Defining and determining load combinations and calculations is essential in the design of structures to ensure their safety and stability. This process involves identifying and analyzing all the loads a structure will be subjected to, and assigning the correct safety factors and loading scenarios to simulation models.
Design loads encompass everything from the self-weight of the structure to the weight of occupants, wind, snow, and seismic forces as well as the weight of any materials, semi-permanent members, and equipment used in construction or operation. Engineers must also determine the appropriate design load combinations to develop strength and serviceability design to withstand impact and prevent fatigue failure over time.
Design codes in varying geographical jurisdictions also have different recommendations when it comes to loading combinations, which must be considered. This is due to specific features of the environment structures in a given jurisdiction will be subject to, as well as the principles and nature of regulators.
In this article, we’ll summarise design load calculations and combinations as recommended by AS/NZS 1170: 2002, ASCE7-10, and EN1991-1-1 to gain an understanding of commonalities and differences in loading requirements in these three major code sets.
Unsurprisingly, design load, load combinations, materials, and structural systems vary significantly between national and local authorities. Nevertheless, engineers need to be familiar with specific design codes and standards that apply to their projects, regardless of location.
What are structural loads, and how do you calculate them?
A structure can be affected by various loads, the nature of which will vary depending on its design, location, usage, and more. In most cases, the maximum loads a structure must withstand are used to define design requirements. Typically loading can be considered as dead loads (DL) or live loads (LL). This is an important distinction, as it determines safety factors applied to the underlying design action.
Dead loads on a structure are always present and cannot be removed. As such, a structure must be designed to safely support the weight of its own dead loads.
Dead loads, also referred to as permanent or static loads, are those expected to remain relatively constant over time. Examples include the self-weight of a building's structural elements, flooring components and finishes, building services and operational equipment, like the HVAC system, non-structural permanent partitions, immovable fixtures, and even built-in cabinets.
Most dead loads can be calculated by examining the specified materials' weights and volumes as depicted on drawings and/or in-situ and factoring them by the area they are imposed over. As a result, it is possible to quite accurately calculate dead loads.
Structural engineers also tend to be conservative in their estimates, minimizing acceptable deflections, allowing for a margin of error, and accounting for potential changes in conditions over time. As a result, design dead loads frequently far exceed actual loads.
Calculating dead loads
By calculating the volume of each member and multiplying it by the unit weight of the materials from which it is composed, an accurate dead load can be determined for each component.
The different components can be added together to determine the total dead load.
Table 1: Typical unit weights for construction materials
In most instances, engineers will translate loads into a UDL (uniformly distributed load) measured and record it in kPa or psf and an equivalent allowable concentrated loads (point loads). Dead load UDLs will be combined with superimposed loads and live loads to calculate ultimate and service loading on the structure.
Superimposed dead loads (SDLs)
Superimposed Dead Loads (SDLs) are permanent loads added to a structure, but not part of the structure itself. Examples include movable partitions, planter boxes, fixed office equipment, and base building equipment such as mechanical and electrical systems. They may seem permanent but can be relocated during renovations.
Table 2: Differences between SDL and DL
Similarly to pure dead loads, SDLs will typically be presented as a UDL used generally in a design. An important departure between pure dead load and SDLs is that the SDLs are sometimes treated as allowable loading for architects and building occupants.
That is to say, the engineers have allowed for 5kPa of SDL in a certain area. Other stakeholders then use this information to design layout etc.
DL and SDL are typically combined and referred to as a total dead load, also sometimes referred to as G, in structural engineering codes and calculations.
Live loads, also known as applied or imposed loads, can change over short timeframes. Live loads on a structure are not always present and can vary in location and magnitude. These loads include the weight of people, furniture, and other objects on the structure. A structure must be designed to safely support the weight of the maximum possible live loads it may be subject to. The weight of the audience in an auditorium, the books in a library, traffic loads, and so on are all examples of typical live loads.
Given the dynamic nature of live loads, it’s rare that an engineer will calculate them from scratch, as they do with dead loads. Rather, rates and allowable loading requirements are determined using design codes.
