Special inspection & soils/foundations (IBC Chapters 17 & 18)—Part 2.
This multi-part series discusses significant structural changes to the 2024 International Building Code (IBC) by the International Code Council (ICC). Part 2 includes an overview of changes to IBC Chapter 17 on special inspection and IBC Chapter 18 on soils and foundations. Only a portion of these chapters’ total number of code changes are discussed in this article. More information on the code changes can be found in the 2024 Significant Changes to the International Building Code, available from ICC (Figure 1).
Metal Building Systems
Metal building systems (Figure 2) are significantly different from other forms of steel construction, especially regarding the shared design responsibilities between the metal building system manufacturer and registered design professional for the project. Adding new metal building system provisions, a definition, and material requirements clarifies the design requirements for the systems (a future article in the series will provide more details). New IBC Section 1705.2.6 provides special inspection provisions for metal building systems.
1705.2.6 Metal building systems. Special inspections of metal building systems shall be performed in accordance with Sections 1705.2.1, 1705.2.3, 1705.2.4 and 1705.2.5 and Table 1705.2.6. The approved agency shall perform inspections of the erected metal building system to verify compliance with the approved construction documents.
Change Significance: With clarification of the design requirements for different metal building system components, special inspection requirements become clearer. Today, many construction documents list nonexistent Metal Building Manufacturer Association standards as the governing design requirements. By placing material and special inspection requirements into the IBC, designers can create more accurate construction documents, and building departments can more easily inspect the construction.
New special inspection requirements complement the new provisions for metal building systems in IBC Section 2210. Metal building systems are typically highly optimized structures heavily dependent on bracing components to function per the design. Some bracing components consist of materials that are not considered to be “structural steel.” As these components didn’t previously have special inspections listed in IBC Section 1705.2, an inspection of the completed installation of these critical components was often overlooked.
Metal building systems often contain assemblies made of various components, such as structural steel, cold-formed steel, and steel cables. While the individual components are often covered by fabricator special inspections and tests found in the subsections of IBC Section 1705.2, the “systems” used in metal building systems are often unique and have not been specifically identified in the steel section. The new language adds explicit requirements for special inspection of commonly used systems not previously identified elsewhere.
Special Inspection Of Reinforcing Bar Welding
IBC Section 1705.3.1 references AWS D1.4 Structural Welding Code – Reinforcing Steel which contains requirements for rebar inspection. The construction documents must indicate the type and location of welded splices and any other welding. Reinforcement must be of a material quality that forms a strong weld where welding is required. Reinforcing steel per ASTM A706 Standard Specification for Deformed and Plain Low-Alloy Steel Bars for Concrete Reinforcement is typically specified. Other rebar must be tested before welding begins to determine whether welding is an option or if mechanical splices are required for the reinforcement. Ideally, rebar weldability is determined before construction begins. Changes to IBC Table 1705.3 coordinate the special inspection provisions for welding of reinforcing steel (Figure 3) with the provisions in Section 26.13.3 of ACI 318-19 Building Code Requirements for Structural Concrete.
Change Significance: IBC Table 1705.3 Item 2(b) adds a new requirement for continuous special inspection of reinforcement welding in special moment frames, boundary elements of special structural walls, and coupling beams as required by ACI 318 Section 26.13.3.2(d). Because of their critical nature, new Item 2(c) was added to require continuous special inspection of all welded reinforcement splices. New item 2(d) for continuous special inspection for welding primary tension reinforcement in corbels highlights another difficult area to weld.
Previous Item 2(b) for periodic inspection of single pass fillet welds is renumbered as Item 2(e). Former Item 2(c) for special inspection of all other welds is renumbered as Item 2(f) and revised to permit these welds to be performed as a periodic special inspection since the critical welds covered by new Items 2(b), 2(c), and 2(d) have been introduced into the table as separate continuous special inspections.
