CELE Structural Theory & Analysis — Loads and Load Combinations (NSCP)Summary

Dead load is the permanent, self-weight of the structure and all fixed components—concrete, steel frame, roof decking, mechanical/electrical equipment, finishes, and built-in partitions. Dead load is calculated directly from material unit weights and geometry and does not vary with use. For example, a reinforced concrete slab 200 mm thick has a dead load of approximately 5.0 kPa (0.2 m × 25 kN/m³). Dead load is considered permanent and highly predictable.

Live load represents occupancy and use loads—people, furniture, movable equipment, and other temporary loads that vary with building occupancy and activity. NSCP 2015 prescribes minimum uniformly distributed live loads by occupancy type: residential floors 1.9 kPa, office buildings 2.4 kPa, classrooms 1.9–2.9 kPa, assembly areas 4.8 kPa, retail stores 3.8 kPa, parking garages 2.4 kPa. The code also specifies concentrated loads (e.g., 1.3 kN per 0.3 m² for office floors) that may govern localized member design. Live load is variable and uncertain in time and location.

Roof live load is a temporary maintenance load on roof surfaces, applied during roof installation, repair, and inspection. NSCP 2015 specifies Lr typically between 0.5 and 1.0 kPa depending on roof slope and access. For sloped roofs (slope > 5°), Lr may be reduced. Roof live load is considered intermittent and does not act simultaneously with full floor live load.

Wind load is the lateral pressure exerted by wind on building surfaces. NSCP 2015 calculates wind pressure from three components: basic wind speed (V, in m/s, obtained from hazard maps for the project location), exposure category (B, C, or D reflecting surface roughness), and design wind pressure P = q(GCp), where q is velocity pressure and GCp is the pressure coefficient. Wind load varies with height, direction, building geometry, and surface properties. Wind loads are typically not symmetric and can create both positive (pressure) and negative (suction) effects.

Earthquake (seismic) load is the inertial force induced by ground motion during an earthquake. The Philippines is in a highly seismic region, and NSCP 2015 Annex 4 specifies seismic design via the spectral acceleration method. Seismic base shear V = CsW (where Cs is the seismic response coefficient and W is total seismic weight) distributes vertically and horizontally. Seismic design requires consideration of soil profile, site hazard, structure period, and ductility. Seismic forces act in horizontal directions and create both flexure and shear in vertical members.

Rain load (R) is the load from accumulated water on roof surfaces, typically controlled by effective roof drainage. In areas without adequate drainage, R may be 1.0 kPa or higher; in well-drained roofs, R is negligible. Snow load is rarely significant in the Philippines except at extreme elevations. Rain and snow loads are treated as roof loads and appear in combinations 2 and 3.

Tributary area is the floor or roof area from which a structural member receives load. The tributary area is bounded by lines halfway between the subject member and adjacent parallel members. For a floor with beams spaced at uniform intervals s (center-to-center), an interior beam has tributary width s (full spacing), while edge beams have tributary width s/2. A column's tributary area is the product of tributary widths in both directions. Tributary area converts area loads (kPa) into line loads (kN/m) for beams or point loads (kN) for columns using: line load w = q × s; point load P = q × At, where At is tributary area in m².

LRFD uses load factors and resistance factors to achieve a target reliability level. NSCP 2015 provides seven principal load combinations: 1. U = 1.4D 2. U = 1.2D + 1.6L + 0.5(Lr or R) 3. U = 1.2D + 1.6(Lr or R) + (1.0L or 0.5W) 4. U = 1.2D + 1.0W + 1.0L + 0.5(Lr or R) 5. U = 1.2D + 1.0E + 1.0L 6. U = 0.9D + 1.0W 7. U = 0.9D + 1.0E Combination 2 governs most gravity load cases in typical buildings. The factor 1.2 on D and 1.6 on L reflect the variability and uncertainty in live load. The 0.5 factors on Lr and R account for the low probability of simultaneous occurrence with primary live loads. Combinations 6 and 7 use reduced dead load (0.9D) because dead load resists wind and seismic overturning—when D opposes W or E, the conservative case uses a lower D.

ASD uses load factors and prescribed safety factors integrated into allowable stress values. NSCP 2015 ASD combinations include: 1. S = D 2. S = D + L 3. S = D + (Lr or R) 4. S = D + 0.75L + 0.75(Lr or R) 5. S = D + 0.6W or D + 0.7E 6. S = D + 0.75L + 0.75(0.6W) + 0.75(Lr or R) 7. S = 0.6D + 0.6W or 0.6D + 0.7E ASD keeps loads at service (working) levels and applies allowable stresses that are a fraction of yield or ultimate strength (typically 0.6 to 0.67 of Fy). The lower factors on W and E (0.6 and 0.7) reflect the low probability of full code-prescribed wind/seismic acting simultaneously with full live load. The 0.75 factors allow modest stress increases when multiple variable loads act.

