Gravity Loads in Structural Engineering: The Essential Guide (1.2G + 1.5Q)

Gravity Loads in Structural Engineering: The Essential Guide (1.2G + 1.5Q)

Gravity loads are the fundamental loads every structure must resist: the weight of the structure itself, the contents it carries, and the people and equipment using it. Understanding how gravity loads are classified, combined and distributed through a structure is a core skill for structural engineers working under the Australian Standard loading code AS/NZS 1170.

The formula 1.2g + 1.5q (also written as 1.2G + 1.5Q) is the primary strength limit state load combination for gravity loads under the Australian loading standard. G is the permanent action (dead load), Q is the imposed action (live load), and the factors 1.2 and 1.5 are load factors that account for variability and uncertainty. This guide covers what the formula means, when it applies, and how to use it alongside the other load combinations required by AS/NZS 1170.0.

What Are Gravity Loads?

Gravity loads are loads that act vertically downward on a structure due to gravity. They are distinct from lateral loads (wind, earthquake, earth pressure) which act horizontally. In most building structures, gravity loads dominate the design of floors, beams, columns and foundations.

The Three Types of Gravity Load

Dead Load (G) — Permanent Actions

Dead load, called Permanent Action (G) in AS/NZS 1170.1, is the weight of all structural and non-structural components that remain fixed throughout the building’s life. This includes:

• Structural elements: concrete slabs, beams, columns, walls, roof structure
• Finishes: floor tiles, carpet, screed, ceiling, cladding, renders
• Permanent services: mechanical and electrical plant that is fixed in place
• Permanent partitions (full-height walls fixed to structure)

Dead loads are typically calculated from the unit weight of materials per AS/NZS 1170.1 Appendix A. For concrete slabs, the self-weight is commonly 24 kN/m³ (normal weight concrete). A 150 mm thick concrete slab, for example, has a self-weight of 0.15 m × 24 kN/m³ = 3.6 kPa.

Superimposed Dead Load (SDL)

Superimposed dead load is the additional permanent weight applied on top of the structural slab or beam — typically finishes, ceilings, services and permanent partitions. SDL is a component of G and is added to the structural self-weight when calculating total permanent action. Typical SDL values for Melbourne commercial buildings range from 1.0 to 2.0 kPa depending on the fit-out.

Live Load (Q) — Imposed Actions

Live load, called Imposed Action (Q) in AS/NZS 1170.1, is the load applied by the use and occupancy of the building. It is variable — it changes over time as people move, furniture is rearranged, and stock levels change. AS/NZS 1170.1 Table 3.1 specifies minimum live load values by building usage:

How Gravity Loads Travel Through a Structure

Every gravity load applied to a building must follow a continuous load path from the point of application down to the foundations and into the ground. Understanding this load path is the starting point for structural design.

Tributary Area and Gravity Load Distribution

The tributary area of a structural member is the area of floor or roof from which it collects gravity loads. For a simple two-way slab supported on four beams, each beam collects the load from half the span on each side. For columns on a regular grid, the tributary area is approximately the area enclosed by lines drawn halfway to adjacent columns.

The total gravity load on a column is calculated as:

What Happens When a Load Path Is Interrupted

When a structural element is removed or fails, the load it was carrying must redistribute to adjacent elements. If those elements do not have the capacity to take the additional load, they also fail — and this progressive collapse can cause significant structural damage.

This is the structural engineering concern when homeowners remove walls without engineering input. If a wall is load-bearing (carrying floors or roof loads above), removing it without a replacement beam and adequate posts to carry the reaction creates an interrupted load path. The structure above deflects, cracks develop at stress concentrations, and in severe cases, partial collapse can occur.

For residential projects in Melbourne, a load bearing wall assessment by a structural engineer before any wall removal is essential to maintain an uninterrupted gravity load path. See the structural engineer inspection service for pre-purchase and condition assessments where load path integrity is checked.

Load Combinations Under AS/NZS 1170.0

Structural design in Australia requires that loads are combined using the load combinations specified in AS/NZS 1170.0 Combination of Actions. The load combinations apply load factors to each action type to account for the statistical likelihood that maximum loads from different sources occur simultaneously, and to provide the required level of structural reliability.