Live load values of common building use
Typical characteristic live loads for different building purposes are determined according to tables given across various standards:
- EN1991-1-1 TABLE 6.1 + Table 6.2
- AS/NZS 1170.1:2002 - Table 3.1
- ASCE/SEI 7-10 Table 4-1
Environmental loads, such as seismic movement, wind, waves, rain, and snow, can impact structures in a short time frame similar to live loads. These loads, however, vary in their application and impact, so they have specific calculation protocols and loading rules. They are considered separate from live or dead loads as they may act horizontally and dynamically.
Regional differences greatly affect environmental loads. Climate, topography, and seismic activity vary from region to region, causing loading requirements to differ. For instance, Christchurch, New Zealand has stricter seismic loading rules than London, England.
A building's snow load is the downward force caused by the weight of accumulated snow and ice on its roof. If the snow load is greater than the building was designed to support, the roof or the entire structure may fail.
Table of clauses referring to Snow Loads and separated by code/jurisdiction
- AS 1170.3-2002
- ASCE 7-16
- EU 1991-1-3
- NBCC 2015-Section 4.1.6
The wind is a mass of air that moves mostly horizontally from a high-pressure region to a low-pressure region. Wind load is the intensity of pressure, i.e. the load, in pounds per square foot, placed on the exterior of a structure by the wind. High winds can cause a lot of damage because they pressure a building's surface.
- The angle at which the wind strikes the structure
- The shape of the structure (height, width, etc.)
- The size of the structure
- Placement of items like antennas on tops of buildings
- The design and construction of buildings that are safer and more wind-resistant require wind load calculations.
Table of clauses referring to Wind Loads and separated by code/jurisdiction
- AS/NSZ 1170.2
- ASCE 7-16 Section 26
- EN 1991-1-4 Section 5
Additionally, for certain structures, a dedicated wind analysis will be undertaken to provide more accurate wind-loading data for structural designers. This may be required due to a structure's importance or risk exposure levels (i.e. a hospital or stadium). And typically involves the use of dedicated specialist CFD (computational fluid dynamics) software or a physical model of the structure being created and run through a wind tunnel experiment.
The application of earthquake-induced agitation to a structure is the focus of seismic engineering. Damages most often occur where the structure touches the ground or another building.
The calculation of a seismic load is included in the international building code. Seismic load tells how much seismic energy (energy waves that travel through the earth) a building would need to withstand in a given area.
The following factors are considered when calculating seismic loads:
- the earthquake parameters at the construction site;
- the kind of materials used to build the structure;
- the quality of a structure's construction.
General steps to determine seismic loading:
The seismic load can be calculated by multiplying the Total Seismic Weight by the seismic response coefficient Cs in a horizontal direction.
- Define building parameters (Geometry, importance level, occupancy category, etc.). The exact metrics will depend on the code you’re using.
- Define site parameters (Seismicity zone, site adjustment factors, importance factor, etc.)
- Define structural design parameters (based on the structural system selected, ductility, diaphragm flexibility, redundancy factors, etc.)
- Based on the above, compute seismic mass and resultant loads. Ensure to include torsional loads as applicable in your structure.
Table of clauses referring to Seismic Loads and separated by code/jurisdiction
- AS/NSZ 1170.4
- ASCE 7 -16 | Table 12.2 - 1
- EN 1998-1
- NBCC-2005, 2010, 2015
To guarantee the structure's safety under a variety of maximum expected loading scenarios, building codes typically specify a variety of load combinations along with load factors (weightings) for each type of load.
Table of Clauses on where to find load combinations for each Standard.
- ASCE 7-10 Section 2.3.2, 2.4
- ACI 318-08 Section 9-2
- EN 1990:2002, Section 6 - Table A1.2 - A1.4
- AS/NZS 1170.0:2002 Clause 4.2.2
AS1170 1170.0 - Clause 4.2.2 for example shows what determines the loading combination used in the analysis for strength (as opposed to serviceability). Clause 4.2.2 lists basic loading combinations encountered in structural design. They involve commonly considered loads such as dead loads, live loads, and wind action. Some loading combinations require a certain load to be multiplied by a combination factor to determine the total factored load. These combined factors are listed in Table 4.1.
Multiple loading combinations are considered for the design load of a structural member. All combinations of relevant loads experienced by the structural member are calculated and the governing design load is the highest calculated load combination.
