Concealed connection systems are often selected for mass timber projects where the wood is a structural element and an architectural feature. Architects and designers often take advantage of the natural beauty of timber by leaving the wood exposed and hiding structural steel connections. Rather than encasing structural members in fireproofing materials such as gypsum board, leaving the wood exposed means that fire protection for the connection systems must be achieved through the combustible wood members themselves.
This blog post provides an overview of design guidelines to achieve fire-rated concealed post-to-beam and beam-to-girder connections using MTC Solutions pre-engineered Beam Hanger Systems.
Approximately 12-minute reading time.
Heavy timber elements have long been recognized for having excellent performance in fire due to the beneficial effects of charring. In a fire event, a layer of char insulates the unburned wood forming an insulating layer that slows down further combustion. Charring of wood can also protect non-combustible steel elements and therefore can achieve Fire Resistance Ratings (FRR). Industry testing has modeled charring rates as predictable functions of time, meaning that sacrificial char layers in beams and columns can be designed such that the reduced effective cross-section can effectively carry the structural load.
Technical Report No. 10
Technical Report No. 10 (TR 10), published by the American Wood Council, is a widely recognized document pertaining to the fire safety design of timber structures in North America. Both the NDS in the US and the CSA O86 in Canada reference this report.
Section 3.1.1 of the TR 10 outlines a method for calculating the effective char depth, achar, for a given FRR.
As an example, a nominal char rate (or nominal char depth over time), βn, of 1.5”/hr corresponds to an effective char rate, βeff, of 1.8”/hr, and an effective char depth, achar, of 1.8” for a 1-hour FRR. This means a designer must add an additional 1.8” to the bottom and side faces of their cross-section to achieve a 1-hour fire-resistance rating (top of members is assumed to be covered with a Cross Laminated Timber Floor panel(CLT panel). According to the TR 10, the nominal char depth is adjusted to account for the pyrolysis zone, the layer of material in the process of charring between unaffected and fully charred wood.
Based on multiple fire test results, the TR 10 explains that charring rates are non-linear and that linear models tend to underestimate the depth of the char layer for exposure times shorter than 1 hour, and overestimate the depth of the char layer for exposure times greater than 1 hour. The non-linear behavior is a result of the insulating properties of the char layer.
Corner Rounding Effect
During a fire event, structural members may be exposed to fire on more than one face, which will result in corners experiencing a faster charring rate. The unidirectional models used to estimate char layer thickness exclude the effect of fire exposure from multiple directions on a wood member. To account for increased charring at corners, corner rounding is considered in structural fire design. As shown in the figure below, the radius of the corner (r) illustrates the additional charring experienced and the increased cover requirements at corners.
This corner rounding effect is especially important when designing concealed beam hanger connections. In the figures below, the undamaged wood represents the section of the beam that will be insulated from a fire event and will be protected by the charred layers. Following regular design requirements outlined in our connection design guides, the dashed red line represents the maximum distance from the bottom of the member cross-section, asec (see MTC Solutions Beam Hanger Design Guide), at which the connector may be installed. MTC Solutions beam hangers consider this requirement to avoid the use of reinforcing screws (perpendicular to grain splitting), and this is still an important detail in fire design. To achieve a proper fire-rated connection, the connector must fit inside this region of undamaged wood and the bottom of the connector must sit below this dashed red line.
Let us assume a designer is working on a beam-to-column connection with a 6-7/8” x 21” beam carrying a shear reaction of 18,000 lbs ASD in the USA (or up to 140 kN LSD in Canada) while achieving a 1-hour fire-resistance rating. In accordance with the TR 10, connections in assemblies tested in accordance with ASTM E119 may be designed at the interface between the connection and the protection to the charring temperature of wood, with the char depth (achar) for an hour is equal to 1.5”.
Consequently, MTC’s Beam Hangers full-scale fully loaded fire tests (Softwood Lumber Board’s Glulam Connection Fire Test Summary Report) were performed according to ASTM E119 “Standard Test Methods for Fire Tests of Building Construction and Materials”. This requirement means either upsizing the beam to gain the necessary cover (15.4” x 31.5”), or utilizing a low-profile connector such as MTC’s RICON S VS 390×80 XL connector, specifically engineered for this challenge of meeting cover requirements while providing an architecturally appealing high-capacity off-the-shelf solution. On the 6-7/8” x 21” beam, the RICON easily meets the cover requirements outlined in the TR 10, leaving 1-7/8” of cover on the sides and 1-5/8” of cover on the bottom of the connector.
The Eurocode 5, Part 1-2: General Structural Fire Design, Clause 220.127.116.11 (3) states that only gaps larger than 1/16” [2 mm] between wood elements require additional protection measures. The North American codes do not mention any tolerance requirements specifically. It may therefore be generally accepted that a connection between two elements with a tolerance gap smaller than 1/16” [2 mm] may not require additional fire protective measures.
Despite the general acceptance that additional measures are unnecessary when gap tolerance requirements are met, the use of common fire caulking may help to ease the approval process with local building authorities. In MTC’s full-scale fully loaded fire tests, Hilti FS-ONE fire caulking was successfully used.
For more information, MTC’s Beam Hanger Design Guide provides complete design tables to easily achieve concealed and fire-rated connections. The guide provides the minimum cross-section required to properly conceal the GIGANT, the RICON S VS and the MEGANT for 1-hour and 2-hour FRR. Moreover, the RICON S VS and the MEGANT were tested following ASTM standards and were awarded a 1.5-hour fire rating certification under a full-scale fully loaded test conducted at the South West Research Institute in San Antonio Texas.
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