Loads and load paths

A structure exists to carry loads to the ground. Every kilogram of concrete, every person on a floor, every centimetre of snow on a roof and every gust of wind against a wall creates a force that must travel through slabs, beams, columns, walls and foundations until it reaches soil or rock that can resist it. That route is the load path. If any link in it is missing, weak or not yet built, the load finds another way down, and that way may be a collapse.

This lesson explains the kinds of loads the code requires engineers to consider, how loads are combined, how they travel through a building and how to estimate the load on a member using tributary areas. These are the tools a construction manager needs to reason about construction loads, stored materials and temporary works.

Types of loads

Codes divide loads into categories because each behaves differently and carries different uncertainty.

• Dead load (D): the weight of the structure itself and everything permanently attached to it, such as floors, walls, finishes, ceilings, mechanical equipment and cladding. Dead loads are predictable because they can be calculated from dimensions and material densities.
• Live load (L): loads from occupancy and use, such as people, furniture, stored goods and movable equipment. The code specifies minimum uniformly distributed values and concentrated loads for each type of occupancy.
• Snow and rain load (S): loads from accumulated snow and the rain that soaks it, including drifts against parapets and higher roofs.
• Wind load (W): pressures and suctions from wind on walls and roofs, which push the building sideways and try to lift roofs off.
• Earthquake load (E): inertial forces produced when the ground shakes and the building's mass resists moving with it.
• Other loads: soil and water pressure on walls below grade, temperature movements, shrinkage and creep, impact from vehicles or cranes, and ice on towers.

During construction there are also construction loads: workers, equipment, stored materials, wet concrete, formwork and the impact of placing concrete or landing loads from a crane. These can be larger and more concentrated than the design live load and act on members that are not yet at full strength.

Densities and dead loads

Dead loads are calculated from the volume of each element and the unit weight of its material. A few values are worth memorizing because they come up constantly on site.

Live loads in the code

The code gives minimum live loads for each occupancy. The values are not predictions of what is usually on a floor; they are deliberately conservative to cover crowding, storage and future changes of use. The engineer may use higher values if the owner expects heavy use.

Live load reduction is allowed for members that support large areas, because it is unlikely that every square metre of a large floor area will be fully loaded at once. That is why a column supporting many floors is not designed for the full live load on every floor.

Snow, wind and earthquake: the environmental loads

In Canada, NBC Part 4 gives climatic data for hundreds of locations in Appendix C, including the ground snow load Ss and the associated rain load Sr in kPa, and hourly wind pressures. The roof snow load is calculated as S = Is[Ss(Cb Cw Cs Ca) + Sr], where Is is an importance factor, Cb a basic roof factor (often 0.8), Cw a wind exposure factor, Cs a slope factor and Ca an accumulation factor for drifts. Seismic design uses spectral accelerations from the Geological Survey of Canada's national seismic hazard model, which is highest in coastal British Columbia and parts of Quebec.

Wind pressure depends on the square of wind speed. The basic relationship is the dynamic pressure q = ½ρV², where ρ is the density of air (about 1.2 kg/m³) and V is the wind speed. Codes add factors for height, exposure, gusts and building shape, but the square law explains why a storm that doubles the wind speed quadruples the pressure on a wall.

For construction managers the practical point is that wind on unbraced walls, formwork panels, hoarding, signs and partly built frames rises quickly in a storm. Temporary works must be designed for wind, and work plans should set wind limits for crane lifts, form handling and masonry.

Load combinations

Loads do not all reach their maximum at once. A building is unlikely to be fully occupied, under a design snow load and hit by a design windstorm at the same moment. Codes use load combinations with load factors that reflect the probability of each combination and the uncertainty in each load. Engineers check every relevant combination and design for the worst.

NBC Part 4 uses limit states design. Typical ultimate limit state combinations include 1.4D; 1.25D + 1.5L + 1.0S (or 0.5S); 1.25D + 1.5S + 1.0L (or 0.5L); and 1.25D + 1.4W + 0.5L or 0.5S, with 0.9D replacing 1.25D where dead load counteracts the other loads, and 1.0D + 1.0E + 0.5L + 0.25S for earthquake.

You do not need to run load combinations as a construction manager, but you should recognize that the numbers on drawings are often specified loads (unfactored) and that the engineer's design includes margins that are not available for unplanned construction loads.

Load paths: gravity and lateral

There are two main load paths in every building. The gravity load path carries vertical loads down: from the floor deck to the joists or beams, from beams to girders, from girders to columns or walls, and from columns to foundations. The lateral load path carries wind and earthquake forces sideways: from the walls into the floor and roof diaphragms, from the diaphragms into the lateral force resisting system (shear walls, braced frames or moment frames) and down to the foundations.

During construction, parts of these load paths are often incomplete. A steel frame may stand before its floor deck is welded, so the diaphragm that will tie it together does not yet exist. A wood-frame wall may be raised before the floor above that braces it is built. Precast panels may be set before their connections are welded. The structure in these states relies on temporary bracing to complete the load path, which is why erection sequences and bracing plans matter so much.

Tributary areas

The simplest way to estimate the load on a beam or column is the tributary area method. Each member carries the load from the area of floor that is closer to it than to any neighbouring support. For a regular grid of columns, an interior column carries the area extending halfway to each adjacent column in both directions. A beam carries a strip of floor extending halfway to the beams on either side.

Construction loads and why they matter

Construction loads are often more severe than service loads. When a 200 mm slab is poured, the wet concrete, the formwork, workers, vibrators, a concrete pump hose and heaped concrete all act on formwork and on the floors below through shores. When drywall is stocked, pallets can create local loads several times the design live load. A crane on a slab, or a concrete truck on a podium, imposes wheel or outrigger loads that the design may never have considered.

In Canada, CSA S269.1 sets design loads for falsework, including minimum construction live loads on formwork in addition to the weight of concrete and forms, and requires consideration of impact and lateral loads. Many engineers also refer to ASCE 37 for construction loads.

The safe approach is simple: treat any load you are about to put on a structure during construction as a question for an engineer unless it is clearly within an approved plan, such as a loading plan on the drawings or a formwork engineer's laydown limits.