2021 IRC Basic Design Criteria

IRC applies to single-family, two-family, and townhouse dwelling

Design Live Loading

• IRC Table R305.1 Minimum Uniformly Distributed Live Loads
• IRC Table R305.7 Allowable Deflection of Structural Members
• Deck Live (snow) Load: 46 psf (P_g)

Snow Load

• Ground Snow Load: 46 psf (P_g)
• Roof Snow Load: 41 psf (P_m)

ARM 24.301.154 (5) Subsection R301.6, Roof Load, is deleted and replaced with the following:

"Snow loads shall be determined by the building official. In areas of the state outside of certified city, county, and town jurisdictions, the design snow load shall be calculated using the 2022 edition of the American Society of Civil Engineer's "Minimum Design Loads and Associated Criteria for Buildings and Other Structures, ASCE/SEI 7-22." The information is available online using the ASCE 7 Hazard Tool. The minimum design roof snow load after allowed reductions shall be 30 psf unless justified by a Montana licensed design professional to the satisfaction of the building official. Coefficients and factors other than those specified in the building code may be used when justified by a Montana licensed design professional to the satisfaction of the building official."

IRC Section R905.1.2 Ice Barrier Required. 

Frost Depth

• One Story: 36 inches minimum bottom of the footing
• Two Story: 48 inches minimum bottom of the footing

Basic Design Wind Speed

Wind Speed: 115 mph

IRC Figures R301.2(2), R301.2.1.1. Exposure category per IRC Section R301.2.1.4

Weathering

Severe - Table R301.2(1)

Seismic Design Category

D₀  - Section R301.2.2.1

Geotechnical (Soils)

• Site-specific Geotechnical Analysis Required at permit application
• Final Observation Report submitted prior to footing inspection

2021 IBC Basic Design Criteria

Design Live Loading

• IBC Table 1607.1
• Deck Live (snow) Load: 46 psf (P_g)

Snow Load

• ASCE 7 Table 7.2-4 for Montana
• Ground Snow Load: 46 psf (P_g)
• Roof Snow Load: 41 psf (P_m)

ARM 24.301.146 (26) Subsection 1608.2, Ground Snow Loads, is deleted and replaced with the following:

"Snow loads shall be determined by the building official. In areas of the state outside of certified city, county, and town jurisdictions, the design snow load shall be calculated using the 2022 edition of the American Society of Civil Engineer's "Minimum Design Loads and Associated Criteria for Buildings and Other Structures, ASCE/SEI 7-22." The information is available online using the ASCE 7 Hazard Tool. The minimum design roof snow load after allowed reductions shall be 30 psf unless justified by a Montana licensed design professional to the satisfaction of the building official. Coefficients and factors other than those specified in the building code may be used when justified by a Montana licensed design professional to the satisfaction of the building official."

Frost Depth

• One Story: 36 inches minimum bottom of the footing
• Two Story: 48 inches minimum bottom of the footing

Basic Design Wind Speed (3-Second gust)

• Wind Speed: 115 mph

Using IBC Figures 1609.3(1), 1609.3(2), 1609.3(3), and 1609.3(4). Exposure category per IBC Section 1609.4

Seismic Design Category: D

Use this basic procedure to determine the Seismic Design Category:

1. Determine the mapped maximum considered earthquake spectral response acceleration at short periods(S_s) and at 1-second period (S₁), for the site location from IBC Figures 1613.2.1(1) through 1613.2.1(8)
2. Determine the (soil) site class in accordance with ASCE 7
3. Determine the site coefficients F_a and F_v from IBC Tables 1613.2.3(1) and 1613.2.3(2), respectively

4. Determine the 5-percent damped design spectral response acceleration at short periods (S_DS) as follows:

   S_DS = (2/3)(S_MS)

   S_D1 = (2/3)(S_M1)

5. Determine the seismic design category as prescribed by Tables 1613.2.5(1) and 1613.2.5(2). The highest of the seismic design categories from the two tables is the category assigned

Geotechnical (Soils)

• Site-specific Geotechnical Analysis Required at permit application
• Final Observation Report submitted prior to footing inspection