Strip Footing Capacity

Output:

Definitions
• ϕ_c = Resistance factor for concrete
• ϕ_s = Resistance factor for reinforcing steel
• f_c = Specified concrete strength (psi)
• f_y = Specified reinforcing steel yield strength (ksi)
• λ = Concrete density modification factor; 1.0 for normal density concrete
• A_s = Area of reinforcing steel (in²)
• α₁ = Stress modification factor for equivalent stress block
• β = Cracked concrete shear factor
• d = Depth to primary reinforcing (in)
• B = Width of footing (in)
• L_f = Footing length (ft)
• t_w = Wall thickness (in)
• Cover = concrete cover to centroid of reinforcing (in)
• q_a = Allowable bearing resistance for service conditions (ksf)
• q_ult = Ultimate bearing resistance (ksf)
• q_s = Service bearing demand (ksf)
• q_u = Factored ultimate bearing demand (ksf)
• d_v = Shear depth, normally taken as 0.9d (in)
Calculate soil bearing stresses
DCR = 100%
• P_s = (D + L)/L_f = (7 + 5) / 1 = 12 kip/ft
• P_u = max[1.4D, 1.25D + 1.5L] / L_f = max[9.8, 16.25] / 1 = 16.25 kip/ft
• q_s = P_s / B = 12 / (36/12) = 4 ksf
• q_u = P_u / B = 16.25 / (36/12) = 5.42 ksf
Calculate shear capacity at critical section, located 'dv' away from the face of the wall
DCR = 26%
• d = h - cover = 18 - 3 = 14.6875 in
• d_v = max[0.9d, 0.72h] = 0.9 (14.6875) = 13.22 in
• β = 0.21 if h < 14in or (B-t_w < 2d_v)). Else β = 230/(1000 + d_v*25.4). --> β = 0.21
• V_u,crit = q_u L_f (B - t_w - d_v) = (5.42) (1) (36 - 12 - 13.22)/12 = 4.87 kip
• V_c = ϕ_c λ β √f_c L_f d_v = (0.75) (1) (0.21) (0.762) (1) (13.22) = 19.03 kip
• V_u / V_r = 4.87 / 19.03 = 0.26 --> okay
Calculate the flexural capacity at the critical section, located at the face of the wall
DCR = 11%
• M_u = q_u L_f (B-t_w)² / 2 = (5.42) (1) ((36 - 12)/12)² / 2 = 130 kip-in = 10.84 kip-ft
• M_r = ϕ_s A_s f_y (d - a/2), where a = (ϕ_s A_s f_y) / (ϕ_c α₁ f_c L_f)
• Mr = (0.9) (1.55) (60) (14.6875 - 0.957/2)
• Mr = (83.7) (14.209)
• Mr = 1189 kip-in = 99.11 kip-ft
• M_u / M_r = 10.84 / 99.11 = 0.11 --> okay
User Notes:
User notes