The 2021 IBC code changes are critical for the Architecture, Engineering and Construction (AEC) industry. As various jurisdictions adopt the latest version, understanding the structural changes and their impacts on building assembly, design costs and project schedules becomes paramount. This article will introduce a few impactful code updates, including:
- Mass timber additions to the building code.
- Updates on construction types and fire resistance.
- Local Code Amendments to the 2021 IBC.
- Other key updates to consider in design.
Mass Timber Addition to the Building Code
Mass Timber has received significant attention in the 2021 IBC updates. While the 2018 IBC allowed Mass Timber Structures up to 85 feet and 6 stories (Type IV HT), the 2021 IBC has introduced four new construction types that extend the potential of Mass Timber up to 18 stories with a total building height of up to 270 feet. The new types are:
- Type IV-HT (Heavy Timber)
- Type IV-C (CLT – Mostly Exposed)
- Type IV-B (CLT – Mostly Protected)
- Type IV-A (CLT – Completely Protected)
Photo Credit: iccsafe.org
In conjunction with the new mass timber construction types, the 2021 IBC provides the following provisions:
- Prescriptive fire-resistance requirements added in IBC 722.7.
- Approval of mass timber connections per IBC 2304.10.1.
- Testing and Sealing requirements for intersections and abutting edges per 703.6 and 703.7.
- Connection Protection Inspection per 110.3.5.
The 2021 IBC Sections 508.4.4.1 and 509.4.1 emphasize fire safety and resistance for mass timber, introducing requirements like ½” gypsum separation for mass timber elements serving as fire barriers in Type IV-B or IV-C constructions. For mass timber buildings over 120 feet tall, redundant fire pump requirements are added in Section 403.3.2. Special inspection requirements are added for mass timber construction, including Section 1705.20, which requires periodic special inspections of sealants or adhesives used in adherence to Section 703.7.
- Roof supports: Fire-resistance ratings of primary structural frame and bearing walls are permitted to be reduced by 1 hour where supporting a roof only.
- Except in Group F-1, H, M and S-1 occupancies, fire protection of structural members in roof construction shall not be required, including protection of primary structural frame members, roof framing and decking where every part of the roof construction is 20 feet or more above any floor immediately below. Fire-retardant-treated wood members shall be allowed to be used for such unprotected members.
- In all occupancies, heavy timber complying with Section 2304.11 shall be allowed for roof construction, including primary structural frame members, where a 1-hour or less fire-resistance rating is required.
- Not less than the fire-resistance rating is required.
- Not less than the fire-resistance rating based on fire separation distance (see Table 705.5).
- Not less than the fire-resistance rating as referenced in Section 704.10.
- Heavy timber bearing walls supporting more than two floors or more than a floor and a roof shall have a fire resistance rating of not less than 1 hour.
Photo Credit: IBC 2021
Construction Types and Fire Resistance Updates
The IBC has regularly expanded and refined its requirements related to construction types and fire resistance. The 2021 code cycle was no exception. The following section describes related updates.
Section 1604.5 has expanded the classification of Risk Category III buildings to include additional qualifiers such as:
- Buildings with one or more public assembly spaces, each with over 300 occupants or a combined public assembly occupancy of over 2,500.
- Buildings with Group E or Group I-4 occupancies with more than 250 occupants.
Section 716.4 has introduced new listing and labeling requirements for fire curtains. Fire curtains are not specifically required or allowed by the code, but if they are used by special approval or trade-off, standards are now provided by this section.
Fire Retardant Treated (FRT) wood is no longer allowed in Group I-2 or Group B Ambulatory Care Facilities shaft enclosures per Section 603.1. This update conforms with applicable Federal Standards and CMS enforcement rules.
Section 704.4 requires fire-protected secondary structural members to be protected by individual encasement protection, and Section 704.6.1 requires any attachment to fire-protected steel members, primary or secondary, to be rated for at least 12 inches away from the structural member.
Local IBC Amendments and Jurisdictional Considerations
With the wide range of code editions currently adopted by local jurisdictions, the importance of being cognizant of the applicable requirements and amendments to the IBC cannot be stressed enough. It is recommended to communicate directly with local authorities to confirm the code edition currently adopted and its amendments before initiating the design phase. This section highlights Utah-specific code requirements.
Utah has amended Section 902 of the 2021 IBC on fire pump and riser room requirements, adding additional sections on fire pump and fire sprinkler rooms. The Utah amendments focus on room access, passageway clearances, and room markings, as well as room temperature and lighting. These amendments can be found on the State’s website at le.utah.gov/xcode/Title15A/Chapter3/15A-3-P1.html.
Along with adopting the IBC 2021, many local jurisdictions have also adopted its counterpart, the IEBC 2021, governing existing buildings. A notable Utah amendment revises the IEBC Section 906.6, removing and replacing the section with the following:
“Bracing requirement for unreinforced masonry parapets and other appendages upon reroofing. Where the intended alteration requires a permit for reroofing and involves removal of roofing materials from more than 25% of the roof area of a building assigned to Seismic Design Category D, E, or F that has parapets constructed of unreinforced masonry or appendages such as cornices, spires, towers, tanks, signs, statuary, etc., the work shall include installation of bracing to resist out-of-plane seismic forces, unless an evaluation demonstrates compliance of such items. For the purpose of design, reduced seismic forces shall be permitted.”
Seismic and Structural Observations
The importance of structural observations has been underscored, with the 2021 IBC now requiring structural observations for all Risk Category III facilities and above, previously only being required for Category IV facilities. Structural observations are now required for the Seismic Design Category E buildings more than two stories.
Where structural observations are required by the code, Section 1704.6 introduces some more definitive language on what should be fulfilled during a structural observation. The code now states that “the observer shall visually observe representative locations of structural system, details and load paths” for conformance. Although the term load paths is not yet defined in the IBC, it is generally used in the engineering community to describe a complete path that loads must take from their source until being resolved into the foundations of the structure. The addition of this term may indicate a trend towards more frequent observation from structural engineers to ensure complete load paths from foundation to roof are observed and conform to the approved documents.
Frost Protection at Exits
Section 1809.5.1 introduces a significant update: the requirement for frost protection for all exterior landings at required means-of-egress doors. This change aims to prevent concrete landings from heaving and compromising the operation of egress doors. Frost protection is only required as needed to prevent the obstruction of exit doors.
ACI 318-19 Adoption and Shear Force Amplification
IBC 2021’s adoption of ACI 318-19 brings in substantial updates to structural components, notably requiring the increased amplification of earthquake shear forces for design and introducing new requirements for shear lugs and anchorage to concrete.
Although amplification of forces is not new to concrete seismic design, ACI 318-19 now introduces amplification factors to shear walls based on wall height to wall length aspect ratios and then again amplified based on the number of stories. As a result, tall and slender shear walls supporting several levels may now be required to resist up to three times the design level forces. This can lead to significant increases in shear wall designs compared to previous code cycles.
Conclusion
As in previous code cycles, architects and their consultants should remain aware of the significant changes to the governing building codes. Adapting to the mass timber additions, staying updated on local amendments, and ensuring effective communication with clients and engineers can prepare for successful project execution.
Chris Bratt is an associate at BHB Structural. He is a licensed professional engineer and has been practicing structural engineering for 12 years. He can be reached at chris.bratt@bhbengineers.com.
