Tools › Civil and Geotechnical › Design Loads › Wind, Snow and Seismic Loads › Example
Two-story 100 × 60 ft building: wind pressures, roof snow with a lower-roof drift, ELF base shear and load combinations
| Tool | Wind, Snow and Seismic Loads |
| Source | American Society of Civil Engineers (2017). Minimum Design Loads and Associated Criteria for Buildings and Other Structures. ASCE/SEI 7-16, Reston, VA (Eqs. 26.10-1, 27.3-1, 30.3-1, 7.3-1, 7.4-1, 7.7-1, Figure 7.6-1, Eqs. 11.4-1 to 11.4-4, 12.8-1 to 12.8-12, Sections 2.3 and 2.4, as mathematics); U.S. Department of Defense (2023). Unified Facilities Criteria: Structural Engineering. UFC 3-301-01 (Tables 2-2, 3-1 and 3-2); Building Seismic Safety Council (2004). NEHRP Recommended Provisions, Part 1. FEMA 450 (Tables 1.4-1, 1.4-2, 5.2-1, 5.2-2). |
| Level | Professional |
Problem
A two-story building 100 ft along the wind by 60 ft across, floor at 14 ft (500 kips) and flat roof at 28 ft (300 kips), stands in Exposure C with a basic wind speed of 115 mph, a ground snow load of 30 psf, an adjacent lower roof 10 ft below the main roof (upper roof 100 ft, lower roof 40 ft), and mapped S_s = 1.0, S_1 = 0.4 on Site Class D (Risk Category II, R = 6, I_e = 1.0). Find the velocity pressure and the MWFRS wall pressures, the story wind forces, the flat-roof snow load with the drift on the lower roof, the design spectral accelerations, the seismic design category, the approximate period, the base shear and its distribution, and the governing load combinations for D = 50, L = 30, L_r = 10, S = 12, W = 20 and Q_E = 15.
Inputs to Enter
- Building: L 100 ft, B 60 ft, mean roof height 28 ft, slope 0°; levels Floor 2 at 14 ft / 500 kip, Roof at 28 ft / 300 kip
- Wind: V 115 mph, Exposure C (alpha 9.5, z_g 900 ft), flat terrain, K_d 0.85, K_e 1.0, G 0.85, enclosed GC_pi ±0.18, C_p 0.8 windward / Figure 27.3-1 leeward / −0.7 side, flat-roof zones −0.9, −0.9, −0.5, −0.3; C&C zones at 10 ft²; sign 10 × 20 ft with C_f 1.8; equipment A_f 24 ft², A_r 30 ft²
- Snow: p_g 30 psf, C_e = C_t = I_s = 1.0, all other surfaces, flat roof; lower roof l_u 100 ft, l_l 40 ft, step h_r 10 ft
- Seismic: S_s 1.0, S_1 0.4, Site Class D, ASCE 7-16 tables (UFC 3-301-01), T_L 8 s, Risk Category II, R 6, Ω_0 2, C_d 5, C_t 0.02, x 0.75, ρ 1.0, drift limit 0.020 h_sx; load effects D 50, L 30, L_r 10, S 12, W 20, Q_E 15
Answers
- K_z(28 ft) = 2.01 (28/900)^(2/9.5) = 0.968, q_h = 0.00256 (0.968)(1.0)(0.85)(1.0)(115)² = 27.86 psf; windward wall at the roof 13.93 psf (+GC_pi) / 23.96 psf (−GC_pi), leeward (C_p −0.367 at L/B = 1.67) −13.70 / −3.67 psf, side walls −21.59 / −11.56 psf, roof zone 0 to h/2 −26.33 / −16.30 psf
- Story wind forces (windward integrated over the tributary band plus leeward suction, × 60 ft): Floor 2 32.10 kip, Roof 11.38 kip, base shear 43.48 kip, overturning 768 kip-ft; C&C wall zone 4 +32.87 / −35.66 psf, roof zone 3 −83.02 psf; sign F = 7.94 kip; equipment 1.27 kip lateral, 1.25 kip uplift
- Snow: p_f = 0.7 (1)(1)(1)(30) = 21 psf, p_m = 20 psf, γ = 0.13 (30) + 14 = 17.9 pcf, h_b = 1.17 ft; drift: h_c = 8.83 ft, leeward h_d = 0.43 ∛100 ⁴√40 − 1.5 = 3.52 ft (windward 3/4 h_d at 40 ft = 1.65 ft), w = 4 h_d = 14.1 ft, p_d = h_d γ = 63.0 psf, total 84.0 psf at the step
- Seismic: F_a = 1.1, F_v = 1.9 (UFC Tables 3-1, 3-2), S_MS = 1.10, S_M1 = 0.76, S_DS = 0.733, S_D1 = 0.507, T_0 = 0.138 s, T_S = 0.691 s, SDC D; T_a = 0.02 (28)^0.75 = 0.243 s, C_u = 1.4; C_s = 0.733/6 = 0.1222 (cap 0.347, minimum 0.0323), V = 0.1222 × 800 = 97.78 kip; k = 1, C_vx 0.5455 / 0.4545, F_x 53.33 / 44.44 kip, story shears 53.33 / 97.78 kip, base overturning 2,116 kip-ft; simplified V = 1.1 (0.933)(800)/6 = 136.9 kip
- Combinations: E_h = 15, E_v = 0.2 (0.733)(50) = 7.33; strength governs by 1.2D + 1.0W + L + 0.5S = 116.0 (seismic 114.7, minimum 0.9D − E_v + E_h = 52.7); allowable stress governs by D + 0.525E_v + 0.525E_h + 0.75L + 0.75S = 93.23
Notes
The tool's default inputs are this case; press Submit. The building is illustrative (stated on the page); every formula is checked against an independent numpy re-implementation and against the published examples of FEMA 451 (Tables 5.1-1 and 5.2-1 to 5.2-3), FEMA P-751 (Table 4.1-4), the FEMA 2019 snowdrift design guide (Design Examples 1 to 3) and FEMA P-55 (Example 8.5, 0.6 q = 29.38 psf) by test_wind_snow_seismic_loads.php (538 checks).