NSCP 2015 Load Combinations Explained: Ultimate (LRFD) and Service Combinations with Examples

By Engr. Ruel H. Cepeda, Structural Engineer

Every structural member must be checked against several simultaneous conditions of dead, live, wind, and earthquake load — never one load type alone. NSCP 2015 Section 203 sets out how to combine these into design-level demands, for both strength design (LRFD, factored) and allowable stress design (ASD, service-level). Skipping a combination is a common cause of undersized members and, worse, undetected uplift or overturning that a gravity-only check would miss. This article covers both sets of combinations, the special f₁ live-load factor, and two worked examples — a beam under wind and a column under seismic axial force.

Where These Combinations Come From

NSCP 2015 Section 203 is the code's general load-combination section — commonly referenced as Section 203.3 for strength design (LRFD) and 203.4 for allowable stress design (ASD). Its strength-design set is numerically consistent with ASCE 7-10, whose wind provisions NSCP 2015 adopts (hence the 1.0W and 0.5W factors), but it is written in the UBC 1997-derived format with an explicit f₁ live-load factor, and its ASD set keeps the UBC-style E/1.4 seismic term rather than ASCE 7's 0.7E. Confirm clause numbering and exact wording against the code copy recognized by the building official of record.

The Load Symbols

• D — Dead load: self-weight plus permanently attached finishes, fixed equipment, and partitions.
• L — Live load: from occupancy and use — people, furniture, movable equipment, stored materials.
• L_r — Roof live load: from maintenance workers, tools, and materials on the roof, distinct from floor L.
• R — Rain load: from ponded rainwater on a roof, especially where drainage is restricted.
• W — Wind load: pressure or suction from the design wind speed per NSCP 2015.
• E — Earthquake load: horizontal (and, where applicable, vertical) seismic force effect per NSCP 2015.

The Seven Ultimate (Strength Design / LRFD) Combinations

These produce the factored demands used to size members against nominal strength reduced by a strength-reduction (φ) factor. The fluid (F), self-straining (T) and lateral earth-pressure (H) terms that appear in the code equations are omitted below for clarity, as is snow (S), which does not apply in the Philippines.

The f₁ Live Load Factor, Explained

Combinations 3, 4, and 5 reduce the coefficient on L from 1.6 to a factor called f₁, because it is unlikely that full live, wind (or seismic), and roof load all peak at once. NSCP 2015 sets:

f₁ = 1.0 for garages, places of public assembly, and any area where L exceeds 4.8 kPa

f₁ = 0.5 for all other occupancies

In these exception categories, a large fraction of live load is plausibly present at peak wind or seismic demand, so the reduction is not justified. For an ordinary office or residential floor at or below 4.8 kPa, f₁ = 0.5 applies. ASCE 7-10 and ACI 318 reach the same numbers by a different route: their live-load factor in these combinations is 1.0, with an exception permitting 0.5 outside garages, places of public assembly, and floors where L exceeds 4.8 kPa (100 psf).

Why 0.9D Matters: Uplift and Overturning

Combinations 6 and 7 look unusual because the dead load factor drops below 1.0. That is deliberate: dead load normally stabilizes a structure against uplift or overturning, and since actual self-weight can run lower than the design value, the code reduces D to 0.9 — a conservatively low restraining force — paired with the full lateral load. A shear wall footing sized only from gravity combinations can lose bearing and go into tension under 0.9D + 1.0E when overturning is large relative to wall weight. Never skip these for a lateral-load-path member.

Service (Allowable Stress Design) Combinations

NSCP 2015 Section 203.4 also lists the basic ASD combinations. They are written in the UBC 1997-derived form the Philippine code has carried through its UBC-based editions, updated for ultimate-level wind (0.6W): the seismic term stays E/1.4 (about 0.71E) where ASCE 7-10 writes 0.7E. The table below again omits F, H, T and S for clarity — confirm the full wording, and the alternate ASD set that the same section also offers (with its permitted one-third increase in allowable stresses), against the code text before use.

Combinations 6 and 7 are the ASD counterpart to the 0.9D uplift check above, with the same conservatively reduced dead load. Note that ASCE 7-10 Section 2.4 additionally carries 0.75-factored combinations that pair reduced live load with reduced wind or seismic load (for example D + 0.75L + 0.75(0.6W) + 0.75(L_r or R)); if you design to ASCE 7 rather than NSCP, use that document's full list.

Worked Example 1 — Beam Under Dead, Live, and Wind

Given: A simply supported beam, span ℓ = 6.0 m, carries D = 15 kN/m and L = 10 kN/m from an ordinary floor (f₁ = 0.5), plus a wind line load transferred from attached cladding, W = ±6 kN/m (positive downward). L_r = R = 0; no seismic force acts on this member, so combinations 5 and 7 are skipped.