Strength Limit State Load Combinations

The strength limit state (SLS — not to be confused with serviceability limit state; also called ULS in some references) ensures the structure does not fail, collapse or become unstable. The following combinations must all be checked for gravity-dominant design:

The Primary Formula: 1.2G + 1.5Q Explained

The formula 1.2G + 1.5Q (or equivalently, 1.2g + 1.5q in lowercase notation) is the most commonly applied load combination in building structural design:

• G (or g) = Permanent Action = the dead load, typically in kPa for floor/roof areas or kN for point loads
• 1.2 = load factor on G, accounting for uncertainty in the actual dead load (materials may be heavier than specified, construction tolerances, etc.)
• Q (or q) = Imposed Action = the live load from AS/NZS 1170.1 Table 3.1
• 1.5 = load factor on Q, reflecting greater uncertainty in live loads compared to dead loads

The result, 1.2G + 1.5Q, is the design action effect (often denoted N*, V* or M* depending on whether it’s an axial force, shear or bending moment). Structural members are then designed so their capacity (φR_n) is greater than or equal to this design action.

Worked Numerical Example: Applying 1.2G + 1.5Q

Consider a 200 mm reinforced concrete flat plate slab in an office building:

• Slab self-weight: 0.200 m × 24 kN/m³ = 4.80 kPa
• Superimposed dead load (floor finish, ceiling, services): 1.50 kPa
• Total permanent action: G = 4.80 + 1.50 = 6.30 kPa
• Imposed action (office use, AS/NZS 1170.1 Table 3.1): Q = 3.00 kPa

Applying the 1.2G + 1.5Q combination:

Now check the 1.35G combination:

The design load of 12.06 kPa is used to calculate the design bending moment and shear in the slab, and the slab reinforcement is sized to provide adequate capacity. In this example, the 1.2G + 1.5Q combination governs, as is typical for office buildings where live load is significant.

Serviceability Load Combinations

The serviceability limit state (SLS) ensures the structure performs adequately in use: deflections are within limits, cracks do not impair function, and vibrations are acceptable. Serviceability combinations use lower or no load factors on the actions, since the concern is performance, not structural failure.

When Each Combination Governs

Understanding when each load combination governs helps engineers work efficiently and check their results:

• 1.2G + 1.5Q governs when live load is significant relative to dead load (offices, retail, residential above first floor) — which is the majority of building floor design
• 1.35G governs when dead load is dominant and live load is small (heavy precast roofs with minimal access, plant rooms with fixed-weight equipment, earth retaining structures)
• 0.9G + W_u governs for wind uplift on roof structures, anchor design in tilt-up construction, overturning of isolated structures
• 1.2G + W_u + ψ_cQ governs for external columns and walls where combined gravity and wind loads produce the critical design condition

Live Load Reduction for Multi-Storey Buildings

AS/NZS 1170.1 permits a reduction in design live load for members supporting large tributary areas, on the basis that the probability of maximum live load occurring simultaneously over a large area is lower than over a small area. This is called the live load reduction factor (ψa) and applies to imposed floor loads where the tributary area exceeds a threshold.

The reduction factor is applied to Q before substituting into the load combinations. For columns in multi-storey buildings, the cumulative live load reduction can be significant (20 to 40% in some cases) and affects the foundation design.

Serviceability Limits Under Gravity Loading

In addition to strength, structural members must satisfy deflection and crack width limits under serviceability load combinations. For gravity-loaded beams and slabs in Australia, the primary deflection limits come from AS 3600 (concrete structures) and AS 4100 (steel structures):

• Total deflection under G + Q: typically L/250 (where L is the span)
• Post-installation deflection (to avoid damage to non-structural elements): typically L/500
• Pre-cambered members: long-term deflection after pre-camber typically limited to L/300

Serviceability often governs the design of longer-span members where strength requirements are easily met but the deflection under sustained loads is the controlling criterion.

Gravity Loads and Melbourne Structural Engineering Projects

In residential structural engineering across Melbourne, gravity load analysis underpins the most common types of structural work:

• Load bearing wall removal: When an internal wall is removed, the gravity loads it was carrying (1.2G + 1.5Q from floors and roof above) must be transferred through a new beam and posts to the foundation. The beam design is governed by the tributary area and the applied load combination.
• House extensions: New floor and roof areas add gravity loads to the existing structure. The existing foundations must be assessed to confirm they can take the increased load without excessive settlement.
• Retaining wall design: Earth pressure acts laterally, but the surcharge from vehicles or structures above the wall adds a gravity component to the design.
• Underpinning: Foundation settlement is often caused by the existing gravity loads (1.2G + 1.5Q applied over the life of the building) exceeding the bearing capacity of reactive clay soils after moisture change.

