13.4 NSCP Minimum Design Loads: Dead, Live, Wind, and Earthquake

13.4 NSCP Minimum Design Loads: Dead, Live, Wind, and Earthquake

Every structural design in the Philippines must comply with the legal mandates of the National Structural Code of the Philippines (NSCP 2015, Volume 1, 7th Edition). Chapter 2 dictates the minimum design loads required to ensure structural safety, serviceability, and life preservation under gravity, extreme typhoons, and severe seismic events.

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1. Dead Loads (NSCP Section 204)

Dead loads consist of the permanent weight of all materials of construction incorporated into the building, including walls, floors, roofs, ceilings, stairways, built-in partitions, finishes, and fixed service equipment.

Material Densities & Component Weights

NSCP 2015 gives Table 204-1 (minimum densities of materials, kN/m³) and Table 204-2 (minimum design dead loads of components, kPa). Representative values:

Movable Partition Allowance

In office buildings and commercial spaces where interior partitions are subject to rearrangement, a minimum uniformly distributed partition live/dead load allowance of $1.0\text{ kPa}$ must be included on the floor slab, regardless of whether partitions are shown on architectural drawings.

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2. Live Loads & Reduction Formulas (NSCP Section 205)

Live loads are transient forces produced by the use and occupancy of the building, excluding environmental loads such as wind and earthquake.

Minimum Uniform Live Loads (Table 205-1)

• Residential (Habitable rooms): $1.9\text{ kPa}$
• Classrooms: $1.9\text{ kPa}$
• Office Floors: $2.4\text{ kPa}$ (Lobbies: $4.8\text{ kPa}$)
• Corridors (above 1st floor): $3.8\text{ kPa}$ (1st floor corridors: $4.8\text{ kPa}$)
• Light Storage Warehouse: $6.0\text{ kPa}$
• Heavy Storage Warehouse: $12.0\text{ kPa}$
• Flat and Pitched Roof Live Load ($L_r$): $1.0\text{ kPa}$

Live Load Reduction

Members supporting large tributary floor areas are statistically unlikely to experience full design live loading across all bays simultaneously. For structural members supporting an influence area $A_I \ge 37.2\text{ m}^2$, the design live load may be reduced:

$L = L_0 \left( 0.25 + \frac{4.57}{\sqrt{A_I}} \right)$

Where:

• $L$ = reduced design live load per square meter.
• $L_0$ = unreduced basic design live load.
• $A_I = K_{LL} A_T$ = live load influence area ($\text{m}^2$).
• $A_T$ = tributary area supported by the member ($\text{m}^2$).
• $K_{LL}$ = live load element factor:
  • $K_{LL} = 4$ for interior columns and exterior columns without cantilever slabs.
  • $K_{LL} = 2$ for interior beams and exterior beams without cantilevers.
  • $K_{LL} = 1$ for two-way slabs.

Absolute Reduction Limits:

1. The reduction factor $\left(0.25 + \frac{4.57}{\sqrt{A_I}}\right)$ shall not be less than $0.50$ for members supporting one floor ($L \ge 0.50 L_0$).
2. The reduction factor shall not be less than $0.40$ for members supporting two or more floors ($L \ge 0.40 L_0$).
3. Ineligibility: Live loads exceeding $4.8\text{ kPa}$ shall not be reduced, except that members supporting two or more floors may have their live loads reduced by up to $20\%$. Areas intended for public assembly and parking garages cannot be reduced.

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3. Basic Load Combinations (NSCP Section 203)

Strength Design / Ultimate Strength Design (USD / LRFD)

Under Strength Design, structural members must resist the most critical factored load combination:

1. $U = 1.4 D$
2. $U = 1.2 D + 1.6 L + 0.5 (L_r \text{ or } R)$
3. $U = 1.2 D + 1.6 (L_r \text{ or } R) + (1.0 L \text{ or } 0.5 W)$
4. $U = 1.2 D + 1.0 W + 1.0 L + 0.5 (L_r \text{ or } R)$
5. $U = 1.2 D + 1.0 E + 1.0 L$
6. $U = 0.9 D + 1.0 W$
7. $U = 0.9 D + 1.0 E$

Note on Seismic Combinations: The load combinations $0.9D + 1.0E$ and $0.9D + 1.0W$ govern foundation uplift and member overturning where dead load resists lateral overturning. For combination 5, the live load factor may be reduced to $0.5 L$ for occupancies where $L_0 \le 4.8\text{ kPa}$, except for garages and public assembly.

Allowable Stress Design (ASD) Alternative

• $D$
• $D + L$
• $D + (L_r \text{ or } R)$
• $D + 0.75 L + 0.75 (L_r \text{ or } R)$
• $D + (0.6 W \text{ or } E/1.4)$
• $0.6 D + (0.6 W \text{ or } E/1.4)$

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4. Wind Loads: MWFRS (NSCP Section 207)

The Philippines experiences the highest frequency of destructive tropical cyclones in the world. Wind load calculations reflect 3-second gust wind speeds at $10\text{ m}$ above ground level in Exposure C.

