Structural changes in the 2012 International Building Code

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    About 100 successful code changes to the structural provisions of the 2009 International Building Code were subsequently incorporated into the current 2012 edition of the IBC. The 2012 edition of the IBC references several hundred national standards, which are listed alphabetically in Chapter 35. The year or edition of the standard is only shown in Chapter 35, not in the body of the code. For example, Sections 1604.10 and 1613 reference “Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7” for determination of wind and seismic load effects, but do not indicate what edition. Chapter 35 shows the edition (year) of the standard being referenced. The main structural standards referenced in the 2012 IBC for loads and materials are shown in the following table.

    SubjectIBC ChapterReferenced Standard in 2012 IBCStructural loads16ASCE/SEI 7-10Concrete19ACI 318-11Aluminum20ADM1-2010Masonry21TMS402-11/ACI 530-11/ASCE 5-11
    TMS602-11/ACI 530.1-11/ASCE 6-11Structural Steel22AISC 360-10 AISC 341-10Cold-formed steel22AISI S100-07/S1-101Wood23AWC NDS-2012
    AF&PA SDPWS-082

    Key referenced structural standards in the 2012 IBC
    1See Chapter 35 for complete list of AISI standards referenced in the IBC.
    2Same as 2009 IBC (no change in 2012).

    Of the structural standards referenced by the 2012 IBC, perhaps most significant is the 2010 edition of “Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7 (ASCE 7-10).” Part I of this two-part article provides a brief overview of the more significant changes to Chapter 16 of the IBC, which covers design loads for structures. For a complete discussion of the significant changes to the 2012 IBC, including both structural and non-structural fire and life safety provisions, refer to Significant Changes to the International Building Code, 2012 Edition, available from the ICC.

    Section 1604.5, Risk Category. The term “occupancy category” has been changed to “risk category” to better reflect the intended meaning and to coordinate the terminology with ASCE 7-10.

    The term “occupancy category” is somewhat misleading because it implies something about the nature of the building occupants and “occupancy” relates primarily to the non-structural fire and life safety provisions, not the risks associated with structural failure. Some structures regulated by the IBC and IEBC are not even occupied but are assigned an occupancy category because their failure could pose a substantial risk to the public.

    Figure 1. Approximate Relationship between Number of Lives Placed at Risk by a Failure and Risk Category (Courtesy of ASCE, ASCE 7-10 Commentary Figure C1-1).

    Although the terminology changed, the classifications continue to reflect the progression of the consequences of failure from the lowest (Risk Category I) to the highest (Risk Category IV) (see Figure 1 on page 40). A detailed discussion of the risk categories is contained in Section C1.5 of the ASCE 7-10 commentary.

    Section 1605, Load Combinations. The strength design and allowable stress design load combinations in the 2012 IBC have been coordinated with the load combinations in Section 2.3 (SD and LRFD) and Section 2.4 (ASD) of ASCE 7-10, respectively. Loads due to fluids, F, and lateral earth pressures, ground water pressures, or the pressure of bulk materials, H, as well as ice loads have been included. The load combinations for “other loads” for LRFD in Section 1605.2.1 and ASD in Section 1605.3.1.2 were modified to include atmospheric ice loads for ice-sensitive structures. These sections reference ASCE 7 Section 2.3.4 for LRFD and ASCE 7 Section 2.4.3 for ASD, respectively. The term “ice-sensitive structure” has been added to Section 202.

    Section 1605.2, Load Combinations Using Strength Design or Load and Resistance Factor Design. The wind design requirements of Section 1609 were extensively revised to update and coordinate them with the latest wind load provisions in ASCE 7-10, which are based on ultimate design wind speeds, Vult. Ultimate design wind speeds produce strength level wind loads similar to seismic load effects. For strength design or LRFD, the load factor on the wind load, W, has been changed to 1.0 to account for the new strength level wind forces in ASCE 7-10.



    Section 1605.3, Load Combinations Using Allowable Stress Design. For ASD, the load factor on the wind load, W, has been changed to 0.6 in both the basic and alternative basic ASD load combinations to account for the new ultimate design wind speed in ASCE 7-10 (WASD = 0.6Wult ). The ω factor in the alternative basic ASD load combinations has been modified to be either 1.3 or 1.0. When allowable stresses have been increased or load combinations have been reduced (as permitted by a material chapter in the code or a referenced standard), the coefficient ω is taken as 1.3. Otherwise ω is to be taken as 1.0. To achieve consistency with the strength design load combinations and ASCE 7-10, earthquake load effect, E, was removed from the basic ASD load combination Equation 16-13, and a new load combination Equation 16-14 was added. This has the effect of retaining roof live load, Lr, and rain load, R, in combination with wind load, W (Equation 16-13), but removed these loads in combination with earthquake load, E, in Equation 16-14. This achieves consistency between the ASD load combinations and the strength design or LRFD load combinations in Equations 16-4 and 16-5.

    Table 1607.1, Minimum Live Loads. Many live loads in Chapter 4 of ASCE 7 were updated in the 2010 edition of the ASCE 7 standard. To coordinate the changes in ASCE 7-10 with the 2012 IBC, the live loads prescribed in IBC Section 1607 and Table 1607.1 have been updated to coordinate them with the live loads of Chapter 4 and Table 4-1 in ASCE 7-10. In some cases, duplicate provisions in the code were deleted because they were incorporated into ASCE 7-10. For example, the detailed loading requirements for vehicle barriers were deleted from the code and replaced with a reference to similar provisions in ASCE 7.

    Section 1607.6, Helipads. The terminology and live load design requirements for helicopter landing areas have been updated and coordinated with ASCE 7-10. The new term “helipad” is now used to describe helicopter landing areas. A helipad is defined as a structural surface used for landing, taking off, taxiing, and parking of helicopters. The previous provisions were unclear as to whether or not helicopter live loads were intended to be separate or in addition to the load combinations required by Section 1605. The helipad loading requirements have been relocated to Section 1607.6 (which prescribes live loads) from Section 1605 (load combinations). The actual loading criteria required to design helipads were updated and coordinated with the helicopter loads specified in ASCE 7-10 and references to dead load, D, and snow load, S, which served no real purpose, were deleted.

    Section 1607.7, Heavy Vehicle Loads. Provisions relating to the design of structures supporting heavy vehicle loads in excess of 10,000 pounds gross vehicle weight have been updated. Structures intended to support heavy vehicle loads are designed in accordance with the same specifications required by the jurisdiction for the design of roadways and bridges, such as AASHTO. The new requirements apply specifically to fire truck and emergency vehicles, and heavy vehicle parking garages. Changes were also made to loading requirements for forklifts and movable equipment.

    Sections 1608.3 and 1611.2, Ponding Instability. A new definition of “susceptible bay” has been added to clarify where ponding must be considered in the design of roof structures to avoid progressive deflection. A susceptible bay is defined as a roof, or portion thereof, with 1) a slope less than 1/4 inch per foot; or 2) on which water is impounded upon and the primary drainage system is blocked. Only those portions of the roof considered susceptible bays must be designed for ponding. Areas of the roof with a slope of 1/4 inch per foot or greater toward points of free drainage are not considered susceptible and need not be designed for ponding.

    For a complete discussion of important provisions in the 2012 IBC, including structural and non-structural fire and life safety requirements, refer to the 2012 International Building Code Handbook, available from ICC.


    John R. Henry, P.E., is the principal staff engineer, International Code Council. Contact him at jhenry@iccsafe.org.