Girts provide lateral support to the wall panel, primarily to resist wind loads, and provide a framework for the thermal control layer.[citation needed][1] Though girts come in a variety of profile shapes, Z-girts predominate wall assembly applications.[2]
A comparable element in roof construction is a purlin.
Stability in steel building construction
[edit]The girt is commonly used as a stabilizing element to a building's primary vertical members. Wall cladding fastened to the girt, or a discrete bracing system which includes the girt, can provide shear resistance, in the plane of the wall, along the length of the primary member. Since the girts are normally fastened to, or near, the exterior flange of a column, stability braces may be installed at a girt to resist rotation of the unsupported, inner flange of the primary member. The girt system must be competent and adequately stiff to provide the required stabilizing resistance in addition to its role as a wall panel support.[citation needed]
Girts can be stabilized by sag rods, angles, or straps as well as by the wall cladding itself. Stabilizing rods are discrete brace members to prevent rotation of an unsupported flange of the girt. Sheet metal wall panels are usually considered providing lateral bracing to the connected, typically exterior flange along the length of the girt. Under restricted circumstances,[3] sheet metal wall panels are also capable of providing rotational restraint to the girt section.[citation needed]
See also
[edit]
References
[edit]- ^ "Product Guides | Understanding Fiberglass Z-Girts and How to Choose the Right Product". acelabusa.com. Retrieved 2025-12-04.
- ^ Finch, Graham (December 2017). "Cladding Attachment Solutions for Exterior Insulated Commercial Walls" (PDF). Rockwool Cladding Support Systems.
- ^ North American Specification for the Design of Cold-Formed Steel Structural Members
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from Grokipedia
Definition and Overview
Definition
A girt is a horizontal structural member that runs parallel to the ground and spans between vertical columns or posts in wall framing systems of buildings.[4] It serves as a secondary framing element, primarily designed to support wall cladding or panels by providing lateral stability against forces such as wind loads.[5] Unlike primary framing components like columns and rafters, which form the main load-bearing skeleton of a structure, girts function as non-load-bearing supports specifically for exterior wall elements.[1] Girts are typically fabricated with cross-sections such as Z-shaped or C-shaped profiles, which offer efficient attachment points for wall panels while optimizing material use and structural performance.[6] These profiles allow girts to transfer lateral loads from the wall sheathing to the primary frame without directly bearing vertical gravity loads from the roof or floors.[7] In building construction, girts are analogous to purlins, which perform a similar supportive role but for roof cladding systems.[8] This distinction underscores their specialized application in vertical wall assemblies, contributing to the overall envelope integrity of framed structures.[2]Terminology and Etymology
The term "girt" is an alteration of "girth", originating from Old Norse gjǫrð meaning girdle or belt, borrowed into Middle English around the 14th century and ultimately derived from the Proto-Indo-European root gher- connoting enclosure or grasping.[3][9] The noun "girt" first appears in English records around 1563.[3] In the context of architecture, this etymological sense of encircling evolved to describe a horizontal structural member that braces vertical elements, akin to a belt securing a framework. By the 19th century, "girt" had become established in building terminology, particularly in timber framing systems like post-and-girt construction, where it denoted horizontal timbers connecting corner posts in house frames to provide stability.[11] In modern construction lexicon, "girt" is differentiated from related horizontal members: unlike a purlin, which spans parallel to the ridge in roof systems to support cladding and transfer loads to primary rafters, a girt runs horizontally along walls to brace panels against lateral forces.[5] It also contrasts with a joist, a horizontal element supporting floor or ceiling loads perpendicular to walls, and a stud, a vertical framing member in light-frame construction that forms the primary wall skeleton.[12] Regional variations highlight "girt" as predominantly North American terminology, especially in pre-engineered metal building contexts originating in the mid-20th century, where it appears in patents and standards for rigid-frame structures developed post-World War II.[13] In British English, the equivalent is often termed a "sheeting rail," emphasizing its role in supporting wall sheeting rather than broader framing.[14][15] This distinction underscores "girt"'s specialization in metal and post-frame systems across North America since the 1940s.[16]Role in Construction
Functions
