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

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

ARM 24.301.154 (5) Subsection R301.6 is deleted - “Snow Loads for Structural Design in Montana", Civil Engineering Department, Montana State University, 2004 revised edition
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 (23) Subsection 1608.2 is deleted - "Snow Loads for Structural Design in Montana", Civil Engineering Department, Montana State University, 2004 Revised Edition.
IBC Section 1507.1.2 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 (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