Guards On Retaining Walls
Guards are systems comprising posts, handrails (where required), and balusters or panels. Guards are required to prevent a person from falling to a lower elevation that might cause injury. The code was silent on requirements for guards on top of retaining walls. These conditions commonly occur in public places such as parks and schools. New IBC Section 1807.2.5 includes provisions for guards on retaining walls (Figure 4) consistent with guard requirements elsewhere in the code. The exception exempts conditions where a retaining wall is not accessible to the public.
1807.2.5 Guards. Guards shall be provided at retaining walls in accordance with Sections 1807.2.5.1 through 1807.2.5.3.
Exception: Guards are not required at retaining walls not accessible to the public.
1807.2.5.1 Where required. At retaining walls located within 36 inches (914 mm) of walking surfaces, a guard shall be required between the walking surface and the open side of the retaining wall where the walking surface is located more than 30 inches (762 mm) measured vertically to the surface or grade below at any point within 36 inches (914 mm) horizontally to the edge of the open side. Guards shall comply with Section 1607.9.
1807.2.5.2 Height. Required guards at retaining walls shall comply with the height requirements of Section 1015.3.
1807.2.5.3 Opening limitations. Required guards shall comply with the opening limitations of Section 1015.4.
Change Significance: IBC Section 1807.2.5.1 parameters for proximity to a walking surface and vertical distance to the surface or grade below are consistent with IBC Section 1015.2 for guards in the interior of a building and IBC Section 105.2 for work exempt from permits. IBC Section 105.2 Item 4 exempts retaining walls less than 4 feet tall from a permit; however, that distance is measured from the bottom of the footing making the grade difference approximately 30 inches. IBC Section 105.2 Item 6 indicates a sidewalk or driveway with more than a 30-inch grade change would require a permit.
IBC Section 1807.2.5.2 height requirement provisions reference IBC Section 1015.3, which mandates a 42-inch-high guard measured vertically from the adjacent walking surface. IBC Section 1807.2.5.3 opening limitation requirements reference IBC Section 1015.4, which restricts openings to the passage of a 4-inch-diameter sphere for the required guard height.
Grade Beams
In high seismic regions, grade beams (Figure 5) must be designed as ductile per ACI 318 unless the beam is strong enough to resist the anticipated maximum earthquake force as determined by the seismic load combination with an overstrength factor from ASCE 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Section 2.3.6 or 2.4.5. In other words, grade beams must be designed for overstrength or ductility. Accordingly, the exception in IBC Section 1810.3.12 for grade beams in deep foundation systems was modified so that only the ductile detailing provisions in ACI 318 Section 18.13.3.1 are exempt when grade beams are designed for the overstrength factor.
Section 1809 Shallow Foundations
1809.14 Grade beams. Grade beams shall comply with the provisions of ACI 318.
Exception: Grade Beams not subject to differential settlement exceeding one-fourth of the thresholds specified in ASCE 7 Table 12.13-3 and designed to resist the seismic load effects including overstrength factor in accordance with Section 2.3.6 or 2.4.5 of ASCE 7 need not comply with ACI 318 Section 18.13.3.1.
Section 1810 Deep Foundations
1810.3.12 Grade beams. Grade beams shall comply with the provisions of ACI 318.
Exception: Grade beams not subject to differential settlement exceeding one-fourth of the thresholds specified in ASCE 7 Table 12.13-3 and designed to resist the seismic load effects including overstrength factor in accordance with Section 2.3.6 or 2.4.5 of ASCE 7 need not comply with ACI 318 Section 18.13.3.1.
Change Significance: This clarification for shallow and deep foundations is needed since ASCE 7 Section 12.13.9.3.1 permits the downdrag of pile design based on the significant differential settlement during an earthquake. Differential settlement exceeding one-fourth of the threshold may impose moments and shears in the grade beam that exceed those computed with seismic load effects, including the overstrength factor, in which case the ductile detailing requirements for grade beams in ACI Section 18.13.3.1 would be required for structures assigned to Seismic Design Category D, E, or F. According to the ACI 318 Commentary for this section, “Grade beams resisting flexural stresses from column moments should have reinforcement details similar to the beams of the frame above the foundation.”
