5.2 Strength and Service Load Combinations

Load combinations model which actions can occur together and how uncertainty is treated. They are not interchangeable lists of coefficients. For a July 2026 PE Civil: Structural problem, use the April 2024 specification and its listed editions: ASCE 7-16 with IBC 2018 for building load combinations, AASHTO LRFD 8th edition with the listed errata for bridges, ACI 318-14 for concrete resistance, the AISC Steel Construction Manual 15th edition for steel, and NDS 2018/SDPWS 2015 using ASD only for wood. Do not import the April 2027 editions.

Classify Nominal Loads Before Combining

Record each nominal action with its symbol and sign: permanent dead load D; floor live load L; roof live load Lr; snow S; rain R; wind W; earthquake effect E; and other actions such as fluid, earth pressure, or temperature when applicable. The governing source defines the symbol and exceptions. Keep vertical and lateral effects separate until the chosen combination directs concurrency.

A sound workflow is:

1. Identify whether the structure is a building or bridge and open that code family's combination provisions.
2. List nominal load cases, including opposite wind or seismic directions, patterned live load, and stabilizing versus destabilizing dead load.
3. Select the required design route: strength/LRFD, ASD, or a stated serviceability check.
4. Generate every applicable combination from the exact exam reference; do not reconstruct factors from memory.
5. Analyze each case with its signs and placement, then envelope the requested response.
6. Compare demand with resistance or a service limit using compatible methods.
7. Label the governing combination and route in the answer.

Three Building Workflows That Must Stay Distinct

Strength or LRFD. ASCE 7-16 strength combinations use factored loads; familiar cases include 1.4D and 1.2D + 1.6L + 0.5(Lr or S or R), subject to the complete provisions and exceptions. Material resistance is also reduced or otherwise formatted for strength. Concrete commonly checks U ≤ φR_n under ACI 318-14. Steel LRFD checks R_u ≤ φR_n under AISC 15th. A factored demand cannot be compared with a raw nominal resistance unless the governing equation explicitly formats it that way.

Allowable stress design. The ASCE 7-16/IBC 2018 ASD combinations use a different factor set and concurrency logic. Wind and earthquake coefficients in this family visibly differ from their strength counterparts, and stabilizing dead load can be reduced for uplift or overturning cases. AISC ASD checks R_a ≤ R_n/Ω. For this NCEES specification, NDS wood design is ASD only, so use the building ASD combinations and NDS adjusted allowable capacities; do not invent a wood LRFD solution because another material permits LRFD.

Serviceability. Deflection, drift, vibration, crack control, or settlement may use nominal-load combinations and stiffness assumptions specified by the applicable standard or problem. “Service” describes performance under use, not a universal coefficient list. A service deflection demand is compared with a deflection limit, not with φR_n; an ASD strength check is not automatically the project's serviceability check merely because its loads are less amplified than a strength case.

IBC 2018 and ASCE 7-16 building provisions are coordinated, but follow the route and exceptions that the problem identifies. Do not splice one line from each as if they were independent factor menus.

Worked Compatibility Example

A steel column has nominal axial effects P_D = 35 kips and P_L = 22 kips. The problem directs evaluation of the building strength case 1.2D + 1.6L and the ASD case D + L. It gives nominal compressive resistance P_n = 120 kips, LRFD factor φ = 0.90, and ASD factor Ω = 1.67.

LRFD route

P_u = 1.2(35) + 1.6(22) = 77.2 kips

φP_n = 0.90(120) = 108 kips

77.2/108 = 0.715, so the stated LRFD check passes.

ASD route

P_a = 35 + 22 = 57.0 kips

P_n/Ω = 120/1.67 = 71.9 kips

57.0/71.9 = 0.793, so the stated ASD check also passes.

These are two complete, valid comparisons. Comparing P_u with P_n/Ω mixes LRFD demand with ASD resistance; comparing P_a with φP_n mixes ASD demand with LRFD resistance. Neither mixed ratio has design meaning, even if its numerical result looks favorable. The example factors are given so the lesson is compatibility, not memorization; actual problems require all applicable combinations, stability provisions, and resistance adjustments.

Concurrency, Direction, and Envelopes

Environmental loads are transient and directional. A combination specifies when companion live, roof, snow, or rain effects appear and at what factors. It may require both signs of wind or seismic response. Reduced dead load can govern uplift because gravity is stabilizing, while larger dead load may govern downward force. Patterned live load can govern a continuous-member moment even when full live load governs total reaction. Generate cases first, then take the maximum positive, maximum negative, or largest absolute response requested. Do not add maxima from different mutually exclusive cases.

Bridges Are a Separate Code Family

AASHTO LRFD organizes bridge actions into named limit states such as Strength, Service, Extreme Event, and Fatigue, with its own load factors, modifiers, multiple-presence rules, and dynamic allowances. Select the AASHTO limit state and resistance provisions together. An ASCE 7 building wind factor or AISC building combination does not replace an AASHTO bridge combination, even when the member material is steel or concrete. Material mechanics may look familiar, but the demand and resistance framework remains the one prescribed for the bridge.

Before selecting an answer, label it building strength, building ASD, serviceability, or AASHTO LRFD; confirm that demand, resistance, code family, and units all match.