For Melbourne homeowners planning structural alterations, a structural engineer report will confirm the existing gravity load conditions and the engineering required for the proposed changes. Principal Built Engineering provides structural engineering services across Melbourne for residential and commercial projects.

Gravity Load: Frequently Asked Questions

What is the 1.2G + 1.5Q load combination?

1.2G + 1.5Q (also written as 1.2g + 1.5q) is the primary gravity load combination for the strength limit state in AS/NZS 1170.0. G is the permanent action (dead load) and Q is the imposed action (live load). The factors 1.2 and 1.5 are load factors that account for uncertainty in the magnitude of each load type. The result is the design action that structural members must be capable of resisting without failure.

When does 1.35G govern instead of 1.2G + 1.5Q?

1.35G governs when the permanent load (dead load) is dominant and the imposed load (live load) is small. This occurs in heavily loaded dead-weight structures such as precast concrete roof elements with minimal access, earthworks retaining structures, or plant rooms with fixed heavy equipment. For most floor members in occupied buildings, 1.2G + 1.5Q governs because live load is significant.

What is the difference between dead load and live load?

Dead load (permanent action, G) is the weight of all permanent components of the structure that remain fixed throughout its life: structural elements, finishes, fixed services and permanent partitions. Live load (imposed action, Q) is the variable load applied by the use of the building: people, furniture, stock, vehicles and movable equipment. Dead loads can be calculated with reasonable precision; live loads are more variable and carry a higher load factor (1.5 vs 1.2) in the AS/NZS 1170 combinations.

How is live load determined for a residential floor?

AS/NZS 1170.1 Table 3.1 specifies the minimum design live load for residential floors as 1.5 kPa. This is applied uniformly over the floor area. Stairs in residential buildings have a minimum live load of 3.0 kPa. These values are minimums — the actual load case used must reflect the realistic use of the space.

Why does removing a load-bearing wall cause damage?

A load-bearing wall carries gravity loads from the floors and roof above (typically the 1.2G + 1.5Q combination applied over the tributary area of the wall). When the wall is removed without a replacement structural element (a beam and adequate posts), the gravity loads have no path to the foundations. The floor or roof structure above deflects, connections at the perimeter develop stress concentrations, and cracking occurs. In severe cases, partial collapse is possible. A structural engineer must design a replacement load path before the wall is removed.

What is tributary area in gravity load design?

Tributary area is the area of floor or roof from which a structural member collects gravity loads. For a beam in a regular grid, the tributary area is the span of the beam multiplied by half the distance to the adjacent beams on each side. The design load on the beam is (1.2G + 1.5Q) × tributary area (for uniformly distributed loads). For columns, the tributary area multiplied by the number of floors above gives the total accumulated gravity load.

What are the serviceability load combinations for gravity loads in Australia?

AS/NZS 1170.0 specifies three serviceability gravity load combinations: G + Q (rare, for maximum short-term deflection), G + ψsQ (short-term frequent, where ψs = 0.7 for most floor uses), and G + ψlQ (long-term quasi-permanent, where ψl = 0.4 for most floor uses). The long-term combination is used to calculate creep deflection in concrete members.

What does ψ (psi) mean in AS/NZS 1170.0 load combinations?

Psi (ψ) in AS/NZS 1170.0 represents combination and probability factors for imposed actions. ψc is the combination factor (typically 0.4 for floors and 0.6 for storage) used when live load accompanies another variable action (wind or earthquake). ψs is the short-term factor (0.7 for most floor uses) and ψl is the long-term factor (0.4 for most floor uses) used in serviceability combinations to represent the fraction of live load that acts frequently or permanently.

What structural engineering services does PBE provide for Melbourne projects?

Principal Built Engineering provides structural engineering services for residential, commercial and industrial projects across Melbourne. For residential projects, the most common services are structural engineer inspections, load bearing wall assessments, and retaining wall design. For commercial projects, PBE provides structural design from concept through to construction documentation. Contact PBE to discuss your project.