Girts, as horizontal structural members in wall framing systems, serve critical roles in enhancing the overall integrity of buildings by providing support and facilitating load management.[5] A primary function of girts is to offer lateral support to wall panels, resisting wind loads and seismic forces through efficient transfer of these forces to the primary framing elements, such as columns.[5][7] This mechanism ensures the walls remain stable under dynamic environmental pressures without compromising the building's envelope.[17] Girts also act as attachment points for wall components, featuring clips or fasteners that secure sheeting, insulation, and cladding to maintain a weather-tight and structurally sound exterior.[7][17] These attachments distribute attachment stresses evenly, preventing localized failures in the panel system.[5] In terms of configuration, girts are typically spaced 5 to 8 feet vertically along the wall height to promote even distribution of loads and optimize panel support.[18][19] This spacing allows for balanced force transmission while accommodating standard panel dimensions and construction practices.[7] Additionally, girts aid in alignment during building erection by providing a framework that helps maintain the plumb and level of wall surfaces, with pre-punched holes ensuring precise positioning and reducing erection errors.[17] This contributes to the overall dimensional accuracy and aesthetic quality of the completed structure.[5]Integration in Wall Systems
Girts integrate into wall systems primarily through horizontal connections to vertical primary framing members, such as columns or rigid frame elements, where they are secured at their ends using bolted or welded joints. These connections enable girts to transfer wind and other lateral loads from the wall panels to the building's main structural frame, ensuring overall stability.[7][20] In modern energy-efficient wall assemblies, girts function as furring strips installed over layers of rigid insulation, creating a supportive framework for exterior cladding while reducing thermal bridging. This layering allows for continuous insulation beneath the girts, with cladding materials such as metal panels or veneers attached directly to the girts, enhancing the building envelope's thermal performance. Girts also serve as attachment points for wall panels, bridging the gap between structural framing and exterior finishes.[2][21] Sidewall girts are typically installed continuously along the building's length, spanning between columns to provide uniform horizontal support for extended wall spans parallel to the roof ridge. In end walls, however, girts run perpendicular to the ridge and are often interrupted or adjusted around architectural openings such as doors and windows to accommodate framing and maintain structural integrity.[22][23] At the eave, the transition between wall and roof, girts coordinate with eave struts to form a continuous support line, where the uppermost girt connects to the eave strut for seamless load transfer and panel attachment across the junction. This integration prevents discontinuities in the structural envelope and supports both wall and roof sheeting effectively.[20][24]Types of Girts
Cold-Formed Girts
Cold-formed girts represent the predominant type of girts in contemporary prefabricated metal buildings, valued for their lightweight construction and cost-effectiveness in providing horizontal support to wall panels. These girts are produced by roll-forming steel sheets into specific profiles that enhance structural efficiency without the need for heavy materials. They serve as secondary framing members attached to primary columns, facilitating the attachment of cladding while minimizing overall building weight.[25] The primary profile shapes for cold-formed girts are Z-girts and C-girts, each suited to particular installation needs. Z-girts, with their offset flanges forming a zigzag configuration, are preferred for continuous spanning across multiple bays due to their superior lapping capabilities, which allow seamless overlap connections that maintain structural continuity and distribute loads effectively. In contrast, C-girts feature a channel-like shape that enables flush mounting directly against framing elements, making them ideal for applications requiring a smooth interior surface. Both profiles typically include stiffened lips on the flanges—formed at angles around 50 degrees—to increase torsional stiffness and prevent local buckling under load.[6][26][27] Cold-formed girts are generally manufactured from steel with thicknesses corresponding to 14- to 16-gauge (approximately 0.075 to 0.060 inches), though ranges up to 20-gauge are available for lighter duties; this gauge selection balances strength with ease of handling. Their high strength-to-weight ratio—often saving up to 40% in material compared to heavier alternatives—enables efficient designs that reduce transportation costs and on-site labor. Additionally, the profiles facilitate easy nesting during shipping, where sections stack compactly to optimize space, and support overlap connections at 12 to 24 inches, enhancing span continuity without excessive material use. These girts comply with the American Iron and Steel Institute (AISI) S100 North American Specification for the Design of Cold-Formed Steel Structural Members, ensuring standardized performance in structural applications.[6][27][28]Hot-Rolled and Other Variations