NSCP 2015 recognizes that not all live load is equally variable or equally likely to reach code-prescribed values simultaneously across large tributary areas. For certain occupancies and when live load exceeds 4.8 kPa, a companion live-load factor f₁ may be reduced from 1.0 to 0.5. Assembly areas and garages carry f₁ = 1.0L (full live load); residential, office, and storage areas carry f₁ = 0.5L when combined with wind or seismic (combos 3–5). Reducing L acknowledges that it is highly improbable for all occupants to apply maximum live load while simultaneously experiencing design wind or earthquake.

For any member, multiple load combinations must be evaluated, and the engineer must select the combination that produces the largest internal force (shear, bending moment, axial load, etc.) or moment. For a gravity-dominated beam in an office building, combination 2 (U = 1.2D + 1.6L) typically governs. For a tall building in a wind-prone region, combination 4 or 6 may govern. For a structure in a seismic zone, combinations 5 or 7 often control lateral members. The 'governing' or 'critical' combination is the one that requires the largest member cross-section and controls final design.

When wind or seismic loads create overturning moments on a structure, the dead load of the structure provides the resisting moment. In combinations 6 (0.9D + 1.0W) and 7 (0.9D + 1.0E), the dead load factor is reduced to 0.9 because dead load is beneficial (resists overturning) in these cases, and conservative design uses the lower D value. Additionally, for columns or foundations experiencing uplift (tension) from wind or seismic load, the reduced 0.9D may result in net tension where D alone would provide compression. These combinations ensure that uplift resistance and anchor capacity are correctly sized.

LRFD (Load and Resistance Factor Design) factors loads upward (1.2D, 1.6L, etc.) and factors resistance downward (φ factor, e.g., φ = 0.9 for concrete bending), aiming for a target probability of failure. Nominal strength is multiplied by φ: φMn ≥ Mu. ASD (Allowable Stress Design) uses service loads and divides nominal strength by a safety factor FS to obtain allowable stress: fa ≤ Fa (actual stress ≤ allowable). LRFD generally allows more efficient design (smaller members) and is favored in modern codes. ASD is simpler conceptually but less efficient and is gradually being phased out. Both approaches must use consistent design formulas and safety criteria from the applicable design standard (ACI 318 for concrete, AISC 360 for steel).

All load types (D, L, Lr, W, E) are first identified per occupancy and hazard. Each type is then distributed via tributary area to specific members (tributary line load for beams, tributary point load for columns). The distributed loads are then factored using LRFD or ASD combination rules to compute ultimate/service design loads, which drive member sizing.

In typical gravity combinations (1–3, and combos 2 in ASD), dead load opposes live load (both compress members). In wind and seismic combinations (6–7 LRFD, 5 and 7 ASD), dead load resists overturning moment. This dual role is why D is factored at 1.2 in gravity combos but reduced to 0.9 in lateral combos—the reduced 0.9D represents the conservative case for overturning resistance.

The live-load companion factor f₁ depends on occupancy and context. Assembly areas (theaters, auditoriums, stadiums) and parking garages use f₁ = 1.0 (full live load) in all combinations, reflecting high occupancy density and predictability. Residential, office, and storage use f₁ = 0.5L in combos 3–5 (wind/seismic), acknowledging that the probability of full occupant load + design wind/seismic is low. This factor is a code-prescribed empirical reduction and is essential for realistic design load calculation.

LRFD factors reflect uncertainty and variability: D is factored at 1.2 (modest variability in dead load due to material density and construction tolerance); L is factored at 1.6 (high variability and low probability of occurring at maximum across a structure); W and E are factored at 1.0 (calculated from deterministic design wind speed or spectral acceleration, low additional uncertainty). The larger factors on more uncertain loads result in rational load combinations aligned with reliability theory.

For any member (beam, column, connection), the engineer computes shear, moment, or axial force for all applicable load combinations and identifies the combination producing the largest internal force. That combination is the 'governing' combination. The member is then designed to resist the internal forces from the governing combination using the applicable design standard (ACI 318 or AISC 360). A change in geometry or load assumptions may shift the governing combination; iterative design is common.

In a multi-story building, roof tributary areas feed roof live and dead loads to roof beams. Roof beams transfer loads to roof-supporting columns. Floor tributary areas feed floor loads to floor beams, which transfer to floor-supporting columns. Each column accumulates tributary loads from all floors above it (stacked). The column at the base carries the cumulative load from all stories; the top-story column carries only its own story. This stacking is critical in column design and often results in 1.2D + 1.6L being the largest combination for lower-story columns.