Engineers need to understand building codes and standards to ensure that the structures they design and build are safe, functional, and meet the needs of the people who will use them. Building codes and standards provide a set of minimum requirements that must be met to protect the public's health, safety, and welfare. They also help ensure that structures are built to last, are energy efficient, and can withstand natural disasters and other hazards.
Engineers who understand building codes and standards can design structures that meet these requirements and anticipate and address potential issues before they arise. This can help to prevent costly mistakes and delays during the construction process and can help ensure that the finished structure is fit for its intended use.
Live loads are always a variable when calculating structural loads. They can vary gradually over the years with the moving of furniture inside a building during renovation or change rapidly within seconds, as with vehicles traveling across a bridge, for example.
As with determining design loads, engineers consider the worst-case scenario on a structural member at any given loading scenario. For live loads, this requires engineers to consider pattern loading, wherein live loads aren’t considered to be evenly distributed.
This is required as varying loading arrangements may increase design loads on structural elements. Some examples of common pattern loads are shown below.
Various pattern loading arrangements
The diagrams above show different loading patterns along a continuous beam with five sections. In each scenario, not every member will be under the same loading at any given time. For a continuous beam, this may increase hogging moments for example, which need to be designed for.
Varying building codes will require different sets of pattern loads and depth of analysis. So engineers should pay attention to the dynamics of expected live loading on structures they design.
Fire rating and impact on loading calculations
The load combinations explained above simulate the expected loadings of a structure under regular circumstances. A bridge is designed to support people and vehicles crossing it, and a dam is built with the expectation of holding back large volumes of water. However, there are extreme cases that need to be accounted for, such as in the events of a fire.
Let’s take a closer look at Australia’s regulations on building fire standards to understand this more.
The Building Code of Australia (BCA) categorizes buildings into three types concerning fire safety, A, B, and C, in order of risk from fires from highest to lowest. Type A buildings are the buildings most at risk of substantial consequences, such as high-rise buildings. Type C buildings are typically one to two-story houses which face lower risks.
Structures are required to remain standing for a designated period throughout a fire. They are also expected to maintain compartmentation such that fires are contained locally within the building to a small group of rooms or floors.
AS 1530.4:2014 Clause 2.12.2 outlines that structural adequacy of elements exposed to fire is recorded for failure in these aspects:
- Excessive deflection of structural elements under heating
- Detachment of structural elements
- Structural collapse
- The failure criteria are provided in Clause 2.13.1.
Based on these failure criteria, the Fire Resistance Level (FRL) is determined by the amount of time in minutes that a structural member can last before it fails the criteria.
The FRL is formatted as ‘adequacy/integrity/insulation’. If a structural element has an FRL of 30/60/90, then it means the element is within the adequacy criteria for 30 minutes before failing, integrity criteria for 60 minutes, and insulation criteria for 90 minutes. If a criterion is inapplicable, the value will take the form of a dash (i.e. –/60/90, 30/–/90, or –/60/90).
While it is important for individual structural elements to withstand extreme loadings applied to them, it is equally important to look at structures as a whole. In construction, robustness describes the ability of an overall structure to withstand events such as extreme loading, fire, explosions, wind, and other loading effects without losing its core functionality.
The relevant building codes for structural robustness are:
- AS1170.0 Section 6
- EN 1991-1-7
- ASCE 7-02
Of these codes, only the Australian Standard provides specific requirements for structural robustness in terms of the load resistance of a structure. The Eurocodes and US codes provide general provisions and design requirements for structures that experience such events without explicit mention.
AS1170.0 Section 6 describes the loading requirements of a structure as a whole to withstand events. The main provision comes from Clause 6.2.2 which requires the lateral resistance of the structure and its connections to be of a specific percentage of the design loads of members at each individual floor. The purpose of these lateral load resistances is to facilitate load paths across the structure to its foundations.
Additionally, for hyper-critical structures, engineers should also understand alternate load paths to avoid systemic collapse. That is to say, in the case of one element failing, it should not cause an entire structure to collapse. Such failures can be caused due to defects in fabrication and manufacturing or construction, with effects as fatal as the pancaking of Christchurch buildings in the 2010 Canterbury Earthquake. Ideally, alternate load paths can meet the required lateral and connectional resistances outlined in AS1170.0 Clause 6.2.2.