Governing: w_u = 34.0 kN/m from combination 2, a gravity-driven case since the assumed wind is modest. Combination 6 with W = −6 kN/m gives the lowest net load, 7.5 kN/m, still downward — no reversal here. Taking w_u = 34.0 kN/m through to design actions for the 6.0 m span:

• Design moment: M_u = w_uℓ²/8 = 34.0 × 6.0² / 8 = 34.0 × 36 / 8 = 153.0 kN·m
• Design shear: V_u = w_uℓ/2 = 34.0 × 6.0 / 2 = 102.0 kN

Check these in seconds with the free simple beam calculator, cross-checked against the sister site's beam calculator on RHCES.

Worked Example 2 — Column Under Seismic Axial Force

Given: An interior column, part of the lateral system, carries D = 900 kN and L = 400 kN (f₁ = 0.5), plus E = ±350 kN from frame overturning. L_r = R = W = 0, so only combinations with D, L, and E apply.

• Combination 1: 1.4D = 1.4(900) = 1260 kN
• Combination 2: 1.2D + 1.6L = 1.2(900) + 1.6(400) = 1080 + 640 = 1720 kN
• Combination 3: 1.2D + f₁L = 1080 + 0.5(400) = 1280 kN
• Combination 5: 1.2D + 1.0E + f₁L = 1080 + 350 + 200 = 1630 kN (E = +350); 1080 − 350 + 200 = 930 kN (E = −350)
• Combination 7: 0.9D + 1.0E = 810 + 350 = 1160 kN (E = +350); 810 − 350 = 460 kN (E = −350)

Maximum compression is P_u = 1720 kN from combination 2 — gravity governs since the seismic force is moderate. Combination 7 with E = −350 kN gives the minimum, P_u = 460 kN — still compression, but exactly the check that flags net tension if overturning were larger: had E exceeded 810 kN, the column would need a tension connection and revised anchorage, a result combination 2 alone would never reveal.

NSCP 2015 vs. ACI 318 — Wording Differences to Note

NSCP 2015 gives the general combinations in Section 203, and its concrete chapter (which follows ACI 318-14) restates an equivalent strength-design set — just as ACI 318-19 Table 5.3.1 lists the ASCE 7 combinations in full, with the same 1.4, 1.2, 1.6, 0.5, 1.0 and 0.9 factors, and its 0.5 live-load reduction is written as a permitted exception rather than an f₁ symbol. The differences that matter sit in the load definitions, not the equations: NSCP 2015 computes W from Philippine wind-speed maps and E from a UBC 1997-style seismic-zone procedure, both different from ASCE 7's US mapped values, but the multipliers applied to them are the values shown above.

Assumptions & Limitations

• Load factors and f₁ follow NSCP 2015 Section 203 (strength-design values consistent with ASCE 7-10; ASD set in the UBC 1997-derived form); F, H, T and S terms are omitted from the tables. Verify against the code edition adopted by the building official of record.
• E already represents the code-defined seismic force effect (including any redundancy factor and vertical component the code requires), not a raw response-spectrum output.
• Snow load S does not apply in the Philippines and is omitted above.
• Both worked examples assume f₁ = 0.5 for ordinary occupancy with L ≤ 4.8 kPa — confirm against the actual occupancy before reuse.
• This article covers load-combination mechanics only, not member capacity design, governed separately by NSCP 2015 and ACI 318-19.

Frequently Asked Questions

Do I need to check all seven ultimate combinations for every member?

In principle, yes — different combinations govern different members and force effects. Software runs every combination and reports the envelope; by hand, shortlist based on which loads act on the member. Never skip the 0.9D pair — they rarely govern maximum load but often govern the uplift check.

What is the difference between NSCP's ultimate combinations and ACI 318's load combinations?

Effectively none in the multipliers. ACI 318-19 Table 5.3.1 lists the ASCE 7 strength-design combinations, and NSCP 2015 Section 203 applies the same 1.2, 1.6, 1.0 and 0.9 factors to D, L, W, and E; ACI writes the reduced live-load factor as a permitted 0.5 where NSCP writes f₁ = 0.5. Differences lie in how each code defines the individual loads, not in the combination equations.

Why does the sign of wind or seismic load matter — shouldn't the larger magnitude always control?

Because the two signs govern different failure modes. The positive sign, with 1.2D, produces maximum bending or compression. The negative sign, with 0.9D, produces minimum compression and uplift. Checking only the larger-magnitude case — as example 2 shows — can hide an uplift problem the 0.9D combination exists to catch.

Getting the combinations right before sizing a member saves rework later. Use the free simple beam calculator to turn a governing w_u into moment and shear, or browse all free web tools. For how these factors compare across editions, see load combinations across ASCE 7 editions.