Basic Wind Speed Maps ($V$)

NSCP 2015 replaced the older three-zone wind map of NSCP 2010 with wind-speed contour maps, Figures 207A.5-1A to 207A.5-1C. Separate maps cover different occupancy categories. The speeds are nominal 3-second gusts at $10\text{ m}$ in Exposure C, derived from PAGASA data. Linear interpolation between contours is allowed, and coastal sites take the last contour.

• Speeds are highest on the typhoon-exposed Pacific seaboard (eastern Luzon, Bicol, Eastern Visayas) and decrease toward the west and southwest.
• Higher occupancy categories, such as essential facilities, use the map with longer return periods and therefore higher speeds.
• Convert to m/s before using the velocity pressure equation: $V\,[\text{m/s}] = V\,[\text{km/h}] / 3.6$.

Terrain Exposure Categories

• Exposure B: Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger ($z_g = 365.76\text{ m}$, $\alpha = 7.0$).
• Exposure C: Open terrain with scattered obstructions having heights generally less than $9\text{ m}$, including flat open country and grasslands ($z_g = 274.32\text{ m}$, $\alpha = 9.5$).
• Exposure D: Flat, unobstructed areas and water surfaces, including smooth salt flats and coastal shorelines in hurricane-prone regions ($z_g = 213.36\text{ m}$, $\alpha = 11.5$).

Velocity Pressure Equation

$q_z = 0.613 K_z K_{zt} K_d V^2 \quad (\text{N/m}^2)$

Where:

• $V$ = basic wind speed in meters per second ($\text{m/s}$).
• $K_z$ = velocity pressure exposure coefficient evaluated at height $z$.
• $K_{zt}$ = topographic factor (accounting for wind speed-up over isolated hills, ridges, or escarpments; $K_{zt} = 1.0$ for level ground).
• $K_d$ = wind directionality factor ($K_d = 0.85$ for Main Wind-Force Resisting Systems and Components/Cladding).

Design Wind Pressure for MWFRS

For rigid enclosed buildings:

$p = q G C_p - q_i (G C_{pi})$

where $q = q_z$ for windward walls and $q_h$ for leeward walls, $G = 0.85$ is the gust effect factor, $C_p = +0.8$ for windward walls and $-0.5$ to $-0.2$ for leeward walls, and $(G C_{pi}) = \pm 0.18$ is the internal pressure coefficient for enclosed buildings ($\pm 0.55$ for partially enclosed buildings).

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5. Earthquake / Seismic Loads: Equivalent Lateral Force Procedure (NSCP Section 208)

The Equivalent Lateral Force (ELF) static procedure models dynamic ground motions as equivalent static horizontal lateral forces applied at each floor level.

Design Total Seismic Base Shear ($V$)

$V = \frac{C_v I}{R T} W$

Upper Bound Limit (Governs for short-period / stiff structures):

$V \le \frac{2.5 C_a I}{R} W$

Lower Bound Limits:

1. For all seismic zones: $V \ge 0.11 C_a I W$
2. In addition, for Seismic Zone 4: $V \ge \frac{0.8 Z N_v I}{R} W$

Where:

• $W$ = total seismic dead weight of the structure (plus $25\%$ of floor live load for storage occupancies, plus partition allowance of $1.0\text{ kPa}$).
• $T$ = fundamental natural period of the building in seconds (Method A: $T = C_t (h_n)^{3/4}$, with $C_t = 0.0731$ for RC moment frames, $0.0853$ for steel moment frames).
• $I$ = Seismic Importance Factor:
  • Essential Facilities (Hospitals, Emergency response, Power stations): $I = 1.50$
  • Hazardous Facilities: $I = 1.25$
  • Special Occupancy (Schools, Assembly $>300$ persons): $I = 1.00$
  • Standard Occupancy (Residential, Commercial): $I = 1.00$
• $R$ = Response Modification Factor (ductility capacity):
  • Reinforced Concrete Special Moment Resisting Frame (RC SMRF): $R = 8.5$
  • RC Intermediate Moment Resisting Frame (RC IMRF): $R = 5.5$
  • RC Ordinary Moment Resisting Frame (RC OMRF): $R = 3.5$
  • Concrete Shear Walls in a Building Frame System: $R = 5.5$

Seismic Zone Factor ($Z$)

• Zone 4 ($Z = 0.40$): Entire Philippine archipelago except Zone 2.
• Zone 2 ($Z = 0.20$): Palawan (except Busuanga), Sulu, and Tawi-Tawi.
• (Note: There are no Zone 1 or Zone 3 designations in NSCP 2015).

