5.6 Loads, Load Combinations & Design Codes

Load Types

ASCE 7 (American Society of Civil Engineers Standard 7, Minimum Design Loads) defines the loads a structure must resist:

• Dead load (D) — permanent self-weight of structure and fixed equipment; predictable, low variability.
• Live load (L) — movable occupancy loads (people, furniture); ASCE 7 tabulates minimum uniform values (e.g., 40 psf residential, 100 psf assembly/corridors).
• Roof live load (Lr), snow (S), rain (R) — roof-specific environmental loads.
• Wind (W) — pressure from wind velocity, a function of basic wind speed, exposure, and building geometry.
• Seismic (E) — inertial earthquake forces, proportional to building mass and site seismicity.

Loads are also classified by direction (gravity vs. lateral) and duration. Because live, wind, and seismic loads carry far more uncertainty than dead load, they receive larger factors in design combinations.

Tributary Area & Load Paths

The load path is the chain that carries load from where it is applied to the ground: slab → beam → girder → column → foundation → soil. Every element in the path must be designed for the loads it accumulates.

The tributary area of a member is the floor or roof region that sheds its load onto that member. For a regular grid:

• A beam picks up a strip equal to its span × half the distance to the adjacent beam on each side (its tributary width).
• A column supports a tributary area extending halfway to each neighboring column in both directions.

Member load = (unit area load) × (tributary area). As load accumulates down the path, ASCE 7 permits live-load reduction for large tributary areas (>400 ft²) because full simultaneous live load over a large area is improbable. A common trap is forgetting that a girder's tributary area includes the reactions of the beams framing into it, not just its own strip.

ASCE 7 Load Combinations & Design Codes

Members are designed for the worst-case load combination. The principal LRFD (strength) combinations from ASCE 7 Section 2.3:

Combinations 6–7 use 0.9D to check uplift/overturning where dead load stabilizes. The governing US design codes: ASCE 7 (loads), AISC 360 (steel), ACI 318 (concrete), and the International Building Code (IBC), which adopts these standards by reference and sets occupancy/risk categories.

Limit States & a Worked Tributary Example

Design addresses two limit states:

• Strength (ultimate) limit states — prevent collapse: yielding, buckling, fracture, overturning. Checked with factored loads (LRFD combinations) against φRn.
• Serviceability limit states — preserve function under normal use: deflection, vibration, cracking, drift. Checked with service (unfactored) loads against limits such as L/360 live-load deflection.

Mixing them is a classic error: never apply load factors to a deflection (serviceability) check.

Worked tributary example: An interior column on a 25 ft × 30 ft bay grid supports a floor with dead load D = 80 psf and live load L = 50 psf.

• Tributary area = 25 × 30 = 750 ft².
• Service loads: P_D = 80 × 750 = 60,000 lb = 60 kip; P_L = 50 × 750 = 37.5 kip.
• Factored axial load: Pu = 1.2D + 1.6L = 1.2(60) + 1.6(37.5) = 72 + 60 = 132 kip.

The column is then sized so φPn ≥ 132 kip.

Lateral Loads, Risk Categories & ASD Combinations

Lateral loads — wind and seismic — control tall or slender buildings and are resisted by a dedicated lateral force-resisting system (braced frames, moment frames, shear walls). They follow a horizontal load path: wind/seismic force → diaphragm → vertical lateral system → foundation. Seismic base shear in ASCE 7 is V = Cs·W, where the seismic response coefficient Cs depends on the response-spectrum acceleration and the response-modification factor R; wind pressure is q = 0.00256·Kz·Kzt·Kd·V² (V = basic wind speed, mph).

The International Building Code (IBC) assigns each building a Risk Category (I–IV) by occupancy and consequence of failure — hospitals and emergency facilities are Category IV with the highest importance factors, increasing snow, wind, and seismic demands.

ASCE 7 also provides ASD (service) load combinations, e.g., D + L; D + 0.75L + 0.75(Lr or S or R); D + 0.6W; these use unfactored loads for the Allowable Strength Design format. The key exam discipline: pick the matching combination set for your design method — never combine LRFD factored loads with an ASD allowable-stress capacity, or vice versa.

** A structure is only as strong as the weakest link in its load path, so every element from slab to soil must be sized for the loads it accumulates — a strong beam on an undersized column or footing still collapses. 5) — so φRn ≥ demand; and (5) verify serviceability under service loads. 3). Loads, analysis, and design are one continuous chain, not separate silos.