Hot-rolled girts consist of structural steel shapes, such as channels or wide-flange beams, formed by heating steel above its recrystallization temperature and shaping it into the desired profile. These girts are employed in high-load wall systems, particularly in industrial buildings where greater spanning capabilities and load-bearing capacity are required compared to lighter alternatives. For instance, they are suitable for supporting heavy cladding or bridging longer distances around large openings like doors and windows, providing robust secondary framing that integrates with primary structural columns.[29][30] Sub-girts serve as vertical or horizontal secondary framing members that attach to primary girts or structural elements, offering support for exterior cladding while accommodating drainage and ventilation in rainscreen or insulated facade systems. In rainscreen applications, sub-girts create an air gap behind the cladding to promote airflow and moisture management, often with depths aligned to insulation thickness for continuous thermal performance. These components, typically made from galvanized steel or other corrosion-resistant materials, can be oriented horizontally for wall panel attachment or vertically to enhance system modularity in multi-story facades.[31][32] Composite girts incorporate hybrid materials to combine structural strength with enhanced thermal properties, such as wood-steel combinations or insulated metal hybrids designed to minimize heat transfer in energy-efficient envelopes. In wood-steel hybrids, timber elements provide natural insulation alongside steel for load support, often used in post-frame structures to meet varying regional building codes. Insulated composite types, like those with embedded foam cores in metal profiles, reduce thermal bridging by up to 90% compared to traditional steel in certain designs, aiding compliance with stringent energy standards such as those in the International Energy Conservation Code.[33][2][34] Historical variations of girts prominently feature timber constructions, where large horizontal wooden beams were placed between vertical posts in early barn and agricultural buildings to stiffen walls and support siding. Dating back to the 18th and 19th centuries, these timber girts were hewn from local trees and joined using traditional mortise-and-tenon connections, forming the backbone of post-and-girt framing systems in North American vernacular architecture. While steel alternatives predominate in industrial applications, timber girts continue to be used in modern timber-frame and sustainable constructions, though preserved in legacy structures for their cultural and structural integrity.[35][36]Design and Engineering
Load-Bearing Considerations
Girts in metal building systems primarily experience transverse loads from wind pressures acting on wall panels, which are transferred as uniformly distributed loads along the girt's span. These transverse forces cause bending in the major axis, with positive and negative pressures leading to biaxial bending when combined with the self-weight of the girts and siding.[37] Axial loads on girts arise from compressive forces due to panel attachments and minor eccentricities, though these are typically secondary to transverse effects; combined loading scenarios require evaluating interaction effects per cold-formed steel design standards.[38] Span lengths for girts are determined by bay widths between primary framing columns, with maximum allowable spans influenced by building height and wind exposure categories as defined in ASCE/SEI 7-22, where higher exposure increases velocity pressure and thus demands shorter effective spans or deeper sections to control deflections. Deflection limits for girts supporting metal panels are generally set at a span-to-deflection ratio of L/120 under service-level (10-year recurrence) wind loads to ensure cladding performance and prevent attachment failures, though stricter limits like L/180 may apply for buildings with sensitive finishes.[39] Factored load combinations for girt design follow ASCE/SEI 7-22 provisions, such as 1.2D + 1.0W + 0.5L for wind-dominant cases, where D represents dead loads (including panel and girt weight), W is wind load, and L is any applicable live load; these ensure capacity against ultimate limit states while incorporating load factors for safety. Building geometry significantly impacts girt loading, as taller wall heights increase tributary areas for wind, necessitating closer vertical girt spacing—typically reducing from 5-7 feet at mid-height to 3-4 feet near eaves—to distribute loads evenly and mitigate buckling risks under compression.[40]Stability and Bracing
Girts play a critical role in the lateral stability of metal buildings by functioning as components of the wall diaphragm system, where they support the cladding panels to collectively transfer shear forces generated by wind or seismic loads to the rigid end frames. This diaphragm action distributes longitudinal shear across the wall length, preventing excessive frame drift and ensuring overall structural integrity.[41] To mitigate buckling risks, particularly lateral-torsional buckling in girts subjected to compressive forces from leeward wind pressures, bracing elements such as sag rods or cross-bracing are installed between adjacent girts, typically at mid-span points. Sag rods provide discrete lateral support to the compression flange, reducing the unbraced length and enhancing torsional restraint, while cross-bracing offers continuous stabilization against out-of-plane deformation. These mechanisms limit the potential for three-wave buckling modes in parallel girt systems by introducing stiffness that counters local web deformations and offsets.