ASCE 7 Section 12.13.9 exempts foundation elements likely to be affected by earthquake-induced settlement from complying with ductility requirements when their deformation is less than limits which vary based on risk category.
Therefore, provisions for grade beams in foundation systems in the IBC were modified so that while grade beams designed with an overstrength factor need not meet the ductile detailing provisions in ACI 318, the exception is only permissible when differential settlements are less than one-fourth of the thresholds in ASCE 7 Table 12.13-3. All other provisions of ACI 318 for strength, durability, reinforcing steel cover, etc., are still applicable.
IBC Section 1809.14 is a new section that adds the grade beam provisions in the deep foundation section to the shallow foundation section. The grade beam must either be ductile or strengthened to resist forces from a maximum considered earthquake by including overstrength in its design.
Deep Foundation Elements
In those sections of IBC Chapter 18 that specifically deal with the types of elements most used in the construction of deep foundations, there are limitations placed on the stresses that can be used in the deep foundation element design. In most cases, the allowable stresses are stated as a percentage of the strength of the element’s material. For example, in the case of piles made of steel, the allowable stresses are prescribed as a percentage of the yield strength of the steel. The allowable design stresses designated in IBC Chapter 18 for each deep foundation type are intended to provide a safety factor against the dynamic forces that may cause damage, for example, from driving a deep foundation element, and to avoid overstress in the element under the design loads and loads induced by subsoil conditions.
1810.3.2 Materials. The materials used in deep foundation elements shall satisfy the requirements of Sections 1810.3.2.1 through 1810.3.2.8, as applicable.
Sections 1810.3.2.1 through 1810.3.2.7 are unchanged and not shown for brevity.
1810.3.2.8 Justification of higher allowable stresses. Use of allowable stresses greater than those specified in Section in Table 1810.3.2.6 that must be justified in accordance with this section shall be permitted where supporting data justifying such higher stresses is filed with submitted to and approved by the building official. Such substantiating data shall include the following:
A geotechnical investigation in accordance with Section 1803.
Load tests in accordance with Section 1810.3.3.1.2, regardless of the load supported by the element.
The design and installation of the deep foundation elements shall be under the direct supervision of a registered design professional knowledgeable in the field of soil mechanics and deep foundations who shall submit a report to the building official stating that the elements as installed satisfy the design criteria.
Change Significance: As previously written, this section could have allowed the override of the allowable stresses in IBC Table 1810.3.2.6 when a pile passed a load test – which is not the intent. Several foundation types in IBC Table 1810.3.2.6 have multiple allowable stresses for the same material type. For example, IBC Table 1810.3.2.6 provides for a higher allowable compressive yield stress of 0.5Fy for steel piles when justified per IBC Section 1810.3.2.8, which requires a geotechnical investigation and a load test. Otherwise, steel piles are limited to 0.35Fy. In other words, the pile test is to justify the higher values in IBC Table 1810.3.2.6, not to allow even higher values than those already tabulated.
Conclusion
Structural engineers should be aware of significant structural changes in the 2024 IBC for special inspections and soils/foundations. New special inspection provisions for metal building systems have been incorporated. Changes to IBC Table 1705.3 coordinate the special inspection provisions for welding of reinforcing steel with the provisions in ACI 318-19. New provisions for guards on retaining walls are added consistent with guard requirements elsewhere in the code. Provisions for grade beams in deep foundation systems were modified so that only the ductile detailing provisions in ACI 318 are exempt when grade beams are designed with an overstrength factor. Finally, new provisions allow tabulated higher allowable stresses for deep foundation elements when evidence supporting the higher capacity is submitted and approved by the building official.■