Soil Profile Types ($S_A$ through $S_F$)

• $S_A$: Hard rock ($v_s > 1500\text{ m/s}$)
• $S_B$: Rock ($760 < v_s \le 1500\text{ m/s}$)
• $S_C$: Very dense soil and soft rock ($360 < v_s \le 760\text{ m/s}$)
• $S_D$: Stiff soil profile ($180 \le v_s \le 360\text{ m/s}$)
• $S_E$: Soft soil profile ($v_s < 180\text{ m/s}$)
• $S_F$: Soil requiring site-specific evaluation (liquefiable soils, collapsible soils, highly sensitive clays).

Near-Source Factors ($N_a$ and $N_v$)

Near-source factors account for localized velocity pulses and high-frequency ground motion amplification within close proximity to active fault lines (Seismic Source Types A, B, C):

• For distance $\le 2.0\text{ km}$ to an active Type A fault: $N_a$ reaches up to $1.50$ and $N_v$ reaches up to $2.00$.
• For distance $\ge 15.0\text{ km}$: $N_a = 1.00$ and $N_v = 1.00$.

Vertical Distribution of Seismic Base Shear

The base shear $V$ is distributed across the building height:

• If the fundamental period exceeds $0.7\text{ seconds}$ ($T > 0.7\text{ s}$), a concentrated top force $F_t$ is assigned to the roof level to account for higher-mode whiplash effects: $F_t = 0.07 T V \le 0.25 V$
• If $T \le 0.7\text{ s}$, $F_t = 0$.
• The remaining shear force $(V - F_t)$ is distributed to each floor level $x$:

$F_x = \frac{(V - F_t) w_x h_x}{\sum_{i=1}^n w_i h_i}$

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6. Comprehensive Worked Example

Worked Example: Seismic Base Shear of an Office Building in Manila

Problem: A 5-story reinforced concrete Special Moment Resisting Frame (SMRF, $R = 8.5$) office building is to be constructed in Quezon City (Seismic Zone 4, $Z = 0.40$).

• Site Data: Soil Profile Type $S_D$ (Stiff soil). The site is located $18.0\text{ km}$ from the West Valley Fault ($N_a = 1.0$, $N_v = 1.0$).
• Occupancy: Standard Office ($I = 1.0$).
• Building Height: Total height $h_n = 17.5\text{ m}$ ($3.5\text{ m}$ per story).
• Seismic Weight: Total dead weight $W = 12,000\text{ kN}$ ($2,400\text{ kN}$ at each floor level).
• Seismic Coefficients: From NSCP 2015 tables for $Z = 0.40$ and Soil $S_D$: $C_a = 0.44 N_a = 0.44$, $C_v = 0.64 N_v = 0.64$.

Determine: (a) fundamental period $T$, (b) design base shear $V$, and (c) vertical distribution of lateral forces.

Solution:

• Step 1: Fundamental Period $T$ (Method A): For reinforced concrete moment frames: $T = C_t (h_n)^{3/4} = 0.0731 (17.5)^{3/4} = 0.0731 \times 8.567 = 0.626\text{ s}$

• Step 2: Calculate Base Shear from Period Formula: $V = \frac{C_v I}{R T} W = \frac{0.64 \times 1.0}{8.5 \times 0.626} \times 12,000 = \frac{0.64}{5.321} \times 12,000 = 0.1203 \times 12,000 = 1443.3\text{ kN}$

• Step 3: Check Governing Upper Limit: $V_{\text{max}} = \frac{2.5 C_a I}{R} W = \frac{2.5 \times 0.44 \times 1.0}{8.5} \times 12,000 = \frac{1.10}{8.5} \times 12,000 = 0.1294 \times 12,000 = 1552.9\text{ kN}$ Since $V_{\text{period}} = 1443.3\text{ kN} \le 1552.9\text{ kN}$, the period formula governs.

• Step 4: Check Lower Limits:

  • Minimum Limit 1: $V_{\text{min1}} = 0.11 C_a I W = 0.11 \times 0.44 \times 1.0 \times 12,000 = 580.8\text{ kN}$
  • Minimum Limit 2 (Zone 4): $V_{\text{min2}} = \frac{0.8 Z N_v I}{R} W = \frac{0.8 \times 0.40 \times 1.0 \times 1.0}{8.5} \times 12,000 = \frac{0.32}{8.5} \times 12,000 = 451.8\text{ kN}$ Since $V = 1443.3\text{ kN}$ is strictly greater than both lower limits ($1443.3 > 580.8$ and $1443.3 > 451.8$), the design base shear is $V = 1443.3\text{ kN}$.

• Step 5: Vertical Force Distribution: Because $T = 0.626\text{ s} \le 0.70\text{ s}$, the top concentrated force is zero: $F_t = 0$. The lateral force is distributed proportionally to $w_x h_x$:

Check: $\sum F_x = 481.1 + 384.9 + 288.7 + 192.4 + 96.2 = 1443.3\text{ kN} = V$.

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7. Licensure Exam Pitfalls & Review Notes