[42] The effective length for flexural buckling of girts is calculated as , where is the unbraced length and is the effective length factor accounting for end restraint conditions. For braced frames, is derived using the alignment chart method, where for sidesway inhibited conditions, K = \frac{\pi}{\sqrt{\frac{\pi^2}{(G_A + G_B) \tan(\pi \sqrt{\frac{G_A G_B}{G_A + G_B}})} + \frac{G_A G_B - 1}{G_A + G_B} + 1}, with representing the stiffness ratio of columns to girders at each end (approximated as 10 for pinned, 1.0 for fixed). This derivation stems from solving the stability equations for subassemblages, balancing rotational stiffness to approximate the true buckling load without full frame analysis. For many secondary members like girts, is conservatively used unless justified by analysis.[43] Design provisions for girt stability and bracing in metal buildings adhere to ANSI/AISC 360-22, particularly Chapters E (compression members) and F (flexure), with Appendix 6 specifying required brace strengths (e.g., 2% of required moment for torsional bracing) and stiffness (e.g., for LRFD). These apply to secondary framing systems, ensuring braces prevent incremental deformations exceeding .[44]Materials and Manufacturing
Common Materials
Girts are predominantly fabricated from steel, with galvanized steel being the most common choice due to its balance of strength, durability, and cost-effectiveness in metal building systems. The primary standard for this material is ASTM A653, which specifies zinc-coated steel sheets with a G90 coating designation providing approximately 0.90 ounces of zinc per square foot for enhanced corrosion resistance, particularly in exposed environments. Steel grades used for girts typically include ASTM A653 Grade 50 or 55, offering yield strengths ranging from 50 to 55 ksi (345 to 380 MPa), which ensures adequate load support without excessive weight.[45] The modulus of elasticity for these steels is consistently 29,000 ksi (200 GPa), contributing to their predictable deformation behavior under stress.[46] Alternative materials are selected based on specific environmental or economic needs. Aluminum girts, often extruded from alloys like 6063-T6, provide lightweight construction and inherent corrosion resistance through oxide formation, making them suitable for coastal or humid areas where steel galvanization may degrade over time. Wood girts, typically dimension lumber such as 2x6 Douglas fir, are used in low-cost agricultural structures like pole barns for their ease of handling and thermal insulation properties, though they require treatment to mitigate rot and insect damage.[47] Composite girts, including composite-only and composite-metal hybrid (CMH) variants, offer solutions for thermal bridging and corrosion challenges while preserving structural performance. Composite-only girts use insulating materials like rigid foam integrated with facing, providing continuous insulation. CMH girts combine steel's strength with insulating cores, such as in GreenGirt systems, to enhance energy efficiency in wall assemblies.[2] From a sustainability perspective, steel girts incorporate high levels of recycled content, with new structural steel averaging 93% recycled material, including post-consumer scrap (typically at least 25% based on industry standards for LEED certification), reducing energy consumption and environmental impact in production.[48][49] This recyclability aligns with green building practices, as steel can be reused indefinitely without loss of quality.[50]Fabrication Processes
The fabrication of girts primarily involves processing steel, the most common material for these structural components, through specialized techniques to achieve the desired profiles such as C- or Z-shapes.[51] Cold-forming is the predominant method for lighter girts, where sheet steel coils are uncoiled and fed into roll-forming mills that bend the material progressively through a series of rollers at room temperature, without applying heat, to create precise, uniform cross-sections.[51][25] This process allows for high-volume production of consistent shapes suitable for secondary framing in buildings.[52] For heavier sections, hot-rolling is employed, beginning with heating steel billets to approximately 1,100–1,300°C in a reheating furnace, followed by passing the softened material through a series of grooved rolls in a steel mill to form the initial shape, and then cooling it to solidify the profile.[25] This method produces robust, standardized shapes but requires additional fabrication steps compared to cold-forming.[25] Finishing operations follow forming to prepare girts for use, including punching holes for bolt connections and other penetrations, typically performed inline during roll-forming or as a post-process using automated punches to ensure accuracy.[51] Corrosion protection is achieved via hot-dip galvanizing, where fabricated girts are cleaned, fluxed, immersed in a bath of molten zinc at around 450°C, and cooled, forming a durable zinc-iron alloy coating that adheres metallurgically to the steel surface. Quality checks during finishing adhere to standards such as ISO 1461 for galvanized coatings, involving visual inspections, thickness measurements via magnetic gauges, and tests for uniformity and adhesion to verify compliance. Customization enhances girt efficiency, with lengths cut to specification—often up to 30 feet for practical handling and transport—using saws or shears integrated into the production line.[53] To optimize shipping, Z-shaped girts are frequently nested by overlapping their profiles into compact bundles, reducing volume and securing them for transit without damage.[54]Installation and Applications
