Tiêu đề 18 Load Computation.pdf ✅

PSAD 18: Load Computation 1. Design Methods There are two design methods that are considered in the National Structural Code of the Philippines (NSCP), as per the latest 2015 provisions, namely, the Allowable Stress Design (ASD) and the Load and Resistance Factor Design (LRFD). 1.1. Allowable Stress Design (ASD) It ensures that the nominal loads of a member do not exceed its allowable stresses with applied factory safety. It limits the design in the elastic region. It is also known as working stress design (WSD). F ≤ Rn Ω F = nominal member load Rn = nominal member capacity Ω = factor of safety (>1) 1.2. Load and Resistance Factor Design (LRFD) In this method, nominal loads are increased using load factors while allowable stresses are reduced through a reduction factor. The magnified loads must not exceed the reduced capacity. This design method considers some member strength in the plastic region. ΓF ≤ φRn F = nominal member load Rn = nominal member capacity Γ = load factor (≥ 1) φ = reduction factor (<1) 2. Load Combinations Various conditions cause simultaneous loads on a structure. The combination of loads with the most critical effect must be considered in design. Allowable Stress Design Load and Resistance Factor Design Section 203.4.1 D + F D + H + F + L + T D + H + F + (Lr or R) D + H + F + 0. 75[L + T + (Lror R)] D + H + F + (0. 6W or E 1. 4 ) Section 203.3.1 1. 4(D + F) 1. 2(D + F + T) + 1. 6(L + H) + 0. 5(Lr or R) 1. 2D + 1. 6(Lr or R) + (f1L or 0. 5W) 1. 2D + 1. 0W + f1L + 0. 5(Lr or R) 1. 2D + 1. 0E + f1L 0. 9D + 1. 0W + 1. 6H 0. 9D + 1. 0E + 1. 6H where: D = dead load L = live load Lr = roof live load E = earthquake load W = wind load F = load due to fluids with well-defined pressures and maximum heights H = load due to lateral pressure of soil and water in soil T = self-straining forces R = rain load f1 = 1.0 for floors in places of public assembly, for live loads in excess of 4.8kPa, and for garage live loads = 0.5 for other live loads

2.4. Earthquake Load This is a type of environmental load that considers a building’s resisting force against earthquake ground motion. The commonly used procedure in the board exam is the Equivalent Static Analysis, also known as the Equivalent Lateral Loads Method. The Detailed Procedure is outlined below: Step 1. Solve for the Structural Period, T T = Ct (hn ) 0.75 Ct = 0.0853 (steel moment resisting frames) = 0.0731 (reinforced concrete moment resisting frames) = 0.0488 (other buildings) hn = height of building Step 2. Solve for the Base Shear, V V = CVIW RT Base shear must not exceed maximum base shear, Vmax Vmax = 2.5CaIW R Base shear must be at least the minimum base shear, Vmin Vmin = 0.11CaIW Minimum Base Shear for Seismic Zone 4, VminZ4 VminZ4 = 0.8ZNvIW R Cv = velocity seismic response coefficient (Table 208-7) I = seismic importance factor (Table 208-1) W = total floor loads = ∑ Wn R = response modification factor = usually, 8.5 for reinforced concrete = usually, 8.0 for steel Ca = acceleration seismic response coefficient (Table 208-7) Z = Z-factor (Table 208-3) = 0.4 for Zone 4 = 0.2 for Zone 2 Nv = velocity near-source factor (Table 208-5) Step 3. Vertical Distribution of Force The base shear is distributed as a set of equivalent lateral forces applied at each floor. Fx = (V − Ft )wxhx ∑ wxhx Fx = lateral force at floor x Ft = whiplash force wx = weight of floor x hx = height of floor x The whiplash force is an additional force applied at the topmost part of the building. Ft = 0.07TV ≤ Ftmax = 0.25V Note: when T ≤ 0.7s → Ft = 0 Step 4. Solve for the Base Moment M = (∑Fxhx) + (Fthtop)