Installation Methods
Steel Girts in Pre-Engineered Metal Buildings
Girts in pre-engineered metal buildings (PEMBs) are typically installed after the primary structural columns have been erected and plumbed to ensure a stable framework for secondary members.[55][56] The process begins in the first braced bay, starting with the base girt and proceeding upward to maintain alignment and prevent instability during erection.[55][57] Temporary bracing, such as wood blocking or steel cables, is applied between girt lines to hold the assembly plumb until the full frame is complete.[55][56] Attachment of girts to columns is achieved using 1/2-inch diameter ASTM A307 bolts through pre-welded clip angles, typically with two to four bolts per connection tightened to a snug-tight condition.[57][55] For girt end overlaps, blind rivets or additional bolts secure the laps at intervals of approximately 20 inches on center to ensure continuity and load transfer.[55] Self-drilling screws may supplement bolts where clips allow, particularly for flush or bypass girt configurations.[56] These connections follow erection drawings to accommodate specific bay spacing, often 20 to 40 feet between columns.[57] Alignment during installation requires the use of levels or transits to verify plumb and straightness at each girt attachment point.[56][55] Safety protocols include OSHA-approved fall protection systems, such as harnesses and netting, for elevated work above six feet, along with hard hats and non-slip footwear.[56][57] Temporary bracing must remain in place until wall panels are attached, providing lateral stability.[55] Installation tolerances limit deviations to a maximum of 1/8 inch from true position for girt alignment, in accordance with industry standards to ensure structural integrity and proper panel fitment.[58] Once girts are erected, wall panels are fastened directly to them using self-drilling screws for a weathertight envelope.[55]Wood and Composite Girts
In post-frame construction, common for agricultural and residential applications, wood girts (typically 2x6 lumber) are installed horizontally by nailing or screwing them to the exterior face of vertical posts, spaced at 24 inches on center.[59] Posts are usually spaced 8 to 12 feet apart, and girts run continuously or lapped between posts, starting from the base and working upward after posts are set and braced. Temporary diagonal bracing stabilizes the frame until sheathing is applied.[60] Composite girts, such as insulated hybrid variants, are installed similarly to steel girts but with manufacturer-specific clips or adhesives to minimize thermal bridging; they attach to framing via self-tapping screws or bolts, often in retrofit scenarios over existing walls.[2]Common Applications
Girts are a fundamental component in pre-engineered metal buildings (PEMBs), where they provide horizontal support for sidewall cladding in structures such as warehouses and factories. These buildings represent a significant portion of low-rise commercial construction in the United States, with the Metal Building Manufacturers Association (MBMA) estimating that approximately 48% of low-rise buildings utilize metal building systems that incorporate girts for load transfer and panel attachment (as of 2017).[61] This application leverages girts' ability to enhance lateral stability, allowing for efficient, clear-span interiors ideal for storage and manufacturing operations.[2] In agricultural settings, girts constructed from wood or steel are widely used to support cladding on barns and silos, enabling durable enclosures for equipment, feed, and livestock. Post-frame designs often employ 2x6 wood girts spaced at 24 inches on center, nailed to posts for straightforward installation and cost-effectiveness in rural environments.[59] Hybrid systems combining steel framing with wood girts further adapt to varying site conditions, providing fire resistance and flexibility for large-scale farm operations.[62] Girts play a key role in retrofitting existing structures, particularly when adding new facades to masonry walls for improved insulation and aesthetics. Adjustable continuous girts facilitate the alignment of exterior cladding over uneven surfaces, minimizing thermal bridging while complying with energy retrofit standards.[63] Metal sub-girts are especially effective in renovations, reinforcing older walls without major structural alterations and supporting modern cladding materials.[64] Emerging applications of girts include their integration into solar panel mounting walls, where Z-section steel girts serve as robust brackets for securing photovoltaic arrays on building facades.[65] Additionally, in hurricane-resistant enclosures, girts are reinforced to meet enhanced wind-load requirements outlined in building codes updated during the 2010s, such as those following major storms, ensuring greater envelope integrity in high-velocity zones.[66]References
- https://en.[wiktionary](/page/Wiktionary).org/wiki/girt